What type of reaction takes place when a halogenoalkane is heated with ethanolic potassium hydroxide?

It is an elimination reaction, in which a molecule of hydrogen halide is removed and a carbon–carbon double bond is formed.

What role does the hydroxide ion play in an elimination reaction?

It acts as a base, removing a hydrogen ion from the carbon atom next to the one that carries the halogen.

Describe, in terms of curly arrows, the mechanism of the elimination of hydrogen bromide from bromoethane.

An arrow from a lone pair on the hydroxide ion to a hydrogen atom on the carbon next to the bromine shows the base removing that hydrogen, a second arrow from that carbon–hydrogen bond into the space between the two carbon atoms shows those electrons forming the new pi bond, and a third arrow from the carbon–bromine bond to the bromine shows it leaving as a bromide ion.

CCEA A-Level Chemistry 1110

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Halogenoalkanes

Topic overview Classification and boiling points of halogenoalkanes Laboratory preparation of a halogenoalkane from an alcohol Nucleophiles and hydrolysis with aqueous alkali Reactions with ammonia and with potassium cyanide Relative rates of hydrolysis and carbon–halogen bond enthalpy Elimination using ethanolic potassium hydroxide CFCs and the ozone layer

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Halogenoalkanes

Elimination using ethanolic potassium hydroxide

3 things to know
Unit AS 2

Overview

Recall questions on the reagent and conditions for elimination, the hydroxide ion acting as a base rather than a nucleophile, the mechanism in curly arrows, the equations for bromoethane and 2-bromopropane, why 2-bromobutane gives two alkenes and which predominates, and how conditions decide between substitution and elimination.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the reagent and conditions used to eliminate a hydrogen halide from a halogenoalkane.

Key term

Write the equation for the reaction of bromoethane with ethanolic potassium hydroxide.

Key term

Name the organic product formed when bromoethane is heated with ethanolic potassium hydroxide.

Questions from this subtopic

Each one with the model answer underneath.

1
State the reagent and conditions used to eliminate a hydrogen halide from a halogenoalkane.
Model answer
The halogenoalkane is heated under reflux with potassium hydroxide dissolved in ethanol.
2
What type of reaction takes place when a halogenoalkane is heated with ethanolic potassium hydroxide?
Model answer
It is an elimination reaction, in which a molecule of hydrogen halide is removed and a carbon–carbon double bond is formed.
3
What role does the hydroxide ion play in an elimination reaction?
Model answer
It acts as a base, removing a hydrogen ion from the carbon atom next to the one that carries the halogen.
4
Write the equation for the reaction of bromoethane with ethanolic potassium hydroxide.
Model answer
The equation is CH₃CH₂Br + KOH → CH₂=CH₂ + KBr + H₂O.
5
Name the organic product formed when bromoethane is heated with ethanolic potassium hydroxide.
Model answer
The organic product is ethene.
6
Describe, in terms of curly arrows, the mechanism of the elimination of hydrogen bromide from bromoethane.
Model answer
An arrow from a lone pair on the hydroxide ion to a hydrogen atom on the carbon next to the bromine shows the base removing that hydrogen, a second arrow from that carbon–hydrogen bond into the space between the two carbon atoms shows those electrons forming the new pi bond, and a third arrow from the carbon–bromine bond to the bromine shows it leaving as a bromide ion.

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