Benzyl

  In organic chemistry, benzyl is the term for the anion C₆H₅CH₂^-, which can be obtained formally by deprotonation of toluene. The benzyl functional group is sometimes abbreviated "Bn", not to be confused with "Bz", which is short for the benzoyl group. For example, benzyl alcohol can be abbreviated to BnOH.

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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.

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
• 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 ^[1]:

The benzyl group can be removed by hydrogenation. PMB ethers can be cleaved by magnesium bromide –dimethyl sulfide, CAN or DDQ ^[2].

One study ^[3] 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 SN1 based on the detection of trace amounts of Friedel-Crafts reaction sideproducts with toluene as a solvent.

References

1. ^ 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
2. ^ Protecting groups Krzysztof Jarowicki and Philip Kocienski J. Chem. Soc., Perkin Trans. 1, 1998, 4005–4037 4005 doi:10.1039/a803688h
3. ^ 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.