Substitution, addition, elimination

Three main families of reactions

In organic chemistry, almost all transformations fall into one of these three families.

1. Substitution

An atom or group of atoms is replaced by another, without changing the carbon skeleton. Example: a haloalkane treated with a hydroxide ion gives an alcohol, with the bromine being replaced by -OH:

CH3-CH2-Br + HO- -> CH3-CH2-OH + Br-

Check: carbon 2=2, hydrogen (5+1)=6 on the left, 6 on the right, bromine 1=1, oxygen 1=1. Charges: -1 on the left (HO-), -1 on the right (Br-). Balanced in elements and charges.

2. Addition

Two atoms or groups add on either side of a multiple bond (double or triple), which becomes a single bond. This is the typical reaction of alkenes.

3. Elimination

This is the reverse of addition: a molecule loses two atoms (or groups) carried by two neighboring carbons, which creates a double bond. Example: the dehydration of an alcohol into an alkene, under the effect of heat and an acid catalyst:

CH3-CH2-OH -> CH2=CH2 + H2O

Check: carbon 2=2, hydrogen 6=(4+2)=6, oxygen 1=1. Balanced.

Key takeaways

• Substitution: a group replaces another, the skeleton's degree of saturation does not change.
• Addition: a multiple bond disappears, the molecule becomes more saturated.
• Elimination: a multiple bond appears, the molecule becomes less saturated.

Common mistake

Do not confuse addition and substitution: in an addition, no atom is expelled from the main molecule (everything combines into a single molecule); in a substitution, a fragment is indeed expelled (here, the bromide ion Br-).