Q.Assertion (A): Among the structural isomers of C₄H₉Br, only 2-bromobutane is optically active. Reason (R): It contains one asymmetric carbon and exists in two enantiomeric forms.

Showing the 12 most recent of 18 on this concept.

1. CBSE 2026Set 56/1/11 markMCQ
   Q.Which of the following is ‘not’ true about enantiomers ? (A) They have the same chemical reactivity. (B) They have the same specific rotation. (C) They have the same melting or boiling point. (D) They have the same refractive index.

   ›Reveal solutionSolution

   Enantiomers are non-superimposable mirror images that differ only in how they rotate plane-polarized light — they have opposite specific rotations. The answer is (B).

   Enantiomers are stereoisomers that stand as mirror images of each other, like your left and right hands. Because they have identical connectivity and the same functional groups in the same spatial arrangement (just reflected), most of their physical and chemical properties are identical. The one property that must differ is their interaction with plane-polarized light.

   When plane-polarized light passes through a solution of a chiral molecule, the molecule rotates the plane of polarization. Enantiomers rotate light by the same magnitude but in opposite directions: one clockwise (dextrorotatory, $+$), the other counterclockwise (levorotatory, $-$). This is the defining optical difference.

   Let's examine each statement:

   1. Chemical reactivity (A): Enantiomers have identical chemical reactivity in an achiral environment. They react at the same rate with achiral reagents because the transition states formed are also mirror images with equal energy. In biological systems or with chiral reagents, reactivity can differ — but the statement as written (without qualification) is generally considered true for standard achiral conditions.

   2. Specific rotation (B): Specific rotation $[\alpha]$ is defined as the observed rotation per unit concentration and path length. For a pair of enantiomers, if one has $[\alpha] = +50°$, the other has $[\alpha] = -50°$. They have equal magnitude but opposite sign. The statement claims they have "the same" specific rotation, which is false — they have opposite specific rotations. …

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2. CBSE 2026Set 56/2/11 markMCQ
   Q.Which of the following molecules is chiral in nature ? (A) Propan-2-ol (B) Butan-2-ol (C) 1-Bromobutane (D) 2-Bromopropane

   ›Reveal solutionSolution

   A molecule is chiral if it has a carbon atom bonded to four different groups. Among the given options, only Butan-2-ol has such a carbon, making it the chiral molecule.

   The Concept: What Makes a Molecule Chiral?

   Chirality in organic chemistry is a property of "handedness" — a molecule is chiral if it is not superimposable on its mirror image. For most simple organic compounds (like the ones in this question), chirality arises from the presence of a stereocenter (often called a chiral centre or asymmetric carbon). This is a carbon atom that is bonded to four different substituents.

   If a carbon has even two identical groups attached, its mirror image can be rotated to match the original — it is achiral. So the task is to check each molecule for a carbon with four distinct groups.

   Watch out

   A common mistake is to look for any carbon with four bonds and assume it's a chiral centre. The key is four different groups — not just four bonds. A carbon with two identical groups (like two hydrogens) is never a chiral centre.

   Step-by-Step Analysis

   Let's examine each molecule one by one.

   1. Propan-2-ol — Structure: $\text{CH}_3-\text{CH}(\text{OH})-\text{CH}_3$

   The central carbon (the one with the -OH group) is bonded to:

   • A hydrogen atom (H)
   • A hydroxyl group (-OH)
   • A methyl group ($\text{CH}_3$)
   • Another methyl group ($\text{CH}_3$)

   Since two of the groups are identical (both are $\text{CH}_3$), this carbon is not a chiral centre. The molecule is achiral.

   2. Butan-2-ol — Structure: $\text{CH}_3-\text{CH}(\text{OH})-\text{CH}_2-\text{CH}_3$

   Look at the carbon that carries the -OH group (the second carbon in the chain). Its four bonds go to:

   • A hydrogen atom (H)
   • A hydroxyl group (-OH)
   • A methyl group ($\text{CH}_3$)
   • An ethyl group ($\text{CH}_2\text{CH}_3$)

   Are all four groups different? Yes — H, -OH, $\text{CH}_3$, and $\text{CH}_2\text{CH}_3$ are all distinct. This carbon is a stereocenter, so Butan-2-ol is chiral.

   Tip

   Butan-2-ol is a classic example of a chiral alcohol. In fact, it is often used in textbooks to introduce optical isomerism because it has exactly one chiral centre and no symmetry.

   3. 1-Bromobutane — Structure: $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2\text{Br}$

   Scan each carbon: …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.Which of the following is optically active?

   (a) CH3CH2Cl

   (b) CH3CHClCH3

   (c) CH2ClCH2Cl

   (d) CH3CH2CHClCH3

   ›Reveal solutionSolution

   A compound is optically active only if it has a chiral carbon — one bonded to four different substituents. Only 2-chlorobutane qualifies here.

   Optical activity requires the molecule to be chiral, most commonly recognised by the presence of an asymmetric (stereogenic) carbon atom — a carbon attached to four DIFFERENT groups. Checking each option:

   • (a) $CH_3CH_2Cl$: the carbon bearing Cl is attached to H, H, Cl, $CH_3$ — two H's are identical, not chiral.
   • (b) $CH_3CHClCH_3$ (2-chloropropane): the C-Cl carbon is attached to Cl, H, and two identical $CH_3$ groups — not chiral.
   • (c) $CH_2ClCH_2Cl$: neither carbon has four different groups — not chiral. …

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4. CBSE 2026Set ANNUAL1 mark
   Q.The stereoisomers related to each other as non-superimposable mirror images are called ______ (fill in the blank).

   ›Reveal solutionSolution

   Stereoisomers that are related as object and mirror image, and cannot be superimposed on one another, are called enantiomers.

   Such molecules are chiral (possess a stereocentre with four different groups, most commonly), rotate plane-polarised light in equal but opposite directions, and are otherwise ide …

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5. CBSE 2026Set ANNUAL1 mark
   Q.Arrange the following compounds in increasing order of reactivity towards SN2 reaction: CH3-CH2-CH(Br)-CH3 , CH3-C(CH3)(Br)-CH3 , CH3-CH2-CH2-CH2-Br

   ›Reveal solutionSolution

   SN2 reactions proceed via a back-side nucleophilic attack through a crowded transition state, so reactivity decreases sharply as more alkyl groups (steric bulk) surround the C-Br carbon: primary > secondary > tertiary.

   • CH3-CH2-CH2-CH2-Br is a primary halide (1-bromobutane) - least hindered, most reactive towards SN2.
   • CH3-CH2-CH(Br)-CH3 is a secondary halide (2-bromobutane) - intermediate. …

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6. CBSE 2026Set ANNUAL1 markMCQ
   Q.Assertion (A): Among the structural isomers of C₄H₉Br, only 2-bromobutane is optically active. Reason (R): It contains one asymmetric carbon and exists in two enantiomeric forms.

   (a) Both A and R are true and R is the correct explanation of A.

   (b) Both A and R are true but R is not the correct explanation of A.

   (c) A is true but R is false.

   (d) A is false but R is correct.

   ›Reveal solutionSolution

   2-Bromobutane is the only C₄H₉Br isomer with a chiral carbon, so it alone is optically active and exists as two enantiomers; A and R are both true and R explains A — option (A).

   The structural isomers of $C_4H_9Br$ are: 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane, and 2-bromo-2-methylpropane.

   A molecule is optically active if it contains an asymmetric (chiral) carbon — a carbon attached to four different groups.

   • In 2-bromobutane, $CH_3-CHBr-CH_2-CH_3$, carbon $C2$ carries $H$, $Br$, $-CH_3$ and $-CH_2CH_3$: four different groups. So $C2$ is a chiral centre.
   • The other three isomers each have a carbon bearing at least two identical groups, so none is chiral. …

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7. CBSE 2025Set 56/4/11 markMCQ
   Q.Out of 2-Bromobutane, 1-Bromobutane, 2-Bromopropane and 1-Bromopropane, the molecule which is chiral in nature is : (A) 2-Bromobutane (B) 1-Bromobutane (C) 2-Bromopropane (D) 1-Bromopropane

   ›Reveal solutionSolution

   A chiral molecule must have a carbon atom bonded to four different substituents. Only 2-Bromobutane has such a carbon (the second carbon), making it the only chiral molecule among the four. The correct option is (A).

   The Concept: What Makes a Molecule Chiral?

   Chirality in organic chemistry is all about asymmetry. A molecule is chiral if it cannot be superimposed on its mirror image — like your left and right hands. The most common cause is a chiral centre (or stereocentre): a carbon atom bonded to four different groups.

   The key test: look for a tetrahedral carbon where no two substituents are identical. If you find one, the molecule is almost certainly chiral. If every carbon has at least two identical groups attached, the molecule is achiral.

   Let’s apply this to each compound.

   Step-by-Step Analysis

   1. 2-Bromobutane

   Structure: $\mathrm{CH_3-CHBr-CH_2-CH_3}$

   Focus on carbon-2 (the one with the bromine). Its four bonds go to:

   • a hydrogen atom ($\mathrm{H}$)
   • a bromine atom ($\mathrm{Br}$)
   • a methyl group ($\mathrm{CH_3-}$)
   • an ethyl group ($\mathrm{CH_3CH_2-}$)

   Are all four different? Yes — $\mathrm{H}$, $\mathrm{Br}$, $\mathrm{CH_3}$, and $\mathrm{CH_3CH_2}$ are all distinct. This carbon is a chiral centre. Therefore, 2-bromobutane exists as a pair of enantiomers and is chiral.

   2. 1-Bromobutane

   Structure: $\mathrm{CH_2Br-CH_2-CH_2-CH_3}$

   Check each carbon:

   • Carbon-1: bonded to two hydrogens, one bromine, and one carbon chain — two hydrogens are identical, so not chiral.
   • Carbon-2: bonded to two hydrogens, one $\mathrm{CH_2Br}$ group, and one $\mathrm{CH_2CH_3}$ group — again two identical hydrogens.
   • Carbons-3 and -4: similarly have at least two identical substituents.

   No carbon has four different groups. The molecule is achiral.

   3. 2-Bromopropane

   Structure: $\mathrm{CH_3-CHBr-CH_3}$

   Carbon-2 is bonded to: one hydrogen, one bromine, and two methyl groups ($\mathrm{CH_3}$). The two methyl groups are identical. So this carbon has only three different substituents — not a chiral centre. The molecule is achiral.

   4. 1-Bromopropane …

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8. CBSE 2025Set 56/5/11 markMCQ
   Q.Which of the following molecules is chiral in nature ? (A) 1-chloropropane (B) 2-chloropropane (C) 1-chlorobutane (D) 2-chlorobutane

   ›Reveal solutionSolution

   A molecule is chiral if it has a carbon atom bonded to four different groups. Among the given options, only 2-chlorobutane has such a carbon, making it the chiral molecule.

   The key idea is the chirality centre (also called an asymmetric carbon or stereocentre). A carbon is chiral if it is sp³-hybridised and carries four different substituents. If any two substituents are the same, the molecule is achiral (it can be superimposed on its mirror image).

   Let’s check each option systematically.

   1. 1-chloropropane ($\mathrm{CH_3CH_2CH_2Cl}$)

      The carbon atoms are:

      • C1 (bonded to Cl, H, H, CH₂CH₃) — two H atoms → not chiral.
      • C2 (bonded to H, H, CH₃, CH₂Cl) — two H atoms → not chiral.
      • C3 (bonded to H, H, H, CH₂CH₂Cl) — three H atoms → not chiral. No carbon has four different groups. Achiral.

   2. 2-chloropropane ($\mathrm{CH_3CHClCH_3}$)

      The central carbon (C2) is bonded to Cl, H, CH₃, and CH₃. Two of the groups are identical methyl groups. So it has a plane of symmetry and is achiral.

   3. 1-chlorobutane ($\mathrm{CH_3CH_2CH_2CH_2Cl}$)

      All carbons are either primary or secondary with at least two identical substituents (usually two H atoms). No carbon has four different groups. Achiral.

   4. 2-chlorobutane ($\mathrm{CH_3CHClCH_2CH_3}$)

      Look at C2: it is bonded to

      • Cl
      • H
      • CH₃ (methyl)
      • CH₂CH₃ (ethyl) …

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9. CBSE 2025Set ANNUAL1 mark
   Q.What are enantiomers?

   ›Reveal solutionSolution

   Enantiomers are mirror-image molecules of each other that cannot be superimposed, arising from a chiral centre.

   Enantiomers are a pair of stereoisomers whose three-dimensional structures are related as an object and its mirror image, and which are not superimposable on one another (in the same way that a left hand and a right hand are mirror images but cannot be perfectly overlapped).

   …

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10. CBSE 2024Set A11 markMCQ
    Q.The stereoisomers related to each other as non-superimposable mirror images are called ;

    (a) Enantiomers

    (b) Diastereomers

    (c) Anomers

    (d) Racemic mixture

    ›Reveal solutionSolution

    Stereoisomers that are non-superimposable mirror images are enantiomers — option (a).

    Enantiomers are a pair of stereoisomers that are mirror images of each other and cannot be superimposed (they are chiral, arising from an asymmetric carbon). Diastereomers are stereoisomers that are NOT mirror images; anomers are a special kind of diastereomer of sugars; and a r …

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11. CBSE 2024Set ANNUAL1 mark
    Q.Draw the structures of the possible enantiomers of 3-methylpent-1-ene.

    ›Reveal solutionSolution

    C3 is bonded to four different substituents, making it a stereocentre; swapping any two groups at C3 gives the mirror-image (enantiomeric) structure.

    In CH₂=CH–CH(CH₃)–CH₂CH₃, the C3 carbon is attached to four different groups — a vinyl group (–CH=CH₂), a methyl group (–CH₃), an ethyl group (–CH₂CH₃), and a hydrogen atom — so C3 is a chiral centre (stereocentre). This gives rise to two enantiomers, which are non-superimposable mirror images of each other, differing only in the 3-D spatial arrangement of these four groups around C3 (assigned as R or S by CIP priority rules): (R)-3-methylpent-1-ene and (S)-3-methylpent-1-ene. Both have the identical connectivity/constitution and identical physical properties (e.g. same boilin …

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12. CBSE 2023Set 56/3/11 markMCQ
    Q.Which of the following is not true about enantiomers? (A) They have the same density. (B) They have the same melting or boiling point. (C) They have the same specific rotation. (D) They have the same chemical reactivity.

    ›Reveal solutionSolution

    Enantiomers are mirror-image molecules that are identical in most physical properties (density, melting/boiling points) and react identically with achiral reagents, but they rotate plane-polarised light in opposite directions — so they do not have the same specific rotation. The false statement is (C).

    Enantiomers are like your left and right hands — mirror images that cannot be superimposed. In chemistry, this “handedness” (chirality) leads to a fascinating split: most physical properties are identical, but interaction with polarised light is opposite. That’s the key to spotting the wrong option here.

    Let’s check each statement one by one.

    1. Density — Enantiomers have the same density.

       Density depends on molecular mass and packing in the bulk state. Since enantiomers are identical in every way except spatial arrangement at a chiral centre, their crystals or liquids pack identically. So density is the same. Option (A) is true.

    2. Melting or boiling point — Again, these are bulk physical properties.

       In a pure sample, enantiomers have identical melting and boiling points. (A mixture of equal amounts, a racemate, may have a different melting point — but the question asks about enantiomers themselves, not their mixture.) So (B) is true.

    3. Specific rotation — This is the trick.

       Specific rotation $[\alpha]$ measures how much a compound rotates plane-polarised light. Enantiomers rotate light by exactly the same magnitude but in opposite directions — one is (+) and the other is (−). So they do not have the same specific rotation; they have equal but opposite values. Option (C) is false — and that’s what we’re looking for.

    4. Chemical reactivity — This needs a careful nuance. …

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