Diphenylmethane Explained

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.

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:[1]

C6H5CH2Cl + C6H6 → (C6H5)2CH2 + HCl

Reactivity of the C-H bond

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]

See also

Notes and References

  1. 10.15227/orgsyn.014.0034 . Diphenylmethane . Organic Syntheses . 1934 . 14 . 34 . W. W. Hartman and Ross Phillips.
  2. William S. . Murphy . Phillip J. . Hamrick . Charles R. . Hauser . Charles R. Hauser . 1,1-Diphenylpentane . . 1968 . 48 . 80 . 10.15227/orgsyn.048.0080.
  3. Charles R. . Hauser . Charles R. Hauser . Phillip J. . Hamrick . . 79 . 12 . 3142-3145 . 1957 . Alkylation of Diphenylmethane with Alkyl Halides by Sodium Amide. Substitution versus β-Elimination. Relative Acidities of Diphenylmethane and Ammonia . 10.1021/ja01569a041.
  4. 10.1021/ja00051a010 . Homolytic bond dissociation energies of the benzylic carbon-hydrogen bonds in radical anions and radical cations derived from fluorenes, triphenylmethanes, and related compounds . Xian-Man . Zhang . Frederick G. . Bordwell . . 1992 . 114 . 25 . 9787–9792.
  5. Blanksby . S. J. . Ellison . G. B. . Bond Dissociation Energies of Organic Molecules . . 36 . 4 . 255–263 . 2003 . 12693923 . 10.1021/ar020230d.
  6. Web site: Table of Bond Dissociation Energies . Streitwieser . Andrew . Andrew Streitwieser . Bergman . Robert G. . Robert G. Bergman . 2018 . . December 30, 2021.