Alcohols, Phenols and Ethers: Classification and Nomenclature of Alcohols (For CBSE, ICSE, IAS, NET, NRA 2022)

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Alcohols

  • Alcohols are organic compounds that have one or more hydroxy groups bonded to the carbon atoms in aliphatic compounds. They occur widely in nature and have many industrial and pharmaceutical applications.
  • For example, methanol and ethanol are two industrially important alcohols.

Classification

Alcohols are classified as primary , secondary or tertiary depending upon whether the number of alkyl groups bonded to the carbon atom bearing the hydroxy group is one, two or three, respectively.

Classification of Alcohol

Nomenclature of Alcohols

According to the IUPAC system of nomenclature, alcohols are called alkanols. They are named as the derivatives of the corresponding alkane in which the of the alkane is replaced by .

The procedure for nomenclature involves the following steps:

  • Step 1: Select the longest carbon chain which contains the carbon atom bearing the group. Count the number of carbon atoms and identify the corresponding alkane. From the name of this alkane, drop the final e and suffix in its place. This gives the root name or the parent name.
  • Step 2: Number the carbon chain starting from the end nearest to the hydroxy group. The number of the carbon atom bearing the hydroxy group is indicated before -ol in the name.
  • Step 3: Number the other substituents according to their position on the chain.
  • Step 4: Write the name of the alcohol by listing the substituents in the alphabetical order along with their position.

Structure

Hydrogen Bonding in Alcohol Molecules

Normally, hydrogen bonding is responsible for higher boiling points of alcohols. The negatively polarized oxygen atom of one alcohol molecule attracts the positively polarized hydrogen atom of the other molecule. Thus, alcohol molecules are associated or are held together. This force of attraction is to be overcome before a molecule is set free from the liquid state and vaporizes. Thus, more heat energy is required to break the hydrogen bonds.

Physical Properties

  • Boiling point of ether is much more lower than there corresponding alcohols because ether does not form intermolecular H-bonding.
  • Slightly soluble in water. Solubility decreases as molecular mass increases.
  • Lower alcohols are liquid at room temperature.
  • The boiling points of alcohols are higher than alkanes and haloalkanes of comparable molecular mass.
Physical Properties

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