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The structure of the benzyl group

In organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C₆H₅CH₂-. The abbreviation "Bn" is frequently used to denote benzyl moieties in nomenclature and structural depictions of chemical compounds. For example, benzyl alcohol can be represented as BnOH. This abbreviation is not to be confused with "Bz", which is the abbreviation for the benzoyl group C₆H₅C(O)-.

The term benzyl is also used in reference to the carbocation^[1], anion, and free radical^[2] moieties featuring a benzene ring attached to a CH₂ group, in which the CH₂ group bears a positive charge, a negative charge, or a single unpaired electron respectively.

In substituent nomenclature, benzyl and phenyl are commonly confused. Benzyl can be seen as a phenyl attached to a CH₂ before attaching to the parent compound.

The term benzylic refers to the position on a carbon skeleton next to a phenyl or other aromatic ring. Because of the aromatic neighbor, benzylic positions are endowed with special reactivity, as in oxidation, free radical halogenation, or hydrogenolysis.

Benzyl protective groups

Benzyl groups are frequently used in organic synthesis as protective group for alcohols and carboxylic acids.

Two common methods for benzyl ether protection:

• reaction of alcohol with benzyl bromide and a strong base such as sodium hydride as in a Williamson ether synthesis^[3]
• reaction of alcohol with an imidate such as benzyl trichloroacetimidate (C₆H₅CH₂OC(CCl₃)=NH) promoted by trifluoromethanesulfonic acid. An example forming a p-methoxybenzyl (PMB) ether in total synthesis:^[4]

The benzyl group can be removed by hydrogenation. PMB ethers can be cleaved by magnesium bromide –dimethyl sulfide, CAN or DDQ.^[5]

One study ^[6] employs a benzyl pyridinium salt as a benzyl donor for alcohols:

The solvent is trifluoromethylbenzene and magnesium oxide is an acid scavenger. The reaction type for this conversion is believed to be S_N1 based on the detection of trace amounts of Friedel-Crafts reaction side-products with toluene as a solvent.

References

1. ^ Carey, F. A., and Sundberg, R. J.; Advanced Organic Chemistry, Part A: Structure and Mechanisms, 5th ed.; Springer: New York, NY, 2008. pp 806–808.
2. ^ Carey, F. A., and Sundberg, R. J.; Advanced Organic Chemistry, Part A: Structure and Mechanisms, 5th ed.; Springer: New York, NY, 2008. pp 312–313.
3. ^ DeSelms, R. H.; Benzyl Phenyl Ether Compounds; Enigen Science Publishing: Washington, DC, 2008.
4. ^ Total synthesis of rutamycin B via Suzuki macrocyclization James D. White, Thomas Tiller, Yoshihiro Ohba, Warren J. Porter, Randy W. Jackson, Shan Wang, and Roger Hanselmann 80 Chem. Commun., 1998 doi:10.1039/a707251a
5. ^ Protecting groups Krzysztof Jarowicki and Philip Kocienski J. Chem. Soc., Perkin Trans. 1, 1998, 4005–4037 4005 doi:10.1039/a803688h
6. ^ K. W. C. Poon and G. B. Dudley (2006). "Mix-and-Heat Benzylation of Alcohols Using a Bench-Stable Pyridinium Salt". J. Org. Chem. 71 (10): 3923–3927. doi:10.1021/jo0602773.