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Alcohols

Alcohols are the compounds of the general formula R – OH, where R is an alkyl or a substituted alkyl group. Thus, alcohols are the derivatives of alkanes in which one or more H atoms are replaced by -OH.

Alcohols are very important industrial chemicals. Ethanol is widely used as an antiseptic in the form of a rectified spirit. It is the main component of all alcoholic beverages, and is used as a solvent for lacquers and varnishes.

Table of Contents

Preparation of Alcohols

Alcohol can be prepared by general methods as well as by industrial processes.

Preparation from Haloalkanes

Haloalkanes, when heated with aqueous alkali, or moist silver oxide, gives the corresponding monohydric alcohols.

By Fermentation of Carbohydrates

Slow decomposition of large organic molecules (carbohydrates, sugar and starch) in the presence of suitable enzymes produces alcohol. This method can be used to prepare ethanol by fermenting glucose.

Read More: Preparation of Alcohols

Properties of Alcohols

The physical and chemical properties of alcohols are primarily due to the presence of a hydroxyl group.

Physical Properties of Alcohols

  1. Alcohols are generally liquid at room temperature.
  2. They generally have higher boiling points in comparison to other hydrocarbons having equal molecular masses. In general, the boiling point of alcohol increases with an increase in the number of carbon atoms in the aliphatic carbon chain.
  3. They are water-soluble. Alcohols with smaller hydrocarbon chains are extremely soluble. The solubility in water decreases as the length of the hydrocarbon chain increases.
  4. Alcohols react with active metals like sodium, potassium, etc., which indicates that alcohols are acidic in nature. The acidic nature is due to the polarity of the bond between the hydrogen and oxygen atoms in the hydroxyl group.

Chemical Properties of Alcohols

Alcohols exhibit a wide range of spontaneous chemical reactions due to the cleavage of the C-O bond and O-H bond. Some prominent chemical reactions of alcohol are:

    1. Oxidation Reaction – Alcohols are oxidised in the presence of an oxidising agent to produce aldehydes and ketones, which can then be oxidised further to produce carboxylic acids.

    1. Reaction with metals – Alcohols react with active metals such as sodium, potassium etc. to form the corresponding alkoxide.

    1. Dehydration of Alcohols – When exposed to an acidic medium, alcohols dehydrate, which results in the formation of an alkene.

    1. Esterification – When alcohol reacts with carboxylic acid in the presence of a catalyst, it results in the formation of a sweet-smelling compound called an ester.

Classification of Alcohols

Alcohols can be classified on the following basis:

i. On the basis of the number of -OH groups present in the molecule.

ii. On the basis of the nature of the carbon atom bonded to the -OH group.

Classification of Alcohols on the basis of -OH groups

  • Monohydric alcohols – These contain one -OH group in their molecules. For example- .
  • Dihydric alcohols – These are the alcohols containing two -OH groups in their molecule. For example- 1,2-Ethandiol.
  • Trihydric alcohols – These contain three -OH groups in their molecule. For example- 1,2,3-Propantriol.
  • Polyhydric alcohols are the alcohols containing four or more OH groups in their molecule.

Classification of Alcohols on the basis of the nature of the carbon atom bonded to the -OH group

Alcohols are classified into three types- Primary, Secondary, and Tertiary based on the number of carbon atoms directly attached to the carbon bonded with the -OH group.

  • Primary Alcohols – The carbon atom having the -OH group is attached to only one carbon atom.
  • Secondary Alcohols – The carbon atom having the -OH group is attached to two carbon atoms.
  • Tertiary Alcohols – The carbon atom having the -OH group is attached to three carbon atoms.

Nomenclature of Alcohols

Alcohols are organic molecules with a functional hydroxyl or –OH group bonded directly to carbon. Alcohol gets its name from the hydrocarbon from which it is derived.

There are three systems of naming alcohols-

  1. Common or trivial system
  2. IUPAC system and
  3. Carbinol system

Trivial or Common System

The common name of alcohol can be obtained by adding the word alcohol after the alkyl group. Alternatively, the common name of alcohol can be obtained by replacing the ending -ane of the parent hydrocarbon by yl, and adding the word alcohol thereafter.

For alcohols containing three or more carbon atoms, there can be more than one location of the -OH group. Such alcohols are named primary, secondary or tertiary alcohols. They are often named using the prefixes as n or iso-.

  • The dihydric alcohols are named glycols. The names are derived from the corresponding alkene.
  • The trihydric alcohols are generally called by their names derived from the source of origin.

Trivial or Common System

IUPAC System

In this system, the name of alcohol is obtained by replacing the last ‘e’ in the name of the parent alkane, akene or alkyne by the suffix ‘ol’. However, the higher and branched-chain compounds follow certain procedures.

IUPAC System

Read More: Nomenclature of Alcohols

Uses of Alcohols

  • The alcohol in alcoholic drinks is ethanol.
  • Alcohol is used as high-efficiency fuel. It gives carbon dioxide and water when it burns.
  • It is also used to make vinegar.
  • It is used as an antiseptic, dressing for wounds, and antidote for snake bites. It is also used as a mild sedative.
  • Alcohol is a common cough remedy ingredient that can be found in most cough syrups.
  • Since alcohol has antifungal and antibacterial properties, it is used in disinfectants.
  • To eliminate microbial contamination, alcohol is used as a cleaning agent in hospitals, medical facilities, and laboratories.
  • Ethanol, Methanol, and Isopropanol are solvents used in analytical chemistry processes, such as chromatography analysis.

Some of the commonly used alcohols are methanol, ethanol, propanol and butanol.

Read More: Uses of Methanol and Ethanol

Frequently Asked Questions on Alcohol

What is the effect of alcohol on litmus?

Alcohols are neutral substances and have no effect on litmus.

How can alcohol be detected?

Test reagents that react with the -OH group can be used to detect the presence of alcohol. The first step in identifying alcohols is to take a neutral liquid devoid of water and add solid phosphorus(V) chloride. The presence of alcohol is indicated by a burst of acidic steamy hydrogen chloride fumes. Following tests are required to differentiate between alcohol classifications.

Give the classification of alcohols based on the number of -OH groups present in the molecule of alcohol.

Alcohols can be classified as monohydric, dihydric, trihydric and polyhydric based on the number of -OH groups present in the molecule of alcohol.

Which alcohol is the most soluble in water.

Methanol is most soluble as it has the smallest chain among all the other compounds of alcohol.

Which alcohol is named glycols?

The dihydric alcohols are named glycols.

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