Introduction:
The bond dissociation energy (BDE) is a measure of the strength of a bond. It represents the energy required to break a bond in a molecule, resulting in two separate radicals. In general, the C-H bond dissociation energy decreases as the number of alkyl substituents on the carbon atom increases. This is due to the increasing electron density around the carbon atom, which stabilizes the C-H bond and makes it harder to break.

Comparison of Compounds:

Toluene:
Toluene is an aromatic compound that contains a methyl group (CH3) attached to the benzene ring. The methyl group is an alkyl substituent. The C-H bond in toluene is relatively weak due to the presence of the electron-donating methyl group. As a result, toluene has a higher C-H bond dissociation energy compared to benzene.

Benzene:
Benzene is an aromatic compound that contains only hydrogen atoms attached to the benzene ring. There are no alkyl substituents in benzene. The C-H bond in benzene is stronger compared to toluene because it lacks the electron-donating effect of alkyl groups. Therefore, benzene has a lower C-H bond dissociation energy compared to toluene.

n-Pentane:
n-Pentane is an aliphatic compound that contains a straight chain of five carbon atoms. It does not have any alkyl substituents or aromatic rings. The C-H bonds in n-pentane are relatively stronger compared to benzene and toluene. This is because the electron density around the C-H bonds in n-pentane is lower than in aromatic compounds or compounds with alkyl substituents.

2,2-Dimethylpropane:
2,2-Dimethylpropane is an aliphatic compound that contains a branched chain of four carbon atoms. It has two methyl groups attached to a central carbon atom. The presence of the two methyl groups increases the electron density around the central carbon atom, which stabilizes the C-H bonds. As a result, 2,2-dimethylpropane has the lowest C-H bond dissociation energy among the given compounds.

Conclusion:
In summary, the C-H bond dissociation energy decreases as the number of alkyl substituents on the carbon atom increases. Among the given compounds, 2,2-dimethylpropane possesses the C-H bond with the lowest bond dissociation energy due to the stabilizing effect of the two methyl groups.