Haloalkanes and Haloarenes

Chemistry

NCERT

1   Among the isomeric alkanes of molecular formula $C_5 H_{12}$ identify the one that one photochemical chlorination yields$\\$ (i) A single monochloride$\\$ (ii) Three isomeric monochlorides.$\\$ (iii) Four isomeric monochlorides.

Solution :

(i) Neopentane$\\$ As all the H-atoms are equivalent, the replacement of any one of them give the same product.$\\$ (ii)$\overset{a}{CH_3} \overset{b}{CH_2} \overset{c}{CH^2} \overset{d}{CH_2} \overset{a}{CH_3}$ n-pentane. a, b, c are the three sets of equivalent hydrogens.$\\$ Therefore, three isomeric monochlorides are possible.$\\$ (iii)iso-pentane, there are four sets of equivalent hydrogens designated as a,b,c,d. Thus, four isomeric monochlorides are possible.

2   Write structures of the following compounds:$\\$ (i) $2 -$ chloro$-3$ methylpentane$\\$ (ii) $1-$ chloro$- 4 -$ ethylcyclohexane$\\$ (iii)$ 4 -$ tert .Butyl$-3-$ iodoheptane$\\$ (iv)$ 1, 4 -$Dibromobut$- 2 -$ ene$\\$ (v) $1-$ Bromo$- 4 -$ sec/ butyl$- 2 -$methylbenzene.

Solution :

3   Why is sulphuric acid not used during the reaction of alcohols with $Kl$?

Solution :

$H_2 SO_4$ is an oxidizing agent. It oxides $Hl$ produced during the reaction to $I _2$ and thus prevents the reaction between an alcohol and $HI$ to from alky iodide. In order to prevent this, a non- oxidising acid like $H _3 PO _3$ is used which prevents the oxidation of $HI$.$\\$ $2 KI + H_2 SO_4 \to 2 KHSO_4 + 2 HI \to I_2$

4   Write structures of different dihalogen derivatives of propane

Solution :

Four isomers are possible. There are:$\\$ $Br- CH_2 CH_2 CH_2 - Br 1,3 ,-$dibromopropane

5   Among the isomeric alkanes of molecular formula $C_5 H_{12}$ identify the one that one photochemical chlorination yields$\\$ (i) A single monochloride$\\$ (ii) Three isomeric monochlorides.$\\$ (iii) Four isomeric monochlorides.

Solution :

(i) Neopentane$\\$ As all the H-atoms are equivalent, the replacement of any one of them give the same product$\\$ (ii) $CH _3 CH _2 CH _2 CH _2 CH _3$ n-pentane. a, b, c are the three sets of equivalent hydrogens.$\\$ Therefore, three isomeric monochlorides are possible.$\\$ (iii)iso-pentane, there are four sets of equivalent hydrogens designated as a,b,c,d. Thus, four isomeric monochlorides are possible.

6   Draw the structures of major monohalo products in each of the following reactions:$\\$

Solution :

7   Arrange each set of compounds in order of increasing boiling points.$\\$ (i)Bromomethane, Bromoform, Chloromethane, Dibromomethane.$\\$ (ii) Chloropropane, Isopropyl chloride, $1 -$Chlorobutane.

Solution :

(i) Chloromethane $<$ Bromomethane $<$ Dibromomethane $< $Bromoform The reason is:$\\$ (a) for same alkyl group, B.Pt increases with size of halogen atom.$\\$ (b) B.Pt increases as number of halogen atoms increase$\\$ (ii) lsopropyl chloride $ < 1 -$ Chloropropane $ < 1 -$ Chlorobutane$\\$ Reason :$\\$ (a) For same halogen, B.Pt. increases as size of alkyl group increases.$\\$ (b) B.Pt. decreases as branching increases.

8   Which alkyl halide from the following pairs would you expect to react more rapidly by an $S N_ 2$ mechanism? Explain your answer.

Solution :

In $S N_ 2$ mechanism, reactivity depends upon the steric hindrance around the C-atom carrying the halogen. Lesser the steric hindrance, faster the reaction.$\\$ (i)$ CH_ 3 CH_ 2 CH_ 2 CH _2 Br 1 ^o$ alkyl halide$\\$ $CH _3 CH _2 CH ( Br ) CH _3 2^ o $alkyl halide$\\$ As steric hindrance in $2^ o $ alkylhalide is more, thus reactivity of$\\$ $CH _3 CH_ 2 CH_ 2 CH_ 2 Br > CH_ 3 CH_ 2 CH ( Br ) CH_ 3$ $\\$ (ii) $CH _3 CH _2 CH ( Br ) CH _3 2^ o$ alkyl halide $ ( CH_ 3 ) CBr 3^ o$ alkyl halide$\\$ As steric hindrance in $( CH _3 )_ 3$ CBr is more, thus it is less reactive than$\\$ $CH _3 CH _2 CH ( Br ) CH_ 3$ $\\$ (iii) Both are $ 2^ o$ alkylhalides but $CH _3$ group at $C _2$ is closer to Br atom than $CH _3 - $group at .$ C_ 3$ as a result $CH_ 3 CH _2 CH ( CH _3 ) CH _2 Br$ suffers greater steric hindrance than$\\$ $CH_ 3 CH ( CH_ 3 ) CH_ 2 CH _2 Br$ and will thus be less reactive in $S _N 2$

9   In the following pairs of halogen compounds, which compound undergoes faster $S _N 1$ reaction?

Solution :

Reactivity of $S _N 1$ is governed by stability of carbocations.

(i) reacts faster due to greater stability of $3 ^o$ carbocation.

(ii)reacts faster due to greater stability of $2^ o$ carbocation over $1^ o$ carbocation

10   Identify $A, B,C,D,E,R$ and $R _1$ in the following:

Solution :

11   Name the following halides according to $ IUPAC $ system and classify them as alkyl, benzyl (primary,secondary ,tertiary),vinyl or aryl halides:$\\$ ( i ) $(CH _3 ) _2 CHCH ( Cl ) CH_ 3$ $\\$ ( ii ) $CH _3 CH _2 CH ( CH_ 3 ) CH (C _2 H _5 ) Cl$ $\\$ ( iii ) $CH_ 3 CH _2 C ( CH _3 )_ 2 CH _2 l$ $\\$ ( iv ) $(CH _3 )_3 CCH _2 CH ( Br ) C _6 H _5$ $\\$ ( v ) $CH _3 CH ( CH _3 ) CH ( Br ) CH _3$ $\\$ ( vi ) $CH _3 C ( C_ 2 H_ 5 ) _2 CH _2 CH _3$ $\\$ ( vii )$ CH_ 3 CH = C ( Cl ) CH _2 CH (CH _3 )_2$ $\\$ ( ix )$ CH _3 CH = CHC (Br )( CH _3 )_ 2$ $\\$ ( x ) $P - ClC _6 H _4 CH _2 C ( CH _3 ) _2$ $\\$ ( xi )$ m - ClCH _2 C _6 H _4 CH _2 C ( CH _3 )_ 3$ $\\$ ( xii ) $o - Br - C _6 H _4 CH(CH _3 ) CH _2 CH _3$

Solution :

(i) $2-$Chloro$-3$methylbutane,$2^ o$ alkyl halide $\\$ (ii)$ 3-$Chloro$-4$methyl hexane,$2 ^o $alkyl halide $\\$ (iii) $1 -$Iodo$-2, 2-$dimethylbutane,$1^ o$ alkyl halide $\\$ (iv) $1-$Bromo$-3, 3-$dimethyl$ -1-$phenylbutane, $2 ^o $benzylic halide $\\$ (v) $2-$Bromo$-3-$methylbutane,$2^ o$ alkyl halide $\\$ (vi)$ 1-$Bromo$-2-$ethyl$-2-$methylbutane,$1 ^o $alkyl halide $\\$ (vii) $3-$Chloro$-3-$methylpentane,$3^ o$ alkyl halide $\\$ (viii)$ 3-$Chloro$-5-$methylhex$-2-$ene, vinylic halide $\\$ (ix)$ 4-$Bromo$-4-$methylpent$-2-$ene, allylic halide $\\$ (x)$1-$Chloro$-4-(2-$methylpropyl) benzene, aryl halide $\\$ (xi)$ l -$Chloromethyl$-3- (2,2-$dimethylpropyl) benzene,$1 ^o$ benzylic halide. $\\$ (xii) $1-$Bromo$-2-(l-$methylpropyl) benzene,aryl halide. $\\$