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
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
|
é CH4 ¾¾–¾H ® CH3 – X ¾¾–¾H ® CH 2 – X 2 ¾¾–¾H ® CH – X3 ¾¾–¾H ® C – X4 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 – C– X
|
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
- a–w 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 derivatives | Common name | IUPAC name |
| CH3Cl
CH3CH2Br CH3CHBrCH3 CH3CH2CH2CH2Cl CH3 CH – CH Br CH3 2 CH3 | CH3 – C– CH3 | Br CH CH Cl 3 Cl CH2Cl.CH2Cl CHCl3 CHI 3 CCl4 |
Methyl chloride | Chloromethane |
| Ethyl bromide | Bromoethane | |
| Isopropyl bromide | 2-Bromopropane | |
| n-Butyl chloride | 1-Chlorobutane | |
| Isobutyl bromide | 1-Bromo -2- methylpropane | |
| Tertiary butyl bromide | 2-Bromo –2-methylpropane | |
|
Ethylidene chloride |
1,1-Dichloroethane |
|
| Ethylene chloride | 1,2,-Dichloroethane | |
| Chloroform | Trichloromethane | |
| Iodoform | Tri-iodomethane | |
| Carbon tetrachloride | Tetrachloromethane |
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 – C– CH2CH3
|
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
|
- With sulphuryl chloride : R – H + SO2Cl2 ¾¾Org¾anic¾per¾oxid¾e(R¾’CO ¾) ® R – Cl + 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 – CH – CH3 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
R – OH+ H – X 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) CH3 CH 2 OH + SOCl2 ¾¾Pyri¾di¾ne ® CH3 CH 2 Cl + SO2 + HCl
Note : ® Reaction takes place through SN 2
mechanism.
- From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation)
(Free radical mechanism)
R – C– O – 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) :
R – X + NaI ¾¾Ace¾to¾ne ® R – I + 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 P¾O4 ¾®
ROH ¾¾X2 +¾(Ph¾O)3¾P ®
Rydon method
- Dihalide
¾¾Zn-¾Cu¾®
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 ® R – R + 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
C – X
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 –
- C– X
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 – :
R – X ¾¾–X¾- ® R+ ¾¾N¾u– ® R – Nu ( S reaction)
Slow
Fast N1
Nu– + R – X ¾¾Slo¾w ® Nu…..R X ¾¾Fa¾st ® Nu – R + 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 ® CH3 CH2 Br ¾¾AgO¾H ® CH3 CH2OH
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® R – O – R+ 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
|
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® R – N
O+ AgX O
Nitro-alkane
|
- 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.)
+ –
(C2 H5 )2 NH + BrC2 H5 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
R‘ COO– .
R‘ COOAg + XR ¾¾® R‘ COOR + 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® R – S – 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.
RX – NaSR‘ ¾¾® R – S – R‘+ NaX
C2 H5 Br + 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.
|
–
..
H H H H H
| | | | |
E1 reaction : R – C – C – H ¾¾Slo¾w ® R – C – C – H ¾¾Fa¾st ® R – C = C – H + B – H
| | – X – | + |
H X H H
B– : H H
| |
BL H H H
| | |
E2 Reaction : R – C – C – H ¾¾Slo¾w ® R – C – C– H ¾¾Fa¾st ® R – C = C– H + B – H + 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.
RCH2CH2 X ¾¾300¾°¾C ® RCH = CH2 + HX ;
Alkene
The decomposition follows the following order,
C2 H5 Br 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® R – H + 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 ® R – Mg – X
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.
2C2 H5 Br + 2Zn ¾¾Eth¾er ®(C2 H5 )2 Zn + ZnBr2
Heat
- When heated with lead-sodium alloy, ethyl bromide gives tetra ethyl lead which is used an antiknock compound in
4C2 H5 Br + 4 Pb(Na) ¾¾®(C2 H5 )4 Pb + 4 NaBr + 3Pb
- Alkyl halides form dialkyl mercury compounds when treated with sodium amalgam.
2C2 H5 Br + Na – Hg ¾¾®(C2 H5 )2 Hg+ NaBr
Diethyl mercury
- Reaction with lithium : Alkyl halides react with lithium in dry ether to form alkyl
RX + 2Li ¾¾Eth¾er ® R – Li + LiX ;
C2 H5 Br + 2Li ¾¾® C2 H5 – 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
C6 H6 + RCl ¾¾AlC¾l3 ®
Benzene
C6H5R + HCl ;
Alkyl benzene
C6 H6 + C2 H5 Br ¾¾AlB¾r3 ® C6 H5C2 H5 + HBr
- Substitution (Halogenation) : Alkyl halides undergo further halogenation in presence of sunlight, heat energy or
C2 H5 Br ¾¾B¾r2 ® C2 H4 Br2 ¾¾B¾r2 ® C2 H3 Br3 …..
hv hv
(1) Methods of preparation of dihalides
(i) Methods of preparation of gemdihalide
X
- From alkyne (Hydrohalogenation) :
R – C º C – H + HX ¾¾® R – C = C – H ¾¾+H¾X ® R –
|
C– CH3
| | |
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 – CH – CH2
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)
|
Chemical properties of dihalide
- Reaction with aqueous KOH :
RCHX2 + 2KOH(aq.) ¾¾¾® RCH(OH)2 ¾¾–H2¾O ® RCHO
- Reaction with alcoholic KOH :
- Reaction with Zn dust
RCH 2
- CHX2
H
|
¾¾Alc.¾KO¾H ® R – C
-(KX + H2O)
X
|
|
= C– H ¾¾¾¾2 ® R – C 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)
|
Hydrolysis
- Other substitution reaction
CH2 – X
- |
CH2 – X
CH2 – X
- |
CH2 – X
CH2 – NH2
¾¾NH¾3 / 3¾73¾K ® |
CH2 – NH2
Ethylene diamine
CH2 –OCOCH3
¾¾2CH¾3 C¾OO¾N¾a ® |
CH2 –OCOCH3
- 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 – Ca – O – 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 /¾H2¾O ® 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
Cl– C – 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 + 2C2 H5OH ¾¾®(C2 H5O)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 /¾H2¾O ® 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.
|
|
OH(aq.)
é OHù O O
H – C
OH(aq.) ¾¾–Na¾Cl ® êHC
OHú ¾¾–H¾2O ® H – C
¾¾NaO¾H ® H – C
OH – H2O
ONa
OH(aq.)
ëê OHúû
Unstable
Formic acid
Sodium formate
(Orthoformic acid)
[So, CHCl3 + 4 KOH(aq.) ¾¾Hyd¾roly¾¾sis ® HCOOK + 3KCl + 2H 2 O ]
- 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
OC2 H5 ¾¾-3N¾a¾Cl ® H – C OC2 H5
OC2 H5
OC2 H5
OC2 H5
Ethyl orthoformate
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