1,2-Addition Definition:

A reaction involving the addition of two groups to a conjugated p system in which one group is installed at the C1
position and the other group is installed at the C2 position.

1,2-Addition Explained:

The term “1,2-addition reaction” refers to two groups added at the first and second carbon (Hence denoted “C1” and “C2”, respectively). Below is an example reaction with HCl:

The first and second carbon reaction with HBr to form a 1,2 bond. The hydrogen bonds to the first carbon, while bromine binds to the second carbon because it is more substituted (may not be the case for certain condition).