Chapter 18- Haloalkanes

Question 1

Nomenclature for haloalkanes

Answer 1

The root name is based on the longest chain containing the halogen.
This root give the alkane part of the name.
The type of halogen defines the halo prefix, e.g. chloro-
The chain is numbered so as to give the halogen the lowest possible number

Question 2

What are the names for the three steps in radical substitution reactions?

Answer 2

Initiation
Propagation
Termination

Question 3

Describe the initiation reaction for the radical chain mechanism for the reaction of methane with Br2

Answer 3

Light energy breaks up Br2 to undergo homolytic cleavage to form two bromine radicals.

Question 4

Describe the propagation reaction for the radical chain mechanism for the reaction of methane with bromine

Answer 4

(a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
(b) The methyl radical abstracts a bromine atom from another molecule of Br2 to form bromomethane and another bromine radical, which can then itself undergo reaction (a) creating a cycle that can repeat.

Question 5

Describe the termination reaction for the radical chain mechanism for the reaction of methane with bromine

Answer 5

Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, bromomethane. These reactions remove radicals and do not perpetuate the cycle.

Question 6

Define the term ‘bimolecular reaction’

Answer 6

A bimolecular reaction involves the collision of two particles. They are common in organic reactions such as nucleophilic substitution.

Question 7

Define the term ‘unimolecular reaction’

Answer 7

A unimolecular reaction occurs when a molecule rearranges itself to produce one or more products.

Question 8

Describe Sn2 reactions

Answer 8

They occur in one step, and both the haloalkane and the nucleophile are involved in the rate determining step. The departure of the leaving group occurs simultaneously with the attack by the nucleophile. The SN2 reaction thus leads to a predictable configuration of the stereocentre - it proceeds with inversion

Question 9

Describe Sn1 reactions

Answer 9

This pathway is a multi-step process with the following characteristics:
step 1: slow loss of the leaving group to generate a carbocation intermediate, then
step 2 : rapid attack of a nucleophile on the electrophilic carbocation to form a new bond

Question 10

Principles determining SN1 versus SN2

Answer 10

Sn2 reactions are favoured with methyl and primary haloalkanes. Sn1 reactions are favoured with tertiary haloalkanes.
Secondary haloalkanes- Sn2 is favoured in aprotic solvents with good nucleophiles, Sn1 is favoured in protic conditions with poor nucleophiles.