Chapter 16 Aldehydes and Ketones Part 2- Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 16 Aldehydes and Ketones Part 2- Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-16-Aldehydes-and-Ketones-Part-2.pdf

 

 

 

Different imine formation with

NH 2Z is given below

Å                                                              R

 

R’ – NH2 /H/D

C = N R’

R

 

Imine from p-amine (Schiff base)

 

R

C = N OH

R

Oxime

 

R

C = N NH2

R

 

O

 

R C R

Hydrazone

 

R

C = N NNHC H

 

6   5

R

Phenylhydrozone (Brown coloured)

 

 

R

C = NNH

R

NO2

NO2

 

2,4-Dinitrophenylhydrazone (Red coloured)

 

 

 

 

 

 

Semicarbazide

O

R

C = N NH – C – NH2

R

Semicarbazone

 

Ketoxime when treated with acid at 0°C it undergoes rearrangement known as Beckmann rearrangement. Thus acid catalysed conversion of ketoximes to N-substituted amides is called Beckmann rearrangement. Acid catalyst used are proton acids(H 2 SO4 , HCl, HPO4 ) and Lewis acids (PCl5 , SOCl2 , PhSO2 Cl, RCOCl, SO3 , BF3 etc.)

 

 

C6 H5

  • C CH3

||

NOH

¾¾(i) P¾C¾l5  ® CH3

  • H2O

O

||

  • C NH C6 H5

 

O

 

CH3

  • C C6 H5

||

¾¾(i) P¾C¾l5  ® C6 H5

  • H2O

||

  • C NH CH3

 

NOH

In short product of the rearrangement can be obtained as follows:

 

R        R

O

Tautomerisation               ||

 

C

||

NOH

¾¾ ¾® R‘- CO H ¾¾¾¾¾¾® R‘ – C NH R

||

RN

 

(3)  Oxidation of carbonyl compounds

 

 

 

  • Oxidation by mild oxidising agents : Mild oxidising agents oxidise only aldehydes into carboxylic They do not oxidises ketones. Main oxidising agents are:

 

  • Fehling solution : It is a mixture of two Fehling solution: Fehling solution 1 : It contains solution and NaOH.

Fehling solution No.2 : It contains sodium potassium tartrate. (Roschelle salt).

  • Benedict’s solution : This solution contains CuSO4 , Na2CO3 and sodium or potassium

CuSO4

 

  • Reacting species of both solutions is Cu ++

oxidation no. of Cu varies from 2 to 1.

 

  • These two oxidising agents oxidise only aliphatic aldehydes and have no effect on any other functional groups:

 

Å

CH3  – CHO + Cu++ ¾¾Red¾¾ox ® CH3COOH + Cu

reaction

(as Cu2O ) (red ppt.)

 

CH 2

= CHCHO ¾¾Cu¾+ +  ® CH

2

D

= CH COOH + Cu2O

 

CH 2

OH – (CHOH)4

  • CHO ¾¾Cu¾++  ® CH

D

2OH – (CHOH)4

  • COOH + Cu2O

 

Benedict’s solution and Fehling solutions are used as a reagent for the test of sugar (glucose) in blood sample.

 

  • Tollens reagent : Tollens reagent is ammonical silver nitrate Its reacting species is
  • It oxidises aliphatic as well as aromatic

Ag Å .

 

RCHO + Ag Å ¾¾Red¾¾ox ® RCOOH + Ag

reaction

(as silver mirror)

 

CHCHO + Ag Å ¾¾® CHCOOH + Ag

  • This reagent has no effect on carbon-carbon multiple

CH 2 = CHCHO + Ag Å ¾¾® CH 2 = CHCOOH + Ag

C6 H5 – CH = CH CHO + Ag Å ¾¾® C6 H5 – CH = CH COOH + Ag

In this reaction the oxidation no. of Ag varies from +1 to 0.

Note : ® Glucose, fructose give positive test with Tollen’s reagents and Fehling solution.

 

C5 H11O5CHO + Cu2O   (or)

AgO ¾¾® CH11OCOOH

Gluconic acid

 

Fructose contain

C = O

(keto) group yet give positive test with Fehling solution due to presence

 

of hydroxyl group. Tollens reagent also gives positive test with terminal alkynes and HCOOH.

Reaction with mercuric chloride solution : R C H+ HgCl2 + H2O ¾¾® R C OH+ HCl + Hg2Cl2 (¯)

 

 

R C H+ Hg 2Cl2 + H 2O ¾¾®

||                                                                ||

O                                                               O

R C OH+ HCl + Hg(¯)

(White)

 

||

O

 

Schiff’s reagent : Megenta dye

 

¾¾SO¾2  ®

||

O

colourless soln

(Black)

 

 

¾¾CH¾3CH¾O ®  pink colour restored.

 

 

 

  • Oxidation by    strong   oxidising    agents   :      Main    strong    oxidising    agents   are

 

KMnO4 / OH / D, KMnO4 / H Å / D, KCr2 O7 / H Å / D

well as ketones.

and conc

HNO3 / D . These agents oxidise aldehydes as

 

  • Oxidation of aldehydes : Aldehydes are oxidised into corresponding

RCHO ¾¾[¾O] ® RCOOH ;     C6 H5 CHO ¾¾KM¾nO/ O¾H /¾D ® C6 H5 COOH

C=n                                           C=n

  • Oxidation of ketones : Ketones undergo oxidation only in drastic conditions. During the oxidation of ketones there is breaking of carbon-carbon bond between acarbon and carbonyl carbon. In this process both carbons convert into carboxylic This leads to the formation of two moles of monocarboxylic acids.

Case I : Oxidation of symmetrical ketones

O

 

CH3

  • CH 2
  • CH 2

||                   

  • C

¯

CH 2

¯a

  • CH 2
  • CH3

¾¾[¾O] ®

 

C = 7

COOH

COOH

CH3 – CH2 – CH2 – COOH+ CH3 – CH2 – COOH

 

C=4                                               C=3

Total number of C’S=4+3=7

Thus number of carbons in any product is less than the number of carbons in ketone.

Case II : Oxidation of unsymmetrical ketones : In case of unsymmetrical ketones acarbon whose bond breaks always belongs to the alkyl group which has more number of carbons. This rule is known as Poff’s rule.

O

||

CH3  – CH 2  – CH 2  – CCH 2  – CH3   ¾¾[¾O] ® CH3  – CH 2  – COOH + CH3  – CH 2  – COOH

 

¯

COOH

¯

COOH

 

Case III : Oxidation of cyclic ketones : Formation of dibasic acid takes place from cyclic ketones. In this case number of carbons in ketone and dibasic carboxylic acid is always same.

O

 

a

¾¾[¾O] ® COOH – (CH2 )4  – COOH

Note : ® If both a-carbons are not identical then bond breaking takes place between carbonyl carbon and

a-carbon which has maximum number of hydrogens.

 

2H  O

a CH3

CH3

 

2
3

1H      ¾¾[¾O] ® COOH – (CH   )

|

  • CH
  • COOH

 

  • Miscellaneous oxidation
  • Haloform Reaction : In this reaction amethyl carbonyl compounds undergo oxidation with

O

 

 

X 2 / O H .

 

||                                

Å

RCCH3  ¾¾(i) X¾2 / O¾H ® RCOOH + CHX3

(ii) H

 

 

 

O

||                                 

 

3

CCH    ¾¾(i) X¾2 / O¾H ®

Å

(ii) H

COOH + CHX3

 

 

C6 H5

O

||

  • CCH3
6

¾¾(i) I2¾/ N¾a2C¾O¾3  ® C

Å

(ii) H

H5COOH + CHI 3

 

  • Oxidation  at   a-CH2 or   CH3 by   SeO2   : SeO2 oxidises

aCH3 – group into aldehydic group.

aCH2 – group into keto group and

 

In this oxidation reactivity of CH2

is more than the CH3

O

group and Oxidation is regio selective in nature.

O

 

CH3

  • CHO ¾¾SeO¾2 ® CHOCHO ;

CH3

||

  • C CH3

¾¾SeO¾2  ® CH3

||

  • C CHO

 

Glyoxal

 

O                                                   O  O

Methylglyoxal

 

CH   CH

||

  • CH

¾¾SeO¾2  ® CH

||     ||

  • – – CH

 

3             2     C             3

3   C     C             3

 

O                                                  O

SeO2

Dimethylglyoxal

O

 

  • Oxidation by organic peracids : Organic peracids oxidise aldehydes into carboxylic acids and ketones into This oxidation is known as Baeyer – Villiger oxidation.

 

O                                                 O

R                        C6 H5COOOH
C
–  C O H

||                                                 ||

–    – H ¾¾¾¾¾® R

O                                                 O

R C R ¾¾¾¾ ¾® R C O H

||                                                 ||

;                   C6 H5COOOH

 

In case of aldehyde there is insertion of atomic oxygen (obtained from peracid) between carbonyl carbon and hydrogen of carbonyl carbon.

In case of ketone, insertion of oxygen takes place between carbonyl carbon and acarbon. Thus the product is ester. This is one of the most important reaction for the conversion of ketones into esters.

Symmetrical ketones :

O                                             O

 

 

CF3COOOH

 

O

O

e-Lactone

O

 

CH3

||

  • C CH3

¾¾C6 H¾5CO¾OO¾H ® CH3

||

  • C O CH3

 

Unsymmetrical ketones : In case of unsymmetrical ketones preference of insertion in decreasing order is as

H > 3°R > 2°R > Ph > 1°R > CH3

 

O

||

CH  –    – C H

¾¾CF ¾COO¾O¾H ® CH

O

||

  • O C H

 

3     C        6     5           3                                        3     C                6     5

CH3    O                                                               CH3          O

 

|

CH3 – C

|

||

–  C CH3

¾¾CFCOO¾O¾H ®

CH3

|

C

|

||

O C CH3

 

CH3

CH3

 

 

Note : ® Vic dicarbonyl compound also undergo oxidation & product is anhydride.

O  O

R                               C6 H5COOOH

||     ||

–    –    – R ¾¾¾¾¾® R CO CR

C C

||             ||

O            O

  • Popoff’s rule : Oxidation of unsymmetrical ketones largely take place in such a way that the smaller alkyl group remains attached to the CO group during the formation of two molecules of acids. This is known as Popoff’s rule

Example :  CH3  – COCH2  – CH3  ¾¾[¾O] ® CH3  – COOH + HOOCCH3

H

|

  • Baeyer- villiger oxidation : HC H + OOCH ¾¾® HCOH

||                                ||                                 ||

O                               O                               O

H

|

CH3 – CH + OO C H ¾¾® CH3 – C OH

||                              ||                                      ||

O                              O                                     O

Note : ® Reaction will be held if the oxidation agent is performic acid.

(4)  Reuction of carbonyl compounds

O

||

  • Reduction of C – group into –CH2 group : Following three reagents reduce carbonyl group into

  • CH 2 – groups: (a) HI / P / D (b) Zn / Hg / Conc. HCl and (c) NH2 – NH2 / OH .

 

HI/P/D

O

R CH2R’

 

||

R C R’

R CH2

 

D                                     R CH2

  • R’

 

  • R’

(Clemmensen reduction)

 

  • Reduction of carbonyl compounds into hydroxy compounds : Carbonyl group converts into

 

  • CHOH – group by

LiAlH4 , NaBH4 , Na / C2 H5OH

and aluminium isopropoxide.

C
|

O                                                        OH

 

RCHO

¾¾¾(i) L¾iAlH¾4

  • NaBH 4

¾® R CH2OH ;

R – || – R‘ ¾¾¾(i) L¾iAlH¾4   ¾® RCH

  • NaBH 4
  • R

 

  • Aluminium isopropoxide (iii) Aluminium isopropoxide

 

NaBH4

group.

is regioselective reducing agent because it reduced only. CHO in the presence of other reducible

 

Example : CH3  – CH  = CHCHO ¾¾NaB¾H¾4  ® CH3  – CH  = CHCH2OH

 

Hydride ion of

NaBH4

attack on carbonyl carbon during reduction.

NaBD4

OH

|

CHCCH

CH

 

 

Example : CH3

OD

|

  • CCH2

|

D

  • CH3

 

¬¾Na¾B¾D4 ¾ 2-Butanone

D2O

H2O

 

 

 

 

NaBH

3                     2            3

|

D

OD

|

 

4                 CH  C CH  CH

 

D2O

3                     2            3

|

H

 

 

 

  • Reductive amination : In this reduction – CO – group converts into – CH NH 2 group as follows:

 

R                                                   R

C = O + NH 3 ¾¾®

R                                                   R

R

C = NH ¾¾H2 ¾/¾Ni ®

R

Im ine

CH NH 2

 

 

CH3

  • CH 2

O

||

  • C CH3

¾¾(i)¾NH¾3¾® CH3

  • CH 2

NH2

|

  • CH
  • CH3

 

  • H2 / Ni

Primary -amine

 

  • Reduction of ketones by Mg or Mg/Hg : In this case ketones undergo reduction via coupling reaction and product is vic cis

 

O            O

R – ||              || – R  ¾¾(i) M¾g / ¾H¾g  ® R

OH OH

|       |

–    – R

 

C            C

||             ||

R             R

C     C

(ii) HOH                              |       |

R      R

 

Vic cis diol

 

When this reaction is carried out in the presence of

Mg / Hg / TiCl4 , the product is vic trans diol.

 

 

2
  • Hg Mg TiCl4

O          (ii) HOH

 

Vic trans diol

  • Reduction of benzaldehyde by Na/C2H5OH : Benzaldehyde undergoes reduction via coupling reaction and product is vic

 

O

||

C H

O

||

  • C H

¾¾(i) N¾a/C¾2 HO¾H  ® C  H

OH   OH

|           |

–        –

  • C H

(Bouveault-blanc reaction)

 

6   5      C          C        6   5

6  5   CH      CH        6   5

 

|            |                             (ii) HOH

H         H

vic diol

 

Note : ® Aldehydes are reduced to 1° alcohols whereas ketones to 2° alcohols. If carbon – carbon double bond is also present in the carbonyl compound, it is also reduced alongwith. However, the use of the reagent 9-BBN (9–borabicyclo (3, 3, 1) nonane) prevents this and thus only the carbonyl group is reduced

 

Example :

CH  = CHCHO ¾¾9-B¾B¾N ® ¾¾HO¾CH¾2 CH¾2 N¾H¾2  ®

Cinnamaldehyde

CH = CHCH2OH

Cinnamyl alcohol

 

  • If reducing agent is NaH, reaction is called Darzen’s reaction, we can also use LiAlH4 in this

 

  • If reducing agent is aluminium iso propoxide

(CH3C HO-)3 Al . Product will be alcohol. This

|

CH3

 

reaction is called Meerwein – pondorff verley reduction (MPV reduction).

  • The percentage yield of alkanes can be increased by using diethylene glycol in Wolf Kishner Then reaction is called Huang – Millan conversion.
  • Hydrazones when treated with base like alkoxide give hydrocarbon (Wolf – Kishner reduction).

 

O

C

R – || – R‘ ¾¾NH¾2 NH¾2  ®

N.NH2

||

R C R

Hydrazone

¾¾RO¾N¾a ® RCH    R

2

D

 

 

 

  • Schiff’s base on reduction gives secondary

RCH  = O ¾¾RN¾H¾2  ® RCH  = NR‘ ¾¾H2 ¾/¾Ni ® RCH 2 NHR

Schiff’s base

(5)  Reactions due to a-hydrogen

  • Acidity of a-hydrogens :
  • ahydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing
  • CO –                              aHydrogen is acidic due to strong –I group; – CO –.

 

H     O

|      ||

C C

|

aCarbon

  • Thus carbonyl compounds having ahydrogen convert into carbanions in the presence of This carbanion is stabilised by delocalisation of negative charge.

 

 

CH3

O

||

CR

 

Base

 

CH 2

O

||

C R ¬¾¾® CH2

O

|

= C R

 

Carbanion (less stable)

Enolate ion (more stable)

 

  • The acidity of ahydrogen is more than pKa value of aldehydes and ketones are generally 19 – 20 where as pKa value of ethyne is 25.
  • Compounds having active methylene or methyne group are even more acidic than simple aldehydes and

O                                                                     O                   O

||                                                                      ||                    ||

 

C6 H5 – CH2 – C CH3         pKa = 15.9   ;   C6 H5 – C CH2 – C CH3

 

pKa = 8.5

 

 

  • Halogenation : Carbonyl compounds having ahydrogens undergo halogenation This reaction is catalysed by acid as well as base.
  • Acid catalysed halogenation : This gives only monohalo

 

O

||

CH  –     – CH

¾¾Br  ¾/ CH¾CO¾O¾H  ® CH

O

||

  • CH Br

 

3     C             3          2          3

3      C            2

 

  • Base catalysed halogenation : In the presence of base all ahydrogens of the same carbon is replaced by

 

O

||                                           

O  X

||     |

 

CH3  – CH 2 – CCH 2  – CH3  ¾¾X2 ¾/ O¾H ®  CH3  – CH2 – CCCH3

 

X2 / OH

O

||

CH3CH C CHCH 3

(Excess)                                              |

X

 

|                     |

X              X

 

 

 

Carbonyl compounds having three ahydrogens give haloform reaction.

O                                                    O                         

 

C

||                                               ||

3

RCCH3  ¾¾X2 /¾O¾H ® R –     – CX    ¾¾O¾H¾®

RCOO

  • CHX3

 

 

  • Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid (DÅ ) as well as base

(OD) . In both the cases all the hydrogens on only one acarbon is replaced by D.

 

 

O                                                            O                                    O

||                              D O / OD                        ||                                     ||

O

Å                       ||

 

RCCH2  – R ¾¾2  ¾¾® RCCD2  – R;   R CCH2  – R ¾¾D2O¾/¾D ® RCCD2  – R

  • Racemisation : Ketones whose acarbon is chiral undergo Racemisation in the presence of acid as well as

 

O     CH3

O   CH3

CH3   O

 

5

||      |

C H  –    –   – C H

HÅ or

||      |

C H   –    –   – C H

|

  • C H

||

  • C H

 

6     5     C   C        2

|

H

¾¾¾®            C    C         2     5

6
5

|

OH                                                     H

2            C       C        6     5

5

|

H

 

Racemic mixture

 

  • Alkylation : Carbonyl compounds having ahydrogens undergo alkylation reaction with RX in the

 

presence of base. This reaction is SN2 reaction. The best result is obtained with elimination in the presence of strong base.

CH3X . Other halides undergo

 

O                  CH3

O               CH3

O   CH3

 

||

CH  –    – CH

¾¾Na¾H ® CH

||      

–    –

¾¾CH¾3 I  ® CH

||      |

  • – – CH

 

3   C

CH3

(Small base)

3   C   C

CH3

3   C   C            3

|

CH3

 

LDA

(Bulky base)

 

 

 

CH3

 

 

 

 

CH I

 

O                   CH

3

||

(Main product)

 

CH2CCH

||

O

CH3

¾¾¾3   ® CH3CH2CCH

 

(Main product)

CH3

 

  • Wittig reaction : Aldehyde and ketones undergo the wittig reaction to form

PhP = CHR1 + CHR2 ¾¾® PhP ÅCHR1 ¾¾® PhPCHR1 ¾¾® PhP + CHR1

 

||                                            |

|       |                                 ||     ||

 

O                                 O –CHR2

OCHR2

O  CHR2

 

  • Condensation reaction of carbonyl compounds : Nucleophilic addition reaction of compounds having carbonyl group with those compounds which have at least one acidic hydrogen at a-carbon is known as condensation In this addition reaction :

Substrate is always an organic compound having a carbonyl group, e.g.

 

O

||

H C H,

O

||

C6 H5 – C H,

O

||

R C H,

O

||

RCR

etc.

 

Addition always takes place on the carbonyl group.

 

 

 

Reagents of the condensation reaction are also organic compounds having at least one hydrogen on acarbon and acarbon should have –I group, e.g.

 

a

CH3 – NO2 ,

a

CH3CH CHO,

|

CH3

a

CH3CH2CN

 

Note : ® If substrate and reagent both are carbonyl compounds then one should have at least one a– hydrogen and other may or may not have a-hydrogen.

Condensation reaction always takes place in the presence of acid or base as catalyst. Best result is obtained with base at lower temp.

 

O

||

R C R + CH3

Å

H     or

Z ¾¾¾®

OH

OH

|

R C CH2Z

|

R

 

Condensation is carried out at lower temperature (£ 20°C) because product of the reaction is alcohol which has strong –I group at bcarbon.

OH

|

R CCH2Z

a  |       β

R

Such type of alcohols are highly reactive for dehydration. They undergo dehydration in the presence of acid as well as base even at 25°C. They also undergo elimination even on strong heating.

OH

 

|

R CCH2Z

              R

¾¾HO¾/ D ®

C = CH Z

 

a  |       β

R

Dehydration R

 

  • Aldol condensation
  • This reaction takes place between two molecules of carbonyl compounds; one molecule should have at least two ahydrogen In this reaction best result is obtained when

Both molecule are the same or

One should have no ahydrogen atom and other should have at least two ahydrogens.

  • These reactions are practical when base is NaOH and reaction temperature is high (³ 100°) .
  • The reaction is two step First step is aldol formation and second step is dehydration of aldol.

é            OH                              ù

     ê            |                                   ú

CH3  – CHO + CH3  – CHO ¾¾NaO¾H /¾O¾H ®êCH3  – CHCH2  – CHOú  ¾¾Deh¾ydr¾ati¾on ® CH3CH  = CHCHO

 

D          ê                                       ú

ë                                       û

a, b -unsaturated aldehyde

 

Due to hyper conjugation in crotonaldehyde further condensed give conjugated alkene carbonyl compound.

CH3CH = CH CHO + CH3CH = CH CHO

NaOH

OH

|

CH3CH = CH CH CH2CH = CH CHO

D –H2O

CH3CH = CH CH = CH CH = CH CHO

 

CH3 – (CH = CH –)3CHO

Condensed compound

 

 

 

The net result can be written as follows]

 

CH3CHO + H2CH CHO

O       H2

OH/D

 

CH3CH = CH CHO

Crotonaldehyde

 

 

C6H5CHO + CH3

O

||

  • CHO

OH/D

H2O

 

 

C6H5CH = CH CHO

Cinnamaldehyde

O

||

 

C H CHO + H CH C CH

OH/D

C H CH = CH C CH

 

6    5                          2                            3

6    5                                              3

Benzalacetophenone

 

Note : ® If product is given then reactants can be known as follows :

b                 a

Suppose structure of product is C6 H5CH = CHCHO

Break carbon-carbon double bond between a and b carbons and attach two hydrogens on a-carbon

 

b

and an oxygen on b-carbon, i.e. C6 H5 – CH

O

a

CHCHO ¾¾® C6 H5 – CHO + CH3 – CHO .

H2

 

 

In aldol condensation, dehydration occurs readily because the double bond that forms is conjugated, both with the carbonyl group and with the benzene ring. The conjugation system is thereby extended.

Crossed aldol condensation : Aldol condensation between two different aldehydes or two different ketones or one aldehyde and another ketone provided al teast one of the components have a-hydrogen atom gives different possible product

 

 

 

 

(a) CHCHO+ CH3  – CH2  – CHO ¾¾dil N¾aO¾H ® CH3

OH

|

  • CH

CH3

|

  • CH

 

  • CHO + CH3

 

  • CH2

 

  • CHOH CH2

 

  • CHO

 

Ethanal                      Propanal

However crossed aldol condensation is important when only it the components has a-hydrogen atom.

CH2O + CH3 CHO ¾¾® CH2 – CH2  – CHO ¾¾D ® CH2  = CHCHO

 

|

OH

(3-hydroxy propanal)

H2O

(Acrolein)

 

Intra molecular aldol condensation : One molecule Intramolecular condensed give aldol compounds

OH

 

Example : O = CH – (CH2 )5  – CHO ¾¾NaO¾H ®

CHO

 

  • Claisen Schmidt reaction : Crossed aldol condensation between aromatic aldehyde and aliphatic ketone or mixed ketone is known as Claisen – Schmidt reaction. Claisen – Schmidt reactions are useful when bases such as sodium hydroxide are used because under there conditions ketones do not undergo self condensation. Some examples of this reaction are :

O                                                                                                     O

||                       OH                                                                            ||

C6 H5 CHO + CH3 – CCH3  ¾¾100¾°C ® C6 H5  – CH  = CHCCH3

4-Phenyl-3-buten-2-one

 

 

C H   CHO + CH

O

||

–    – C H

¾¾OH¾/¾D ® C  H

O

||

CH = CH –       – C H

 

6    5                             3     C        6    5                          6    5                               C        6    5

 

 

||

CHO                      O

1,3-Diphenyl – 2-propene-1- one

 

O

||

CH = CH C CH3

 

+   CH3C CH3        OH

 

Geranial                                                                                      Pesudoionone

TestAldehydesKetones
1.         With Schiff’s reagentGive pink colour.No colour.
2.         With Fehling’s solutionGive red precipitate.No precipitate is formed.
3.         With Tollen’s reagentBlack precipitate of silver mirror is formed.No black precipitate or silver mirror is formed.
4.         With saturated sodium bisulphite solution in waterCrystalline compound (colourless) is formed.Crystalline compound (colourless) is formed.
5.         With 2 : 4-dinitrophenyl hydrazineOrange-yellow or red well defined crystals with melting points characteristic of individual aldehydes.Orange-yellow or red well defined crystals with melting points characteristic of individual ketones.
6.         With sodium hydroxideGive brown resinous mass (formaldehyde does not give this test).No reaction.
7.         With sodium nitroprusside and few drops of sodium hydroxideA deep red colour (formaldehyde does not respond to this test).Red colour which changes to orange.

 

 

 

 

Formaldehyde : Formaldehyde is the first member of the aldehyde series. It is present in green leaves of

 

plants where its presence is supposed to be due to the reaction of chlorophyll.

CO2 + H 2O ¾¾® HCHO + O2

CO2

with water in presence of sunlight and

 

Traces of formaldehyde are formed when incomplete combustion of wood, sugar, coal, etc., occurs.

(1)  Preparation

 

  • By oxidation of methyl alcohol

2CH3 OH + O2  ¾¾Plat¾inise¾d a¾sbes¾t¾os ® 2HCHO + 2H 2 O 300-400°C

 

CH3 OH + [O] ¾¾K2C¾r2O¾7  ® HCHO + H 2 O

H2SO4

  • By dehydrogenation of methyl alcohol

 

CH 3 OH ¾¾Cu ¾or A¾g ® HCHO + H 2

300-400°C

 

  • By heating calcium formate :

Ca(HCOO)2  ¾¾He¾at ® CaCO3  +

Calcium formate

HCHO

Formaldehyde

O

 

 

 

 

  • By ozonolysis of ethylene :

CH 2

= CH 2

  • O3

¾¾®

H2C                  CH2

¾¾H¾2  ® 2HCHO + H2O

Pd

 

 

  • Manufacture :

CH4  + O2  ¾¾Mo-¾oxi¾de ®

HCHO

  • H2O

O Ozonide O

 

Methane

Catalyst

Formaldehyde

 

It is also prepared by passing water gas at low pressure through an electric discharge of low intensity.

CO + H 2  ¾¾Elec¾. dis¾cha¾r¾ge ® HCHO

(2)  Physical properties

  • It is a colourless, pungent smelling
  • It is extremely soluble in Its solubility in water may be due to hydrogen bonding between water molecules and its hydrate.
  • It can easily be condensed into The liquid formaldehyde boils at – 21°C.
  • It causes irritation to skin, eyes, nose and
  • Its solution acts as antiseptic and
  • Chemical properties : Formaldehyde is structurally different from other aldehydes as it contains no alkyl group in the Though it shows general properties of aldehydes, it differs in

certain respects. The abnormal properties of formaldehyde are given below

  • Reaction with ammonia : Like other aldehydes, formaldehyde does not form additon product but a crystalline compound, hexamethylene tetramine, with

6HCHO + 4 NH 3 ¾¾® (CH 2 )6 N 4 + 6H 2 O

 

Formaldehyde

Urotropine (Hexamethylene tetramine)

 

 

 

Urotropine

 

 

 

Hexamethylene tetramine has a cyclic structure. It is used as medicine in case of urinary troubles under the name of Urotropine or hexamine.

  • Reaction with sodium hydroxide (Cannizzaro’s reaction) : It does not form resin with sodium hydroxide like acetaldehyde but when treated with a concentrated solution of sodium hydroxide, two molecules of formaldehyde undergo mutual oxidation and reduction forming formic acid salt and methyl alcohol (Disproportionation).

 

2HCHO + NaOH ¾¾® HCOONa +

CH3 OH

 

Formaldehyde

Sod. Formate

Methyl alcohol

 

This transformation is known as Cannizzaro’s reaction.

Tischenko’s reaction : This is a modified form of cannizzaro’s reaction. All aldehydes undergo cannizzaro’s reaction in presence of aluminium ethoxide. The acid and alcohol formed react together to give the ester.

2CH3 CHO ¾¾(CH5O¾)3 ¾Al ®[CH3 COOH + C2 H5 OH] ¾¾® [CH3 COOC2 H5 ]

Ethyl acetate

 

CH3

CH3

CHCHO  ¾¾Al +¾Bu¾toxi¾de ®

CH3

CH3

CH CH 2OH + HOOC CH

 

¯

CH 3

CH3

CH3

 

 

CH3

 

CH3

CH CH 2OOC CH

CH3

 

  • Aldol condensation : Formaldehyde in presence of a weak base undergo repeated aldol condensation to give formose (a– acrose).

 

6HCHO  ¾¾Ca(¾OH¾)2  ®

Formaldehyde

C6 H12 O6

Formose (hexose)

 

  • Condensation with phenol : Formaldehyde condenses with phenol to give a synthetic plastic, bakelite. The condensation occurs in presence of dilute sodium hydroxide or ammonia at 80 – 90° Bakelite is used for preparing electrical insulators, electric switches, toys, etc.

 

 

 

OH

 

+

Phenol

 

O

||

H C H

Formaldehyde

 

 

 

 

 

Base

dil. K2CO3

OH

 

 

 

CH2

 

 

 

OH

CH2             CH2

 

OH OH

 

CH2             CH2

Bakelite

OH

 

 

 

CH2

 

 

 

OH

 

 

Bakelite is electrical and thermal resistant so it is used in formation of electrical appliances. This reaction is called Lederer- Manasse reaction.

  • Condensation with urea : Formaldehyde also condenses with urea in acidic solution to form a plastic like

 

 

 

|

CH2

|                   |

 

mH 2 NCONH 2 +

Urea

 

nCH 2 O

Formaldehyde

¾¾®

  • CH 2
  • CH 2

NCON

CH 2N CO N

CH 2N CO N

 

|                    |

CH2

| Formaldehyde- urea plastic

  • Reaction with alcohol : Formaldehyde reacts with methyl alcohol in presence of dry hydrogen chloride or fused calcium chloride forming methylal which is used as

 

 

H 2 C = O +

Formaldehyde

H OCH3

+

H OCH3

¾¾® HC

OCH3

OCH3

  • H 2 O

 

Methyl alcohol

Methylal (Dimethoxy methane)

 

  • Polymerisation : Formaldehyde readily undergoes
  • Paraformaldehyde : When an aqueous solution of formaldehyde is evaporated to dryness, a white crystalline solid with fishy odour is It is a long chain polymer.

 

nHCHO

Formaldehyde

(CH 2O)n

Para-formaldehyde

n = 6

to 50

 

On rapid heating it gives back gaseous formaldehyde.

 

When a formaldehyde solution is treated with con. are formed.

H 2 SO4 , a white solid, polyoxy methylenes (CH 2 O)n .H 2 O

 

nHCHO

Conc. H2SO4

 

heat

(CH 2O)n .H 2O ; n > 100

Polyoxy methylene

 

This on heating gives back formaldehyde.

  • Metaformaldehyde : On allowing formaldehyde gas to stand at room temperature, it slowly polymerises to metaform, (HCHO)3 . It is a white solid (pt. 61 – 62°C). This on heating gives back gaseous formaldehyde.

O CH 2

 

3HCHO

Formaldehyde

(HCHO)3         or

Meta- formaldehyde or trioxane

CH 2                                O

O CH 2

Trioxy methylene (trioxan)

 

  • Reaction with grignard reagent : Formaldehyde forms primary alcohols with Grignard

 

R

C

HC = O + RMgI ¾¾Eth¾er ® H –  | – OMgI ¾¾HO¾H ® RCH

OH

2OH + Mg

 

H                                                       H

|                                                          |                                      Primary alcohol                     I

 

Formaldehyde does not react with chlorine and phosphorus pentachloride. It does not give iodoform test.

(4)  Uses

  • The 40% solution of formaldehyde (formalin) is used as disinfectant, germicide and antiseptic. It is used for the preservation of biological
  • It is used in the preparation of hexamethylene tetramine (urotropine) which is used as an antiseptic and

 

 

 

  • It is used in silvering of
  • It is employed in manufacture of synthetic dyes such as para-rosaniline, indigo,
  • It is used in the manufacture of formamint (by mixing formaldehyde with lactose) – a throat
  • It is used for making synthetic plastics like bakelite, urea-formaldehyde resin,
  • Rongalite – a product obtained by reducing formaldehyde sodium bisulphite derivative with zinc dust and ammonia and is used as a reducing agent in vat
  • As a methylating agent for primary and secondary amines, g.,

C2 H5 NH 2 + 2HCHO ¾¾® C2 H5 NH CH3 + HCOOH

Ethylamine                                             Ethyl methylamine

  • If aqeous solution of formaldehyde is kept with lime water in dark room for 5 – 6 days then it converts into a sweet solution called formose or a It is an example of linear polymer.

 

6HCHO ¾¾CaOH¾)2 / B¾a(O¾H¾)2  ®

Dark 5-6 days

C6 H12O6

Formose / a -acrose

Acetaldehyde

 

Acetaldehyde is the second member of the aldehyde series. It occurs in certain fruits. It was first prepared by Scheele in 1774 by oxidation of ethyl alcohol.

  • Preparation : It may be prepared by any of the general The summary of the methods is given below
    • By oxidation of ethyl alcohol with acidified potassium dichromate or with air in presence of a catalyst like silver at 300°C.
    • By dehydrogenation of ethyl The vapours of ethyl alcohol are passed over copper at 300°C.
    • By heating the mixture of calcium acetate and calcium
    • By heating ethylidene chloride with caustic soda or caustic potash
    • By the reduction of acetyl chloride with hydrogen in presence of a catalyst palladium suspended in barium sulphate (Rosenmund’s reaction).

 

  • By the reduction of CH3CN

with stannous chloride and HCl in ether and hydrolysis (Stephen’s method).

 

  • By hydration of acetylene with

HSO4

and

HgSO4

at 60°C.

 

  • By ozonolysis of butene-2 and subsequent breaking of
  • Laboratory preparation : Acetaldehyde is prepared in the laboratory by oxidation of ethyl alcohol with acidified potassium dichromate or acidified sodium

K2Cr2O7 + 4 H2 SO4 ¾¾® K2 SO4 + Cr2 (SO4 )3 + 4 H2O + 3[O] [CH3CH2OH + O ¾¾® CH3CHO + H2O] ´ 3

K2Cr2O7 + 3CH3CH2OH+ 4 H2 SO4   ¾¾® K2 SO4 + Cr2 (SO4 )3 + 3CH3CHO+ 7H2O

 

Potassium dichromate

Ethyl alcohol

Sulphuric acid

Potassium sulphate

Chromic sulphate

Acetaldehyde

Water

 

To recover acetaldehyde, the distillate is treated with dry ammonia when crystallised product, acetaldehyde ammonia, is formed. It is filtered and washed with dry ether. The dried crystals are then distilled with dilute sulphuric acid when pure acetaldehyde is collected.

 

 

 

 

 

CH CHO + NH

 

¾¾® CH

OH

|

  • NH

 

¾¾H S¾O¾® CH

 

CHO + (NH

 

) SO

 

3                        3                      3     CH              2          2      4                      3

4 2       4

 

Acetaldehyde ammonia                                 Acetaldehyde

  • Manufacture : Acetaldehyde can be manufactured by one of the following methods:
  • By air oxidation of ethyl alcohol : Ethyl alcohol vapours and limited amount of air are passed over heated silver catalyst at 300°C.

2CH3CH2OH + O2  ¾¾A¾g ® 2CH3CHO + 2H2O

300°C

  • By dehydrogenation of alcohol : Vapours of ethyl alcohol are passed over heated copper at 300°C.

CH3CH2OH ¾¾C¾u ® CH3CHO

300°C

  • By hydration of acetylene : Acetylene is passed through water containing 40% sulphuric acid and 1% mercuric sulphate at 60°C when acetaldehyde is

CH  º CH + H2O ¾¾HgS¾O4 ¾,(1%¾),60¾°¾C ® CH3CHO

H2SO4 (40%)

  • From ethylene (Wacker process) : Ethylene is passed through an acidified aqueous solution of palladium chloride and cupric chloride, when acetaldehyde is

CH2  = CH2  + PdCl2  + H2O ¾¾CuC¾l2  ® CH3CHO + Pd + 2HCl

H +

 

Pd + 2CuCl2 ¾¾® PdCl2 + 2CuCl

 

2CuCl + 2HCl + 1 O

2   2

¾¾® 2CuCl2

  • H2O

 

 

 

CH2

= CH2

  • 1O

2   2

¾¾® CH

3CHO

 

Ethylene                                     Acetaldehyde

 

(So

H2C = CH2  + O2  ¾¾PdC¾l2 ,C¾uC¾l2  ® H3CCHO )

H2O

 

(2)  Physical properties

  • Acetaldehyde is a colourless volatile It boils at 21°C.
  • It has a characteristic pungent
  • It is soluble in water, chloroform, ethyl alcohol and Its aqueous solution has a pleasant odour. In water, it is hydrated to a considerable extent to form ethylidene glycol.

CH3CHO + H2O ¾¾® CH3CH(OH)2

  • Chemical properties : It gives all characteristic reactions of Besides general reactions, acetaldehyde shows the following reactions also.

 

  • Haloform reaction : It responds to iodoform reaction due to the presence of CH3CO
  • Tischenko’s reaction : It forms ethyl acetate in presence of aluminium 2CH3CHO ¾¾(CH5 O¾)3 ¾Al ® CH3COOC2H5

Ethyl acetate

group.

 

 

 

  • Chlorination : Hydrogen atoms of the methyl group are substituted by chlorine atoms when acetaldehyde is treated with

CH3CHO + 3Cl2 ¾¾® CCl3CHO+ 3HCl

Chloral

  • Polymerisation : Acetaldehyde undergoes polymerisation forming different products under different
  • Paraldehyde : It is formed, when anhydrous acetaldehyde is treated with sulphuric acid.

 

 

 

3CH3CHO

Acetaldehyde

(CH3 CHO)3

Paraldehyde, (trimer)

 

 

 

CH                                            CH

 

It is a pleasant smelling liquid (b.pt. 124°C). It has cyclic structure and when heated with dilute sulphuric acid it changes again into acetaldehyde. It is used as a hypnotic and soporific (sleep producing).

 

H

|                H

Reaction with NH3 : CH3 – C = O +

H

|

NH ¾¾- H¾O ® CH3 – C = NH

 

H                               2                        Acetaldimine

 

 

CH

3

|

 

NH

||

CH3 – CH

NH

CH–CH3+3H2O

.    3H2O

 

 

 

Trimethyl hexa hydro triazine [Trihydrate]

 

  • Metaldehyde : Acetaldehyde on treatment with hydrogen chloride or sulphur dioxide is converted into

 

metaldehyde

(CH3CHO)4 . It is a white solid (m. pt. 246°C). On heating it sublimes but changes again into

 

acetaldehyde when distilled with dilute sulphuric acid. It is used as a solid fuel.

CH3CH O CH CH3

|                      |

4CH3CHO ¾¾®               O                 O

Acetaldehyde                                |                      |

CH3CH O CH CH3

Metaldehyde (textramer)

 

It is used for killing slugs and snails.

  • Uses : Acetaldehyde is used :

 

 

 

  • In the preparation of acetic acid, acetic anhydride, ethyl acetate, chloral, 1,3-butadiene (used in rubbers), dyes and
  • As an antiseptic inhalent in nose
  • In the preparation of paraldehyde (hypnotic and sporofic) and metaldehyde (solid fuel).
  • In the preparation of acetaldehyde ammonia (a rubber accelerator).

Comparative study of formaldehyde and acetaldehyde

S.No.    Reaction                                                           Formaldehyde HCHO                                  Acetaldehyde CH3CHO

 

  1. Similarty

Addition of hydrogen

  • H2 in presence of catalyst, Ni, Pd or Pt

Forms methyl alcohol

HCHO + H2 ¾¾® CH3OH

 

Forms methane

Forms ethyl alcohol

 CH3CHO + H2 ¾¾® CH3CH2OH  

Forms ethyl alcohol Forms ethane

 

  • LiAlH4 (ether)

HCHO + 4 H ¾¾® CH + H O

 CH CHO + 4H ¾¾® C H + H O

 

  • Amalgamated zinc + HCl (Clemmensen reduction)

4        2                                 3

2    6       2

 

  1. Addition of NaHSO3 solution

Forms bisulphite addition product

 HCHO + NaHSO3 ¾¾® CH2(OH)SONa

Forms         bisulphite         addition product

CH3CHO + NaHSO3 ¾¾®

 

 

 

 

  1. Addition of HCN
HCHO + HCN ¾¾® CH2(OH)CN

Forms formaldehyde cyanohydrin

 CH3CH(OH)SO3 Na

Forms acetaldehyde cyanohydrin

CH3CHO + HCN ¾¾®

 

 

 

 

  1. Addition of      Grignard     reagent followed by hydrolysis

Forms ethyl alcohol

 

HCHO + CH3MgI ¾¾® CH2

 

¾¾HO ® CH3CH2OH

Mg(OH)I

 

OMgI CH3

 CH3CH(OH)CN  

Forms isopropyl alcohol

CH3CHO + CH3MgI ¾¾®

 

 

 

 

 

  1. With hydroxylamine

 

NH 2 OH

 

Forms formaldoxime

CH 2  = O + H 2 NOH ¾¾H¾2¾O ®

CH 2 = NOH

CH3CH OH

|

CH3

Forms acetaldoxime

CH3 CH  = O + H 2 NOH ¾¾H¾2¾O ®

CH3 CH = NOH

 

  1. With hydrazine (NH 2 NH 2 )

Forms formaldehyde hydrazone

CH 2 O + H 2 N  NH 2  ¾¾H¾2¾O ®

CH 2 = NNH 2

Forms acetaldehyde hydrazone

CH3 CH  = O + H 2 NNH 2  ¾¾H¾2¾O ®

CH3CH = NNH2

 

 

 

  1. With phenyl             hydrazine

(C6 H5 NHNH 2 )

Forms            formaldehyde         phenyl hydrazone

CH 2  = O + H 2 NNHC6 H5  ¾¾H¾2¾O ®

CH 2 = NNHC6 H5

Forms        acetaldehyde          phenyl hydrazone

CH3 CH = O + H 2 NNHC6 H5

¾¾ H¾2¾O ® CH3 CH  = NNHC6 H5

 

 

 

 

  1. With semicarbazide

(H 2 NNHCONH 2 )

Forms formaldehyde semicarbazone

 CH 2  = O + H 2 NNHCONH 2  ¾¾H¾2¾O ®

 CH 2 = NNHCONH 2

Forms                             acetaldehyde semicarbazone

 CH3CH = O + H 2 NNHCONH 2

¾¾ H¾2¾O ® CH3 CH  = NNHCONH 2

 

  1. With alcohol presence of acid

(C2H5OH)

in     Forms ethylal

 H 2 C = O + 2C2 H5 OH ¾¾H¾Cl ®

Forms acetaldehyde diethyl acetal

 CH3 CHO + 2C2 H5 OH ¾¾H¾Cl ®

 

 

 

  1. With thioalcohols presence of acid

(C2 H5 SH) in

Forms thio ethylal

 H 2C = O + 2C2 H5 SH ¾¾®

Forms        acetaldehyde         diethyl thioacetal

 CH3CH = O + 2C2 H5 SH ¾¾®

 

 

 

  1. Oxidation with acidified K2Cr2O7

Forms formic acid

 HCHO + O ¾¾® HCOOH

Forms acetic acid

 CH3CHO + O ¾¾® CH3COOH

 

  1. With Schiff’s reagent Restores pink colour of Schiff’s reagent
  2. With Tollen’s reagent Gives black precipitate of       Ag or silver             mirror

 Ag2O + HCHO ¾¾® 2Ag + HCOOH

Restores pink colour of Schiff’s reagent

Gives black precipitate of Ag or silver mirror

 Ag2O + CH3CHO ¾¾®

 

 

 

 

  1. With Fehling’s        solution        or Benedict’s solution

Gives red precipitate of cuprous oxide

2CuO + HCHO ¾¾® Cu2O + HCOOH

2Ag + CH3COOH

Gives red precipitate of cuprous oxide

2CuO + CH3CHO ¾¾®

 

 

 

 

  1. Polymerisation Undergoes polymerisation

 Cu2O + CH3COOH

Undergoes polymerisation

 

 nHCHO

Evaporation

3CH3CHO

H2SO4Conc.

 

(HCHO)n 

ParaformaldehydReoom temp.

dil. H2SO4. distill

 

3HCHO

(HCHO)3

Metaformaldehyde

heat

4CH3CHO

 

 

 

 

Difference

With PCl5                                                         No reaction

Forms ethylidene chloride

 

 

 

 

  1. With chlorine No reaction

+ POCl3

Forms chloral

 CH3CHO + 3Cl2 ¾¾® CCl3CHO

 

+3HCl

 

 

 

Inter conversion of formaldehyde and acetaldehyde

(1) Ascent of series : Conversion of formaldehyde into acetaldehyde

 

(i)

HCHO

¾¾H2 ¾/¾Ni ® CH3 OH ¾¾PC¾l5  ® CH3 Cl ¾¾A¾lc. ® CH3 CN ¾¾Na ¾/ Alc¾oh¾ol ® CH3 CH 2 NH 2  ¾¾NaN¾O¾2  ®

 

Formaldehyde

Methyl alcohol

Methyl chloride

KCN

Methyl cyanide

Ethyl amine

HCl

 

 

CH3CH2OH ¾¾H2 S¾O(d¾il.) ® CH3CHO

 

 

(ii)

HCHO

3

Formaldehyde

¾¾CH¾3 M¾gI ® CH

Ether

3CH

OMgI ¾¾HO ® CH

3CH

2OH ¾¾C¾u ® CH

300°C

Ethyl alcohol

 

3CHO

K2Cr2O7

Acetaldehyde

 

Ethyl alcohol                        Acetaldehyde

 

 

2

(iii)

HCHO

¾¾K2C¾r2O¾7  ® HCOOH ¾¾Ca(¾OH¾)2  ®(HCOO)2 Ca ¾¾(CH¾3CO¾O)¾2 C¾a ® CH3 CHO

 

Formaldehyde

H2SO4

Formic acid

Calcium formate

heat

Acetaldehyde

 

  • Descent of series : Conversion of acetaldehyde into formaldehyde
    • CH3 CHO ¾¾K2C¾r2O¾7 ® CH3 COOH ¾¾NH¾3  ® CH3 COONH4  ¾¾He¾at ® CH3 CONH 2  ¾¾Br/ KO¾H ®

 

Acetaldehyde

H2SO4

Acetic acid

Amm. acetate

Acetamide

 

CH3 NH 2  ¾¾NaN¾O¾2  ® CH3 OH ¾¾C¾u ®

HCHO

 

Methyl amine

HCl

300°C

Formaldehyde

 

 

 

  • CH3CHO ¾¾K2C¾r2O¾7 ® CH3COOH ¾¾NaO¾H ® CH3COONa ¾¾Sod¾alim¾e ® CH4   ¾¾C¾l2  ® CH3Cl ¾¾AgO¾H ®

 

Acetaldehyde

HSO4

Acetic acid

Sod.acetate

 

 

 

 

Acetone

heat

Methane

hv

 

CH3OH ¾¾C¾u ®

300°C

HCHO

Formaldehyde

 

It is a symmetrical (simple) ketone and is the first member of the homologous series of ketones. In traces, it is present in blood and urine.

  • Laboratory preparation : Acetone is prepared in laboratory by heating anhydrous calcium

(CH3COO)2 Ca ¾¾® CaCO3 + CH3COCH3

Calcium acetate                                                   Acetone

The retort is heated slowly when acetone distills over and collected in the receiver.

The distillate is shaken with saturated solution of sodium bisulphite when colourless crystals are formed. These are filtered and distilled with saturated solution of sodium carbonate. The aqueous solution of acetone is dried over

 

anhydrous calcium chloride and redistilled to obtain pure acetone. The fraction is collected between 55 to (b.pt. pure acetone 56o C ).

57o C

 

CH3

 

CH3

C = O + NaHSO3 ¾¾®

CH3

 

CH3

OH

C

SO3 Na

 

 

CH3

 

CH3

 

Acetone

OH

C

SONa

 

+ Na2CO3 ¾¾®

 

Acetone sodium bisulphite

CH3

C = O + NaHCO3 + Na2SO3

CH3

Acetone

Tags:

Leave a Reply

Your email address will not be published. Required fields are marked *