Chapter 10 Halogen Derivatives Part 1 – Chemistry free study material by TEACHING CARE online tuition and coaching classes

Chapter 10 Halogen Derivatives Part 1 – Chemistry free study material by TEACHING CARE online tuition and coaching classes

File name : Chapter-10-Halogen-Derivatives-Part-1.pdf

 

 

Compounds derived from hydrocarbons by the replacement of one or more hydrogen atoms by the corresponding number of halogen atoms are termed as halogen derivatives. The halogen derivatives of the hydrocarbons are broadly classified into three classes:

  • Halogen derivatives of saturated hydrocarbons (Alkanes)- Halo-alkanes.

·            Halogen derivatives of unsaturated hydrocarbons (Alkenes and alkynes)-Halo-alkene or alkyne.

  • Halogen derivatives of aromatic hydrocarbons (Arenes)-Halo-arenes.

 

 

  • Organic compounds in which halogen atom

(F, Cl, Br, I)

is directly linked with saturated carbon atom are

 

known as halo-alkanes. General formula is

n = no. of carbon atoms.

Cn H 2n+2-m Xm

( X = F, Cl, Br, I ) and

m = no.of halogen atom;

 

  • Depending on the number of halogen atoms present in the halogen derivative, these are termed as mono-, di-, tri-, tetra-, and polyhalogen
ê

é CH4   ¾¾¾H ® CH3  – X ¾¾¾H ® CH 2  – X 2  ¾¾¾H ® CHX3  ¾¾¾H ® CX4 ù

 

ëMethane     + X

Mono           + X

Di               + X

Tri

  • X Tetra  úû

 

  • Monohalogen derivatives are termed as alkyl

 

Example :

CH3Cl

Methyl chloride

C2 H5 Br

Ethyl bromide

C3 H7 I

Propyl iodide

 

Monohalogen derivatives or alkyl halides are classified as primary (1°), secondary (2°) or tertiary (3°) depending upon whether the halogen atom is attached to primary, secondary or tertiary carbon atoms.

 

H       Primary carbon

|

R‘     Secondary carbon

|

R‘     Tertiary carbon

|

 

R CX

|

H

Primary alkyl halide

R C X

|

H

Secondary alkyl halide

R C X

|

R

Tertiary alkyl halide

 

(ii)       The dihalogen derivatives are mainly of three types

  • Gem-dihalides : In these derivatives both the halogen atoms are attached to the same carbon These are also called alkylidene halides.

 

CH CH

Cl ;

CH   CBr

  • CH

 

3         Cl

3             2             3

 

Isopropylidene bromide

Ethylidene chloride

  • Vic-dihalides : In these derivatives, the halogen atoms are attached to adjacent (Vicinal) carbon These are also termed as alkylene halides.

 

CH2Cl.CH2Cl   ;

Ethylene chloride

CH3CHCl.CH 2Cl

Propylene chloride

 

  • aw halides (Terminal dihalides) : In these derivatives, the halogen atoms are attached to terminal carbon These are also called polymethylene halides.

 

CH 2 BrCH 2 CH 2 Br  ;

Trimethylene bromide

Cl CH2  – CH2  – CH 2  – CH 2  – Cl

Tetra-methylene chloride

 

(iii)     The tri-halogen derivatives are termed as halo-forms

 

Example :

CHCl3       ;

Chloroform

CHBr3        ;

Bromoform

CHI 3

Iodoform

 

 

 

  • In tetra-halogen derivatives all the four halogen atoms are attached to the same carbon atom in derivatives of

 

Example :

CCl4               ;

Carbon tetrachloride

CBr4

Carbon tetrabromide

 

In other derivatives, the four halogen atoms are attached to different carbon atoms, e.g.,

 

 

 

  • The common and IUPAC names of some halogen derivatives are listed

CHCl2

|

CHCl2

Acetylene tetrachloride or 1,1,2,2- tetrachloroethane

 

 

Formula of halogen derivativesCommon nameIUPAC name
 CH3Cl

 CH3CH2Br

 CH3CHBrCH3

 CH3CH2CH2CH2Cl

CH3            CH CH  Br

CH3                                     2

CH3

|

CH3 – CCH3

|

Br

CH CH        Cl

3              Cl

 CH2Cl.CH2Cl

 CHCl3

 CHI 3

 CCl4

Methyl chlorideChloromethane
Ethyl bromideBromoethane
Isopropyl bromide2-Bromopropane
n-Butyl chloride1-Chlorobutane
Isobutyl bromide1-Bromo -2- methylpropane
Tertiary butyl bromide2-Bromo –2-methylpropane
 

Ethylidene chloride

 

1,1-Dichloroethane

Ethylene chloride1,2,-Dichloroethane
ChloroformTrichloromethane
IodoformTri-iodomethane
Carbon tetrachlorideTetrachloromethane

Usually, the simple and lower members are called by common names and higher members are given IUPAC names.

  • Higher members of alkyl halides show following types of isomerism,

CH3

|

  • Chain isomerism : CH3 – CH2 – CH2 – CH 2 – X ¬¾® CH3 – CH CH 2 – X

1-Halo butane                                                1-Halo – 2-methylpropane

 

  • Position isomerism : CH3 – CH2 – CH2 – X ¬¾® CH3 –

1-Halopropane

C H

|

X

CH3

 

2-Halopropane

  • Optical isomerism : This is due to the presence of asymmetric carbon atom in secondary butyl

H

|

CH3 – CCH2CH3

|

Br

(2-Bromobutane)

 

The total number of isomers in alkyl halides are: Propyl four isomers, and Pentyl (C5 H11 – X) has eight isomers.

(C3 H7  – X)

has two isomers, Butyl (C4 H9 – X) has

 

 

 

  • Halo-alkanes contain sp3 hybridised carbon atom bonded to halogen atom or

 

 

(1)  From alkanes

  • By halogenation : C2 H6

(Excess) + Cl2 ¾¾hv ®

C2 H5Cl             + HCl

 

Ethane

Cl2

Ethyl chloride (Major product)

 

CH3CH2CH3  ¾¾UV ¾lig¾ht  ® CH3CH2CH2Cl  + CH3CHCH3

 

Propane

1-Chloropropane (45%)

|

Cl

  • Chloropropane (55%)

 

This reaction proceed through free radical mechanism.

 

Note : ® Order of reactivity of

X2 for a given alkane is,

F2 > Cl2 > Br2 > I2 .

 

  • The reactivity of the alkanes follows the order : 3°alkane > 2°alkane > 1°alkane.

hv

2 2
  • With sulphuryl chloride : RH + SO2Cl2 ¾¾Org¾anic¾per¾oxid¾e(R¾’CO ¾)   ® RCl + SO2  + HCl

Note : ® In presence of light and trace of an organic peroxide the reaction is fast.

  • From alkenes (Hydrohalogenation)

CH3 – CH = CH CH3 + HBr ¾¾® CH3CH2 – CHCH3 ¾¾® Electrophillic addition.

 

But- 2-ene

|

Br

2-Bromobutane

 

Note : ® Addition of HBr to alkene in the presence of organic peroxide take place due to peroxide effect or Kharasch’s effect.

  • This addition take place by two mechanism, Peroxide initiates free radical mechanism. Markownikoff’s addition by electrophillic
  • From alkyne we cannot obtain mono alkyl

 

  • The order of reactivity of halogen acids is,

(3)  From alcohols

(i)        By the action of halogen acids

HI > HBr > HCl .

 

Groove’s process

ROH+ HX ¾¾Anh¾y. Z¾nC¾l2  ®

RX     + H2O

 

Alcohol

300°C

Haloalkane

 

Note : ® The reactivity order of HX in the above reaction is : HI > HBr > HCl > HF .

  • Reactivity order of alcohols 3° > 2° > 1° > MeOH .
  • 2° and 3° alcohols undergo SN1 ; where as 1° and MeOH undergo SN 2
  • Concentrated HCl + ZnCl2 is known as lucas reagent.

 

  • Using PCl5 and PCl3 : CH3CH2OH +

PCl5

Phosphorus pentachloride

¾¾® CH3CH2Cl+

Chloroethane

POCl3 + HCl

Phosphorus Oxychloride

 

3CH3CH2OH + PCl3 ¾¾® 3CH3CH2Cl+

Chloroethane

H3 PO3

Phosphorus acid

 

Note : ® Bromine and iodine derivatives cannot be obtain from the above reaction, because are unstable.

PBr5

or PI 5

 

  • This method gives good yield of primary alkyl halides but poor yields of secondary and tertiary alkyl

 

 

 

(iii)     By the action of thionyl chloride

(Darzan’s process)  CHCHOH + SOCl2  ¾¾Pyri¾di¾ne ® CHCHCl + SO2  + HCl

 

Note : ® Reaction takes place through SN 2

mechanism.

 

  • From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation)

 

(Free radical mechanism)

R CO Ag + Br Br

||

O

¾¾CC¾l4  ®

Decarboxylation

R Br + CO2 ­ + AgBr ¯

 

Note : ® The reactivity of alkyl group is 1° > 2° > 3°

  • Not suitable for chlorination because yield is
  • In this reaction iodine forms ester instead of alkyl halide and the reaction is called Birnbourn- Simonini reaction, 2R COOAg + I 2 ¾¾® RCOOR¢+ 2CO2 + 2AgI .

 

  • From alkyl halide (Halide exchange method) :

RX + NaI ¾¾Ace¾to¾ne ® RI + NaX(X  = Cl, Br)

Reflux

 

Note : ® Alkyl fluorides can not be prepared by this method. They can be obtained from corresponding

 

chlorides by the action of

Hg2 F2 or antimony trifluoride.

 

2CH3 Cl + Hg 2 F2 ® 2CH3 F + Hg 2 Cl2

Methyl fluoride

(6)  Other method

 

(i)

(ii)

ROH ¾¾KI,H¾3 P¾O¾®

ROH ¾¾X2 +¾(Ph¾O)3¾P ®

Rydon method

 

  • Dihalide

¾¾ZnCu¾®

HCl

R X

 

  • RMgX ¾¾X2¾¾¾®

 

ROR ¾¾PCl¾5

¾ ¾®

 

 

(1)  Physical properties

 

(i)

CH3F, CH3Cl, CH3 Br

and

C2 H5Cl

are gases at room temperature. The alkyl halides upto

C18 are

 

colourless liquids while higher members are colourless solids.

  • Alkyl halides are insoluble in water but soluble in organic
  • They burn on copper wire with green edged flame (Beilstein test for halogens).
  • Alkyl bromides and iodides are heavier than Alkyl chlorides and fluorides are lighter than water.
  • Alkyl iodides become violet or brown in colour on exposure as they decompose in

2RI ¾¾Lig¾ht ® RR + I2

 

  • For a given alkyl group, the boiling points of alkyl halides are in the order

RI > RBr > RCl > RF

and for a

 

given halogen the boiling points of alkyl halides increase with the increase of the size of the alkyl group.

  • Alkyl halides are in general toxic compounds and bring unconsciousness when inhaled in large
  • Chemical properties : The alkyl halides are highly reactive, the order of reactivity is, Iodide > Bromide > Chloride (Nature of the halogen atom)

Tertiary > Secondary > Primary (Type of the halogen atom)

 

 

Amongst the primary alkyl halide, the order of reactivity is : CH3 X > C2 H5 X > C3 H7 X , etc.

 

The high reactivity of alkyl halides can be explained in terms of the nature of

CX

bond which is highly

 

polarised covalent bond due to large difference in the electronegativities of carbon and halogen atoms. The halogen is far more electronegative than carbon and tends to pull the electrons away from carbon, i.e., halogen acquires a small negative charge and carbon a small positive charge.

d +       d 

  • CX

This polarity gives rise to two types of reactions,

  • Nucleophilic substitution reactions (ii) Elimination reactions

 

  • Nucleophilic substitution reactions : The Cd +

site is susceptible to attack by nucleophiles (An electron

 

rich species).

: Nu + R X ¾¾® Nu R + X :

RX ¾¾X¾-  ® R+ ¾¾N¾u–  ® RNu  ( S      reaction)

 

Slow

Fast                            N1

 

Nu + RX ¾¾Slo¾w ® Nu…..R      X ¾¾Fa¾st ® NuR + X

Transition state

( SN 2

reaction)

 

Examples of SN reactions,

  • Hydrolysis : Alkyl halides are hydrolysed to corresponding alcohols by moist silver oxide (AgOH) boiling with aqueous alkali solution (NaOH or KOH). The attacking nucleophile is OH .

or by

 

RX

Alkyl halide

+ AgOH ¾¾® ROH+ AgX ;

Alcohol

RX + KOH(aq) ¾¾® ROH + KX

 

Note : ® With the help of this reaction an alkene can be converted into alcohol. Alkene is first reacted with

HBr to form alkyl bromide and then hydrolysis is done.

CH2  = CH2  ¾¾H¾Br ® CHCHBr ¾¾AgO¾H ® CHCH2OH

 

Ethylene

Ethyl bromide

Ethyl alcohol

 

  • Reaction with alkoxides or dry silver oxide : Ethers are formed by heating alkyl halides with sodium or

 

potassium alkoxides or dry silver oxide. The attacking nucleophile is OR

(Williamson’s synthesis).

 

RX + NaOR‘ ¾¾He¾at ®

ROR

Unsym. ether

+ NaX  ;

2RX + Ag2O ¾¾® ROR+ 2AgX

Sym. ether

 

  • Reaction with sodium or potassium hydrogen sulphide : Alkyl halides form thioalcohols with aqueous

 

alcoholic sodium hydrogen sulphide or potassium hydrogen sulphide. The nucleophile is

SH .

 

RX +

NaSH

Sodium hydrogen

sulphide

¾¾®

RSH

Thioalcohol or Alkanethiol

or Alkyl mercaptan

  • NaX

 

  • Reaction with alcoholic potassium cyanide and silver cyanide : Alkyl cyanides are formed as the main product when alkyl halides are heated with alcoholic potassium The nucleophile is CN .

 

RX + KCN ¾¾Alco¾h¾ol ®

RCN

Alkyl cyanide or Alkane nitrile

  • KX

 

  • Reaction with potassium nitrite or silver nitrite : On heating an alkyl halide with potassium nitrite in an aqueous ethanolic solution, alkyl nitrite is obtained as the main product though some nitro alkane is also
2

The nucleophile is NO .

RX + K O N = O ¾¾® R O N = O+ KX

Alkyl nitrite

 

 

 

However, when alkyl halide is heated with silver nitrite in an aqueous ethanolic solution, nitro-alkane is the main product. Some alkyl nitrite is also obtained.

 

RX + AgNO2

¾¾® RN

O+ AgX O

 

Nitro-alkane

2
  • Reaction with ammonia : On heating with aqueous or alcoholic solution of ammonia in a sealed tube at

 

100°C, alkyl halides yield a mixture of amines and quaternary ammonium salt. The nucleophile is reaction.

NH

in the first

 

C2 H5 Br + H NH2 ¾¾® C2 H5 NH2 + HBr ; C2 H5 NH2 + BrC2 H5 ¾¾® C2 H5 NHC2 H5 + HBr

 

Ethylamine(p.)

Diethylamine(sec.)

+   –

 

(CH5 )2 NH + BrCH5 ¾¾®

(C2 H5 )3 N

Triethylamine(tert.)

  • HBr ;

(C2 H5 )3 N + BrC2 H5 ¾¾® (C2 H5 )4  NBr

Tetraethyl ammonium bromide(Quaternary)

 

  • Reaction with silver salts of fatty acids : On heating with silver salts of fatty acids in alcoholic solution, alkyl

 

halides yield esters. The nucleophile is

RCOO .

 

RCOOAg + XR ¾¾® RCOOR + AgX

Ester

  • Reaction with sodium acetylide : Alkyl halides react with sodium acetylide to form higher The

 

nucleophile is

CH º C .

RX + NaC º CH ¾¾® R C º CH+ NaX

 

Sodium acetylide                             Alkyne

  • Reaction with sodium or potassium sulphide : Alkyl halides react with sodium or potassium sulphide in alcoholic solution to form

2RX + Na2S ¾¾® RS R+ 2NaX

Thioether

Thioethers can also be obtained by heating alkyl halides with alcoholic solution of sodium mecaptide

(NaSR‘) , i.e., metallic derivative of a thioalcohol.

RXNaSR‘ ¾¾® RSR‘+ NaX

C2 HBr + NaSCH 3 ¾¾® C2 H5 – S CH3 + NaBr

Ethyl methyl thioether

  • Reaction with halides : Alkyl chlorides react with sodium bromide or sodium iodide to form alkyl bromide or alkyl Similarly, alkyl bromides react with sodium iodide in acetone or methanol to form alkyl iodides.

 

RCl

Alkyl chloride

  • NaBr ¾¾®

RBr

Alkyl bromide

¾¾N¾aI ®

RI

Alkyl iodide

 

  • Elimination reactions : The positive charge on carbon is propagated to the neighbouring carbon atoms by inductive effect. When approached by a strongest base (B), it tends to lose a proton usually from the b-carbon Such reactions are termed elimination reactions. They are also E1 and E2 reactions.
B

..

H  H                                     H      H                                             H

|       |                                       |        |                                               |

E1 reaction :  RCCH ¾¾Slo¾w ® R –  CCH ¾¾Fa¾st ® RC  = CH + BH

|      |                – X –                          |       +                                                       |

H     X                                     H                                            H

 

 

 

 

B :  H   H

|       |

BH   H                                               H

|       |                                              |

 

E2 Reaction :    R  CCH ¾¾Slo¾w ® RCCH ¾¾Fa¾st ® RC  = CH + BH + X

|       |                                      |       |                                     |

H     X                                    H     X                                   H

Transiton state

 

As the above reactions involve leaving of

X ,

the reactivity of alkyl halides (Same alkyl group, different

 

halogens) should be limited with C X

Type of bond

bond strength.

C I

C Br

C Cl

 

Bond strength (kcal/mol)           45.5            54              66.5

Bond strength increases

 

The breaking of the bond becomes more and more difficult and thus, the reactivity decrease.

The order of reactivity (Tertiary > Secondary > Primary) is due to +I effect of the alkyl groups which

 

increases the polarity of C X

R

bond.

R

 

R          C             X,               CH R                                             R

X,    R

CH2         X

 

The primary alkyl halides undergo reactions either by

SN 2 or

E2 mechanisms which involve the formation of

 

transition state. The bulky groups cause steric hinderance in the formation of transition state. Therefore, higher homologues are less reactive than lower homologues. CH3 X > C2 H5 X > C3 H7 X , etc.

Example of elimination reaction

  • Dehydrohalogenation : When alkyl halides are boiled with alcoholic potassium hydroxide, alkenes are

 

Cn H2n+1 X +

KOH

(Alcoholic)

¾¾® Cn H2n + KX + H2O Alkene

 

In this reactions, ether is a by-product as potassium ethoxide is always present in small quantity.

C2 H5 Br + KOC2 H5 ¾¾® C2 H5 – O C2 H5 + KBr

  • Action of heat : Alkyl halides when heated above 300°C, tend to lose a molecule of hydrogen halide forming alkenes.

 

RCH2CHX ¾¾300¾°¾C ® RCH  = CH2 + HX ;

Alkene

The decomposition follows the following order,

CHBr ¾¾300¾°¾C ® CH2  = CH2 + HBr

Ethene

 

Iodide > Bromide > Chloride (When same alkyl group is present) and Tertiary > Secondary > Primary (When same halogen is present).

(iii)     Miscellaneous reactions

  • Reduction : Alkanes are formed when alkyl halides are reduced with nascent hydrogen obtained by

 

Zn / HCl

or sodium and alcohol or Zn/Cu couple or LiAlH4 .

RX + 2H ¾¾® RH + HX

 

Reaction is used for the preparation of pure alkanes

  • Wurtz reaction : An ether solution of an alkyl halide (Preferably bromide or iodide) gives an alkane when heated with metallic

2RX + 2Na ¾¾® R R + 2NaX

 

 

 

  • Reaction with magnesium : Alkyl halides form Grignard reagent when treated with dry magnesium powder in dry

 

Rx +

Mg

(Powder )

¾¾Dry¾eth¾er ® RMgX

Grignard reagent

 

Grignard reagents are used for making a very large number of organic compounds.

  • Reaction with other metals : Organometallic compounds are formed.
    • When heated with zinc powder in ether, alkyl halides form dialkyl zinc These are called Frankland reagents.

2CHBr + 2Zn ¾¾Eth¾er ®(CH5 )2 Zn + ZnBr2

Heat

  • When heated with lead-sodium alloy, ethyl bromide gives tetra ethyl lead which is used an antiknock compound in

4CHBr + 4 Pb(Na) ¾¾®(CH5 )4 Pb + 4 NaBr + 3Pb

  • Alkyl halides form dialkyl mercury compounds when treated with sodium amalgam.

2CHBr + Na Hg ¾¾®(CH5 )2 Hg+ NaBr

Diethyl mercury

  • Reaction with lithium : Alkyl halides react with lithium in dry ether to form alkyl

 

RX + 2Li ¾¾Eth¾er ® RLi + LiX ;

CHBr + 2Li ¾¾® CH5 – Li + LiBr

Ethyl bromide

 

Alkyl lithiums are similar in properties with Grignard reagents. These are reactive reagents also.

  • Friedel-Craft’s reaction : Alkyl halides react with benzene in presence of anhydrous aluminium halides to form a homologue of

 

CH6 + RCl ¾¾AlC¾l3  ®

Benzene

C6H5R + HCl ;

Alkyl benzene

CH6  + CHBr ¾¾AlB¾r3  ® CH5CH5  + HBr

 

  • Substitution (Halogenation) : Alkyl halides undergo further halogenation in presence of sunlight, heat energy or

CHBr ¾¾B¾r2  ® CHBr2  ¾¾B¾r2  ® CHBr3 …..

hv                                                 hv

(1)  Methods of preparation of dihalides

(i)        Methods of preparation of gemdihalide

X

 

  • From alkyne (Hydrohalogenation) :

RC º CH + HX ¾¾® RC  = CH ¾¾+H¾X ® R

|

CCH3

 

|        |                                       |

X      H                                     X

  • From carbonyl compound : RCHO + PCl5 ¾¾® RCHCl2 + POCl3

[Terminal dihalide]

Note : ® If ketone is taken internal dihalide formed.

(ii)       Methods of preparation of vicinal dihalide

  • From alkene [By halogenation] : R CH = CH2 + Cl2 ¾¾® R CHCH2

 

 

R CH OH

|           |

Cl        Cl

R CH Cl

 

  • From vicinal glycol :

|

CH2 –OH

+ 2PCl5 ¾¾®

|

CH2 –Cl

  • 2HCl + 2POCl3

 

 

 

(2)  Properties of dihalides

(i)        Physical properties

  • Dihalide are colourless with pleasant smell Insoluble in water, soluble in organic solvent.
  • P and B.P µ -molecular mass.
  • Reactivity of vicinal dihalides > Gem

(ii)

KX

Chemical properties of dihalide

 

  • Reaction with aqueous KOH :

RCHX2  + 2KOH(aq.) ¾¾¾® RCH(OH)2  ¾¾HO ® RCHO

 

 

  • Reaction with alcoholic KOH :
  • Reaction with Zn dust

RCH 2

  • CHX2

H

|

¾¾Alc.¾KO¾H ® RC

-(KX + H2O)

X

NaNH

|

= CH ¾¾¾¾2  ® RC º CH

-( NaX + NH3 )

 

  • Gem halide (di) form higer symmetrical
  • Vicinal dihalide form respective

 

  • Reaction with KCN :

R CHX2

  • 2KCN ¾¾-2K¾X ® RCH(CN)2

¾¾H3O¾Å  ® RCH(COOH)

2

Hydrolysis

 

  • Other substitution reaction

 

CH2 – X

  • |

CH2 – X

CH2 – X

  • |

CH2 – X

CH2 – NH2

¾¾NH¾3  / 3¾73¾K  ® |

CH2 – NH2

Ethylene diamine

CH2OCOCH3

¾¾2CH¾3 C¾OO¾N¾a ® |

CH2OCOCH3

 

 

 

  • 2NaX.

 

 

Chloroform or trichloromethane, CHCl3

It is an important trihalogen derivative of methane. It was discovered by Liebig in 1831 and its name chloroform was proposed by Dumas as it gave formic acid on hydrolysis. In the past, it was extensively used as anaesthetic for surgery but now it is rarely used as it causes liver damage.

(1)  Preparation

  • Chloroform is prepared both in the laboratory and on large scale by distilling ethyl alcohol or acetone with bleaching powder and water. The yield is about 40%. The available chlorine of bleaching powder serves both as oxidising as well as chlorinating

 

CaOCl2

Bleaching powder

  • From alcohol

+ H2O ¾¾® Ca(OH)2 + Cl2

 

  • Alcohol is first oxidised to acetaldehyde by

[Cl2 + H2O ¾¾® 2HCl + O];     CH3CH2OH+ O ¾¾® CH3CHO+ H2O

Ethyl alcohol                                Acetaldehyde

  • Acetaldehyde then reacts with chlorine to form chloral (Trichloro acetaldehyde).

CH3CHO+ 3Cl2 ¾¾® CCl3CHO+ 3HCl

Acetaldehyde                                        Chloral

[So Cl2 acts both as an oxidising and chlorinating agent] Chloral, thus, formed, is hydrolysed by calcium hydroxide.

 

 

 

 

CCl

3

CHO

+

OHC

CCl3

¾¾Hyd¾roly¾¾sis ® 2CHCl3 + (HCOO)2 Ca

 

H        O Ca O –       H

Chloroform

Calcium formate

 

  • From acetone : Acetone first reacts with chlorine to form trichloro

CH3 – CO CH3 + 3Cl2 ¾¾® CCl3COCH3 + 3HCl

Trichloro acetone

  • Trichloro acetone is then hydrolysed by calcium

 

CCl3

COCH3

+

H3C.CO

CCl3

¾¾Hyd¾roly¾¾sis ® 2CHCl3 + (CH3COO)2 Ca

 

H             O CaO –           H

Chloroform

Calcium acetate

 

  • From carbon tetrachloride : Now-a-days, chloroform is obtained on a large scale by the reduction of carbon tetrachloride with iron fillings and This method is used in countries like U.S.A.

CCl4  + 2H ¾¾Fe /¾HO ® CHCl3  + HCl

This chloroform is not pure and used mainly as a solvent.

  • Pure Chloroform is obtained by distilling chloral hydrate with concentrated sodium hydroxide

CCl3CH(OH)2 + NaOH ¾¾® CHCl3 + HCOONa + H2O

Chloral hydrate

Note : ® Chloral hydrate is a stable compound inspite of the fact that two – OH                          H

Cl                      O

 

groups are linked to the same carbon atom. This is due to the fact that intramolecular

ClC C H

 

hydrogen bonding exists in the molecule between chlorine and hydrogen atom of group.

(2)  Physical properties

  • It is a sweet smelling colourless
  • It is heavy Its density is 1.485. It boils at 61°C.
  • OH Cl        H          O

 

  • It is practically insoluble in water but dissolves in organic solvents such as alcohol, ether,
  • It is non-inflammable but its vapours may burn with green
  • It brings temporary unconsciousness when vapours are inhaled for sufficient

(3)  Chemical properties

  • Oxidation : When exposed to sunlight and air, it slowly decomposes into phosgene and hydrogen

 

Cl      C

Cl+ [O] ¾¾Ligh¾t an¾d ¾air ® Cl         C

Cl  ¾¾® HCl + Cl

C = O

 

Cl                H

Chloroform

Cl                OH

Cl

Phosgene

 

or éCHCl

  • 1O

¾¾Ligh¾t an¾d ¾air ® COCl

  • HClù

 

ëê        3      2    2                                         2              úû

Phosgene is extremely poisonous gas. To use chloroform as an anaesthetic agent, it is necessary to prevent the above reaction. The following two precautions are taken when chloroform is stored.

  • It is stored in dark blue or brown coloured bottles, which are filled upto the
  • 1% ethyl alcohol is This retards the oxidation and converts the phosgene formed into harmless ethyl carbonate.

 

 

 

COCl2  + 2CH5OH ¾¾®(CH5O)2 CO+ 2HCl

Ethyl carbonate

  • Reduction : When reduced with zinc and hydrochloric acid in presence of ethyl alcohol, it forms methylene chloride.

CHCl3  + 2H ¾¾Zn /¾H¾Cl ® CH2Cl2  + HCl

(alc.)

When reduced with zinc dust and water, methane is the main product.

CHCl3  + 6H ¾¾Zn /¾HO ® CH4  + 3HCl

  • Chlorination : Chloroform reacts with chlorine in presence of diffused sunlight or UV light to form carbon

 

CHCl3  + Cl2  ¾¾UV ¾lig¾ht ®

CCl4

Carbon tetrachloride

  • HCl

 

  • Hydrolysis : Chloroform is hydrolysed when treated with hot aqueous solution of sodium hydroxide or potassium The final product is sodium or potassium salt of formic acid.

 

ê
ú

OH(aq.)

é           OHù                              O                                            O

 

H C

OH(aq.) ¾¾Na¾Cl ® êHC

OHú ¾¾H¾2O ® HC

¾¾NaO¾H ® HC

OH        H2O

ONa

 

OH(aq.)

ëê           OHúû

Unstable

Formic acid

Sodium formate

 

(Orthoformic acid)

[So, CHCl3  + 4 KOH(aq.) ¾¾Hyd¾roly¾¾sis ® HCOOK + 3KCl + 2HO ]

  • Nitration : The hydrogen of the chloroform is replaced by nitro group when it is treated with concentrated nitric acid. The product formed is chloropicrin or trichloronitro methane or nitro chloroform. It is a liquid, poisonous and used as an insecticide and a war

 

CHCl3 + HONO2 ¾¾®

Nitric acid

CNO2 Cl3

Chloropicrin (Tear gas)

  • H 2 O

 

  • Heating with silver powder : Acetylene is formed when chloroform is heated at high temperature with silver

H C Cl3 + 6 Ag + Cl3 – C H ¾¾® CH º CH+ 6 AgCl

Acetylene

  • Condensation with acetone : Chloroform condenses with acetone on heating in presence of caustic The product formed is a colourless crystalline solid called chloretone and is used as hypnotic in medicine.

 

Cl3CH + O = C

CH3

CH3

¾¾( Na¾O¾H) ®

HO Cl3C

C

Chloretone

CH3

CH3

 

(1,1,1- Trichloro- 2- methyl – 2- propanol)

  • Reaction with sodium ethoxide : When heated with sodium ethoxide, ethyl orthoformate is

 

 

H C

OC2 H5

OCH5  ¾¾-3N¾a¾Cl ® HC OC2 H5

OC2 H5

OC2 H5

OC2 H5

 

Ethyl orthoformate

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