Benzyl Alcohol

Benzyl alcohol with the chemical formula \[C_{6} H_{5} CH_{2} OH\] is an aromatic alcohol. The "Bn" group of benzyls is often abbreviated (not to be mistaken with "Bz" used for benzoyl), and benzyl alcohol is referred to as BnOH. Benzyl alcohol is a colourless liquid with a faint aromatic scent. Its polarity, low toxicity, and low vapour pressure make it a useful solvent. Benzyl alcohol has modest water solubility (4 g/100 mL), and alcohol and diethyl ether are miscible. The anion formed by alcohol group deprotonation is called benzylate, or benzyl oxide.

IUPAC Name: Phenyl methanol

Synonyms:

• Phenylcarbinol

• Benzenemethanol

Chemical Formula:  \[C_{7}H_{8}O\]

Molecular Weight: 108.14 g/mo

Occurrence: Most plants produce benzyl alcohol naturally, and it is commonly found in fruits and teas. It is present also in a variety of essential oils like jasmine, hyacinth, and ylang-ylang. It is also contained in the castoreum of the beaver's castor sacs.

Benzyl Alcohol Production Process

Benzyl alcohol is manufactured industrially from toluene via hydrolyzed benzyl chloride.

\[C_{6} H_{5} CH_{2} Cl + H_{2}O \rightarrow  C_{6} H_{5} CH_{2} OH + HCl\]

Another route involves benzaldehyde hydrogenation, a by-product of the toluene to benzoic acid oxidation.

Benzyl alcohol is currently produced mainly by a chemical synthetic method from benzyl chloride and sodium hydroxide. In the case of a chemical synthetic method, benzyl chloride (synthetic benzyl chloride) is prepared by a Grignard reaction and then benzyl alcohol is obtained from the synthetic benzyl chloride through reduction, hydrolysis, and dehydrogenation.

However, in the case of a chemical synthetic method, a large amount of benzyl chloride, which is a material that is difficult to handle, is used and, as a result, the problem of secondary pollution is caused. An alternative to the chemical synthetic method is a biological method.

known as Phenylethanol, is the simplest aromatic alcohol and is the primary source of phenyl in phenol-formaldehyde resin, the primary ingredient of the lacquer industry. It is also the most commonly used material in microencapsulation. 

Benzyl alcohol is used in many chemical reactions, such as hydrogenation to produce aniline or as a chiral auxiliary ligand for palladium complexes. It is a very important product for the plastics industry, and for many other industries including the manufacture of caprolactam, phenylethylamine, and phenyl methanol. 

Benzyl alcohol is used as a standard in the testing of wines, in particular for the evaluation of residual sugar in Madeira wine.

Structure and Reactions

The structure of benzyl alcohol is characterized by two phenyl rings connected through an alcohol, which may be in the trans (Z) or cis (E) conformation, depending on the presence of an additional \[CH_{2}CH(OH)\] group. Benzyl alcohol is a precursor to the drug thalidomide and is used as a reactant in chemical synthesis. Benzyl alcohol serves as a chiral auxiliary in asymmetric synthesis, particularly for the conversion of aldehydes to acids. 

The conversion of the carbonyl group to alcohol can be accomplished through a hydroxylation reaction, for example in the presence of rhodium hydroxide on alumina, using N-bromosuccinimide to promote the transformation. The product formed is the benzyl alcohol of α- or β-configuration, depending on the chiral auxiliary used. Other chiral auxiliary compounds commonly used in the aldehyde-alcohol condensation include α-phenyl-γ-butyrolactone, α-benzoyloxyazetidine, 1-benzyl-2-azetidine carboxylic acid, and γ-butyrolactone. The reaction is applicable to the condensation of other aldehydes, including aldehydes containing enantioenriched or racemic carbon-carbon double bonds and with carbonyl groups other than carboxyl.

Oligomerization

Benzyl alcohol, either in its aldehyde or hydrate form, is able to oligomerize in the presence of excess hydrogen halides, for example, HF, to form dibenzyl ethers. Reactions are catalyzed by a palladium or platinum complex, e.g. \[Pd(dppf)Cl_{2}\], and the selectivity of oligomerization is governed by the nature of the anion. Benzyl alcohol forms homopolymers in anhydrous solvents such as benzene, and poly(2-methylbutanal) has been prepared. The monomer has been used in solution polymerization to form poly benzyl ethers.

Benzyl alcohol can form hydrogen-bonded aggregates in the solid state, with hydroxyl groups arranged to provide intermolecular hydrogen bonds. Oligomers have been prepared by the condensation of benzyl alcohol and water. The oligomerization takes place by either mechanism involving carboxylate anions or by the addition of hydrogen halides.

Benzyl alcohol polymerization with ethylene leads to polyethylene. The polymerization of ethylene on a small scale using benzyl alcohol as a catalyst was discovered by M. O. Wallin and J. W. Adams, in their study on polymers produced by olefins. This is a chain-growth polymerization and is thought to be initiated by the formation of active cationic centers such as carbocations, or by a radical mechanism. Polyethylene can be prepared by polymerizing ethylene and benzyl alcohol, and a range of polyethylenes can be prepared by changing the monomer ratio. Benzyl alcohol polymerization is a homogeneous catalytic process, as the catalyst is soluble in the reaction mixture. The main advantage of benzyl alcohol polymerization is that it is a non-migrating polymerization, which leads to a polymer with greater purity than polymers prepared by free-radical polymerization.

The mechanism of polymerization is thought to be initiated by a nucleophilic attack of the alcohol onto a carbon atom of the growing chain, which in turn triggers more chain growth. The polymer chains terminate with alcohol groups which can undergo further cyclization reactions to form cyclohexylidene ketals or benzofuranones. Polyethylene formed by this method is linear, having short polymer chains. Linear polyethylenes produced by this mechanism are normally called poly(ethylene-co-1-hexene) (PE-1-hexene), as the hexene is incorporated into the chain.

In addition to the main chain, terminal hydroxyl groups are also produced, and these undergo cyclization reactions to form the cyclohexylidene ketals or benzofuranones. The degree of cyclization can be controlled by the ratio of ethylene and alcohol in the feed.

Ethylene polymerization can be performed at lower temperatures (25-55 °C) compared with typical Ziegler–Natta, and metallocene catalysts (160-350 °C), which makes it advantageous in commercial operations.  It can also be run at a lower pressure than metallocene and Ziegler–Natta catalysts, at atmospheric pressure.

The mechanism of initiation and propagation in benzyl alcohol polymerization is still not entirely clear. It is thought to be initiated by heterolytic cleavage of the benzyl group, followed by cyclization to form an open-chain species. This is followed by recombination to form a propagating polyethylene chain. This chain may be terminated by a different type of initiation, which is thought to be the formation of a cyclic ketal, which undergoes ring opening to form a linear polyethylene. However, many possible mechanisms are possible, and further study is required.

It was found that for certain reactions, such as the polymerization of benzyl alcohol, the addition of methyl bromide results in a higher initial conversion rate. This is likely due to the methyl bromide reacting with the alkoxy groups of the benzyl alcohol to form alkyl bromide species.

Properties

Physical Properties

• It appears as a colourless liquid

• It has a slightly aromatic odour.

• It has a density of 1.044 g cm−3

• It has a melting point of −15.2 °C (4.6 °F; 257.9 K)

• The boiling point of benzyl alcohol is 205.3 °C (401.5 °F; 478.4 K)

Solubility

• It is soluble in water (3.50 g/100 mL (20 °C); 4.29 g/100 mL (25 °C))

• It is soluble in benzene, methanol, ether, acetone, chloroform, ethanol.

Chemical Properties

This interacts with carboxylic acids to form esters just like most alcohols. Benzyl esters are common protecting groups in organic synthesis because they can be extracted by mild hydrogenolysis.

Benzyl alcohol reacts to N-benzyl acrylamide by administering acrylonitrile. This is a case in point of a Ritter reaction.

Benzyl Alcohol is Synthesised for Laboratory Use:

Benzyl alcohol is also produced by the Grignard reaction of phenyl magnesium bromide (\[C_{6} H_{5} Mg Br\]) with formaldehyde and the benzaldehyde Cannizzaro reaction. The latter also gives benzoic acid, an example of a reaction by organic disproportion.

Uses

• Benzyl alcohol is used as a common solvent for the decoration of tints, waxes, shellacs, oils, lacquers, and epoxy resin.

• Benzyl alcohol is used in intravenous medicines, cosmetics, and topical products as a bacteriostatic preservative at low concentrations.

• 5% solution of benzyl alcohol for the treatment of head lice in people 6 months of age and older. 

• It affects the spirals of the louse, preventing them from closing.

• These then become clogged with water or mineral oil or other matter and cause asphyxiation to cause the insect to die.

• Benzyl alcohol is commonly used as the active ingredient in lotion products containing 5 percent benzyl alcohol for curing lice infestations.

• It is used to determine the presence of contaminants and the quality of quartz.