But-2-ene, CH3CH=CHCH3, is an unsaturated hydrocarbon that can be converted into butane by hydrogenation. (a) Identify the type of reaction that converts but-2-ene into butane, and state the reagent and conditions required. [3] (b) Write a balanced equation for this reaction. [1…

Answer: ((a)) Addition (hydrogenation): the reagent is hydrogen gas, H2, and the reaction requires a nickel (or platinum/palladium) catalyst at a temperature of around 150°C. ((b)) CH3CH=CHCH3 + H2 → CH3CH2CH2CH3 ((c)) Hydrogenation of C=C double bonds is used to convert unsaturated liquid vegetable oils into saturated, solid or semi-solid fats, for example in the manufacture of margarine. ((d)) But-2-ene contains a reactive π bond, part of its C=C double bond, across which atoms can add; butane is fully saturated, with only strong C-C and C-H single () bonds and no π bond available for further addition.. Part (a): Identifying the reaction, reagent and conditions Converting but-2-ene into butane is an addition reaction, specifically hydrogenation: hydrogen atoms add across the C=C double bond. The reagent required is hydrogen gas, H2, and t…

AS Chemistry 9701: Hydrogenation of But-2-ene to Butane

Part (a): Identifying the reaction, reagent and conditions

Converting but-2-ene into butane is an addition reaction, specifically hydrogenation: hydrogen atoms add across the C=C double bond. The reagent required is hydrogen gas, $\text{H}_2$ , and the reaction needs a nickel catalyst (platinum or palladium would also work) at a temperature of around 150°C.

Part (b): The equation

$\text{CH}_3\text{CH}=\text{CHCH}_3 + \text{H}_2 \xrightarrow{\text{Ni, } 150°\text{C}} \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$

Check (atom balance): C: $4=4$ . H: $8+2=10$ on the left; $\text{C}_4\text{H}_{10}$ has $10$ hydrogen atoms on the right. Both balance.

Part (c): Industrial application

Hydrogenation of C=C double bonds is used industrially to convert unsaturated liquid vegetable oils (which contain several C=C double bonds) into saturated, solid or semi-solid fats, for example, in the manufacture of margarine from vegetable oil.

Part (d): Why but-2-ene reacts further but butane does not

But-2-ene contains a reactive carbon-carbon double bond: one of the two bonds joining these carbons (the $\pi$ bond) has electron density exposed above and below the plane of the molecule, making it available to react with an added atom on each carbon. Butane, in contrast, is fully saturated. Every carbon is already bonded to four other atoms by strong single ( $\sigma$ ) bonds only, with no $\pi$ bond present, so there is no multiple bond across which further atoms could add.

Final answers

(a) Addition (hydrogenation); reagent $\text{H}_2$ ; nickel catalyst, around 150°C.
(b) $\text{CH}_3\text{CH}=\text{CHCH}_3 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$
(c) Hardening of unsaturated vegetable oils into margarine (or an equivalent solid-fat example).
(d) But-2-ene has a reactive $\pi$ bond available for addition; butane has only single bonds and is fully saturated, so it cannot react further with hydrogen.