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Benzyl group

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Formula
  
C6H5CH2-

Benzyl group httpsuploadwikimediaorgwikipediacommonsthu

The difference between phenyl and benzyl groups


In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure C6H5CH2–. Benzyl features a benzene ring attached to a CH2 group.

Contents

Benzyl group Illustrated Glossary of Organic Chemistry Benzyl group

Nomenclature

Benzyl group Illustrated Glossary of Organic Chemistry Benzyl group

In IUPAC nomenclature the prefix benzyl refers to a C6H5CH2 substituent, for example benzyl chloride or benzyl benzoate. Benzyl is not to be confused with phenyl with the formula C6H5. The term benzylic is used to describe the position of the first carbon bonded to a benzene or other aromatic ring. For example, the molecule, is referred to as a "benzylic" carbocation. The benzyl free radical has the formula C
6H
5CH•
2. The benzylium carbocation has the formula C
6H
5CH+
2; the carbanion has the formula C
6H
5CH−
2. None of these species can be formed in significant amounts under normal conditions, but they are useful referents for discussion of reaction mechanisms.

Abbreviations

Benzyl group FileBenzylgrouppng Wikimedia Commons

The abbreviation "Bn" is frequently used to denote benzyl groups 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)−, or the phenyl group C6H5, abbreviated "Ph".

Reactivity of benzylic centers

Benzyl group FileBenzyl groupsvg Wikimedia Commons

The enhanced reactivity of benzylic positions is attributed to the low bond dissociation energy for benzylic C−H bonds. Specifically, the bond C6H5CH2−H 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.

Benzyl group FileBenzylgroupsvg Wikimedia Commons

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:

CH3C6H4CH3 + 3 O2 → HO2CC6H4CO2H + 2 H2O.

Millions of tonnes of terephthalic acid are produced annually by this method.

Alcohol protection

Benzyl, abbreviated as Bn, is commonly used in organic synthesis as a robust protecting group for alcohols and carboxylic acids.

Most common protection methods

  • Treatment of alcohol with a strong base such as powdered potassium hydroxide or sodium hydride and benzyl halide (BnCl or BnBr)
  • Monobenzylation of diols can be achieved using Ag2O in dimethylformamide (DMF) at ambient to elevated temperatures
  • Primary alcohols can be selectively benzylated in presence of phenol functional groups using Cu(acac)2

    • Most common deprotection methods

    Benzyl ethers can be removed under reductive conditions, oxidative conditions, and the use of Lewis Acids.

    Reductive conditions
  • Removed using hydrogenolysis
  • Single electron process with Na/NH3 or Li/NH3
    • Oxidative conditions
  • Benzyl protecting groups can be removed using a wide range of oxidizing agents including:
  • CrO3/acetic acid at ambient temperature
  • Ozone
  • N-Bromosuccinimide (NBS)
  • N-Iodosuccinimide (NIS)
    • Lewis acid-based
  • Trimethylsilyl iodide (Me3SiI) in dichloromethane at ambient temperature (selectivity can be achieved under specific conditions)

    • The p-methoxybenzyl protecting group

    p-Methoxybenzyl (PMB) is used as a protecting group for alcohols in organic synthesis.

    Most common protection methods

  • Strong base such as powdered potassium hydroxide or sodium hydride and p-methoxybenzyl halide (chloride or bromide)
  • 4-methoxybenzyl-2,2,2-trichloroacetimidate can be used to install the PMB group in presence of:
  • Scandium (III) triflate (Sc(OTf)3) in toluene at 0 °C
  • Trifluoromethanesulfonic acid (TfOH) in dichloromethane at 0 °C

    • Most common deprotection methods

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

    • Amine protection

    The benzyl group is largely used as a protecting group for amines in organic synthesis.

    Most common amine protection methods

  • Aqueous potassium carbonate and benzyl halide (BnCl, BnBr) in methanol
  • Benzaldehyde, 6 M HCl and NaBH3CN in methanol

    • Most common amine deprotection methods

  • Hydrogenation in the presence of the palladium catalyst

    • References

    Benzyl group Wikipedia
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