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JEE Mains Chemistry · Organic Chemistry - Some Basic Principles and Techniques

Stereoisomerism and Conformations

Stereoisomers have the same bonds but a different arrangement in space: cis and trans forms about a C=C that cannot rotate, mirror-image enantiomers from a carbon with four different groups, and conformations that differ only by rotation about a single bond.

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Why this matters

Thirty-two PYQs, sixteen of them asking for a number, and three from 2026. Fourteen decide whether a carbon or a molecule is chiral: counting chiral compounds in a list, spotting a meso form, telling identical drawings from enantiomers, or finding an optical purity. Nine count all the stereoisomers of a structure or the products of monochlorination. Nine test geometrical isomerism: five on cis and trans alkenes, four on the staggered and eclipsed conformations.

Concept 1 of 3: Geometrical isomerism and conformations

A C=C bond cannot rotate at room temperature. If each doubly bonded carbon carries two different groups, the groups can sit on the same side (cis) or on opposite sides (trans), and these are two separate compounds. A C–C single bond does rotate freely; the shapes it passes through are conformations, which cannot be separated.

Definition

  • Condition: each carbon of the C=C must carry two DIFFERENT groups. A carbon with two identical groups, such as a terminal =CH2\mathrm{{=}CH_2} or a =C(CH3)2\mathrm{{=}C(CH_3)_2}, rules it out.
  • cis: like groups on the same side; trans: on opposite sides. When all four groups differ, use E and Z by CIP priority.
  • A cis alkene has a dipole moment; in the trans form the two bond moments cancel. The trans form is more stable and packs better in a crystal, so it usually melts higher; the cis form usually boils higher.
  • The isomers do not interconvert at room temperature.
  • A C=C from a ring carbon to an outside carbon (an alkylidene ring) shows it only if the outside carbon carries two different groups AND the two ring arms differ: a substituent at C-3 of a cyclohexane ring makes them differ, one at C-4 does not.
  • Conformations of ethane: staggered (dihedral angle 60°) and eclipsed (0°). Staggered is more stable, with less torsional strain. The two are rotamers, not isomers that can be isolated.
  • Butane: the anti form, with the two CH3\mathrm{CH_3} groups at 180°, is the most stable; gauche has them at 60°; the fully eclipsed form (0°) is the least stable.

Condition for cis–trans isomerism

abC=Ccd:a≠b and c≠d\mathrm{abC{=}Ccd}:\quad a \neq b \ \text{and}\ c \neq d

Worked example

Which of these show geometrical isomerism: pent-2-ene, 2-methylbut-1-ene, 1,2-dichloroethene and 1,1-dichloroethene?
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The same idea in a real exam question:

JEE Mains · 2025 · 22 Jan 2025 · Q45Moderate

Example 1 · Organic Chemistry - Some Basic Principles and Techniques · Stereoisomerism and Conformations

The incorrect statements regarding geometrical isomerism are : (A) Propene shows geometrical isomerism. (B) Trans isomer has identical atoms/groups on the opposite sides of the double bond. (C) Cis-but-2-ene has higher dipole moment than trans-but-2-ene. (D) 2-methylbut-2-ene shows two geometrical isomers. (E) Trans-isomer has lower melting point that cis isomer. Choose the CORRECT answer from the options given below :

Two identical groups on one carbon cancel it

In 2-methylbut-2-ene, (CH3)2C=CHCH3\mathrm{(CH_3)_2C{=}CHCH_3}, C-2 carries two methyl groups. Swapping them changes nothing, so there are no cis and trans forms.

The trans isomer usually melts higher

Trans alkenes pack more neatly in a crystal, so they generally have the HIGHER melting point. The cis isomer has the dipole moment and usually the higher boiling point.

Conformations are not separable isomers

Staggered and eclipsed ethane interconvert by rotation about a single bond. They are called rotamers or conformers, never enantiomers or geometrical isomers.
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Concept 2 of 3: Chiral centres, meso compounds and optical purity

A carbon with four different groups has a mirror image that cannot be laid on top of it, like a left and a right hand. A molecule built on such a carbon is chiral and rotates plane-polarised light. A molecule with two chiral centres can still have a mirror plane; then it is its own mirror image, a meso compound, and it is optically inactive.

Definition

  • Chiral (asymmetric) carbon: four different groups. Isotopes count as different, so a carbon carrying both H and D can be chiral.
  • Enantiomers: non-superimposable mirror images, with equal and opposite rotations. Diastereomers: stereoisomers that are not mirror images.
  • Meso compound: two or more chiral centres and an internal mirror plane, so it is optically inactive (meso-tartaric acid, cis-1,2-dimethylcyclopropane).
  • To decide whether two drawings are identical or enantiomers, assign R or S to each centre in both drawings; never judge by eye.
  • A racemic mixture (50 : 50) shows no rotation.
  • Optical purity equals the enantiomeric excess: the observed rotation as a percentage of the pure enantiomer's rotation.

Optical purity

optical purity (%)=observed rotationrotation of the pure enantiomer×100\text{optical purity}\ (\%) = \dfrac{\text{observed rotation}}{\text{rotation of the pure enantiomer}} \times 100

Worked example

How many chiral carbons does 2-chloro-3-methylpentane, CH3−CHCl−CH(CH3)−CH2CH3\mathrm{CH_3{-}CHCl{-}CH(CH_3){-}CH_2CH_3}, have?
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The same idea in a real exam question:

JEE Mains · 2023 · 25 Jan 2023 · Q134Moderate

Example 2 · Organic Chemistry - Some Basic Principles and Techniques · Stereoisomerism and Conformations

The isomeric deuterated bromide with molecular formula C4H8DBrC_{4}H_{8}DBr having two chiral carbon atoms is

Deuterium is different from hydrogen

A CH2D\mathrm{CH_2D} group is not a CH3\mathrm{CH_3} group, and a carbon carrying H and D can be a chiral centre. Treat D as a fourth, distinct group.

Two chiral centres do not guarantee optical activity

If the two halves of the molecule are identical and a mirror plane cuts between them, the compound is meso and optically inactive, as in meso-2,3-dibromobutane.

Compare configurations, not drawings

Two drawings that look like mirror images may be the same molecule turned round. Assign R or S to every centre in both before calling them enantiomers.
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Concept 3 of 3: Counting stereoisomers

Each stereogenic unit, a chiral carbon or a C=C that shows cis and trans forms, has two settings. With n such units there are at most 2ⁿ stereoisomers. Symmetry lowers the count, because a meso form stands in for what would otherwise be a pair.

Definition

  • Stereogenic units: chiral carbons, and C=C bonds with two different groups on each carbon.
  • The maximum is 2n2^n. When the molecule has two identical halves, subtract the repeats: two identical chiral centres give 3 (a pair of enantiomers and one meso form).
  • Rings count too: cis and trans ring substituents are stereoisomers. 1,2-Dimethylcyclopropane has a meso cis form and a pair of trans enantiomers.
  • Monochlorination: list each distinct type of hydrogen (one product each), then count each product with a new chiral carbon twice. Butane gives 1-chlorobutane and 2-chlorobutane; the second is chiral, so there are 3 isomers in all.

Maximum number of stereoisomers

N≤2n,n=chiral centres+stereogenic C=C bondsN \le 2^{n}, \qquad n = \text{chiral centres} + \text{stereogenic C=C bonds}

Worked example

How many stereoisomers does 2,3-dibromobutane, CH3CHBr−CHBrCH3\mathrm{CH_3CHBr{-}CHBrCH_3}, have?
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The same idea in a real exam question:

JEE Mains · 2025 · 24 Jan 2025 · Q122Moderate

Example 3 · Organic Chemistry - Some Basic Principles and Techniques · Stereoisomerism and Conformations

The possible number of stereoisomers for 5-phenylpent-4-en-2-ol is ____\_\_\_\_ .

2ⁿ is a ceiling, not the answer

Check for identical halves before writing 2ⁿ. Tartaric acid has two chiral centres but only 3 stereoisomers, because the meso form is its own mirror image.

A double bond is a stereogenic unit too

A C=C with two different groups on each carbon doubles the count, just like a chiral carbon. Counting only the chiral carbons halves the answer.

Include stereoisomers only when asked

When a question says 'including stereoisomers', a chiral monochloro product counts as two. When it asks only for structural isomers, it counts as one.
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Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

  • Geometrical isomerism and conformations

    Condition for cis–trans isomerism

    abC=Ccd:a≠b and c≠d\mathrm{abC{=}Ccd}:\quad a \neq b \ \text{and}\ c \neq d
  • Chiral centres, meso compounds and optical purity

    Optical purity

    optical purity (%)=observed rotationrotation of the pure enantiomer×100\text{optical purity}\ (\%) = \dfrac{\text{observed rotation}}{\text{rotation of the pure enantiomer}} \times 100
  • Counting stereoisomers

    Maximum number of stereoisomers

    N≤2n,n=chiral centres+stereogenic C=C bondsN \le 2^{n}, \qquad n = \text{chiral centres} + \text{stereogenic C=C bonds}

Watch out for (9)

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