Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated CH2Ph2). 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:[2]
- 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.[3]
- (C6H5)2CH2 + NaNH−2 → (C6H5)2CHNa + NH3
The resulting carbanion can be alkylated. For example, treatment with n-bromobutane produces 1,1-diphenylpentane in 92% yield.[4]
- (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][4]
The acidity of the methylene group in diphenylmethane is due to the weakness of the (C6H5)2CH−H, which has a bond dissociation energy (BDE) of 340 kJ/mol (82 kcal/mol).[5] This is well below the published bond dissociation energies for comparable C−H bonds in propane, where the BDE of (CH3)2CH−H is 413 kJ/mol (98.6 kcal/mol), and toluene, where the BDE of (C6H4)CH2−H is 375 kJ/mol (89.7 kcal/mol).[6][7]
See also
- Benzhydryl compounds
- Toluene (methylbenzene, phenylmethane)
- Triphenylmethane
- Tetraphenylmethane
References
- ↑ International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 452. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
- ↑ Hartman, W.W.; Phillips, R. (1934). "Diphenylmethane". Organic Syntheses. 14: 34. doi:10.15227/orgsyn.014.0034; Collected Volumes, vol. 2, 1943, p. 232.
- 1 2 Hauser, Charles R.; Hamrick, Phillip J. (1957). "Alkylation of Diphenylmethane with Alkyl Halides by Sodium Amide. Substitution versus β-Elimination. Relative Acidities of Diphenylmethane and Ammonia". J. Am. Chem. Soc. 79 (12): 3142–3145. doi:10.1021/ja01569a041.
- 1 2 Murphy, William S.; Hamrick, Phillip J.; Hauser, Charles R. (1968). "1,1-Diphenylpentane". Organic Syntheses. 48: 80. doi:10.15227/orgsyn.048.0080; Collected Volumes, vol. 5, 1973, p. 523.
- ↑ Zhang, Xian-Man; Bordwell, Frederick G. (1992). "Homolytic bond dissociation energies of the benzylic carbon-hydrogen bonds in radical anions and radical cations derived from fluorenes, triphenylmethanes, and related compounds". J. Am. Chem. Soc. 114 (25): 9787–9792. doi:10.1021/ja00051a010.
- ↑ Blanksby, S. J.; Ellison, G. B. (2003). "Bond Dissociation Energies of Organic Molecules". Accounts of Chemical Research. 36 (4): 255–263. doi:10.1021/ar020230d. PMID 12693923.
- ↑ Streitwieser, Andrew; Bergman, Robert G. (2018). "Table of Bond Dissociation Energies". University of California, Berkeley. Retrieved December 30, 2021.