Haloalkanes and Haloarenes TEXTBOOK Solution

Question 1.

Write the structures of the following compounds :

(i) 2-Chloro-3-methylpentane
(ii) 1-Chloro-4-ethylcydohexane
(iii) 4-terf-Butyl-3-iodoheptane
(iv) 1, 4-Dibromobut-2-ene
(v) 1 -Bromo-4-sec-butyl-2-methylbenzene

Solution:

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 1

Question 2.

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

Solution:

H2SO4 is a strong oxidising agent. Therefore, when it is used in presence of KI, it tends to convert KI to HI and finally oxidises it to I2.

Question 3.

Write structures of different dihalogen derivatives of propane.

Solution:

The structures of all possible dihalogen derivatives of propane are

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 2

Question 4.

Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields

(i) A single monochloride
(ii) Three isomeric monochlorides
(iii) Four isomeric monochlorides.

Solution:

An alkane with molecular formula C5H12 can exist in the following isomeric forms :

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 3

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 4
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 5

Question 5.

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

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 6
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 7

Solution:

The major haloderivatives formed in the given reactions are

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 8

Question 6.

Arrange each set of compounds in order of increasing boiling points.

(i) Bromomethane, bromoform, chloromethane, dibromomethane.
(ii) 1 – Chloropropane, isopropyl chloride, 1 – chlorobutane.

Solution:

The boiling points of organic compounds depend on the strength of the intermolecular forces in them. These forces are :

(a) van der Waals forces and
(b) dipole-dipole interactions These forces are dependent on the

(i) molecular mass and
(ii) surface area of the molecules

(i) As the molecular mass of the compound increases, the boiling point also increases.

Therefore the correct order is
chloromethane < bromomethane < dibromomethane < bromoform

(ii) Amongst molecules with same mass, it is the size of the molecule that determines the boiling point. Branched compounds are more compact and therefore have less surface area as compared to their straight chain counterparts and therefore lower boiling point.

The order of boiling point is
iso-propyl chloride < 1-chloropropane < 1-chlorobutane

Question 7.

Which alkyl halide from the following pairs would you expect to react more rapidly by an SNmechanism? Explain your answer.

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 9

Solutions:

We know that SN2 mechanism involves a transition state wherein both, the incoming nucleophile as well as the leaving group are present around the carbon atom. There are 5 atoms simultaneously bonded to it.

Thus, for such a transition state to be possible, there should be minimum steric hindrance. Hence, 1° alkyl halides are most reactive towards SN2 followed by 2° and finally 3°.

1° RX > 2° RX > 3° RX

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 10
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 11

Question 8.

In the following pairs of halogen compounds, which compound undergoes faster SN1 reaction?

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 63

Solutions:

SN1 reaction proceeds via the formation of a carbocation intermediate. This intermediate is formed by the cleavage of the C — X bond. More stable is the resultant carbocation faster is the SN1 reaction.
Order of stability of carbocation is
3° carbocation > 2° carbocation >1° carbocation
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 12

Question 9.

Identify A, B, C, D, E, R and R’ in the following.

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 13

Solution:

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 14
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 15
NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 16

Question 10.

A hydrocarbon C5H10 does not react with chlorine in dark but gives a single monochloro compound C5H9Cl in bright sunlight. Identify the hydrocarbon.

Solution:

The hydrocarbon with molecular formula C5H10 can either a cycloalkane or an alkene.Since the compound does not react with Cl2 in the dark, therefore it cannot be an alkene but must be a cycloalkane.

Since the cycloalkane reacts with Cl2 in the presence of bright sunlight to give a single monochloro compound, C5H9Cl, therefore, all the ten hydrogen atoms of the cycloalkanes must be equivalent. Thus, the cycloalkane is cyclopentane.

NCERT Solutions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes 64

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