
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 matter, organic materials, i.e., matter in its various forms that contain ...
, benzyl is the
substituent
In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule.
The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...
or molecular fragment possessing the structure . Benzyl features a
benzene
Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
ring () attached to a
methylene group ().
Nomenclature
In
IUPAC nomenclature
IUPAC nomenclature is a set of recommendations for naming chemical compounds and for describing chemistry and biochemistry in general. The International Union of Pure and Applied Chemistry (IUPAC) is the international authority on chemical nomenc ...
, the prefix benzyl refers to a substituent, for example
benzyl chloride
Benzyl chloride, or α-chlorotoluene, is an organic compound with the formula . This colorless liquid is a reactive organochlorine compound that is a widely used chemical building block.
Preparation
Benzyl chloride is prepared industrially ...
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 , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
with the formula .
The term benzylic is used to describe the position of the first carbon bonded to a benzene or other
aromatic
In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
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
Biogerontology
Biological processes
Causes of death
Cellular processes
Gerontology
Life extension
Metabolic disorders
Metabolism
...
has the formula . The benzyl cation or phenylcarbenium ion is the
carbocation
Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...
with formula ; the benzyl anion or phenylmethanide ion is the
carbanion
In organic chemistry, a carbanion is an anion with a lone pair attached to a tervalent carbon atom. This gives the carbon atom a negative charge.
Formally, a carbanion is the conjugate base of a carbon acid:
:
where B stands for the base (chemist ...
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 mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...
s and may exist as reactive intermediates.
Abbreviations
Benzyl is most commonly abbreviated Bn. For example,
benzyl alcohol
Benzyl alcohol (also known as α-cresol) 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 a ...
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 the
benzoyl group . Likewise, benzyl should not be confused with the phenyl group , abbreviated Ph.
Reactivity of benzylic centers
The enhanced reactivity of benzylic positions is attributed to the low
bond dissociation energy
The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical ...
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 or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
,
free radical halogenation
In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform ...
, or
hydrogenolysis
Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes lysis (breakdown) by hydrogen.Ralph Connor, Homer Adkins. Hydrogenolysis Of Oxygenated Organic Compounds. J. Am. Chem. Soc. ...
. 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 su ...
oxidizes exclusively at the benzylic positions to give
terephthalic acid
Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tons are produced annuall ...
:
:
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 . It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.
This compound is a dark-purple solid ...
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
2-Iodoxybenzoic acid (IBX) is an organic compound used in organic synthesis as an oxidizing agent. This periodinane is especially suited to oxidize alcohols to aldehydes. IBX is most often prepared from 2-iodobenzoic acid and a strong oxidant su ...
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
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
s and
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
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 utili ...
or
sodium hydride and benzyl halide (
BnCl or
BnBr)
*:

* Monobenzylation of
diol
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol may also be called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting gro ...
s can be achieved using
Ag2O in
dimethylformamide
Dimethylformamide, DMF is an organic compound with the chemical formula . Its structure is . Commonly abbreviated as DMF (although this initialism is sometimes used for 2,5-dimethylfuran, dimethylfuran, or dimethyl fumarate), this colourless liqui ...
(DMF) at ambient to elevated temperatures
*
Primary alcohol
A primary alcohol is an alcohol in which the hydroxy group is bonded to a primary carbon atom. It can also be defined as a molecule containing a “–CH2OH” group.
In contrast, a secondary alcohol has a formula “–CHROH” and a tertiary ...
s 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
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
conditions'', and the use of ''Lewis acids''.
* Removed using
hydrogenolysis
Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes lysis (breakdown) by hydrogen.Ralph Connor, Homer Adkins. Hydrogenolysis Of Oxygenated Organic Compounds. J. Am. Chem. Soc. ...
*:

* Single electron process with
Na/
NH3 or
Li/NH
3
* 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 compone ...
at ambient temperature
**
Ozone
Ozone () (or trioxygen) is an Inorganic compound, 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 , break ...
**
''N''-Bromosuccinimide (NBS)
**
''N''-Iodosuccinimide (NIS)
*
Trimethylsilyl
A trimethylsilyl group (abbreviated TMS) is a functional group in organic chemistry. This group consists of three methyl groups bonded to a silicon atom minus;Si(CH3)3 which is in turn bonded to the rest of a molecule. This structural group i ...
iodide (Me
3SiI) in
dichloromethane
Dichloromethane (DCM, methylene chloride, or methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odor is widely used as a solvent. Although it is not miscible with ...
at ambient temperature (selectivity can be achieved under specific conditions)
The ''p''-methoxybenzyl protecting group
''p''-Methoxybenzyl (PMB) is used as a
protecting group
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
In man ...
for
alcohols
In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...
in
organic synthesis
Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
(
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 utili ...
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
Triflic acid, the short name for trifluoromethanesulfonic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest known acids. Triflic acid is mainly used in research as a catalyst for este ...
(TfOH) in
dichloromethane
Dichloromethane (DCM, methylene chloride, or methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odor is widely used as a solvent. Although it is not miscible with ...
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
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
In man ...
for
amine
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
s in
organic synthesis
Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
. Other methods exist.
* Aqueous
potassium carbonate
Potassium carbonate is the inorganic compound with the formula . It is a white salt, which is soluble in water and forms a strongly alkaline solution. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used ...
and benzyl halide (
BnCl,
BnBr) in
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
*:

*
Benzaldehyde
Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is among the simplest aromatic aldehydes and one of the most industrially useful.
It is a colorless liquid with a characteristic almond-li ...
, 6 M
HCl and
NaBH3CN in
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
*:
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 redox, reduce or Saturated ...
in the presence of the
palladium
Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...
catalyst
See also
*
Benzylamine
Benzylamine, also known as phenylmethylamine, is an Organic chemistry, organic chemical compound with the Structural formula#Condensed formulas, condensed structural formula C6H5CH2NH2 (sometimes abbreviated as Phenyl group#Nomenclature, PhCH2NH2 ...
References
External links
*
*
{{functional group, state=expanded
Aryl groups
Protecting groups