Section IB 1

1.a) Halogenoalkanes undergo nucleophilic substitution reactions. The rates and mechanisms of these reactions depend on whether the halogenoalkane is primary secondary or tertiary.

i) Explain the term nucleophilic substitution.

ans (i)

Nucleophilic substitution is when the organic molecule is attacked at the positive atom of a dipole by a reagent that has a lone pair of electrons, resulting in replacement (substitution) of an atom or group by another.

ii) The formula C4H9Br represents more than one compound. Using this formula draw a structure (showing all bonds) to represent a halogenoalkane that is:

Primary
Secondary
Tertiary

ans (ii)


Primary	Secondary	Tertiary

iii) The stoichiometric equation for a nucleophilic substitution reaction is given below:
(CH₃)₃CBr + OH- (CH₃)₃COH + Br-
The reaction takes place via a two step mechanism. Write an equation for each step.

ans (iii)

Step 1: (CH₃)₃CBr (CH₃)₃C + Br

Step 2: (CH₃)₃C + OH (CH₃)₃COH

b) Define the following terms

(i) Molecularity

ans (b)

The molecularity is the number of particles involved in the rate determining step of the reaction mechanism.

(ii) Rate determining step

ans (c)

The rate determiniing step is the slowest step of the mechanism.

d) Identify the rate determining step in the mechanism above

ans (d)

The rate determining step is the first step.