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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
, benzyl is the
substituent A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and '' functional group'', as well as '' ...
or molecular fragment possessing the structure . Benzyl features a
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...
ring () attached to a
methylene group In organic chemistry, a methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom ma ...
() group.


Nomenclature

In IUPAC nomenclature, the prefix benzyl refers to a substituent, for example benzyl chloride or benzyl benzoate. Benzyl is not to be confused with
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydroge ...
with the formula . The term benzylic is used to describe the position of the first carbon bonded to a benzene or other
aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...
ring. For example, is referred to as a "benzylic" carbocation. The benzyl
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...
has the formula . The benzyl cation or phenylcarbenium ion is the carbocation with formula ; the benzyl anion or phenylmethanide ion is the carbanion with the formula . 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

The abbreviation "Bn" denotes benzyl. For example,
benzyl alcohol Benzyl alcohol is an aromatic alcohol with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn" (not to be confused with "Bz" which is used for benzoyl), thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liqui ...
can be represented as BnOH. This abbreviation is not to be confused with "Bz", which is the abbreviation for the benzoyl group , or the phenyl group , 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 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 in
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
, free radical halogenation, or hydrogenolysis. As a practical example, in the presence of suitable catalysts, ''p''-
xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...
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.


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: (). Any non-tertiary benzylic alkyl group will be oxidized to a carboxyl group by aqueous potassium permanganate () or concentrated nitric acid (): (). Finally, the complex of
chromium trioxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple ...
and
3,5-dimethylpyrazole 3,5-Dimethylpyrazole is an organic compound with the formula (CH3C)2CHN2H. It is one of several isomeric derivatives of pyrazole that contain two methyl substituents. The compound is unsymmetrical but the corresponding conjugate acid (pyrazolium ...
() will selectively oxidize a benzylic methylene group to a carbonyl: (). 2-iodoxybenzoic acid in DMSO performs similarly.


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.


Alcohol protection

Benzyl is commonly used in organic synthesis as a robust protecting group for alcohols and
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyl ...
s. * Treatment of alcohol with a strong base such as powdered
potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...
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


Deprotection methods

Benzyl ethers can be removed under '' reductive conditions'', '' oxidative conditions'', and the use of ''Lewis Acids''. * Removed using hydrogenolysis *: * Single electron process with Na/ NH3 or Li/NH3 * Benzyl protecting groups can be removed using a wide range of oxidizing agents including: ** CrO3/
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
at ambient temperature **
Ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
** ''N''-Bromosuccinimide (NBS) ** ''N''-Iodosuccinimide (NIS) * 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 Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
( 4-Methoxybenzylthiol is used to protect thiols). * Strong base such as powdered
potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...
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 **:


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 occasionally used as a protecting group for
amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...
s in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. Other methods exist. * Aqueous potassium carbonate and benzyl halide ( BnCl, BnBr) in methanol *: * Benzaldehyde, 6 M HCl and NaBH3CN in methanol *:


Deprotection methods

*
Hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate org ...
in the presence of the palladium catalyst


See also

*
Benzylamine Benzylamine is an organic chemical compound with the condensed structural formula C6H5CH2NH2 (sometimes abbreviated as PhCH2NH2 or BnNH2). It consists of a benzyl group, C6H5CH2, attached to an amine functional group, NH2. This colorless wate ...


References


External links

* * {{functional group, state=expanded Aryl groups Protecting groups