Benzyl group

In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure C6H5CH2–. Benzyl features a benzene ring attached to a CH2 group.[1]


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, (C6H5)(CH3)2C+ is referred to as a "benzylic" carbocation. The benzyl free radical has the formula C
. The benzyl cation or phenylcarbenium ion is the carbocation with formula C
; the benzyl anion or phenylmethanide ion is the carbanion with the formula C
. 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.


The abbreviation "Bn" denotes benzyl. 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". Confusingly, in old literature, "Bz" was also used for benzyl.

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 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.

Bond Bond Bond-dissociation energy Comment
(kcal/mol) (kJ/mol)
C6H5CH2−H benzylic C−H bond 90 377 akin to allylic C−H bonds
such bonds show enhanced reactivity
H3C−H methyl C−H bond 105 439 one of the strongest aliphatic C−H bonds
C2H5−H ethyl C−H bond 101 423 slightly weaker than H3C−H
C6H5−H phenyl C−H bond 113 473 comparable to vinyl radical, rare
CH2=CHCH2−H allylic C–H bond 89 372 such bonds show enhanced reactivity

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.[2]

Functionalization at the benzylic position

In a few cases, these benzylic transformations occur under conditions suitable for synthetic conditions. The Wohl-Ziegler reaction will brominate a benzylic C–H bond: (ArCHR2 → ArCBrR2).[3] Any non-tertiary benzylic alkyl group will be oxidized to a carboxy group by aqueous potassium permanganate (KMnO4) or concentrated nitric acid (HNO3): (ArCHR2 → ArCOOH).[4] Finally, the complex of chromium trioxide and 3,5-dimethylpyrazole] (CrO3–dmpyz) will selectively oxidize a benzylic methylene group to a carbonyl: (ArCH2R → ArC(O)R).[5] More recently, 2-iodoxybenzoic acid in DMSO has been reported to perform the same transformation.[6]

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.[7]

Alcohol protection

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

Deprotection methods

Benzyl ethers can be removed under reductive conditions, oxidative conditions, and the use of Lewis Acids.[7]

The p-methoxybenzyl protecting group

p-Methoxybenzyl (PMB) is used as a protecting group for alcohols in organic synthesis (4-Methoxybenzylthiol is used to protect thiols).

Deprotection methods

  • 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ)[16]
  • 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.[7]

Deprotection methods

See also


  1. Carey, F. A.; Sundberg, R. J. (2008). Advanced Organic Chemistry, Part A: Structure and Mechanisms (5th ed.). New York, NY: Springer. pp. 806–808, 312–313. ISBN 9780387448978.
  2. Sheehan, Richard J. "Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a26_193.
  3. C., Vollhardt, K. Peter (2018-01-29). Organic chemistry : structure and function. Schore, Neil Eric, 1948- (8e ed.). New York. ISBN 9781319079451. OCLC 1007924903.
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  5. Johnston, Jeffrey N. (2001), "Chromium(VI) Oxide-3,5-Dimethylpyrazole", Chromium(VI) Oxide–3,5-Dimethylpyrazole, Encyclopedia of Reagents for Organic Synthesis, American Cancer Society, doi:10.1002/047084289x.rc170, ISBN 9780470842898
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  7. Wuts, Peter G. M.; Greene, Theodora W. (2006). Greene's Protective Groups in Organic Synthesis (4th ed.). Wiley Online Library. doi:10.1002/0470053488. ISBN 9780470053485.
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  10. Sirkecioglu, Okan; Karliga, Bekir; Talinli, Naciye (2003-11-10). "Benzylation of alcohols by using bis[acetylacetonato]copper as catalyst". Tetrahedron Letters. 44 (46): 8483–8485. doi:10.1016/j.tetlet.2003.09.106.
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