This can be derived by calculating the maximum number of Hydrogens (Hmax) using the formula below. Then the double bond equivalents can be calculated using the second formula
Hmax= 2C + 2 + N;
DBE = (Hmax- H)/2
- H = No. hydrogens
- C = No. carbons
- N = No. nitrogens
Why express the number of double bonds as DBEs? If you calculated the DBE for any given formula, it doesn’t mean that all DBEs are double bonds! If you look at the cycloalkanes, their general formulae follow CnH2n(n>3), i.e. it seems like they all have one double bond!
The formation of a ring results in the ‘loss’ of two hydrogens, just like a C=C bond does. So a single ring system has (at least) one DBE