Benzyl group


In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure. Benzyl features a benzene ring attached to a methylene group.

Nomenclature

In IUPAC nomenclature, the prefix benzyl refers to a substituent, for example benzyl chloride or benzyl benzoate. Benzyl is not to be confused with phenyl with the formula.
The term benzylic is used to describe the position of the first carbon bonded to a benzene or other aromatic ring. For example, is referred to as a "benzylic" carbocation. The benzyl free radical has the formula. The benzyl cation or phenylcarbenium ion is the carbocation with formula ; the benzyl anion or phenylmethanide ion is the carbanion with the formula. None of these species can be formed in significant amounts in the solution phase under normal conditions, but they are useful referents for discussion of reaction mechanisms and may exist as reactive intermediates.

Abbreviations

Benzyl is most commonly abbreviated Bn. For example, benzyl alcohol can be represented as BnOH. Less common abbreviations are Bzl and Bz, the latter of which is ambiguous as it is also the standard abbreviation for the benzoyl group. Likewise, benzyl should not be confused with the phenyl group, abbreviated Ph.

Reactivity of benzylic centers

The enhanced reactivity of benzylic positions is attributed to the low bond dissociation energy for benzylic C−H bonds. Specifically, the bond is about 10–15% weaker than other kinds of C−H bonds. The neighboring aromatic ring stabilizes benzyl radicals. The data tabulated below compare benzylic C−H bond to related C−H bond strengths.
The weakness of the C−H bond reflects the stability of the benzylic radical. For related reasons, benzylic substituents exhibit enhanced reactivity, as in oxidation, free radical halogenation, or hydrogenolysis. As a practical example, in the presence of suitable catalysts, p-xylene oxidizes exclusively at the benzylic positions to give terephthalic acid:
Millions of tonnes of terephthalic acid are produced annually by this method.

Functionalization at the benzylic position

In a few cases, these benzylic transformations occur under conditions suitable for lab synthesis. The Wohl-Ziegler reaction will brominate a benzylic C–H bond:. Any non-tertiary benzylic alkyl group will be oxidized to a carboxyl group by aqueous potassium permanganate or concentrated nitric acid :. Finally, the complex of chromium trioxide and 3,5-dimethylpyrazole will selectively oxidize a benzylic methylene group to a carbonyl:. 2-iodoxybenzoic acid in DMSO performs similarly.

As a protecting group

Benzyl groups are occasionally employed as protecting groups in organic synthesis. Their installation and especially their removal require relatively harsh conditions, so benzyl is not typically preferred for protection.

Alcohol protection

Benzyl is commonly used in organic synthesis as a robust protecting group for alcohols and carboxylic acids.
Benzyl ethers can be removed under reductive conditions, oxidative conditions, and the use of Lewis acids.
p-Methoxybenzyl is used as a protecting group for alcohols in organic synthesis.
  • Strong base such as powdered potassium hydroxide or sodium hydride and p-methoxybenzyl halide
  • 4-methoxybenzyl-2,2,2-trichloroacetimidate can be used to install the PMB group in presence of:
  • *Scandium triflate in toluene at 0 °C
  • * Trifluoromethanesulfonic acid in dichloromethane at 0 °C
  • *:

    Deprotection methods

  • 2,3-Dichloro-5,6-dicyano-p-benzoquinone
  • :
  • Conditions for deprotection of benzyl group are applicable for cleavage of the PMB protecting group

    Amine protection

The benzyl group is occasionally used as a protecting group for amines in organic synthesis. Other methods exist.