InIUPAC nomenclature, the prefixbenzyl refers to aC6H5CH2 substituent, for examplebenzyl chloride orbenzyl benzoate. Benzyl is not to be confused withphenyl with the formulaC6H5. The termbenzylic is used to describe the position of the first carbon bonded to a benzene or otheraromatic ring. For example,(C6H5)(CH3)2C+ is referred to as a "benzylic" carbocation. The benzylfree radical has the formulaC6H5CH2•. The benzyl cation or phenylcarbenium ion is thecarbocation with formulaC6H5CH+2; the benzyl anion or phenylmethanide ion is thecarbanion with the formulaC6H5CH−2. None of these species can be formed in significant amounts in the solution phase under normal conditions, but they are useful referents for discussion ofreaction mechanisms and may exist as reactive intermediates.
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 thebenzoyl groupC6H5C(O)−. Likewise, benzyl should not be confused with the phenyl groupC6H5−, abbreviated Ph.
The enhanced reactivity of benzylic positions is attributed to the lowbond dissociation energy for benzylic C−H bonds. Specifically, the bondC6H5CH2−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.
The weakness of the C−H bond reflects the stability of the benzylic radical. For related reasons, benzylic substituents exhibit enhanced reactivity, as inoxidation,free radical halogenation, orhydrogenolysis. As a practical example, in the presence of suitable catalysts,p-xylene oxidizes exclusively at the benzylic positions to giveterephthalic acid:
Millions of tonnes of terephthalic acid are produced annually by this method.[4]
In a few cases, these benzylic transformations occur under conditions suitable for lab synthesis. TheWohl-Ziegler reaction will brominate a benzylic C–H bond: (ArCHR2 → ArCBrR2).[5] Any non-tertiary benzylic alkyl group will be oxidized to a carboxyl group by aqueous potassium permanganate (KMnO4) or concentrated nitric acid (HNO3): (ArCHR2 → ArCOOH).[6] Finally, the complex ofchromium trioxide and3,5-dimethylpyrazole (CrO3−dmpyz) will selectively oxidize a benzylic methylene group to a carbonyl: (ArCH2R → ArC(O)R).[7]2-iodoxybenzoic acid in DMSO performs similarly.[8]
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.[9]
^Xue, Xiao-Song; Ji, Pengju; Zhou, Biying; Cheng, Jin-Pei (2017). "The Essential Role of Bond Energetics in C–H Activation/Functionalization".Chemical Reviews.117 (13):8622–8648.doi:10.1021/acs.chemrev.6b00664.PMID28281752.
^Zhang, Xian-Man; Bordwell, Frederick G. (1992). "Homolytic bond dissociation energies of the benzylic carbon-hydrogen bonds in radical anions and radical cations derived from fluorenes, triphenylmethanes, and related compounds".Journal of the American Chemical Society.114 (25):9787–9792.doi:10.1021/ja00051a010.
^C., Vollhardt, K. Peter (2018-01-29).Organic chemistry : structure and function. Schore, Neil Eric, 1948- (8e ed.). New York.ISBN9781319079451.OCLC1007924903.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: multiple names: authors list (link)
^Chandler), Norman, R. O. C. (Richard Oswald (1993).Principles of organic synthesis. Coxon, J. M. (James Morriss), 1941- (3rd. ed.). London: Blackie Academic & Professional.ISBN978-0751401264.OCLC27813843.{{cite book}}: CS1 maint: multiple names: authors list (link)
^Johnston, Jeffrey N. (2001), "Chromium(VI) Oxide–3,5-Dimethylpyrazole",Encyclopedia of Reagents for Organic Synthesis, American Cancer Society,doi:10.1002/047084289x.rc170,ISBN9780470842898
^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.ISSN0002-7863.PMID11457049.
^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.
^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.ISSN0022-3263.
^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.
^Smith, Amos B.; Zhu, Wenyu; Shirakami, Shohei; Sfouggatakis, Chris; Doughty, Victoria A.; Bennett, Clay S.; Sakamoto, Yasuharu (2003-03-01). "Total Synthesis of (+)-Spongistatin 1. An Effective Second-Generation Construction of an Advanced EF Wittig Salt, Fragment Union, and Final Elaboration".Organic Letters.5 (5):761–764.doi:10.1021/ol034037a.ISSN1523-7060.PMID12605509.
^Marco, José L.; Hueso-Rodríguez, Juan A. (1988-01-01). "Synthesis of optically pure 1-(3-furyl)-1,2-dihydroxyethane derivatives".Tetrahedron Letters.29 (20):2459–2462.doi:10.1016/S0040-4039(00)87907-1.
^Takaku, Hiroshi; Kamaike, Kazuo; Tsuchiya, Hiromichi (1984-01-01). "Oligonucleotide synthesis. Part 21. Synthesis of ribooligonucleotides using the 4-methoxybenzyl group as a new protecting group for the 2′-hydroxyl group".The Journal of Organic Chemistry.49 (1):51–56.doi:10.1021/jo00175a010.ISSN0022-3263.
^Hanessian, Stephen; Marcotte, Stéphane; Machaalani, Roger; Huang, Guobin (2003-11-01). "Total Synthesis and Structural Confirmation of Malayamycin A: A Novel BicyclicC-Nucleoside from Streptomyces malaysiensis".Organic Letters.5 (23):4277–4280.doi:10.1021/ol030095k.ISSN1523-7060.PMID14601979.
^Kuehne, Martin E.; Xu, Feng (1993-12-01). "Total synthesis of strychnan and aspidospermatan alkaloids. 3. The total synthesis of (±)-strychnine".The Journal of Organic Chemistry.58 (26):7490–7497.doi:10.1021/jo00078a030.ISSN0022-3263.
^Cain, Christian M.; Cousins, Richard P. C.; Coumbarides, Greg; Simpkins, Nigel S. (1990-01-01). "Asymmetric deprotonation of prochiral ketones using chiral lithium amide bases".Tetrahedron.46 (2):523–544.doi:10.1016/S0040-4020(01)85435-1.
^Zhou, Hao; Liao, Xuebin; Cook, James M. (2004-01-01). "Regiospecific, Enantiospecific Total Synthesis of the 12-Alkoxy-Substituted Indole Alkaloids, (+)-12-Methoxy-Na-methylvellosimine, (+)-12-Methoxyaffinisine, and (−)-Fuchsiaefoline".Organic Letters.6 (2):249–252.doi:10.1021/ol0362212.ISSN1523-7060.PMID14723540.
^Rong, Yi; Al-Harbi, Ahmed; Parkin, Gerard (2012). "Highly Variable Zr–CH2–Ph Bond Angles in Tetrabenzylzirconium: Analysis of Benzyl Ligand Coordination Modes".Organometallics.31 (23):8208–8217.doi:10.1021/om300820b.
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