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

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Different imine formation with

NH ₂ – Z is given below

Å R

R’ – NH₂ /H/D

C = N – R’

Imine from p-amine (Schiff base)

C = N – OH

Oxime

C = N – NH₂

R – C – R

Hydrazone

C = N – NNHC H

6 5

Phenylhydrozone (Brown coloured)

C = N – NH

NO₂

2,4-Dinitrophenylhydrazone (Red coloured)

Semicarbazide

C = N – NH – C – NH₂

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, H3 PO4 ) and Lewis acids (PCl5 , SOCl2 , PhSO2 Cl, RCOCl, SO3 , BF3 etc.)

C6 H5

C – CH3

N–OH

¾¾(i) P¾C¾l5 ® CH3

H₂O

C – NH – C₆ H₅

CH3

C – C6 H5

¾¾(i) P¾C¾l5 ® C6 H5

H₂O

C – NH – CH₃

N–OH

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

R R‘

Tautomerisation ||

N–OH

¾¾ ¾® R‘- C– O – H ¾¾¾¾¾¾® R‘ – C – NH – R

R–N

(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:

CH3 – CHO + Cu⁺⁺ ¾¾^Red¾¾^ox ® CH3COOH + Cu

reaction

(as Cu₂O ) (red ppt.)

CH 2

= CH – CHO ¾¾^C^u¾+ + ® CH

= CH – COOH + Cu₂O

CH 2

OH – (CHOH)₄

CHO ¾¾^C^u¾++ ® CH

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 .

R – CHO + Ag ^Å ¾¾^Red¾¾^ox ® RCOOH + Ag

reaction

(as silver mirror)

C6 H5 CHO + Ag ^Å ¾¾® C6 H5 COOH + Ag

This reagent has no effect on carbon-carbon multiple

CH 2 = CH – CHO + Ag ^Å ¾¾® CH 2 = CH – COOH + 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)

Ag 2 O ¾¾® C5 H11O5 COOH

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)

Schiff’s reagent : Megenta dye

¾¾^S^O¾2 ®

colourless soln

(Black)

¾¾^C^H¾3CH¾^O ® pink colour restored.

Oxidation by strong oxidising agents : Main strong oxidising agents are

KMnO4 / OH ^ / D, KMnO4 / H ^Å / D, K2 Cr2 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^[¾^O^] ® RCOOH ; C6 H5 CHO ¾¾^K^M¾ⁿ^O4¾^/ ^O¾^H ^/¾^D ® 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 a–carbon 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

CH3

CH 2

CH 2

CH 2

¯a

CH 2

¾¾^[¾^O^] ®

C = 7

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 a–carbon whose bond breaks always belongs to the alkyl group which has more number of carbons. This rule is known as Poff’s rule.

CH3 – CH 2 – CH 2 – C – CH 2 – CH3 ¾¾^[¾^O^] ® CH3 – CH 2 – COOH + CH3 – CH 2 – 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.

¾¾^[¾^O^] ® 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

1H ¾¾^[¾^O^] ® COOH – (CH )

COOH

Miscellaneous oxidation

Haloform Reaction : In this reaction a–methyl carbonyl compounds undergo oxidation with



X 2 / O H .

|| 

R – C – CH3 ¾¾⁽ⁱ⁾ ^X¾2 / O¾^H ® RCOOH + CHX3

(ii) H

|| 

C – CH ¾¾⁽ⁱ⁾ ^X¾2 / O¾^H ®

(ii) H

COOH + CHX₃

C6 H5

C– CH3

¾¾(i) I2¾/ N¾a2C¾O¾3 ® C

(ii) H

H₅COOH + CHI ₃

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

a – CH3 – group into aldehydic group.

a – CH₂ – group into keto group and

In this oxidation reactivity of CH₂

is more than the CH₃

group and Oxidation is regio selective in nature.

CH3

CHO ¾¾^S^e^O¾2 ® CHO – CHO ;

CH3

C – CH3

¾¾^S^e^O¾2 ® CH3

C – CHO

Glyoxal

O O O

Methylglyoxal

CH – CH

– CH

¾¾^S^e^O¾2 ® CH

|| ||

– – CH

3 2 C 3

₃ C C 3

O O

SeO₂

Dimethylglyoxal

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 – 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 a–carbon. 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

e-Lactone

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

CH – – C H

¾¾^C^F ¾^CO^O¾^O¾^H ® CH

– O – C H

3 C 6 5 3 3 C 6 5

CH3 O CH3 O

CH3 – C

– C – CH3

¾¾CF3¾COO¾O¾H ®

CH3

– C–

O – C – CH₃

CH3

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

O O

R C6 H5COOOH

|| ||

– – – R ¾¾¾¾¾® R – C– O – C– R

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 – CO – CH2 – CH3 ¾¾^[¾^O^] ® CH3 – COOH + HOOCCH3

Baeyer- villiger oxidation : H – C – H + O – O – C – H ¾¾® H – C – OH

|| || ||

O O O

CH3 – C– H + O– O – 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

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

R – CH₂ – R’

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.

O OH

R – CHO

¾¾¾(i) L¾iAlH¾4

NaBH ₄

¾® R – CH₂OH ;

R – || – R‘ ¾¾¾⁽ⁱ⁾ ^L¾^iAlH¾4 ¾® R – CH

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 = CH – CHO ¾¾^N^a^B¾^H¾4 ® CH3 – CH = CH – CH2OH

Hydride ion of

NaBH4

attack on carbonyl carbon during reduction.

NaBD₄

CH – C – CH

– CH

Example : CH₃

C– CH2

¬¾^N^a¾^B¾^D4 ¾ 2-Butanone

D2O

H₂O

NaBH

3 2 3

4 CH – C – CH – CH

D₂O

3 2 3

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

R R

C = O + NH ₃ ¾¾®

R R

C = NH ¾¾^H2 ¾^/¾^Nⁱ ®

Im ine

CH – NH 2

CH3

CH 2

C – CH3

¾¾⁽ⁱ⁾¾^N^H¾3¾® CH3

CH 2

NH2

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.

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

C H –

– C H

¾¾(i) N¾a/C¾2 H5¾O¾H ® 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 = CH – CHO ¾¾⁹^-B¾^B¾^N ® ¾¾^H^O¾^C^H¾2 CH¾2 N¾^H¾2 ®

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

(CH₃ – C H – O-)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).

R – || – R‘ ¾¾^N^H¾2 NH¾2 ®

N.NH2

R – C – R‘

Hydrazone

¾¾^R^O¾^N¾^a ® R – CH – R

Schiff’s base on reduction gives secondary

R – CH = O ¾¾^R^‘^N¾^H¾2 ® R – CH = NR‘ ¾¾^H2 ¾^/¾^Nⁱ ® R – CH 2 NHR

Schiff’s base

(5) Reactions due to a-hydrogen

Acidity of a-hydrogens :

a–hydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing
CO – a–Hydrogen is acidic due to strong –I group; – CO –.

H O

| ||

– C – C –

a–Carbon

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

CH3

– C – R

Base



CH 2

– C – R ¬¾¾® CH₂



= C – R

Carbanion (less stable)

Enolate ion (more stable)

The acidity of a–hydrogen 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 a–hydrogens undergo halogenation This reaction is catalysed by acid as well as base.

Acid catalysed halogenation : This gives only monohalo

CH – – CH

¾¾Br ¾/ CH¾CO¾O¾H ® CH

– CH Br

3 C 3 2 3

3 C 2

Base catalysed halogenation : In the presence of base all a–hydrogens of the same carbon is replaced by

|| 

O X

|| |

CH3 – CH 2 – C– CH 2 – CH3 ¾¾^X2 ¾^/ ^O¾^H ® CH3 – CH2 – C– C– CH3



X2 / OH

O

CH₃ – CH – C – CH– CH ₃

(Excess) |

| |

X X

Carbonyl compounds having three a–hydrogens give haloform reaction.

O  O 

|| ||

R – C– CH3 ¾¾^X2 /¾^O¾^H ® R – – CX ¾¾^O¾^H¾®



RCOO

CHX3

Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid (D^Å ) as well as base



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

O  O O

|| D O / OD || ||

Å ||

R – C– CH2 – R ¾¾2 ¾¾® R – C– CD2 – R; R – C– CH2 – R ¾¾^D2O¾^/¾^D ® R – C– CD2 – R

Racemisation : Ketones whose a–carbon is chiral undergo Racemisation in the presence of acid as well as

O CH3

CH3 O

|| |

C H – – – C H

HÅ or

|| |

C H – – – C H

C H –

– C H

6 5 C C 2

¾¾¾® C C 2 5



OH H

2 C C 6 5

Racemic mixture

Alkylation : Carbonyl compounds having a–hydrogens 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.

CH₃ – X . Other halides undergo

O CH3

CH – – CH

¾¾^N^a¾^H ® CH

|| 

– –

¾¾^C^H¾3 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

(Main product)

CH₂ – C– CH

CH3

¾¾¾3 ® CH₃CH₂ – C– CH

(Main product)

CH3

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

Ph3 P = CHR¹ + CHR² ¾¾® Ph3 P ^Å – CHR¹ ¾¾® Ph3 P– CHR¹ ¾¾® Ph3 P + CHR¹

|| |

| | || ||

O O –CHR2

O – CHR2

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.

H – C – H,

C6 H5 – C – H,

R – C – H,

R – C – R

etc.

Addition always takes place on the carbonyl group.

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

CH3 – NO2 ,

CH₃ – CH – CHO,

CH3

CH₃ – CH₂ – CN

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.

R – C – R + CH₃

H or

– Z ¾¾¾®



R – C – CH₂ – Z

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

R – C– CH₂ – Z

a | β

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.

R – C– CH₂ – Z

 R

¾¾^HO¾^/ ^D ®

C = CH – Z

a | β

Dehydration R

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

Both molecule are the same or

One should have no a–hydrogen atom and other should have at least two a–hydrogens.

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 ¾¾^N^a^O¾^H ^/¾^O¾^H ®êCH3 – CH– CH2 – CHOú ¾¾^Deh¾^ydr¾^ati¾^on ® CH₃ – CH = CH – CHO

^D ê ú

ë û

a, b -unsaturated aldehyde

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

CH₃ – CH = CH – CHO + CH₃ – CH = CH – CHO

NaOH

CH₃ – CH = CH – CH – CH₂ – CH = CH – CHO

D –H₂O

CH₃ – CH = CH – CH = CH – CH = CH – CHO

CH₃ – (CH = CH –)₃ – CHO

Condensed compound

The net result can be written as follows]

CH₃ – CHO + H₂CH – CHO

– O – H₂



OH/D

CH₃ – CH = CH – CHO

Crotonaldehyde

C₆H₅CHO + CH₃

CHO



OH/D

– H₂O



C₆H₅ – CH = CH – CHO

Cinnamaldehyde

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 H₅ – CH = CH– CHO

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

CH– CHO ¾¾® C6 H5 – CHO + CH3 – CHO .

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) CH3 CHO+ CH3 – CH2 – CHO ¾¾^dⁱ^l ^N¾^a^O¾^H ® CH3

CH3

CHO + CH₃

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 ¾¾^D ® CH2 = CH – CHO

(3-hydroxy propanal)

–H2O

(Acrolein)

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

Example : O = CH – (CH2 )5 – CHO ¾¾^N^a^O¾^H ®

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 – C – CH3 ¾¾100¾°C ® C6 H5 – CH = CH – C – CH3

4-Phenyl-3-buten-2-one

C H – CHO + CH

– – C H



¾¾^O^H¾^/¾^D ® C H

– CH = CH – – C H

6 5 3 C 6 5 6 5 C 6 5

CHO O

1,3-Diphenyl – 2-propene-1- one

CH = CH – C – CH₃



+ CH₃ – C – CH₃ OH

Geranial Pesudoionone

*Test*	*Aldehydes*	*Ketones*
1. With Schiff’s reagent	Give pink colour.	No colour.
2. With Fehling’s solution	Give red precipitate.	No precipitate is formed.
3. With Tollen’s reagent	Black precipitate of silver mirror is formed.	No black precipitate or silver mirror is formed.
4. With saturated sodium bisulphite solution in water	Crystalline compound (colourless) is formed.	Crystalline compound (colourless) is formed.
5. With 2 : 4-dinitrophenyl hydrazine	Orange-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 hydroxide	Give brown resinous mass (formaldehyde does not give this test).	No reaction.
7. With sodium nitroprusside and few drops of sodium hydroxide	A 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 ¾¾^Plat¾^inise¾^d ^a¾^sbes¾^t¾^os ® 2HCHO + 2H 2 O 300-400°C

CH3 OH + [O] ¾¾^K2C¾^r2O¾7 ® HCHO + H 2 O

H2SO4

By dehydrogenation of methyl alcohol

CH 3 OH ¾¾^C^u ¾^or ^A¾^g ® HCHO + H 2

300-400°C

By heating calcium formate :

Ca(HCOO)2 ¾¾^He¾^at ® CaCO3 +

Calcium formate

HCHO

Formaldehyde

By ozonolysis of ethylene :

CH 2

= CH 2

¾¾®

H₂C CH₂

¾¾^H¾2 ® 2HCHO + H₂O

Manufacture :

CH4 + O2 ¾¾Mo-¾oxi¾de ®

HCHO

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⁽^C2¾^H5O¾⁾3 ¾^A^l ®[CH3 COOH + C2 H5 OH] ¾¾® [CH3 COOC2 H5 ]

Ethyl acetate

CH3

CH – CHO ¾¾^A^l ⁺¾^B^u¾^t^o^xⁱ¾^d^e ®

CH3

CH – CH ₂OH + HOOC – CH

CH 3

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 ¾¾^C^a⁽¾^O^H¾⁾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.

Phenol

H – C – H

Formaldehyde

Base

dil. K₂CO₃

CH₂

CH₂ CH₂

OH OH

CH₂ CH₂

Bakelite

CH₂

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

N – CO – N

CH ₂ – N – 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

¾¾® H 2 C

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

H – C = O + RMgI ¾¾^Eth¾^er ® H – | – OMgI ¾¾^H^O¾^H ® RCH

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 ¾¾Ca(¾OH¾)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

H 2 SO4

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

– NH

¾¾^H ^S¾^O¾® 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 ¾¾^A¾^g ® 2CH3CHO + 2H2O

300°C

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

CH3CH2OH ¾¾^C¾^u ® 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 ¾¾^H^g^S¾^O4 ¾^,(1%¾^),60¾^°¾^C ® 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 ¾¾^C^u^C¾^l2 ® CH3CHO + Pd + 2HCl

H +

Pd + 2CuCl2 ¾¾® PdCl2 + 2CuCl

2CuCl + 2HCl + 1 O

2 2

¾¾® 2CuCl₂

CH2

= CH2

2 2

¾¾® CH

₃CHO

Ethylene Acetaldehyde

(So

H2C = CH2 + O2 ¾¾^P^d^C¾^l2 ,C¾^u^C¾^l2 ® H3C – CHO )

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⁽^C2¾^H5 O¾⁾3 ¾^A^l ® 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

Reaction with NH₃ : CH3 – C = O +

N – H ¾¾- H¾O ® CH3 – C = NH

H 2 Acetaldimine

CH₃ – CH

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.

CH₃ – CH – O – CH – CH₃

| |

4CH₃CHO ¾¾® O O

Acetaldehyde | |

CH₃ – CH – 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

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

Addition of NaHSO₃ solution

Forms bisulphite addition product

HCHO + NaHSO3 ¾¾® CH2(OH)SO3 Na

Forms bisulphite addition product

CH₃CHO + NaHSO₃ ¾¾®

Addition of HCN

HCHO + HCN ¾¾® CH₂(OH)CN

Forms formaldehyde cyanohydrin

CH₃CH(OH)SO₃ Na

Forms acetaldehyde cyanohydrin

CH₃CHO + HCN ¾¾®

Addition of Grignard reagent followed by hydrolysis

Forms ethyl alcohol

HCHO + CH₃MgI ¾¾® CH₂

¾¾^H2¾^O ® CH3CH2OH

– Mg(OH)I

OMgI CH3

CH₃CH(OH)CN

Forms isopropyl alcohol

CH₃CHO + CH₃MgI ¾¾®

With hydroxylamine

NH 2 OH

Forms formaldoxime

CH 2 = O + H 2 NOH ¾¾^–^H¾2¾^O ®

CH ₂ = NOH

CH₃ –CH – OH

CH3

Forms acetaldoxime

CH3 CH = O + H 2 NOH ¾¾^–^H¾2¾^O ®

CH₃ CH = NOH

With hydrazine (NH 2 NH 2 )

Forms formaldehyde hydrazone

CH 2 O + H 2 N NH 2 ¾¾^–^H¾2¾^O ®

CH 2 = NNH 2

Forms acetaldehyde hydrazone

CH3 CH = O + H 2 NNH 2 ¾¾^–^H¾2¾^O ®

CH₃CH = NNH₂

With phenyl hydrazine

(C6 H5 NHNH 2 )

Forms formaldehyde phenyl hydrazone

CH 2 = O + H 2 NNHC6 H5 ¾¾^–^H¾2¾^O ®

CH 2 = NNHC6 H5

Forms acetaldehyde phenyl hydrazone

CH3 CH = O + H 2 NNHC6 H5

¾¾^– ^H¾2¾^O ® CH3 CH = NNHC6 H5

With semicarbazide

(H ₂ NNHCONH ₂ )

Forms formaldehyde semicarbazone

CH 2 = O + H 2 NNHCONH 2 ¾¾^–^H¾2¾^O ®

CH ₂ = NNHCONH ₂

Forms acetaldehyde semicarbazone

CH₃CH = O + H ₂ NNHCONH ₂

¾¾^– ^H¾2¾^O ® CH3 CH = NNHCONH 2

With alcohol presence of acid

(C2H5OH)

in Forms ethylal

H 2 C = O + 2C2 H5 OH ¾¾^H¾^C^l ®

Forms acetaldehyde diethyl acetal

CH3 CHO + 2C2 H5 OH ¾¾^H¾^C^l ®

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®

Oxidation with acidified K2Cr2O7

Forms formic acid

HCHO + O ¾¾® HCOOH

Forms acetic acid

CH₃CHO + O ¾¾® CH₃COOH

With Schiff’s reagent Restores pink colour of Schiff’s reagent
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®

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®

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

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

¾¾^H2 ¾^/¾^Nⁱ ® CH3 OH ¾¾^P^C¾^l5 ® CH3 Cl ¾¾^A¾^lc. ® CH3 CN ¾¾^N^a ¾^/ ^Alc¾^oh¾^ol ® CH3 CH 2 NH 2 ¾¾^N^a^N¾^O¾2 ®

Formaldehyde

Methyl alcohol

Methyl chloride

KCN

Methyl cyanide

Ethyl amine

HCl

CH3CH2OH ¾¾^H2 S¾^O4¾^(d¾^il.) ® CH3CHO

(ii)

HCHO

Formaldehyde

¾¾^C^H¾3 M¾^g^I ® CH

Ether

₃CH

OMgI ¾¾^H+¾^O ® CH

₃CH

₂OH ¾¾^C¾^u ® CH

300°C

Ethyl alcohol

₃CHO

K2Cr2O7

Acetaldehyde

Ethyl alcohol Acetaldehyde

(iii)

HCHO

¾¾^K2C¾^r2O¾7 ® HCOOH ¾¾^C^a⁽¾^O^H¾⁾2 ®(HCOO)2 Ca ¾¾⁽^C^H¾3CO¾^O⁾¾2 C¾^a ® CH3 CHO

Formaldehyde

H2SO4

Formic acid

Calcium formate

heat

Acetaldehyde

Descent of series : Conversion of acetaldehyde into formaldehyde
- CH3 CHO ¾¾^K2C¾^r2O¾7 ® CH3 COOH ¾¾^N^H¾3 ® CH3 COONH4 ¾¾^He¾^at ® CH3 CONH 2 ¾¾^B^r2¾^/ ^K^O¾^H ®

Acetaldehyde

H2SO4

Acetic acid

Amm. acetate

Acetamide

CH3 NH 2 ¾¾^N^a^N¾^O¾2 ® CH3 OH ¾¾^C¾^u ®

HCHO

Methyl amine

HCl

300°C

Formaldehyde

CH3CHO ¾¾^K2C¾^r2O¾7 ® CH3COOH ¾¾^N^a^O¾^H ® CH3COONa ¾¾^Sod¾^alim¾^e ® CH4 ¾¾^C¾^l2 ® CH3Cl ¾¾^A^g^O¾^H ®

Acetaldehyde

H 2 SO4

Acetic acid

Sod.acetate

Acetone

heat

Methane

CH3OH ¾¾^C¾^u ®

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 56^o C ).

57^o C

CH3

C = O + NaHSO₃ ¾¾®

CH3

SO3 Na

CH3

Acetone

SO3 Na

+ Na₂CO₃ ¾¾®

Acetone sodium bisulphite

CH3

C = O + NaHCO₃ + Na₂SO₃

CH3

Acetone

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(3) Oxidation of carbonyl compounds

Case I : Oxidation of symmetrical ketones

Symmetrical ketones :

(4) Reuction of carbonyl compounds

(5) Reactions due to a-hydrogen

O

X X

b

(1) Preparation

(2) Physical properties

(4) Uses

Acetaldehyde

(2) Physical properties

Comparative study of formaldehyde and acetaldehyde

Inter conversion of formaldehyde and acetaldehyde

Acetone

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