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Benzyl group and derivatives: Benzyl group, benzyl radical, benzyl amine, benzyl bromide, benzyl chloroformate, and benzyl methyl ether. R = heteroatom, alkyl, aryl, allyl etc. or other substituents.

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" 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". Confusingly, in old literature, "Bzl" was also used for benzyl.

Reactivity of benzylic centersEdit

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 positionEdit

In a few cases, these benzylic transformations occur under mild enough conditions to be exploited synthetically. 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, chromium trioxide-3,5-dimethylpyrazole complex (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 groupEdit

Alcohol protectionEdit

Benzyl group protecting an alcohol

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

Most common protection methodsEdit

Most common deprotection methodsEdit

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

Reductive conditionsEdit
Oxidative conditionsEdit
Lewis acid-basedEdit

The p-methoxybenzyl protecting groupEdit

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

The p-methoxybenzyl group

Most common protection methodsEdit

Most common deprotection methodsEdit

  • 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 protectionEdit

Benzyl group protecting an amine

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

Most common amine protection methodsEdit

Most common amine deprotection methodsEdit

See alsoEdit


  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.
  4. ^ Chandler), Norman, R. O. C. (Richard Oswald (1993). Principles of organic synthesis. Coxon, J. M. (James Morriss), 1941- (3rd. ed.). London: Blackie Academic & Professional. ISBN 978-0751401264. OCLC 27813843.
  5. ^ Johnston, Jeffrey N. (2001), "Chromium(VI) Oxide–3,5-Dimethylpyrazole", Encyclopedia of Reagents for Organic Synthesis, American Cancer Society, doi:10.1002/047084289x.rc170, ISBN 9780470842898 |chapter= ignored (help)
  6. ^ Baran, Phil S.; Zhong, Yong-Li (2001-04-01). "Selective Oxidation at Carbon Adjacent to Aromatic Systems with IBX". Journal of the American Chemical Society. 123 (13): 3183–3185. doi:10.1021/ja004218x. ISSN 0002-7863.
  7. ^ a b 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.
  8. ^ Fukuzawa, Akio; Sato, Hideaki; Masamune, Tadashi (1987-01-01). "Synthesis of (±)-prepinnaterpene, a bromoditerpene from the red alga Yamada". Tetrahedron Letters. 28 (37): 4303–4306. doi:10.1016/S0040-4039(00)96491-8.
  9. ^ Van Hijfte, Luc; Little, R. Daniel (1985-10-01). "Intramolecular 1,3-diyl trapping reactions. A formal total synthesis of (±)-coriolin". The Journal of Organic Chemistry. 50 (20): 3940–3942. doi:10.1021/jo00220a058. ISSN 0022-3263.
  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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  16. ^ Hanessian, Stephen; Marcotte, Stéphane; Machaalani, Roger; Huang, Guobin (2003-11-01). "Total Synthesis and Structural Confirmation of Malayamycin A: A Novel Bicyclic C-Nucleoside from Streptomyces malaysiensis". Organic Letters. 5 (23): 4277–4280. doi:10.1021/ol030095k. ISSN 1523-7060. PMID 14601979.
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