Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated). The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. It is a white solid.
Diphenylmethane is a common skeleton in organic chemistry. The diphenylmethyl group is also known as benzhydryl.
It is prepared by the Friedel–Crafts alkylation of benzyl chloride with benzene in the presence of a Lewis acid such as aluminium chloride:[1]
C6H5CH2Cl + C6H6 → (C6H5)2CH2 + HCl
The methylene group in diphenylmethane is mildly acidic with a pKa of 32.2, and so can be deprotonated with sodium amide.
(C6H5)2CH2 + NaNH2- → (C6H5)2CHNa + NH3
The resulting carbanion can be alkylated. For example, treatment with n-bromobutane produces 1,1-diphenylpentane in 92% yield.[2]
(C6H5)2CH- + CH3CH2CH2CH2Br → (C6H5)2CHCH2CH2CH2CH3 + Br-
Alkylation of various benzhydryl compounds has been demonstrated using the corresponding alkyl halides, both primary (benzyl chloride, β-phenylethyl chloride, and n-octyl bromide) and secondary (benzhydryl chloride, α-phenylethyl chloride, and isopropyl chloride), in yields between 86 and 99%.[3]
The acidity of the methylene group in diphenylmethane is due to the weakness of the (C6H5)2CH - H bond, which has a bond dissociation energy of 82 kcal mol-1 (340 kJ mol-1).[4] This is well below the published bond dissociation energies for comparable C - H bonds in propane, where BDE((CH3)2CH - H)=98.6 kcal mol-1, and toluene, where BDE(C6H5CH2 - H)=89.7 kcal mol-1.[5] [6]