Addition to a double bond

Adding across a double bond

The C=C double bond of an alkene contains a π bond, which is weaker than a single bond and opens easily to accommodate two new atoms.

Hydrogenation: addition of dihydrogen

In the presence of a metal catalyst (nickel or platinum), an alkene adds dihydrogen to give the corresponding alkane:

CH2=CH2 + H2 -> CH3-CH3   (catalyst: Ni)

Check: carbon 2=2, hydrogen (4+2)=6 on the left, 6 on the right. Balanced. It is this reaction, applied to vegetable oils rich in double bonds, that produces certain fats that are solid at room temperature.

Hydration: addition of water

In the presence of an acid catalyst, an alkene adds a water molecule to give an alcohol:

CH2=CH2 + H2O -> CH3-CH2-OH   (acid catalyst)

Check: carbon 2=2, hydrogen (4+2)=6 on the left, 6 on the right (3 in CH3, 2 in CH2, 1 in OH), oxygen 1=1. Balanced. This is one of the industrial routes for synthesizing ethanol.

A longer alkene: but-1-ene

Addition is not limited to ethene. But-1-ene CH2=CH-CH2-CH3, when reacted with dihydrogen, gives butane CH3-CH2-CH2-CH3: the double bond, wherever it is located on the chain, becomes single, and each carbon that carried it gains an extra hydrogen atom.

Common mistake

Do not place the new atoms just anywhere on the chain: in an addition, they necessarily attach to the two carbons that carried the double bond, never elsewhere on the molecule.