Mr Cole Chemistry

Elimination Reactions Questions

Elimination Reactions

Halogenoalkanes Worksheet

1-chloro-2-methylpropane

1. 1-chloro-2-methylpropane can react with sodium hydroxide in different conditions, giving different products.
a) If the reaction is completed under aqueous conditions:
ii) Write an equation for the reaction that takes place.
CH2ClCH(CH3)2 + NaOH
→
CH2(OH)CH(CH3)2 + NaCl
iii) Outline the mechanism for the reaction.
Nucleophilic Substitution Mechanism
b) If the reaction takes place under ethanolic conditions:
i) State the role of the OH– ions.

Base (it accepts a proton/H+ from the alkyl chain).

ii) Write an equation for the reaction that takes place.
CH2ClCH(CH3)2 + NaOH
→
CH2=C(CH3)2 + H2O + NaCl
iii) Outline the mechanism for the reaction.
Elimination Mechanism

2-chlorobutane

2. 2-chlorobutane can also react with NaOH in the same way.
a) Write the equation for the reaction that occurs when 2-chlorobutane reacts with NaOH in aqueous conditions.
CH3CHClCH2CH3 + NaOH
→
CH3CH(OH)CH2CH3 + NaCl
b) When reacting under ethanolic conditions, a mixture of products can be formed.
i) Draw the skeletal formula for the two possible structural isomers and name them.
But-2-ene and But-1-ene structures

But-2-ene and But-1-ene.

ii) Explain why there are two possible products from this reaction and show that this is the case by outlining the mechanism.

In elimination reactions, the base removes a hydrogen atom from a carbon atom adjacent to the C-Cl bond.

In 2-chlorobutane, there are two different adjacent carbons (C1 and C3). Removing a hydrogen from C1 forms But-1-ene, while removing a hydrogen from C3 forms But-2-ene.

Elimination mechanism showing two products

Challenge Question

3. This question is for anyone who has studied geometric isomerism in alkenes already. 2-bromo-butan-1-ol can react with NaOH under ethanolic conditions to produce a mixture of isomeric products. Draw all the possible products and state their names.

The reaction eliminates HBr to form a double bond. The hydrogen can be removed from C3 or C1.

  • Major Product (Elimination from C3): Forms But-2-en-1-ol. This molecule has a chiral center and a double bond where each carbon has two different groups attached. It exists as E/Z isomers.
  • Minor Product (Elimination from C1): Forms But-1-en-1-ol. Each carbon of the double bond is attached to two different groups (C1: -H, -OH; C2: -H, -Ethyl), so this also exists as E/Z isomers.
Isomeric products of elimination

Extension Note: Tautomerisation

The product But-1-en-1-ol is an enol (an alkene with an OH group on the double bond). Enols are unstable and spontaneously rearrange into their structural isomers (aldehydes or ketones) via a process called Tautomerisation. In this case, it rearranges to form Butanal.

Find out more about Tautomerisation here
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