Theo Nash
There are several methods to obtain alcohol, a common chemical component with a wide range of applications. Alcohols can be made through the hydration of alkenes or the reduction of aldehydes, ketones, acids, and esters. Aldehydes can be converted to primary alcohols, and ketones to secondary alcohols, through catalytic hydrogenation or chemical reducing agents such as lithium aluminium hydride. Alcohols can also be obtained through the use of Grignard reagents, which are highly reactive and can react with numerous inorganic and organic compounds. The type of alcohol obtained through Grignard synthesis depends on the carbonyl compound used in the reaction. Additionally, alkenes treated with diborane form alkyl boranes, which, when oxidised with alkaline hydrogen peroxide, produce alcohol. Fermentation, a process of culturing yeast under favourable thermal conditions, is another method to produce alcohol, specifically ethanol.
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What You'll Learn
Oxidation
The oxidation reactions involve the formation of a carbon-to-oxygen double bond. The carbon atom bearing the OH group must be able to release one of its attached atoms to form the double bond. The carbon-to-hydrogen bonding is easily broken under oxidative conditions, but carbon-to-carbon bonds are not. Therefore, tertiary alcohols, where the carbon atom carrying the OH group does not have a hydrogen atom attached but is instead bonded to other carbon atoms, are resistant to oxidation.
Primary and secondary alcohols are readily oxidized. For instance, methanol and ethanol are oxidized by liver enzymes to form aldehydes. The oxidation of primary alcohols forms aldehydes, and further oxidation forms carboxylic acids. On the other hand, the oxidation of secondary alcohols yields ketones.
Alcoholic fermentation, which involves the conversion of sugar into ethyl alcohol by yeast, is another process that involves oxidation. During this process, certain inorganic substances like (NH4)2SO4 or phosphates are added as food for the fermenting cells.
Additionally, the oxidation of alkylboranes, which are formed when alkenes are treated with diborane, can produce alcohol.
Furthermore, Grignard reagents and carbonyl compounds can be used to synthesize three different forms of monohydric alcohols (primary, secondary, and tertiary alcohols). Lithium aluminium hydride (LiAlH4) is also a reagent that can convert acids into alcohols.
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Dehydration
The dehydration reaction is typically carried out by warming the alcohol in the presence of a strong dehydrating acid, such as concentrated sulfuric acid. The alcohol undergoes protonation, which allows the hydroxyl group to detach as a water molecule. This leaves a carbocation, a species with a carbon atom bearing three bonds and a positive charge. The stability of the carbocation is enhanced by the removal of a proton from an adjacent carbon atom, resulting in the formation of the alkene.
The ease and speed of alcohol dehydration are influenced by the stability of the carbocation intermediate. More highly substituted carbocations exhibit greater stability, which makes them more susceptible to dehydration. This relationship is reflected in Saytzeff's rule, which states that the major product of dehydration is typically the compound with the most highly substituted double bond.
One notable application of alcohol dehydration is in the preparation of ethyl ether, also known as diethyl ether. This important industrial solvent is most economically produced through the intermolecular dehydration of simple alcohols, such as methanol and ethanol. However, it is crucial to carefully control the reaction conditions to achieve this desired outcome.
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Substitution
Alcohols are versatile compounds that can undergo various reactions, including substitution reactions. Substitution reactions involving alcohols can lead to the formation of different products, depending on the reactants and reaction conditions. One common substitution reaction involving alcohols is the formation of alkyl halides.
When alcohols react with hydrogen halides, such as HCl, HBr, or HI, a substitution reaction occurs, resulting in the formation of an alkyl halide and water. The order of reactivity of the alcohols follows the pattern: 3° > 2° > 1° methyl. Similarly, the reactivity of hydrogen halides decreases in the order: HI > HBr > HCl. This reaction is acid-catalyzed and involves the breaking of the O-H bond in the alcohol, leading to the substitution of the halide ion.
Another substitution reaction involving alcohols is the conversion of an alcohol to a tosylate or mesylate. During this process, the configuration of the alcohol starting material is retained, resulting in a product with opposite stereochemistry. This reaction is followed by an SN2 reaction, causing an inversion of configuration.
The Grignard reagent, an organometallic compound, plays a crucial role in the synthesis of alcohols. It reacts with various inorganic and organic compounds, including water, carbon dioxide, and oxygen. The type of alcohol obtained from a Grignard synthesis depends on the carbonyl compound used in the reaction. Formaldehyde yields primary alcohols, while aldehydes and acetic acid yield secondary alcohols. Ketones, on the other hand, give rise to tertiary alcohols.
In summary, substitution reactions involving alcohols are diverse and depend on the specific reactants and reaction conditions employed. These reactions play a significant role in the synthesis and transformation of alcohols, contributing to their importance in organic chemistry.
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Esterification
The Fischer Esterification Process is a famous reaction that uses an alcohol and a carboxylic acid to form an ester. This process involves breaking the O-H bond of the alcohol, not the C-O bond, which means that the absolute configuration of the carbon atom attached to the hydroxyl group remains unchanged throughout the reaction. This allows for control over the stereochemistry in an organic synthesis.
Alcohols can also be converted to alkyl sulfonates, which are sulfonic acid esters. These esters are formed by reacting an alcohol with a suitable sulfonic acid. For example, methyl tosylate, a typical sulfonate, is formed by reacting methyl alcohol with tosyl chloride.
In addition, alcohols can be converted to metal salts, alkyl halides, aldehydes, ketones, and carboxylic acids. The most common reactions involving alcohols can be classified into five categories: oxidation, dehydration, substitution, esterification, and reactions of alkoxides.
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Reactions of alkoxides
The most common reactions of alcohols can be classified as oxidation, dehydration, substitution, esterification, and reactions of alkoxides.
Alkoxides are formed by the reaction between alcohols and metallic sodium. For example, when a small piece of sodium is dropped into ethanol, bubbles of hydrogen gas are given off, and a colourless solution of sodium ethoxide is left behind. If this solution is evaporated carefully, sodium ethoxide is left behind as a white solid.
The oxygen atom of an alcohol is nucleophilic, meaning it is prone to react with electrophiles. The resulting "onium" intermediate then loses a proton to a base, forming the substitution product. If a strong electrophile is not present, the nucleophilicity of the oxygen may be enhanced by conversion to its conjugate base (an alkoxide). This powerful nucleophile then reacts with weak electrophiles.
Alkyl substitution of the hydroxyl group creates ethers. This reaction provides examples of both strong electrophilic substitution and weak electrophilic substitution. The latter SN2 reaction is known as the Williamson ether synthesis and is generally only used with 1º alkyl halide reactants because the strong alkoxide base leads to E2 elimination with 2º and 3º alkyl halide.
Other Ways to Obtain Alcohols
Dehydration
Converting an alcohol to an alkene requires the removal of the hydroxyl group and a hydrogen atom on the neighbouring carbon atom. This reaction is called dehydration because the elements of water are removed. Dehydrations are most commonly carried out by warming the alcohol in the presence of a strong dehydrating acid, such as concentrated sulfuric acid.
Oxidation
Alcohols may be oxidized to give aldehydes, ketones, and carboxylic acids. The oxidation of organic compounds generally increases the number of bonds from carbon to oxygen and may decrease the number of bonds to hydrogen. Tertiary alcohols (R3COH) are resistant to oxidation because the carbon atom carrying the OH group does not have a hydrogen atom attached but is instead bonded to other carbon atoms. Primary and secondary alcohols are readily oxidized.
Substitution
When alcohols react with a hydrogen halide, a substitution takes place, producing an alkyl halide and water. The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive). The reaction is acid-catalysed.
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Frequently asked questions
There are several methods to obtain alcohol, including:
- Fermentation: The conversion of sugar into ethyl alcohol by yeast.
- Reduction: Using reagents like Lithium Aluminium Hydride (LiAlH4) to reduce an acid to an alcohol.
- Hydrolysis: The addition of water in the presence of a catalyst causes direct hydration, while indirect hydration is achieved by adding sulphuric acid to an alkane.
- Grignard Synthesis: Using Grignard reagents and carbonyl compounds to synthesise primary, secondary, and tertiary alcohols. Formaldehyde yields primary alcohols, aldehydes yield secondary alcohols, and ketones yield tertiary alcohols.
The most common reactions involving alcohols include:
- Oxidation: Alcohols can be oxidised to form aldehydes, ketones, and carboxylic acids.
- Dehydration: Alcohols can undergo dehydration to form alkenes or ethers.
- Substitution: Alcohols can react with hydrogen halides to produce alkyl halides and water.
- Esterification: Alcohols can be converted to esters by reacting with carboxylic acids.
The choice of method depends on the desired type of alcohol (primary, secondary, or tertiary) and the specific reactants available. Grignard synthesis, for example, is highly reactive and versatile, making it a popular choice. Other factors include the availability of starting materials and the desired yield and purity of the final product.
Yes, safety is a critical aspect when working with alcohols. Ethanol, for instance, is toxic and can cause intoxication or poisoning. Proper safety equipment, ventilation, and handling procedures are essential to minimise risks. Additionally, some reactants and products may have specific hazards that require special handling and disposal procedures.
