Alcohol, Phenols and Ethers

Alcohols are chemical compounds in which a hydroxyl group replaces the hydrogen atom of an aliphatic carbon. As a result, an alcohol molecule is made up of two components. The first has an alkyl group, while the second has a hydroxyl group.

They feature a unique set of physical and chemical properties and have a sweet odor. The existence of a hydroxyl group is the most important aspect in defining an alcohol's characteristics.

Alcohol are derivatives of hydrocarbons whose functional group is -OH as -OH has replaced a hydrogen atom. Depending on the presence of hydroxyl groups in the compound  there are different types of alcohol i.e primary alcohol, secondary alcohol, tertiary alcohol. Functional group of alcohol is known as -OH( Hydroxyl group). Nature of alcohol is mainly covalent in nature as the -OH group is attached to the carbon by covalent bond. Ethyl alcohol is considered as primary alcohol or one of the main alcohol and ethyl alcohol is also considered as ethanol. General formula of alcohol is CnH2n + 1OH.

Physical Properties of Alcohol

1. The Boiling Point of Alcohols

Alcohols have higher boiling points than other hydrocarbons with equivalent molecular weights. This is responsible for the formation of intermolecular hydrogen bonds between the hydroxyl groups of alcohol molecules. Furthermore, as the number of carbon atoms in the aliphatic carbon chain grows, the boiling point of alcohol rises.

Boiling points of alcohol are generally higher when compared to other hydrocarbons having the same molecular mass. The concept behind the higher boiling point of alcohols is the presence of intermolecular hydrogen bonding between hydroxyl groups of alcohol molecules. With the increase in a carbon chain of aliphatic alcohol boiling point increases whereas with an increase in the branching of alcohol boiling point decreases.

2. The Solubility of Alcohols

The hydroxyl group determines the solubility of alcohol in water. Intermolecular hydrogen bonds are formed by the hydroxyl group of alcohol. Alcohol is water-soluble due to hydrogen interaction between water and alcohol molecules. Because of the hydrophobic nature of the alkyl group, the solubility of alcohol reduces as the size of the alkyl group increases.

The solubility property of alcohol is determined by the presence of hydroxyl groups. Hydroxyl groups help in the formation of intermolecular hydrogen bonding as the hydrogen bond formed between water and alcohol molecules make them soluble in water, where the alkyl group is hydrophobic in nature. Thus solubility is directly related to the size of the alkyl group.

3. The Acidity of Alcohols

Alkoxides are formed when alcohols react with active metals such as sodium, potassium, and other elements. These reactions demonstrate the acidic nature of alcohol. The polarity of the –OH bond determines how acidic alcohol is. The acidity of alcohol decreases when an electron-donating group is added to the hydroxyl group. This is because it increases the electron density of the oxygen atom.

Alcohol reacts with metals to form the corresponding alkoxide. Example ethanol reacts with sodium metal to form Sodium ethoxide. This reaction shows the acidic property of alcohol as the -OH bond shows the polarity of alcohol. Their acidic property decreases when the electron-donating group is attached to the hydroxyl group.

Chemical Properties of Alcohol

1.  Oxidation of Alcohol

Aldehydes and ketones are generated when alcohols are oxidized in the presence of an oxidizing agent, and these can then be further oxidized to form carboxylic acids.

2. Dehydration of Alcohol

Alcohol dehydrates (loses a molecule of water) when exposed to protic acids, resulting in alkenes.

3. Catalytic Reduction of Butanal

Butanol is generated when butanal is dissolved. This is caused by a hydrogenation process. The hydrogens are added to the carbon-oxygen double bond, which is then changed to a carbon-oxygen single bond, resulting in the carboxyl oxygen group being transformed into a hydroxyl group.

A reduction process, also known as catalytic hydrogenation, is the addition of hydrogen to a carbon-carbon double bond to generate an alkane. The hydrogenation of a double bond is advantageous thermodynamically because it yields a more stable (lower energy) product.

Preparation of Alcohol

There are several methods for the preparation of alcohol, some of these methods are given below:

Hydrolysis of Halides: When alkyl halide is boiled with an aqueous solution of an alkali hydroxide, they form alcohol due to the nucleophilic substitution mechanism. Under this reaction, primary and secondary alcohols are formed.

Reaction:

R-X + KOH → R-OH + KX

Hydration of Alkanes: Under this reaction there occurs direct hydration of alkanes in the presence of a catalyst. 

Hydroboration of Alkenes: Under this reaction, an alkene is treated with diborane to form alkyl boranes, further alkyl boranes on oxidation with alkaline hydrogen peroxide to give alcohol as a final product. 

 Phenols

Phenols are chemical compounds that contain a benzene ring as well as a hydroxyl group. Carbolic acids are another name for them. They have special physical and chemical features due to the presence of a hydroxyl group.

It is an aromatic compound. It consists of phenyl groups attached to each other. Phenol is crystalline in nature having white color.

Physical and Chemical Properties of Phenol

1. Boiling Point of Phenols

Phenols have higher boiling points than other hydrocarbons with identical molecular weights. The primary explanation for this is the presence of intermolecular hydrogen bonding between the hydroxyl groups of phenol molecules. In general, as the number of carbon atoms increases, the boiling point of phenols rises.

2. Solubility of Phenols

The hydroxyl group determines phenol's water solubility. The development of intermolecular hydrogen bonds in phenol is due to the hydroxyl group. As a result, hydrogen bonds develop between water and phenol molecules, making phenol water-soluble.

3. Acidity of Phenols

When phenols combine with active metals like salt or potassium, they produce phenoxide. The acidic character of phenols is indicated by these reactions. The electron-withdrawing group in phenol is the sp2 hybridized carbon of the benzene ring linked directly to the hydroxyl group.

As a result, the electron density of oxygen is reduced. Phenoxide ions are more stable than alkoxide ions due to the delocalization of the negative charge in the benzene ring. As a result, we might conclude that phenols are acidic in comparison to alcohol.

4. Chirality of Phenols

Catechin, for example, is a phenol containing chirality inside its molecules. The lack of planar and axial symmetry in the phenol molecule accounts for this chirality.

Preparation of Phenol

1. From Haloarenes

Haloarenes include chemicals like chlorobenzene. The monosubstitution of a benzene ring yields chlorobenzene. We get sodium phenoxide when chlorobenzene reacts with sodium hydroxide at 623K and 320 atm. Finally, phenols are produced when sodium phenoxide is acidified.

2. From Benzene Sulphonic Acid

By reacting benzene with oleum, we can make benzene sulphonic. The resulting benzene sulphonic acid is treated with molten sodium hydroxide at a high temperature. Sodium phenoxide is formed as a result of this mechanism. Finally, phenols are produced when sodium phenoxide is acidified.

This reaction is the very first commercial step of phenol synthesis. In this process, sodium benzene sulphonate is fused with sodium hydroxide to form sodium phenoxide, which further undergoes acidification to yield phenol.

3. From Diazonium Salts

We may easily generate diazonium salts by treating an aromatic primary amine with nitrous (NaNO2 + HCl) acid at 273–278 K. In nature, these diazonium salts are quite reactive. When heated with water, these diazonium salts hydrolyze to phenols. We can make phenols by treating diazonium ions with dilute acids.

When a diazonium salts solution is treated with steam distilled or is added to boiling dil.H2SO4, it forms phenol as a final product.

4. From Cumene

Cumene is an organic chemical made by alkylating benzene with propylene in the Friedel-Crafts reaction. Cumene hydroperoxide is formed when cumene (isopropylbenzene) is oxidized in the presence of air.

Ethers

They belong to organic compounds that have an oxygen atom attached to two same or different alkyl or aryl groups. The general formula of ether is R-O-R, R-O-Ar or Ar-O-Ar.

Physical Properties of Ethers

1. Ethers have a comparable boiling point as alkanes. When compared to alcohols of comparable molecular mass, however, it is significantly lower. This is true despite the polarity of the C-O bond.

2. Ethers are water-miscible in the same way as alcohols are. Ether molecules are miscible in water. This is because, like alcohols, the oxygen atom of ether may form hydrogen bonds with a water molecule.

Chemical Properties of Ethers

1. Cleavage of C-O bond: Ethers are normally non-reactive. Cleavage of the C-O bond occurs when an excess of hydrogen halide is added to the ether. Alkyl halides are formed as a result of this reaction.

The following is the order of reactivity:

HI > HBr > HCl

R-O-R  +  HX  →  RX  +  R-OH

2. Electrophilic substitution: For electrophilic substitution, the alkoxy group in ether activates the aromatic ring at ortho and para locations. Halogenation, Friedel Crafts reaction, and other electrophilic substitution reactions are common.

3. Ether halogenation: Aromatic ethers undergo halogenation, such as bromination, when a halogen is added in the presence or absence of a catalyst.

4. Aromatic ethers undergo Friedel Crafts reaction, which involves the addition of an alkyl or acyl group when they are introduced to an alkyl or acyl halide in the presence of a Lewis acid as a catalyst.

Preparation of Ethers

There are several methods for the preparation of ether, some of these methods are given below:

Preparation of Ether by Dehydration of Alcohol: This reaction takes place in the presence of protic acid i.e sulphuric acid alcohol undergoes dehydration to produce alkenes and ether as their minor and major products. This reaction occurs at approx 443K. This is the ideal method of preparation of ether.

Williamson’s Synthesis: Under this reaction, alkyl halide reacts with sodium alkoxide and ether is formed as the main product. This reaction generally follows the SN2 mechanism for the formation of primary alcohol.