AS Chemistry 9701: Classifying Addition, Elimination, Substitution and Condensation Reactions

A technician carries out a sequence of four organic reactions.

(a) Ethanol, $\text{CH}_3\text{CH}_2\text{OH}$ , is heated with an excess of concentrated sulfuric acid, causing it to lose a molecule of water and form ethene, $\text{CH}_2=\text{CH}_2$ . State the type of reaction (addition, substitution, elimination, oxidation, reduction, hydrolysis or condensation) and give a reason for your classification based on what happens to the molecule. [2]

(b) The ethene formed in (a) is then reacted with bromine, $\text{Br}_2$ , forming 1,2-dibromoethane, $\text{CH}_2\text{BrCH}_2\text{Br}$ . State the type of reaction and give a reason for your classification. [2]

(c) The 1,2-dibromoethane from (b) is warmed with aqueous sodium hydroxide, and each bromine atom is replaced by a hydroxyl group, forming ethane-1,2-diol, $\text{CH}_2(\text{OH})\text{CH}_2(\text{OH})$ . State the type of reaction and give a reason for your classification. [2]

(d) The ethane-1,2-diol from (c) is then reacted with ethanoic acid, $\text{CH}_3\text{COOH}$ ; the two molecules join together, losing one small molecule overall, to form an ester linkage. State the type of reaction and name the small molecule that is lost. [1]

Part (a): Formation of ethene from ethanol

$\text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}$

Only one molecule (ethanol) starts the reaction, and a small molecule, water, is removed from two adjacent carbon atoms. Nothing takes the place of the atoms lost; instead, the electrons left behind form a new C=C double bond. Removing a small molecule from a single larger molecule, without replacing it, to create unsaturation is the defining feature of an elimination reaction.

Part (b): Reaction of ethene with bromine

$\text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br}$

Here, all of the atoms present in the reactants end up in the single product: both bromine atoms of $\text{Br}_2$ become attached, one to each carbon, and the C=C double bond is used up to accommodate them. No atom that was originally part of ethene or bromine is lost. Two atoms or groups joining across a double bond, with nothing lost and the double bond disappearing, is the defining feature of an addition reaction.

Part (c): Hydrolysis of 1,2-dibromoethane

$\text{CH}_2\text{BrCH}_2\text{Br} + 2\text{NaOH} \rightarrow \text{CH}_2(\text{OH})\text{CH}_2(\text{OH}) + 2\text{NaBr}$

In this step, each bromine atom is directly replaced by a hydroxyl group. One atom/group leaves and a different atom/group takes its place on the same carbon, rather than being simply added on top of an existing double bond or removed with nothing replacing it. Exchanging one atom or group for another in this way is a substitution reaction; because the incoming nucleophile is derived from water (via aqueous hydroxide ions), this particular substitution is more specifically described as hydrolysis.

Part (d): Ester formation from ethane-1,2-diol and ethanoic acid

$\text{CH}_2(\text{OH})\text{CH}_2(\text{OH}) + \text{CH}_3\text{COOH} \rightarrow \text{ester} + \text{H}_2\text{O}$

Two separate molecules join together to form a single larger product (an ester linkage), and in doing so a small molecule, water, is lost overall. Two molecules combining into one, with loss of a small molecule, is the defining feature of a condensation reaction (esterification is a specific example of condensation).

Final answers

(a) Elimination. Water is removed from one molecule with nothing replacing it, forming a C=C bond
(b) Addition. Br2 adds across the C=C double bond, nothing is lost, the double bond disappears
(c) Substitution (hydrolysis). Each Br is directly replaced by an OH group
(d) Condensation, two molecules join with loss of water