benzyl n : the univalent radical derived from toluene [syn: benzyl group, benzyl radical]

English

Pronunciation

Etymology

From benzoic + -yl.

Noun

1. A compound radical, C6H5.CH2, related to toluene and benzoic acid; -- commonly used adjectivally.

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, anion, and free radical 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.

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 :

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

One study 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