Reduction of Alkenes and Alkynes
Quick Notes
- Unsaturated compounds contain C=C (alkenes) or C≡C (alkynes) bonds.
- Hydrogenation = addition of hydrogen (H2) across multiple bonds.
- Alkenes + H2 → alkanes.
- Alkynes + H2 → alkenes, then alkanes (with excess H2).
- These reactions reduce the compound and lower the degree of unsaturation.
Full Notes
As covered previously, reduction often means the addition of hydrogen atoms to an organic molecule (see R3.2.10).
For unsaturated hydrocarbons (alkenes and alkynes), this means:
- Breaking π (pi) bonds in double or triple bonds
- Adding hydrogen atoms to form single (σ) bonds
This process decreases the degree of unsaturation – which is a count of double/triple bonds or rings in a molecule (see below).
Hydrogenation of Alkenes
Alkenes contain one π bond (C=C) and can be hydrogenated to alkanes.
- Requires a metal catalyst (e.g. Ni or Pt)
- Exothermic reaction
- Degree of unsaturation decreases by 1
Example Ethene to Ethane
CH2=CH2 + H2 → CH3–CH3
Hydrogenation of Alkynes
Alkynes have two π bonds (C≡C) and are more unsaturated than alkenes.
The hydrogenation occurs in two steps:
- CH≡CH + H2 → CH2=CH2 (alkyne to alkene)
- CH2=CH2 + H2 → CH3–CH3 (alkene to alkane)
With each hydrogenation step, the degree of unsaturation decreases by 1.
What Is Degree of Unsaturation (DU)?
The degree of unsaturation (also called index of hydrogen deficiency, IHD) tells you how many π bonds or rings are in a molecule.
Each π bond or ring reduces the number of hydrogen atoms in a compound compared to a fully saturated hydrocarbon.
Key contributions:
- C=C double bond → 1 degree
- C≡C triple bond → 2 degrees
- Ring → 1 degree
- Ignore O atoms (they don’t affect DU)
- For halogens (F, Cl, Br, I): count as hydrogen atoms
- For N atoms: subtract 1 H for each N
Formula to Calculate DU
where:
- C = number of carbon atoms
- H = number of hydrogen atoms
- N = number of nitrogen atoms
- X = number of halogen atoms
- Oxygen atoms ignored
Example Ethene (C2H4)
DU = (2×2 + 2 – 4) / 2 = 1 → one double bond
Example Benzene (C6H6)
DU = (2×6 + 2 – 6) / 2 = 4 → 1 ring + 3 double bonds
Example Propanoic Acid (C3H6O2)
DU = (2×3 + 2 – 6) / 2 = 1 → one degree → C=O bond
Summary
- Hydrogenation adds H2 across C=C or C≡C bonds.
- Alkenes reduce to alkanes, alkynes reduce to alkenes then alkanes.
- Degree of unsaturation counts π bonds and rings.
- Classification depends on whether the reaction changes oxidation state.
Linked Course Question
Why are some reactions of alkenes classified as reduction reactions while others are classified as electrophilic addition reactions?
Alkenes always react by breaking their C=C double bond and forming new bonds – but how we classify the reaction depends on what gets added to the carbon atoms:
- Electrophilic addition: an electrophile (e.g. HBr or H2O) is added. There is no change in the oxidation state of the carbon atoms, so it is not a redox reaction.
- Reduction: hydrogen (H2) is added, typically with a Ni catalyst. The carbon atoms gain hydrogen and their oxidation state decreases, so this is classified as a reduction.