Skip to content

Chemistry · Ch 13 — Hydrocarbons

Isomerism

13.3.3

Isomerism

Isomerism in Alkenes

Alkenes exhibit two distinct types of isomerism: structural isomerism and geometrical isomerism. This arises because the carbon-carbon double bond is both a site of branching (like alkanes) and a rigid, non-rotating unit that locks atoms in fixed spatial positions.

Structural Isomerism in Alkenes

Just as with alkanes, the simplest alkenes — ethene (C2H4C_2H_4) and propene (C3H6C_3H_6) — can exist in only one structural form. However, for alkenes with four or more carbon atoms, multiple structural arrangements become possible.

Consider the molecular formula C4H8C_4H_8. Three distinct structural isomers exist:

I. CH2=CH−CH2−CH3CH_2 = CH - CH_2 - CH_3 — But-1-ene

II. CH3−CH=CH−CH3CH_3 - CH = CH - CH_3 — But-2-ene

III. CH2=C(CH3)−CH3CH_2 = C(CH_3) - CH_3 — 2-Methylprop-1-ene

Between these structures, two types of structural isomerism appear:

  • Chain isomerism: Structures I and III differ in the carbon skeleton (straight chain vs. branched). Structures II and III also show chain isomerism.
  • Position isomerism: Structures I and II differ only in the location of the double bond along the same four-carbon chain.
Watch out

Do not confuse chain isomerism with position isomerism. Chain isomers have different carbon skeletons; position isomers have the same skeleton but the functional group (here, the double bond) is at a different location.

Problem 9.9: Structural Isomers of C5H10C_5H_{10}

For pentene (C5H10C_5H_{10}), five structural isomers exist:

StructureIUPAC Name
CH2=CH−CH2−CH2−CH3CH_2 = CH - CH_2 - CH_2 - CH_3Pent-1-ene
CH3−CH=CH−CH2−CH3CH_3 - CH = CH - CH_2 - CH_3Pent-2-ene
CH3−C(CH3)=CH−CH3CH_3 - C(CH_3) = CH - CH_32-Methylbut-2-ene
CH3−CH(CH3)−CH=CH2CH_3 - CH(CH_3) - CH = CH_23-Methylbut-1-ene
CH2=C(CH3)−CH2−CH3CH_2 = C(CH_3) - CH_2 - CH_32-Methylbut-1-ene

Geometrical Isomerism

Geometrical isomerism is a type of stereoisomerism — isomers that have the same structural formula but differ in the spatial arrangement of atoms. It arises specifically because rotation about the carbon-carbon double bond is restricted.

The Condition for Geometrical Isomerism

For geometrical isomerism to occur, each of the two doubly bonded carbon atoms must be attached to two different atoms or groups. Consider a general alkene of the form:

∣XYC=C∣XY\underset{\text{Y}}{\overset{\text{X}}{\phantom{|}}}C = C\underset{\text{Y}}{\overset{\text{X}}{\phantom{|}}}

Here, each carbon has two different substituents (X and Y). This molecule can exist in two distinct spatial arrangements:

(a) The two identical groups (both X or both Y) lie on the same side of the double bond.

(b) The two identical groups lie on opposite sides of the double bond.

These two arrangements are not superimposable — they are stereoisomers. They would be identical if free rotation around the C=C bond were possible, but such rotation is restricted.

Tip

To visualise restricted rotation: take two pieces of stiff cardboard and join them with two nails (representing the sigma and pi bonds). Hold one piece still and try to rotate the other. The nails prevent rotation — exactly as the pi bond prevents rotation around the C=C axis.

Cis and Trans Isomers

The two geometrical isomers are named:

  • cis isomer: The two identical atoms or groups lie on the same side of the double bond (arrangement (a)).
  • trans isomer: The two identical atoms or groups lie on opposite sides of the double bond (arrangement (b)).

Cis and trans isomers have the same structural formula but different configurations — the fixed arrangement of atoms in space. Because of this different spatial arrangement, they differ in physical properties such as melting point, boiling point, dipole moment, and solubility.

Example: But-2-ene

The geometrical isomers of but-2-ene are:

  • cis-but-2-ene: Both methyl groups are on the same side of the double bond.
  • trans-but-2-ene: The methyl groups are on opposite sides of the double bond.
Polarity Difference Between Cis and Trans Isomers

The cis form of an alkene is more polar than the trans form. For but-2-ene:

  • Dipole moment of cis-but-2-ene = 0.33 Debye
  • Dipole moment of trans-but-2-ene ≈ 0 (essentially non-polar)
Important

The trans isomer is non-polar because the two C−CH3C-CH_3 bond dipoles point in exactly opposite directions and cancel each other. In the cis isomer, these dipoles are at an angle and do not cancel, giving a net dipole moment.

Identifying Compounds That Show Geometrical Isomerism …
Start your 14-day free trial to unlock the full solution →
← Previous · 13.3.2Example 9.8 — Calculate number of sigma (σ) and pi (π) bonds in the above structures…Next · 13.3.3 →Example 9.9 — Write structures and IUPAC names of different structural isomers of al…