Isomers of Butane, Constitutional, Conformational, Staggered, Questions (For CBSE, ICSE, IAS, NET, NRA 2022)

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Isomers

  • Isomers are molecules with the same molecular formula but different structural formulas. This phenomenon is called as isomerism. Due to different structure, isomers may have different chemical and physical properties also.
  • There are four carbon atoms in the given molecular formula. So, these four carbon atoms can arrange in two different manners. They can either arrange in the straight chain of four carbon atoms or they can form a chain of 3 carbon atoms with one side chain.
Structural Formula
Structural formulaName of isomer
n-butane
2-methylpropane or isobutane

How Many Isomers Does Butane Have?

Butane is an alkane with four carbon atoms so molecular formula is C4H10. It has two isomers; n-butane and isobutane.

Butane & Isobutane

Butane & Isobutane

Here n-butane is a straight-chain compound with four carbon atoms bonded with single covalent bonds.

N-Butane Straight Chain Isomer

n-butane Straight Chain Isomer

Another isomer is isobutane or 2-methylpropane in which three carbon atoms from the parent chain and one carbon atom is placed as the side chain at C-2 of the parent chain. All carbon atoms have 4 valencies which are satisfied either by carbon atoms or hydrogen atoms.

Isobutane also called 2 – Methylpropane

Isobutane Also Called 2 – Methylpropane

The molecular formula for butane is . There are two possible isomers with this molecular formula. First one is n-butane which has all four carbon atoms in the parent chain with structural formula as

Another isomer is isobutane with three carbon atoms in the parent chain and one in side-chain as a methyl group.

The Molecular Formula

Constitutional Isomers of Butane

  • Constitutional isomers can be defined as the compounds that have the same molecular formula but different structural formulas. In other words, constitutional isomers have different connectivity of atoms in molecules. To determine that two molecules are constitutional isomers of each other or not, we have to count the number of each atom in both molecules.
  • The molecular formula must be the same for both the molecules but the arrangement of atoms should be different.
  • Both of these molecules have the same molecular formula that is . The connectivity of atoms (carbon atoms) . In the case of n-butane, all carbon atoms are in straight-chain whereas, in the case of isobutane, there is a side chain in the molecule. So, they have different connectivity of atoms and are constitutional isomers of each other.

Conformational Isomers of Butane

  • Stereoisomers which can be converted into one another by rotation around a single bond are called as conformational isomers.
  • Alkanes usually show conformational isomerism because of the presence of C-C bonds.
  • For example, when we rotate the molecule of butane at the axis of C-C bond, we get eclipsed, gauche, and anti-butane conformational isomers.
  • Here eclipsed conformation has identical groups directly in-line with one another that makes it unstable.
  • Gauche conformation stands for the presence of identical groups at 60 degrees from one another.
  • It is more stable compared to eclipse confirmation due to less steric hindrance between same molecules.
  • In anti-conformation, identical groups are 180 degrees from one another which makes it the most stable form.
  • Each conformer is interconverted by rotating around the central carbon single bond like eclipsed conformer can convert into gauche conform by rotating 60 degrees.
  • Rotation of eclipsed by 180-degree forms anti- conformer. Rotation of anti-butane by 120 degrees forms gauche conformer.

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