Benzyl
A benzyl group is a substituent or molecular fragment possessing the structure C6H5CH2-. The abbreviation “Bn” is commonly used in nomenclature and structural depictions of chemical compounds.
The term is also used in reference to the anion, carbocation, and free radical moieties featuring a benzene ring attached to a CH2 group, in which the CH2 group bears a negative charge, a positive charge, or a single unpaired electron respectively. In each case, the charge or radical electron is delocalized throughout the aromatic ring. The corresponding species is therefore much more stable than that of an ordinary primary anion, carbocation, or free radical.
This enhanced stability is observed in the Finkelstein reaction. Benzyl chloride has the same rate of reaction toward iodide as methyl chloride despite that methyl chloride is significantly more susceptible to SN2 nucleophilic attack. While both benzyl chloride and n-butyl chloride are primary alkyl halides, the rate of reaction of benzyl chloride is 179 times greater.
Protective Groups
Benzyl groups frequently can be introduced to alcohol and carboxylic acids and subsequently removed easily and in high yield; therefore they are frequently used in organic synthesis as protective groups.
The following are two common methods for the protection of alcohols as the corresponding benzyl ethers:
- reaction of an alcohol with benzyl bromide and a strong base via Williamson ether synthesis.
- reaction of alcohol with an imidate such as benzyl trichloroacetimidate (C6H5CH2OC(CCl3)=NH) promoted by trifluoromethanesulfonic acid.
The following is an example of the use of a p-methoxybenzyl (PMB) ether in total synthesis:
The group can be removed readily by hydrogenation or by using CAN, DDQ, or magnesium bromide–dimethyl sulfide.
The following example demonstrates the use of a benzyl pyridinium salt as a benzyl donor for alcohols:
The solvent is α,α,α-trifluorotoluene and MgO is an acid scavenger. The reaction is believed to proceed via an SN1 mechanism because Friedel-Crafts reaction side products are observed when toluene is used as a solvent.