Theo Nash
Methyl alcohol, also known as methanol, is a highly toxic organic chemical compound. It is a colourless, flammable liquid with a distinctive alcoholic odour. Due to its toxicity, it is often used as a denaturant additive for ethanol, which is then known as denatured alcohol or methylated spirit. Methanol is also used as a solvent, an antifreeze, and as fuel for alcohol lamps, portable fire pits, and camping stoves. The process of removing methyl alcohol and adding a methyl group to an alcohol involves several chemical reactions and considerations, such as using nucleophiles and electrophiles, converting the alcohol to a halide, or utilising specific reagents and reaction conditions.
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What You'll Learn
Use a nucleophile like Me-Li
To remove an alcohol and add a methyl group, one method is to first convert the alcohol to a ketone, then perform the Wittig reaction, and finally hydrogenate the product. Another possible reaction sequence involves first converting the alcohol into a ketone, then using a Grignard reagent, and finally reducing the alcohol.
One user suggests that the first step they considered was to use TsCl to create a good leaving group, followed by a Me nucleophile. However, they realized that this would not work because Li-Me/MeMgBr would displace the leaving group.
Instead, it is proposed that the alcohol be converted to a halide, creating a Grignard reagent, which can then be reacted with MeBr. This reaction sequence can successfully remove an alcohol and add a methyl group.
The use of Me-Li is specifically mentioned as a way to add the methyl group after converting the alcohol to a ketone. This reaction would leave an OH group, which can be removed by adding TsCl and a weak reducing agent, such as NaB(CN)H3.
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Convert alcohol to a halide
To convert an alcohol to a halide, the hydroxyl group must be protonated to convert it to a stable leaving group. The choice of reagent defines the stereochemistry. For example, thionyl chloride inverts the chiral configuration of the native alcohol, whereas tosyl chlorides retain the configuration. The most common methods for converting primary and secondary alcohols to the corresponding chloro and bromo alkanes are treatments with thionyl chloride (SOCl2) and phosphorus tribromide (PBr3), respectively. These reagents are generally preferred over the use of concentrated HX due to the harsh acidity of hydrohalic acids and the carbocation rearrangements associated with their use. Both of these reagents form an alkyl halide through an SN2 mechanism.
Tertiary alcohols react reasonably rapidly with HX (HCl, HBr, or HI), but for primary or secondary alcohols, the reaction rates are too slow. For the reactions that do occur, bubbling HX into an alcohol solution yields a haloalkane or alkyl halide.
Another method for converting alcohols into the corresponding iodides is through the use of elemental iodine (I2). Alcohols can also be converted to alkyl bromides with PBr3.
In summary, the process of converting an alcohol to a halide involves protonating the hydroxyl group to form a stable leaving group. This can be achieved through the use of various reagents such as thionyl chloride, phosphorus tribromide, or elemental iodine, depending on the specific alcohol and desired halide. The choice of reagent determines the stereochemistry and mechanism of the reaction, with some reagents inverting the chiral configuration while others retain it.
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Use an organocuprate as a nucleophile
Organocuprates, also known as Gilman reagents, are versatile compounds that can be used as nucleophiles in various organic synthesis reactions. They are particularly useful when you want to selectively form ketones or control the site of nucleophilic attack.
One of the key reactions of organocuprates is their ability to form new carbon-carbon bonds by reacting with alkyl halides. For example, dimethylcuprate ((CH3)2CuLi) can react with an alkyl bromide, resulting in the methyl group from the cuprate acting as a nucleophile. This nucleophilic methyl group replaces the bromine in the alkyl bromide, forming a new carbon-carbon bond. This reaction is often referred to as a coupling reaction, specifically known as the Corey-Posner/Whitesides-House reaction.
Organocuprates also exhibit interesting behaviour in their reactions with epoxides. They selectively attack the less substituted carbon atom of the epoxide ring, leading to the formation of an alkoxide intermediate. This intermediate then undergoes further reactions, such as treatment with water or acidic workup, to yield the final product, which is an alcohol.
Another important aspect of organocuprate chemistry is their reactivity with α, β–unsaturated carbonyl compounds. Unlike Grignard reagents, organocuprates exhibit a preference for conjugate (1,4) addition to the β-position of the carbonyl group. This results in the formation of an enolate intermediate, which can be further manipulated through subsequent reactions. For instance, performing a second alkylation before the acidic workup allows for the construction of more complex molecules in fewer synthetic steps.
In summary, organocuprates are valuable tools in organic synthesis due to their unique reactivity profiles. They can be used as nucleophiles to selectively form carbon-carbon bonds, ketones, and alcohols, making them versatile reagents in a range of synthetic applications.
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Use a ketone to add methyl
To remove an alcohol and add a methyl group, one can use a ketone. Ketones are organic compounds containing the carbonyl group (C=O). The reduction of ketones leads to secondary alcohols. Similarly, the reduction of aldehydes results in primary alcohols.
Ketones can be reduced to the corresponding methylene compound using the combination of PMHS and FeCl3. This method is convenient and inexpensive. Another option is to use a tandem catalyst composed of heterogeneous Pd/TiO2 and homogeneous FeCl3. This method enables the chemoselective deoxygenation of various aromatic ketones and aldehydes using polymethylhydrosiloxane (PMHS) as a green hydrogen source.
Ketones can also be reduced using sodium borohydride (NaBH4). This reaction can be carried out in water with added sodium hydroxide to make it alkaline. An intermediate product is formed, which can be converted into the final product by adding a dilute acid. Alternatively, the reaction can be carried out in an alcohol solution such as methanol, ethanol, or propan-2-ol. This results in an intermediate that can be converted into the final product by boiling it with water.
Another method involves the use of catalytic Pd(OAc)2 and polymethylhydrosiloxane (PMHS) for the chemo-, regio-, and stereoselective deoxygenation of benzylic oxygenated substrates. This reaction is carried out in the presence of aqueous KF and a catalytic amount of an aromatic chloride, involving palladium-nanoparticle-catalyzed hydrosilylation followed by C-O reduction.
It is important to note that the removal of alcohol resulting from the reduction of a ketone can be challenging due to the volatility difference between the alcohol and the corresponding carbonyl compound.
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Use a demethylating column
Demethylation is a chemical process that removes a methyl group (CH3) from a molecule. This process is often used to convert biomass into useful chemicals. For example, in the pulp and paper industry, lignin is digested using aqueous sodium sulfide, which partially depolymerizes lignin and removes methyl groups in the process.
A demethylizer column is a plate column with 77 plates. It is used to separate ethanol from water, as ethanol forms an azeotropic solution with water, while methanol does not. The raw distillate is fed into the column about 2/3rds of the way up, with hot water fed to the column top and steam injected at the column bottom. This allows a low-strength watery solution (15% ABV ethyl alcohol) to collect in the column bottom and a methanol/ethanol mixture to collect at the top. The watery solution is then re-distilled, yielding a 94%+ clean demethylized product.
The process of demethylation can also be used to remove methyl groups from organic molecules, which is a key aspect of diverse biological processes. For example, demethylation of lanosterol is catalyzed by a specific cytochrome P450 enzyme, which sequentially oxidizes the methyl group to an aldehyde.
Another method for removing methyl groups from aryl methyl ethers is to heat the ether in a solution of hydrogen bromide or hydrogen iodide, sometimes with acetic acid. This process can also be carried out using cyclohexyl iodide in N,N-dimethylformamide, which generates a small amount of hydrogen iodide in situ. Boron tribromide is a more specialized reagent for demethylation of aryl methyl ethers, which can be used at room temperature or below.
It is important to note that while demethylation can be a useful process in certain contexts, it is not possible to completely remove methanol from alcoholic drinks, according to an EU study. Additionally, methanol is a highly toxic substance that can cause blindness or death if ingested or absorbed through the skin or inhalation.
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