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C H A P T e r 10 u Arenes
4. Characteristic relations of aromatic hydrocarbons are initiated by
(a) electrophiles (b) nucleophiles
(c) Free radicals (d) uncharged molecules
5. For reactions of ethylbenzene, the ethyl group is considered as an
(a) ortho director (b) ortho–para director
(c) meta director (d) ortho–meta director
6. Toluene reacts with chlorine in the presence of AlCl3 to give
(a) o-Chlorotoluene (b) o- and p-Chlorotoluene
(c) m-Chlorotoluene (d) o- and m-Chlorotoluene
7. Oxidation of cumene with acidic K2Cr2O7 gives
(a) Phenylacetic acid (b) Benzaldehyde
(c) Benzyl alcohol (d) Benzoic acid
8. Benzene undergoes substitution reaction more easily than addition reaction because
(a) it has a cyclic structure (b) it has three double bonds
(c) it has six hydrogen atoms (d) there is delocalization of electrons
391
9. Which of the following agents is used in order to make benzene react with acetyl chloride to give
acetophenone?
(a) ultraviolet light (b) AlCl3 catalyst
(c) nickel catalyst (d) Al2O
3
10. Which of the following substituents in an ortho and para director and ring deactivating?
(a) –nH2 (b) –Cl
(c) –OCH3 (d) –OH
11. Which of the following will undergo substitution in the ortho and para positions rather than in the
meta position?
(a) nitrobenzene (b) Benzoic acid
(c) Acetenilide (d) Benzaldehyde
12. electron-withdrawing groups are meta directors because
(a) the carbonium ion intermediate has a negative charge on the meta position
(b) the more stable resonance hybrid occurs with meta attachment of the electrophile
(c) the less stable resonance hybrid occurs with meta attachment of the electrophile
(d) the carbonium ion intermediate has a positive charge on the meta position
392
PHArmACeuTiCAl OrgAniC CHemiSTry
13. Compared to benzene, nitration of toluene take place at
(a) same rate (b) toluene
(c) slower rate (d) cannot predict
14. Which of the following compound undergoes nitration most readily?
(a) Benzene (b) Benzoic acid
(c) Toluene (d) nitrobenzene
15. Which of the following compounds is most readily sulphonated?
(a) Benzene (b) Chlorobenzene
(c) Toluene (d) nitrobenzene
16. p-nitrotoluene on treatment with chlorine in the presence of FeCl3 gives
(a) m-Chlorotoluene (b) 2-Chloro-4-nitrotoluene
(c) p-Chlorotoluene (d) 2-nitro-4-chlorotoluene
ANSwERS
1. (a) 2. (c) 3. (c) 4. (a) 5. (b) 6. (b) 7. (a) 8. (d)
9. (b) 10. (b) 11. (c) 12. (b) 13. (b) 14. (c) 15. (c) 16. (b)
ALKYL HALIDES
5
11
11.1 Introduction, 11.2 Classification, 11.2.1 Nomenclature of Halogen Derivatives, 11.3 Isomerism in Alkyl Halides, 11.4 General Methods
of Preparation of Alkyl Halides, 11.5 Polar Nature of Alkyl Halides, 11.6 Physical Properties, 11.7 Chemical Properties, 11.8 Nucleophilic
Substitution Reactions, 11.9 Elimination Reactions, 11.10 Miscellaneous Reactions, 11.10.1 Wurtz Reaction (Formation of Higher
Alkanes), 11.10.2 Corey–House Reaction, 11.10.3 Reaction with Magnesium (Formation of Grignard reagent), 11.10.4 Formation of TEL,
11.10.5 Reduction (Formation of Alkanes), 11.10.6 Rearrangement on Heating (Isomerization), 11.11 Uses of Alkyl Halides
11.1 INTRODUCTION
The important thing in science is not so
much to obtain new facts as to discover
new ways of thinking about them.
–William Lawrence Bragg
Chapter Outline
Hydrocarbons are the parent family of organic compounds. Alkyl halides are halogen derivatives of
hydrocarbons. These compounds are obtained by replacement of one or more hydrogen atoms of a hydrocarbon
by an equal number of halogen (X) atoms. These do not occur in nature and are entirely of synthetic origin.
some of the important aspects of the chemistry of the halogen compounds.
11.2 CLASSIFICATION
hydrogen atoms of an alkane replaced by halogen atoms.
1. Monohalogen derivatives: One halogen atom is attached to carbon atom in these compounds. These
a. Alkyl halides nH
stands for an alkyl group and X stands for the halogen atom. These may be obtained from an
alkane by replacement of one hydrogen atom by a halogen atom.
RH
Alkane
– H
+ X
RX
Alkyl halide
–X or R–X where R
2n+1
394
halogen atom attached to primary, secondary and tertiary carbon atoms, respectively. For
CH
3
CH3 — CH2 — Br; CH3 — CH — Br; CH3 — C — Br
(Bromoethane) (2-Bromo prepane)
Ethy bromide Isopropyl bromide (2-Bromo-2-methylpropane)
(1° alkyl halide) (2° alkyl halide) tert-Butyl bromide
(3° alkyl halide)
CH
CH
3
3
b. Alkenyl halides or haloalkenes: These are the halogen derivatives of alkenes. Their general
1 2 3
CH2 CHCl CH2 CH CH2 Cl
(Chloro ethene) (3- Chloro prop-1-ene)
Vinyl chloride Allyl chloride
4 3 2 1 1 2 3
CH3CH CH CH2 Cl C6H5 CH CH CH2 Cl
(1-Chloro but-2-ene) (3-Chloro-phenyl prop-1-ene)
Crotyl chloride Cinnamyl chloride
nH2n–1
c. Alkynyl halides or haloalkynes: Alkynyl halides or haloalkynes are the halogen derivatives of
3 2 1
CH C Cl Cl CH2 C CH
(Chloro ethyne) (3-Chloro prop-1-yne)
Chloro accetylene Propargyl chloride
nH2n
REMEMBER
An allyl group is CH2 = CH–CH2–
while the vinyl group is CH
=CH–.
2
2. Dihalogen derivatives: The dihalogen derivatives of alkanes are derived by replacement of two
nH
2
X2.
n
a. Gem-dihalides: In these derivatives, both the halogen atoms are attached to the same (geminal)
carbon atom. These are also called alkylidene halides.
b. Vic-dihalides: In these derivatives, the two halogen atoms are attached to adjacent (vicinal)
carbon atoms. These are also termed alkylene halides.
Alkyl Halides
CH2Cl CH2Cl; CH3CHCl CH2Cl
Ethylene chloride Propylene chloride
c. a - ω halides (terminal dihalides): In these derivatives, the halogen atoms are attached to
terminal carbon atoms. These are also called polymethylene halides.
CH2BrCH2CH2Br; Cl CH2 CH2 CH2 CH2 Cl
Trimethylene bromide Tetramethylene chloride
Trihalogen derivatives: Trihalogen derivatives are derived by replacement of three hydrogen atoms
nH
. The trihalogen
2n–1X
CHCl3;
(Chloroform) (Bromoform) (Iodoform)
CHBr3; CHl
3
These compounds are termed as haloforms.
4. Tetrahalogen derivatives: The tetrahalogen derivatives are derived by the replacement of four
hydrogen atoms from alkanes by four halogen atoms.
CCl4;
Carbon tetrachoride Carbon tetrabromide Acetylene tetrachloride
(Tetrachloromethane) (Tetra bromomethane) or Westron
CBr4; CHCl2 CHCl
2
395
11.2.1 Nomenclature of Halogen Derivatives
Structural formula Common name IUPAC name
Methyl chloride
2H5
2 2
n
Isopropyl chloride
222 n
hloropropane
(Contd.…)
396
Structural formula Common name IUPAC name
2 Vinyl chloride
2
2
Isobutyl chloride
sec
tert
2
2 2
Westrosol
2
4
2
2
Trichloromethane
Tetrachloromethane
Acetylene tetrachloride or Westron
11.3 ISOMERISM IN ALKYL HALIDES
Alkyl halides show the following three types of isomerism.
1. Chain isomerism: This type of isomerism is due to difference in the arrangement of carbon atoms in the
CH
3
(CH3)2CH CH2Cl or CH3 C CH2Cl
a.
H
Isobutyl chloride 1-Chloro-2-methyl propane
Alkyl Halides
CH
3
397
(CH3)3C Cl or H3C C CH
b.
3
Cl
tert-Butyl chloride 2-Chloro-2-methyl propane
2. Position isomerism: In this type of isomerism, isomers differ in the position of the halogen atom.
a. CH
b. H
3CH2CH2CH2
C
3
|
Cl
c. (CH
CH CH2Cl or CH3 C CH2Cl
3)2
Isobutyl chloride
d.
(CH3)3C Cl or H3C C CH
tert-Butyl chloride
CH2 CH CH3 sec-Butyl chloride
Cl n-Butyl chloride
CH
H
H
Cl
3
C
3
3
4H9
Optical isomerism: This is a type of isomerism exhibited by alkyl halides having at least one chiral
carbon atom.
sec
C
2H5
H C Cl
CH
3
C
2H5
Cl C H
CH
3
sec
X that has eight isomers.
5H11
4H9X that has four isomers
H7
11.4 GENERAL METHODS OF PREPARATION OF ALKYL HALIDES
1. From alcohols: The hydroxy group (–OH) of alcohol may be replaced by the reagents such as halogen
acids, phosphorus halides or thionyl chloride.
398
a. By the action of halogen acids
i. Alkyl chlorides
2
2
H242.
2 is not required.
C2H5OH + HCl(g)
Ethyl alcohol
(Ethanol)
(CH3)2CHOH + HCl(g)
Isopropyl
(2-propanol)
(CH3)3C–OH + HCl
tert-Butyl alcohol
(2-Methyl-2-propanol)
• •
R O + ZnCl
• •
H H
• •
C–l + R O+ Z–nCl
H
[Zn(OH)Cl2]
–
+ H
Anhyd. ZnCl
Anhyd. ZnCl
Room temp.
2
2
+
2
C2H5Cl + H2O
Ethyl chloride
(Chloroethane)
3
(CH3)2CHCl + H2O
Isopropyl chloride
(2-Chloropropane)
(CH3)3CCl + H2O
tert-Butyl chloride
(2-Chloro-2-methylpropane)
R O
Cl R + [Zn(OH)Cl2]
ZnCl2 + H2O
• •
2ZnCl
2
–
ion is a better
This difference in the mechanism is due to the fact that a tertiary carbonium ion (R
ly more stable and is formed more readily than a primary carbonium ion (RH
–
– ion.
+) is relative
+). The behaviour of
2
secondary alcohol is intermediate between the two, i.e. primary and tertiary alcohols.
ii. Alkyl bromides can be prepared by heating alcohol with potassium bromide or sodium bromide and
concentrated sulphuric acid.
C2H5OH + KBr + H2SO
Ethyl alcohol
(Ethanol)
4
C
Br + KHSO4 + H2O
2H5
Ethyl bromide
(Bromoethane)
Alkyl Halides
∆
∆
CH2 � CH2 + HBr CH3CH2B
r
AlBr
3
CH3OH + Kl + H3PO
4 CH3
–l + KH2PO4 + H2O
�
H2SO
C2H5OH + KBr
Ethyl alcohol
(Ethanol)
4
C2H5Br + H2O
Ethyl bromide
(Bromoethane)
24 on
399
.
24 cannot be
used in place of H
as HI formed is oxidized by H24 to I2
PO4
3° > 2° > 1°
400
1.
2.
HI > HBr > HCl
The reasons for the above order are explained with the help of the mechanism of this reaction
With tertiary alcohols, the second step follows an SN1 mechanism
In case of primary alcohols, the reaction is carried out in the presence of anhydrous zinc chloride. Zinc
electrons on the oxygen atom. This provides a better leaving group for the reaction than OH2.
b. By the action of phosphorous halides
i. Alkyl chlorides
rous trichlorides.
ROH + PCl5 RCl + HCl + POCl
Alcohol Alkyl Phosphoryl
chloride chloride or
Phosphorous
oxychloride
3ROH + PCl3 3RCl + H3PO
Alcohol Alkyl Phosphorous
chloride acid
3
3
ii. Alkyl bromides or alkyl iodides
on alcohols. These phosphorous trihalides are less stable and, therefore, are always prepared with
in the reaction mixture by adding bromine or iodine to a mixture of alcohol and red phosphorous
(in situ).