Benzyl group

Source: Wikipedia, the free encyclopedia.
Not to be confused with
phenyl
.
allyl
etc. or other substituents.

In organic chemistry, benzyl is the substituent or molecular fragment possessing the structure R−CH2−C6H5. Benzyl features a benzene ring (C6H6) attached to a methylene group (−CH2−) group.[1]

Nomenclature

In

phenyl
with the formula C6H5. The term benzylic is used to describe the position of the first carbon bonded to a benzene or other
free radical has the formula C6H5CH2•. The benzyl cation or phenylcarbenium ion is the carbocation with formula C6H5CH+2; the benzyl anion or phenylmethanide ion is the carbanion with the formula C6H5CH−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 of reaction mechanisms
and may exist as reactive intermediates.

Abbreviations

Benzyl is most commonly abbreviated Bn. For example,

benzoyl
group C6H5C(O)−. Likewise, benzyl should not be confused with the phenyl group C6H5−, 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 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[2][3] 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 similar to benzylic C-H
(C6H4)2CH−H
fluorenyl
C–H bond 		80 more activated vs diphenylmethyle (
pKa
= 22.6)
(C6H5)2CH−H diphenylmethyl C–H bond 82 "doubly benzylic" (
pKa
= 32.2)
(C6H5)3C−H
trityl
C–H bond 		81 339 "triply benzylic"

The weakness of the C−H bond reflects the stability of the benzylic radical. For related reasons, benzylic substituents exhibit enhanced reactivity, as in

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.[4]

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: (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 of chromium trioxide and 3,5-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]

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.[9]

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

oxidative conditions, and the use of Lewis acids.[9]

The p-methoxybenzyl protecting group

p-Methoxybenzyl (PMB) is used as a

alcohols in organic synthesis (4-Methoxybenzylthiol
is used to protect thiols).

The p-methoxybenzyl group

Deprotection methods

  • 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ)[18]
  • 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.[9]

Deprotection methods

Structure of tetrabenzylzirconium with H atoms omitted for clarity.[22]

See also

References

  1. ISBN 9780387448978
    .
  2. PMID 28281752
    .
  3. doi:10.1021/ja00051a010
    .
  4. ISBN 978-3527306732
    .
  5. OCLC 1007924903.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: multiple names: authors list (link
    )
  6. OCLC 27813843.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  7. ISBN 9780470842898
  8. PMID 11457049
    .
  9. ^
    S2CID 83393227
    .
  10. doi:10.1016/S0040-4039(00)96491-8
    .
  11. ISSN 0022-3263
    .
  12. doi:10.1016/j.tetlet.2003.09.106
    .
  13. PMID 12605509
    .
  14. doi:10.1016/S0040-4039(00)87907-1
    .
  15. ISSN 0022-3263
    .
  16. PMID 17985906
    .
  17. ISSN 1521-3765
    .
  18. PMID 14601979
    .
  19. ISSN 0022-3263
    .
  20. doi:10.1016/S0040-4020(01)85435-1
    .
  21. PMID 14723540
    .
  22. doi:10.1021/om300820b
    .

External links

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