This type of isomerism arises from the difference in the structure of carbon chain which forms the nucleus of the molecule. It is, therefore, named as chain, nuclear isomerism or skeletal isomerism. For example, there are known two butanes which have the same molecular formula (`C_4H_10`) but differ in the structure of the carbon chains in their molecules (fig-1).
While n-butane has a continuous chain of four carbon atoms, isobutane has a branched chain. These chain isomers have somewhat different physical and chemical properties, n-butane boiling at `-0.5^(o)` and isobutane at `-10.2^(o)`. This kind of isomerism is also shown by other classes of compounds.

It is the type of isomerism in which the compounds possessing same molecular formula differ in their properties due to the difference in the position of either the functional group or the multiple bond or the branched chain attached to the main carbon chain. For example, n-propyl alcohol and isopropyl alcohol are the positional isomers. See fig.(ii)

Butene also has two positional isomers (fig.(iii))

`1` -Chrobutane and `2`-Chlorobutane are also the positional isomers (fig.(iv))

Methylpentane,too,has two positional isomers (fig.(v))

In the aromatic series, the di-substituted products of benzene also exhibit positional isomerism due to different relative positions occupied by the two substituents on the benzene ring. Thus Xylene, `C_6H_4(CH_3)_2`, exists in the following three forms which are positional isomers. (fig.(vi))

When any two compounds have the same molecular formula but possess different functional groups, they are called functional isomers and the phenomenon is termed functional isomerism. In other words substances with the same molecular formula but belonging to different classes of compounds exhibit functional isomerism. Thus,

a.) Diethyl ether and butyl alcohol both have the molecular formula `C_4H_6O`, but contain different functional groups.

`undersettext(diethyl ether)(C_2H_5-O - C_2H_5)` `undersettext(butyl alcohol)(C_4H_9- OH)`

The functional group in diethyl ether is (`- O- `), while in butyl alcohol it is ( `-OH`).

b.) Acetone and propionaldehyde both with the molecular formula `C_3H_6O` are functional isomers.

`undersettext(acetone)(CH_3- CO-CH_3)` `undersettext(propionaldehyde)(CH_3- CH_2- CHO)`

In acetone the functional group is (`- CO-` ), while in propionaldehyde it is ( `-CHO`).

c.) Cyanides are isomeric with isocyanides :

`undersettext(Alkyl cyanide)(RCN)` `undersettext(Alkyl isocyanide)(RNC)`

d.) Carboxylic acids are isomeric with esters.

`undersettext(Propanoic acid)(CH_3CH_2COOH)` `undersettext(Methyl ethanoate)(CH_3COOCH_3)`

e.) Nitroalkanes are isomeric with alkyl nitrites :

`undersettext(Nitroalkane)(R-NO_2)` `undersettext(Alkyl nitrite)(R-O-N=O)`

f.) Sometimes a double bond containing compound may be isomeric with a triple bond containing compound. This also is called as functional isomerism. Thus, butyne is isomeric with butadiene (molecular formula `C_4H_6`).

`undersettext(1-butyne)(CH_3 CH_2 CequivCH)` `undersettext(1,3-butadiene)(CH_2= CH-CH=CH_2)`

g.) Unsaturated alcohols containing three or more carbon atoms isomeric to aldehydes as well as ketones :

`undersettext(Allyl alcohol)(CH_2=CHCH_2OH)` `undersettext(propanaldehyde)(CH_3CH_2CHO)` `undersettext(acetone)(CH_3COCH_3)`

h.) Aromatic alcohols may be isomeric with phenols :

i.) Primary, secondary and tertiary amines of same molecular formula are also the functional isomers :

`underset[text(n-propyl amine)(1^o)](CH_3CH_2CH_2NH_2)` `underset[text(N-ethylmethylamine)(1^o)](CH_3NHC_2H_5)`


`underset[text(trimethylamine) (3^o)][(CH_3)_3N]`

This type of isomerism is due to the unequal distribution of carbon atoms on either side of the multivalent functional group (i.e. `-O-, - S- , -NH -, -CO -` etc.) in the molecule of compounds belonqinq to the same class. For example, methyl propyl ether and oielhyl ether both have same molecular formula

(a) `undersettext(Pentan-3-one)(CH_3CH_2COCH_2CH_3)` is a metamer of `undersettext(Pentan-2-one)(CH_3COCH_2CH_2CH_3)`

These two are also related as position isomers as the position of `CO` in the two isomers is different.

(b) `undersettext(Methyl n -propyl ether)[undersettext(Methoxy propane)(CH_3-O-CH_2CH_2CH_3)]` is a metameter of `undersettext
(Diethyl ether)[undersettext( Ethoxy ethane)(CH_3CH_2-O-CH_2CH_3)]`

(c) `undersettext(Diethyl amine)(CH_3CH_2 - NH - CH_2CH_3)` is a metamer of `undersettext (Methyl n-propyl amine)(CH_3 - NH - CH_2CH_2CH_3)`



.

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.

Alkenes are isomeric with cycloalkanes : See fig.1

Alkynes and alkadienes are isomeric with cycloalkenes : See fig.2

`text(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 degree of unsaturation or double bond equivalents in that compound. The most general type of formula for any organic species is (`C_aH_bN_cO_d`).

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 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 to determine the degree of unsaturation or double bond equivalents.

It is a special type of functional isomerism in which an `alpha`-hydrogen atom is shifted from one position to another. This shift is referred as `1,3`-shift. Such shifts are common between a carbonyl compound containing an `alpha`-hydrogen atom and its enol form. See fig.1.

In most cases, the equilibrium lies towards the left. Thus, the keto form is thermodynamically more sable than enol form by about `12 kcal//mol`. The term tautomerism is used for isomers that are fairly readily interconvertible and that differ from each other only

(a) in electron distribution and (b) in the position of a relatively mobile atom or group,

The mobile atom is generally hydrogen and the phenomenon is then called as `text(prototropy)`.

Both acids and bases catalyse such interconversions. Possible limiting mechanisms are those

(a) in which proton removal and proton acceptance (from the solvent) are separate operations and a carbanion intermediate is involved. i.e. an intermolecular pathway and

(b) in which one and the same proton is transferred intramolecularly.

See fig.2

Mostly the keto form is more stable than enol form but in certain cases, enol form can become the predominant form. The enol form is predominant in following cases:

(i) 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 : See fig.3.

(ii) Molecules, which contain two bulky aryl groups : See fig.4.

In the keto form of `2,2`-dimesitylethanal, the `Ar-C-Ar` bond angle is `109^(o)`, 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^(o)`.

(iii) Highly fluorinated enols : See fig.5.

Because of the greater acidity of `alpha`-hydrogen atom (due to the presence of strongly electron withdrawing fluorines), the conversion to its enol form is high.

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. This is because water reduces the enol content by hydrogen bonding with the carbonyl group, making this group less available for intramolecular hydrogen bonding. The effectiveness of intramolecular hydrogen-bonding in stabilizing the enol, with respect to the keto form is seen on varying the solvent and particularly on transfer to a hydroxylic solvent with `MeCOCH_2COMe`. See Table 1.

Also, the enol content of pentan-`2,4`-dione (`CH_3COCH_2COCH_3`) is found to be `95%` and `45%` at `27.5^o` and `275^oC` respectively.

When a strong base is added to a solution of a ketone with `alpha`-hydrogen atom, both the enol and keto form can lose a proton.
The resulting anion is same in both the cases as they differ only in the placement of electrons. They are not tautomers but canonical forms. See fig.6.

(i) Phenol - Keto tautomerism: See fig.1.

In this case, enol form is more stable than keto form because of the aromatic stabilization. The keto form becomes predominant when second `OH` group or an `N=O` group is present. For example, See fig.2.

(ii) Nitroso-Oxime tautomerism : See fig.3.

This equilibrium lies far to the right and as a rule nitroso compounds are stable only when there is no `alpha`-hydrogen atom.

(iii) Nitro - Aci tautomerism: Aliphatic nitro compounds are in equilibrium with the aci forms. See fig.4.

The nitro form is much more stable than the aci form because nitro group has resonance. Aci form of nitro compounds is also called nitronic acids.

(iv) Imine-Enamine tautomerism : See fig.5.

Imine form predominates generally. Enamines are stable only when there is no hydrogen atom attached to nitrogen.

(v) Hydrocyanic acid, `H-Cequiv N` and lsohydrocyanic acid `H- NequivC`are also tautomeric isomers or tautomers.

a) In tautomerism, an atom changes place but resonance involves a change of position of pi-electrons or unshared electrons.

b) Tautomers are different compounds and they can be separated by suitable methods but resonating structures cannot be separated as they are imaginary structures of the same compound.

c) Two tautomers have different functional groups but there is same functional group in all canonical structures of a resonance hybrid.

d) Two tautomers are in dynamic equilibrium but in resonance only one compound exists.

e) Resonance in a molecule lowers the energy and thus stabilises a compound and decreases its reactivity. But no such effects occur in tautomerism.

f) In resonance, bond length of single bond decreases and that of double bond increases e.g. all six `C-C` bonds in benzene are equal and length is in between the length of a single and a double bond.

g) Resonance occurs in planar molecules but atoms of tautomers may remain in different planes as well.

h) Tautomers are indicated by double arrow = in between the two isomers but double headed single arrow is put between the canonical (resonating) structures of a resonating molecules.