IB DP Chemistry - R3.2.11 Reduction of unsaturated compounds - Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry -R3.2.11 Reduction of unsaturated compounds – Study Notes – New Syllabus
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Reactivity 3.2.11 — Reduction of Unsaturated Compounds
Reactivity 3.2.11 — Reduction of Unsaturated Compounds
Unsaturated organic compounds contain carbon-carbon multiple bonds, such as double bonds (alkenes) and triple bonds (alkynes). These compounds can undergo reduction via the addition of hydrogen (H2) across the multiple bonds — a process called hydrogenation. This reduces the degree of unsaturation by converting multiple bonds into single bonds.
Hydrogenation Reactions
These reactions are usually carried out in the presence of a metal catalyst, such as nickel (Ni), platinum (Pt), or palladium (Pd), under heat and pressure.
- Alkenes (C=C) → Alkanes (C–C)
- Alkynes (C≡C) → Alkenes → Alkanes (stepwise or complete)
1. Hydrogenation of Alkenes
General Reaction:
\( \text{RCH=CH}_2 + \text{H}_2 \xrightarrow{\text{Ni}, \ \Delta} \text{RCH}_2\text{CH}_3 \)
Example:
\( \text{CH}_2=CH_2 + \text{H}_2 \xrightarrow{\text{Ni}, \ \Delta} \text{CH}_3\text{CH}_3 \)
(Ethene → Ethane)
2. Hydrogenation of Alkynes
Full hydrogenation (two moles of H2):
\( \text{RC≡CH} + 2\text{H}_2 \xrightarrow{\text{Ni}, \ \Delta} \text{RCH}_2\text{CH}_3 \)
Example:
\( \text{CH}≡\text{CH} + 2\text{H}_2 \xrightarrow{\text{Ni}, \ \Delta} \text{CH}_3\text{CH}_3 \)
(Ethyne → Ethane)
Partial hydrogenation (stopped at alkene):
With special catalysts (e.g. Lindlar’s catalyst), alkynes can be reduced only to cis-alkenes.
\( \text{CH}≡\text{CH} + \text{H}_2 \xrightarrow{\text{Lindlar’s catalyst}} \text{CH}_2=CH_2 \)
Degree of Unsaturation
Hydrogenation reduces the degree of unsaturation by one unit per π bond. This means:
- Each double bond removed = 1 degree of unsaturation reduced
- Each triple bond removed = 2 degrees of unsaturation reduced (if fully hydrogenated)
Example
Write the equation for the hydrogenation of but-2-ene and name the product.
▶️Answer/Explanation
Equation: \( \text{CH}_3\text{CH=CHCH}_3 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)
Product: Butane
Example
Deduce the number of hydrogen molecules required to fully hydrogenate 1 mole of but-1-yne.
▶️Answer/Explanation
But-1-yne has one triple bond → 2 π bonds
So, 2 moles of H2 are needed for full hydrogenation:
\( \text{CH}≡\text{CH}_2\text{CH}_2 + 2\text{H}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)
Example
A compound with molecular formula \( \text{C}_5\text{H}_8 \) undergoes complete hydrogenation to form \( \text{C}_5\text{H}_{12} \) Identify the type of compound and deduce the number of double/triple bonds present.
▶️Answer/Explanation
The saturated hydrocarbon with 5 carbon atoms is pentane: \( \text{C}_5\text{H}_{12} \).
The compound before hydrogenation has the formula \( \text{C}_5\text{H}_8 \), which is 4 hydrogens fewer than an alkane → this corresponds to 2 degrees of unsaturation.
Thus, it likely contains one triple bond (≡) or two double bonds (=).
Example compound: 1-pentyne.
Equation: \( \text{CH}_3\text{CH}_2\text{C≡CHCH}_3 + 2\text{H}_2 \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3 \)