Mr Cole Chemistry

Nucleophilic Substitution 2 Questions

Nucleophilic Substitution 2

Halogenoalkanes Worksheet

Formation of Diethylamine

1. Diethylamine, pictured below, can be formed from a nucleophilic substitution reaction between a halogenoalkane and an amine.
Diethylamine Structure
a) State the names of the two reactants that can be used in this reaction.

Chloroethane and Ethylamine

b) Outline the mechanism for the formation of diethylamine.
Mechanism forming Diethylamine

Chloroethane and Ammonia

2. Chloroethane and ammonia are another pair of compounds that can undergo a nucleophilic substitution reaction.
a) Explain why ammonia is a nucleophile.

The nitrogen atom has a lone pair of electrons that can be donated.

b) Outline the mechanism for the reaction to form ethylamine from chloroethane and ammonia.
Mechanism forming Ethylamine
c) Write a chemical equation for the reaction to form ethylamine from chloroethane and ammonia.
CH3CH2Cl + 2NH3
→
CH3CH2NH2 + NH4Cl
d) Explain why the reaction between chloroethane and ammonia produces a low yield of ethylamine, and state how you can improve the yield.

Reason: Ethylamine is a nucleophile itself (often stronger than ammonia). It can react further with chloroethane to form diethylamine, triethylamine, and finally a quaternary ammonium salt (tetraethylammonium chloride).

Improve Yield: Use a large excess of ammonia to minimize further substitution.

e) Outline the mechanism of the reaction that reduces the yield of ethylamine (formation of diethylamine).
Further substitution mechanism

Rates of Reaction

3. Different halogenoalkanes will react at different rates. For each of the following pairs, state which will react more quickly, and explain why that is the case.
a) Bromoethane and chloroethane.

Bromoethane will react faster.

The C-Br bond is weaker (lower bond enthalpy) than the C-Cl bond because the Br atom is larger, resulting in a longer bond length. This makes it easier to break.

b) 1-chlorobutane and 2-chloro-2-methylpropane.

2-chloro-2-methylpropane will react faster.

It is a tertiary halogenoalkane. Tertiary carbocations are very stable, allowing the C-Cl bond to break spontaneously (SN1 mechanism), which is generally faster than the SN2 mechanism experienced by primary halogenoalkanes like 1-chlorobutane.

c) Chloroethane and 1-chloropentane.

Chloroethane will react more quickly.

The longer alkyl group in 1-chloropentane exerts a stronger positive inductive effect, stabilising the carbon in the C-Cl bond and making the carbon atom less positive (less $\delta+$). This makes it slightly less attractive to nucleophiles compared to chloroethane.

Applications

4. State one use for an alkyl ammonium salt, like the one made in question 2.
Answer

They can be used as cationic surfactants (commonly found in fabric conditioners and hair products).

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