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Structural Isomerism

Two or more compounds with the same molecular formula, which differ in the arrangement of atoms within the molecule, are called structural isomers, and the phenomenon is called structural isomerism. This is further subdivided into:

Chain isomerism

The chain isomers have same molecular formula, but they differ in the length of carbon chain (straight or branched).

Position isomerism

The position isomers have same molecular formula, but they differ in the position of either substituent or functional group on the same carbon skeleton.

Functional group isomerism

The isomers having same molecular formula but different functional groups in the molecule are called functional isomers. For example, alcohols and ethers are functional group isomers, and aldehydes and ketones are functional isomers. They are also functionally isomeric to unsaturated ethers, unsaturated alcohols, cyclic ethers, and cyclic alcohols; carboxylic acids and esters also share functional group isomerism; dienes, allenes, and alkynes are functional isomers; nitroalkanes and alkyl nitrites are functional group isomers; primary, secondary, and tertiary amines share functional group isomerism; cyanides and isocyanides are functional group isomers.

Degree of unsaturation: Deficiency of two hydrogen atoms in a molecule is a result of either a pi-bond or a ring in the structure of that molecule. The sum of pi-bonds and rings in the structure of a compound collectively is called the degree of unsaturation or double bond equivalents in that compound. The most general type of formula for any organic species is (CaHbNcOd). If the compound contains other atoms also, the tetravalent atoms are replaced by carbon, monovalent atoms are replaced by hydrogen, divalent atoms are replaced by oxygen, and trivalent atoms are replaced by nitrogen. Then all oxygen and all nitrogen atoms are removed from the formula. However, for the removal of each N atom, one H atom is also removed from the molecular formula. As a result of all these operations, we will get a hydrocarbon. Now this concluded hydrocarbon is compared with saturated alkane containing the same number of carbon atoms to determine the degree of unsaturation or double bond equivalents.
For example, consider C12H16N2OCl2. C12H16N2OCl2 will give C12H18N2O after replacing Cl atoms by H atoms, and C12H16 after removing O and N.
Corresponding saturated alkane should be C12H26.
Double bond equivalents = Description: 42803.png

Catalytic hydrogenation (hydrogenation using H2 in the presence of Ni or Pd at room temperature) data are used to distinguish between the unsaturation due to a π-bond and unsaturation due to a ring. Compounds having unsaturation due to π-bonds absorb 1 mol of H2 for each mole of a π-bond, while compounds having unsaturation due to rings do not absorb any hydrogen at room temperature. However, compounds such as cyclopropane and cyclobutane do absorb 1 mol of H2 at 120°C and 200°C, respectively, because these rings are highly strained and in order to get relieve from strain, they do get cleaved by H2 at elevated temperatures. Cyclopentane and higher rings to not absorb any H2 at the experimental temperatures (200–300°C) as such rings are quite stable.
Description: 43007.png


Compounds having same molecular formula but different number of carbon atoms (or alkyl groups) on either side of the multivalent functional group (i.e.,  O , S , NH , CO , etc.) are called metamers, and the phenomenon is called metamerism.


It is a special type of functional isomerism in which an α-hydrogen atom is shifted from one position (atom 1) to another (atom 3). This shift is referred to as 1,3-shift. Such shifts are common between a carbonyl compound containing an α-hydrogen atom and its enol form.
Description: 43016.png
Generally, keto form is more stable than enol form by 12 kcal. So, in most cases, the equilibrium lies towards the left. However, in certain cases, enol form can become the predominant form. The enol form is predominant in following cases:
  1. Molecules in which the enolic double bond is in conjugation with another double bond/phenyl ring. In such cases, sometimes intramolecular hydrogen bonding also stabilizes the enol form.
    Description: 43025.png
  2. Molecules which contain two bulky aryl groups.
    Description: 43036.png
    where Ar = Description: 43045.png
    In the keto form of 2,2-dimesitylethanal, the Ar – – C – – Ar bond angle is 109°, whereby the bulky aryl groups experience greater steric repulsion. This steric repulsion eases off when the keto form transforms to enol form, where the Ar – – C – – Ar bond angle widens to 120°.
  3. When the enol is aromatically stabilized.
    Description: 43054.png
    The extent of enolization is also affected by the solvent, concentration, and temperature. Thus, acetoacetic ester has an enol content of 0.4% in water and 19.8% in toluene. Also, the enol content of pentan-2, 4-dione (CH3COCH2COCH3) is found to be 95% and 45% at 27.5°C, respectively.

Ring-chain isomerism

Compounds having same molecular formula but possessing open chain and cyclic structures are called ring-chain isomers, and the phenomenon is called ring-chain isomerism.

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