What is Diol?

In chemistry there are so many chemical compounds that have a hydroxyl group but a chemical compound having two hydroxyl groups (-OH) is termed as a diol. It also implies the presence of two alcohols whereas polyols contain two or more than two hydroxyl groups (-OH). Glycol or 1,2-diol is an aliphatic diol that has two alcohol groups on adjacent carbon atoms. In industries, the most commonly used diol is ethylene glycol. 

Classes of Diols

As we all know diols are those chemical compounds that contain two hydroxyl groups (-OH groups). So based on the position of the hydroxyl groups, they are categorized into different classes:

Geminal diols

In this compound, two hydroxyl groups are bound at the same atom. Formation of this chemical compound occurs by hydration of the carbonyl compounds. 

Examples:

• Methanediol is considered to be the simplest diol. 
• Dihydroxymalonic acid (HOOC)₂C(OH)₂.
• Dihydroxycyclopentane (C(OH)₂) ₅.
• Chloral hydrate (Cl₃C)HC(OH)₂.

Physical properties

They are highly unstable in nature.

Vicinal diols

In this, the chemical compound has two hydroxyl groups occupying vicinal position, i.e., they are attached to adjacent atoms. These compounds are also called glycol.

Ethylene glycol is a kind of vicinal diol that is majorly used as a co-monomer in polymerization reactions during the process of formation of polymers. 

Examples:

• 1,2-ethanediol
• Ethylene glycol HO−(CH2)₂−OH.

Physical properties

• It has high melting and boiling points when compared to analogous alkanes.
• It has a high solubility in aqueous medium.

1, 3-diol

In this compound, one hydrogen from each methyl group is substituted by a hydroxyl group. 

Examples:

• 2-methyl-2-propyl-1,3-propanediol. 
• Neopentyl glycol.

 Physical properties

• They are colourless and viscous in nature. 
• They are easily soluble in liquid with a high (210°C) boiling point. 

Synthesis of Diol

Synthesis of germinal diol

• When the keto group combines with water molecules, they lead to the synthesis of the geminal hydroxyl group. 

Example:

When formaldehyde (H₂C=O) is dissolved in water, germinal diol (H₂C(OH)₂) is formed.

This reaction of synthesis of germinal diol is reversible and removal of the water molecule during a reaction can lead to the conversion of a gem-diol back to the corresponding carbonyl.

Synthesis of vicinal diol

• Vicinal diol can be synthesized by oxidation of alkenes usually with dilute acidic potassium permanganate.

• Sharpless asymmetric dihydroxylation reaction: In this reaction, an alkene is reacted with osmium tetroxide in the presence of chiral quinine ligand for the formation of vicinal diol. 

• On a commercial scale, vicinal diol is prepared by the hydrolysis of epoxides and the formation of the epoxides is done by epoxidation process of the alkene.

Synthesis of 1,3-diol

• It can be prepared by α,β-unsaturated ketones and aldehydes. The formed keto-alcohol is further hydrogenated. 
• Prinz reaction: 1,3-diol is synthesized by Prinz reaction also. In this reaction, electrophilic addition of an aldehyde or ketone to an alkene or alkyne occurs; followed by the capture of a nucleophile or elimination of an H⁺ ion. 

Different Reactions of Diols

• Diols undergo reaction with alcohols by the esterification process and lead to the formation of ether. 
• The C-C bond present in vicinal diol undergoes glycol cleavage and leads to the formation of ketone or aldehyde functional groups. 
• Geminal diols undergo hydration process and lead to the formation of carbonyl group compounds. Example: carbonic acid ((HO)₂C=O) is unstable and has a tendency to convert to carbon dioxide (CO₂).
• Vicinal diol undergoes cleavage oxidation reaction in the presence of periodic acid or HIO₄ to form two carbonyl compounds. This reaction can be used as a functional group test.

Uses of Diols

• Used in manufacturing of polyesters, particularly PET (polyethylene terephthalate).
• Vicinal diol is used in the food and medicine industry.
• Vicinal diol is also used as a relatively non-poisonous antifreezing product.
• Ethane 1,2-diol is used as a coolant in cars.
• It is also used in avoiding corrosion activity in iron.

Factors Affecting Stability of the Geminal Diol Equilibrium

The hydration ability of aldehyde and ketone is dependent on the stability of gem-diol; whereas stability of gem-diols depends on the following factors: 

• Steric hindrance by +I group around α-carbon decreases the stability of gem-diols.
• Stability of gem-diol is also dependent on the presence of –I group on α-carbon.
• Intramolecular hydrogen bonding present in the gem-diol increases its stability.  

Context and Applications

This chemistry topic is important for both undergraduate exams like NEET, JEE, PAT and other competitive exams and courses like Bachelors and Masters in Chemistry and Biochemistry.

Related Concepts

• Polyol
• Alcohol
• Synthesis of alcohol