Diphenylmethane
# Diphenylmethane

Diphenylmethane is an organic compound with the formula $(C_6H_5)_2CH_2$ (often abbreviated CH $_2Ph$ $_2).$ 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.

## Synthesis

It is prepared by the Friedel–Crafts alkylation of benzyl chloride with benzene in the presence of a Lewis acid such as aluminium chloride:

    $C_6H_5CH_2Cl$ + $C_6H_6$ → $(C_6H_5)_2CH_2$ + HCl

## Reactivity of the C-H bond

The methylene group in diphenylmethane is mildly acidic with a $pK_a$ of 32.2, and so can be deprotonated with sodium amide.

    $(C_6H_5)_2CH_2$ + $NH_2^−$ → $(C_6H_5)_2CH^−$ + $NH_3$

The resulting carbanion can be alkylated. For example, treatment with n-bromobutane produces 1,1-diphenylpentane in 92% yield.

    $(C_6H_5)_2CH^−$ + $CH_3CH_2CH_2CH_2Br$ → $(C_6H_5)_2CHCH_2CH_2CH_2CH_3$ + $Br^−$

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%.

The acidity of the methylene group in diphenylmethane is due to the weakness of the $(C_6H_5)_2CH–H$ bond, which has a bond dissociation energy of 82 kcal $mol^{−1}$ (340 kJ $mol^{−1}).$ This is well below the published bond dissociation energies for comparable C–H bonds in propane, where $BDE((CH_3)_2CH–H)=98.6$ kcal $mol^{−1},$ and toluene, where $BDE(C_6H_5CH_2–H)=89.7$ kcal $mol^{−1}.$

