WAEC 2021 Chemistry · Question 18

The formation of ethene from dehydration of ethanol can be described as-------

Options

A.an addition reaction
B.an elimination reaction
C.an oxidation reaction
D.a substitution reaction

Answer: B. an elimination reaction

Explanation written with AI and checked against the answer key

Analyze the question: It asks what type of reaction occurs when ethanol is dehydrated to form ethene.

Concept: Dehydration means removing water. Ethanol is heated with excess concentrated H₂SO₄ at about 170 °C (or passed over heated alumina). An –OH group and an H atom on adjacent carbons are removed, and a C=C double bond forms.

Reaction:

CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄, 170 °C)

Checking each option:

A. Addition reaction: Two molecules combine into one, usually by breaking a double bond. Here a double bond is formed, not broken, so this is the opposite of addition.
B. Elimination reaction: A small molecule (H₂O) is removed from a larger one, and a double bond forms in the process. This matches exactly.
C. Oxidation reaction: There is no gain of oxygen and no loss of hydrogen as in oxidation. Water is lost, and the carbon skeleton is not oxidised. This does not fit.
D. Substitution reaction: One atom or group is replaced by another. Nothing is replaced here. This does not fit.

Answer: The reaction is an elimination reaction, option B.

Exam tip: Remember the pattern:

Addition: a double bond is lost (alkene → saturated compound).
Elimination: a double bond is gained (saturated compound → alkene, with a small molecule such as H₂O or HX lost).
Substitution: one group is swapped for another.

Whenever you see "dehydration of an alcohol to an alkene", think elimination.

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