There are three types of C-H bonds in cyclohexene, and table 5.3 gives an idea of their relative strengths. Although a typical secondary alkyl C-H bondhas a strength of of about 400 kj/mol and a typical vinylic C-H bond has a strength of 445 kj/mol an allylic C-H bond has a strength of only about 360 kj/mol. An allylic radical is therefore more stable than atypical alkyl radical by about 40 kj/mol