Lena Savoie
Benzyl alcohol is an organic compound with a variety of applications, including as a solvent, in the synthesis of esters and ethers, and as an active ingredient in lotions and shampoos for treating lice infestations. It is produced industrially from toluene via benzyl chloride, which is hydrolyzed. Another method for producing benzyl alcohol involves the oxidation of benzene using an oxidizing agent like $KMnO_4$, followed by the reduction of benzoic acid using a reducing agent such as $LiAlH_4$. Additionally, benzyl alcohol can be synthesized through benzylic substitution, utilizing various catalytic reactions, including photocatalysis, oxyalkylation, and hydroxysulfenylation.
| Characteristics | Values |
|---|---|
| Industrial production | From toluene via benzyl chloride, which is hydrolyzed |
| Alternative industrial production | Through hydrogenation of benzaldehyde, a by-product of toluene oxidation |
| Laboratory production | Grignard reaction of phenylmagnesium bromide (C6H5MgBr) with formaldehyde |
| Laboratory production | Cannizzaro reaction of benzaldehyde |
| Reaction | Like most alcohols, it reacts with carboxylic acids to form esters |
| Organic synthesis | Benzyl esters are popular protecting groups due to their removability by mild hydrogenolysis |
| Reaction | Benzyl alcohol reacts with acrylonitrile to give N-benzylacrylamide (Ritter reaction) |
| Synthesis by benzylic substitution | Three-component oxychalcogenation reaction of alkenes, diselenides/thiophenols, and H2O/alcohols |
| Synthesis by benzylic substitution | Visible light-promoted hydroxysulfenylation of alkenes with thiophenols |
| Synthesis by benzylic substitution | Esterification of primary benzylic C-H bonds with carboxylic acids using di-tert-butyl peroxide as an oxidant |
| Synthesis by benzylic substitution | Rhenium-catalyzed oxyalkylation of alkenes |
| Benzene to benzyl alcohol | Use oxidizing agents like \({{KMn}}{{{O}}_4}\) |
| Benzene to benzyl alcohol | Reduce benzoic acid using a reducing agent like \({{LiAL}}{{{H}}_4}\) |
| Toluene to benzyl alcohol | Chlorinate toluene and heat with aqueous sodium hydroxide or potassium hydroxide |
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What You'll Learn
Using benzene as a starting material
Benzene is a very stable organic compound, making the direct substitution of the hydroxyl group difficult. Therefore, converting benzene to benzyl alcohol involves several steps and the formation of different compounds.
The first step is to perform the Friedel-Crafts alkylation reaction of benzene. This is done by reacting benzene with $CH_3Cl$ in the presence of aluminium chloride as a catalyst. This results in a methyl group being substituted into the benzene ring, forming toluene.
The next step is to oxidize the methyl group outside the benzene ring to form a carboxylic acid. This reaction must be carried out in a basic medium, followed by the addition of acidic hydrogen ions. While any oxidizing agent can be used, $KMnO_4$ is typically used for the oxidation of benzene.
The resulting benzoic acid must then be reduced to obtain benzyl alcohol. This can be achieved using a suitable reducing agent like $LiAlH_4$.
Another method to convert benzene to benzyl alcohol involves first chlorinating toluene and then heating it with aqueous sodium hydroxide or potassium hydroxide. The chlorination of toluene forms benzyl chloride, which then reacts with sodium hydroxide or potassium hydroxide to form benzyl alcohol.
Additionally, benzyl alcohol can be synthesized through the Grignard reaction of phenylmagnesium bromide ($C6H5MgBr$) with formaldehyde or the Cannizzaro reaction of benzaldehyde. These reactions are commonly used for laboratory-scale production.
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Employing benzylic substitution reactions
Benzylic substitution reactions are an important tool in organic chemistry, enabling the preparation of various aromatic compounds. The benzylic position refers to a carbon atom directly bonded to a benzene ring. This position is highly reactive due to the resonance stabilisation of the benzylic carbon, which facilitates nucleophilic substitution and elimination reactions.
One example of a benzylic substitution reaction is the conversion of benzene to benzyl alcohol. This process involves the oxidation of benzene using an oxidising agent like ${{KMn}}{{{O}}_4}$, forming benzoic acid. Subsequently, a suitable reducing agent such as ${{LiAL}}{{{H}}_4}$ is employed to reduce benzoic acid and obtain benzyl alcohol.
Another method involves the chlorination of toluene, followed by heating with aqueous sodium hydroxide or potassium hydroxide to produce benzyl alcohol. Benzyl chlorides can also be utilised for benzylic substitution reactions, providing functionalised allylbenzene derivatives through nickel-catalysed intermolecular benzylation.
Benzylic oxidation is a valuable technique for preparing substituted benzoic acids. This process involves the Friedel-Crafts reaction, followed by oxidation, as an alternative strategy for converting benzene or its derivatives into benzoic acids. The Grignard reaction between phenylmagnesium halides and carbon dioxide is another approach to preparing benzoic acids.
Additionally, benzylic substitution reactions can be employed to introduce specific functional groups to aromatic rings. For instance, the nitro group can be easily added to a benzene ring through nitration with a strong acid. Subsequently, under various conditions, the nitro group can be reduced to an amino group, forming aniline. This process is particularly useful for adding amino groups to aromatic rings.
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Oxidising benzene with KMnO4
To add a benzyl group to an alcohol, you will need to perform multiple steps. Firstly, you must oxidise benzene to benzoic acid. One method of doing this is by using KMnO4, a powerful oxidising agent. KMnO4 is commonly used in organic chemistry textbooks and is applicable in a wide range of reactions.
KMnO4 will oxidise the benzene ring and cleave the alkene carbon-carbon bonds. The benzene ring is inert to strong oxidising agents such as KMnO4, but the presence of the aromatic ring dramatically affects the reactivity of alkyl side chains. These side chains react quickly with oxidising agents and are converted into carboxyl groups, forming benzoic acid.
KMnO4 oxidises organic molecules and will proceed until the formation of carboxylic acids. Therefore, alcohols will be oxidised to carbonyls (aldehydes and ketones), and aldehydes will be oxidised to carboxylic acids. Primary alcohols can be oxidised by KMnO4 in the presence of basic copper salts, but the product is predominantly octanoic acid, with only a small amount of aldehyde.
KMnO4 is not considered well-suited for the conversion of alcohols to aldehydes or ketones, and careful control of reaction conditions is necessary. Under mild conditions, KMnO4 can convert alkenes to glycols, but it can also further oxidise the glycol and cleave the carbon-carbon bond.
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Reducing benzoic acid with LiAlH4
Lithium aluminum hydride (LiAlH4) is a strong reducing agent that can be used to reduce benzoic acid to benzyl alcohol. This reaction is an effective way to add a benzyl group to an alcohol.
LiAlH4 is a strong reducing agent that is similar to, but stronger than, sodium borohydride (NaBH4). It is known to reduce carboxylic acids, such as benzoic acid, to primary alcohols. In the case of benzoic acid, the benzene ring is not affected by the reaction; only the carboxylic acid group is reduced to an alcohol group.
The reaction between benzoic acid and LiAlH4 can be represented by the following equation:
> C6H5COOH + 4 LiAlH4 → C6H5CH2OH + 4 LiAlO2 + 2 H2
In this reaction, LiAlH4 acts as a reducing agent, donating four hydride ions (H-) to the carboxylic acid group (-COOH) of benzoic acid. This reduces the carboxylic acid group to an alcohol group (-CH2OH), forming benzyl alcohol. The other products of the reaction are lithium aluminum oxide (LiAlO2) and hydrogen gas (H2).
It is important to note that acid-base reactions of LiAlH4 tend to be violently exothermic and generate flammable hydrogen gas. Therefore, extreme caution is necessary when handling LiAlH4.
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Using Grignard reaction with phenylmagnesium bromide
Grignard reagents are formed from the reaction of an alkyl halide with magnesium metal in anhydrous ether. They are excellent carbon-based nucleophiles and strong bases. The Grignard reaction involves the addition of a carbanion nucleophile (R:−+MgX) to a carbonyl compound, yielding alcohols.
Phenylmagnesium bromide is a Grignard reagent that can be used to produce benzoic acid or triphenylmethanol. The reaction with triphenylmethanol involves reacting phenylmagnesium bromide with benzophenone. This reaction must be completed on the same day that the Grignard reagent is prepared.
To begin, dissolve 2.00 g (~11.0 mmol) of benzophenone in 15 mL of anhydrous diethyl ether in a clean and dry 100-mL round-bottom flask containing a clamshell-shaped stirring bar. Place the flask over a stirring hot plate (no heat) and stir until all the solid benzophenone has dissolved. Attach a Claisen adapter and place a condenser column into the adapter directly above the flask.
Place a separatory funnel containing 15 mL of phenylmagnesium bromide (in THF) into the side arm of the Claisen adapter. Start adding the Grignard reagent drop-wise from the separatory funnel. The reaction is complete when the red color disappears. When the reaction is complete, cool the tube in ice and add 2 mL of 3M HCl drop-wise with stirring.
It is important to note that Grignard reagents are highly reactive and sensitive to air and water. Therefore, great care must be taken to keep the reactions as isolated as possible from these elements. All glassware must be meticulously cleaned and dried before use, as water can destroy the reagent. Diethyl ether, which is commonly used in this reaction, is volatile and can cause dizziness and lightheadedness. It should always be handled in a closed or covered container.
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Frequently asked questions
Benzyl alcohol is an organic compound in which a hydroxyl group is attached to a $- {CH_2}$ group, which is attached to a benzene ring.
There are several methods to add a benzyl to an alcohol, including:
- Using a Grignard reaction of phenylmagnesium bromide ($C6H5MgBr$) with formaldehyde.
- Using the Cannizzaro reaction of benzaldehyde, which also yields benzoic acid.
- Oxidizing benzene to benzoic acid and then reducing it using a suitable reducing agent like $LiAlH_4$.
Some examples of benzyl alcohol synthesis by benzylic substitution include:
- A three-component oxychalcogenation reaction of alkenes, diselenides/thiophenols, and H2O/alcohols, utilizing tetrabutylammonium tribromide (TBATB) and dimethylsulfoxide (DMSO) as catalysts.
- A rhenium-catalyzed oxyalkylation of alkenes, using hypervalent iodine(III) reagents as both an oxygenation and alkylation source.
Benzyl alcohol has various applications, including:
- As a general solvent for inks, waxes, shellacs, paints, lacquers, and epoxy resin coatings.
- In the synthesis of esters and ethers, which are used in the soap, perfume, and flavor industries.
- As a local anesthetic, especially when combined with epinephrine.
