Organic Chemistry: Some Basic Principles and Techniques NEET PYQs (2005–2026) with Solutions

10 solved Organic Chemistry: Some Basic Principles and Techniques PYQs

Real NEET and AIPMT questions from this chapter, newest first. Try each one, then open the answer and the step-by-step explanation.

1. Question 1 (NEET 2026)

   NEET 2026MediumCarbocation stability, resonance and hyperconjugation

   The following carbocation is stabilized by the interaction of the empty $p$ orbital with $\mathrm{C_6H_5\overset{+}{C}H{-}CH_3}$

   1. AOption A: filled $\sigma$ and filled $\pi$ orbitals
   2. BOption B: empty $\sigma$ and empty $\pi^*$ orbitals
   3. COption C: empty $\sigma^*$ and filled $\pi$ orbitals
   4. DOption D: empty $\sigma^*$ and empty $\pi^*$ orbitals

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   Correct answer: (A) filled $\sigma$ and filled $\pi$ orbitals

   Explanation

   The cation is the 1-phenylethyl (benzylic) carbocation, $\mathrm{C_6H_5\overset{+}{C}HCH_3}$. Its positive carbon is $sp^2$ hybridised with an empty $p$ orbital. Resonance: the empty $p$ orbital overlaps with the filled $\pi$ orbitals of the benzene ring, so the positive charge is delocalised over the ring. Hyperconjugation: the empty $p$ orbital also overlaps with the filled $\sigma$ (C–H) bonding orbitals of the adjacent $\mathrm{CH_3}$ group. Stabilisation needs electron donation into the empty orbital, so it must come from filled orbitals. Empty $\sigma^*$ or $\pi^*$ orbitals cannot donate electrons. So the answer is option (A).

2. Question 2 (NEET 2026)

   NEET 2026EasyStructural Representation & Isomerism

   The pair of molecules that are metamers among the following is:

   1. AOption A: Option 1
   2. BOption B: Option 2
   3. COption C: Option 3
   4. DOption D: Option 4

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   Correct answer: (D) Option 4

   Explanation

   Metamerism occurs due to different alkyl groups on either side of the same functional group. CH₃OCH₂CH₂CH₃ and CH₃CH₂OCH₂CH₃ are ethers with different alkyl groups and are metamers. Hence option (D) is correct.

3. Question 3 (NEET 2026)

   NEET 2026EasyClassification & Nomenclature

   The correct IUPAC name of the following compound is:

   1. AOption A: 3-ethyl-5-methylheptane
   2. BOption B: 2,4-diethylhexane
   3. COption C: 3-methyl-5-ethylheptane
   4. DOption D: 3,5-diethylhexane

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   Correct answer: (A) 3-ethyl-5-methylheptane

   Explanation

   The longest carbon chain contains 7 carbons (heptane). Substituents are ethyl at C-3 and methyl at C-5. Prefixes are written alphabetically: ethyl before methyl. Therefore the name is 3-ethyl-5-methylheptane. Hence option (A) is correct.

4. Question 4 (NEET 2026)

   NEET 2026EasyLassaigne's test for sulphur

   The amino acid that gives a red-blood colour on treating its sodium fusion extract with sodium nitroprusside is

   1. AOption A: leucine
   2. BOption B: threonine
   3. COption C: methionine
   4. DOption D: serine

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   Correct answer: (C) methionine

   Explanation

   When an organic compound is fused with sodium, any sulphur in it is converted to sodium sulphide, $\mathrm{Na_2S}$. In Lassaigne's test, the sodium fusion extract is treated with sodium nitroprusside, and a coloured complex shows that sulphur is present. Only a sulphur-containing amino acid can give this test. Methionine, $\mathrm{CH_3SCH_2CH_2CH(NH_2)COOH}$, contains sulphur. Leucine, threonine and serine contain only C, H, N and O, so their extracts give no sulphide test. So the answer is methionine, option (C).

5. Question 5 (NEET 2026)

   NEET 2026EasyQualitative & Quantitative Analysis

   During Lassaigne’s test, the elements present in an organic compound are converted from:

   1. AOption A: Ionic form to ionic form
   2. BOption B: Covalent form to ionic form
   3. COption C: Covalent form to covalent form
   4. DOption D: Ionic form to covalent form

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   Correct answer: (B) Covalent form to ionic form

   Explanation

   In Lassaigne’s test, covalently bonded elements (N, S, halogens) are converted into ionic sodium salts for detection. Hence option (B) is correct.

6. Question 6 (NEET 2026)

   NEET 2026EasyConjugated dienes and IUPAC names

   Among the following, the compound having conjugated double bonds is

   1. AOption A: hepta-1,3-diene
   2. BOption B: hepta-1,4-diene
   3. COption C: hepta-1,5-diene
   4. DOption D: hepta-1,6-diene

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   Correct answer: (A) hepta-1,3-diene

   Explanation

   Double bonds are conjugated when they alternate with a single bond (C=C–C=C), so that their $\pi$ electrons can be delocalised. Hepta-1,3-diene, $\mathrm{CH_2{=}CH{-}CH{=}CH{-}CH_2CH_2CH_3}$, has double bonds at C1 and C3, separated by one single bond, so they are conjugated. In hepta-1,4-, 1,5- and 1,6-diene, the double bonds are separated by one or more $sp^3$ $\mathrm{CH_2}$ groups, so they are isolated, not conjugated. So the answer is option (A).

7. Question 7 (NEET 2025)

   NEET 2025MediumPurification Techniques

   Match List I with List II Choose the correct answer from the options given below :

   1. AOption A: A-IV, B-III, C-I, D-II
   2. BOption B: A-IV, B-III, C-II, D-I
   3. COption C: A-III, B-IV, C-I, D-II
   4. DOption D: A-III, B-IV, C-II, D-I

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   Correct answer: (A) A-IV, B-III, C-I, D-II

   Explanation

   CHCl3 and aniline can be separated by simple distillation due to sufficient difference in boiling points. Crude oil is separated into fractions using fractional distillation. Glycerol from spent-lye is purified by distillation under reduced pressure because glycerol decomposes at high temperature. Aniline and water are separated using steam distillation since aniline is steam volatile. Hence the correct matching is A-IV, B-III, C-I, D-II.

8. Question 8 (NEET 2024)

   NEET 2024HardOrganic Reactions and Reagents

   Match List I with List II.

   1. AOption A: A-IV, B-I, C-II, D-III
   2. BOption B: A-I, B-IV, C-II, D-III
   3. COption C: A-IV, B-I, C-III, D-II
   4. DOption D: A-III, B-I, C-II, D-IV

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   Correct answer: (A) A-IV, B-I, C-II, D-III

   Explanation

   A involves ozonolysis of fused cyclohexene derivative giving cyclohexanone, hence reagent IV. B represents Friedel-Crafts acylation, hence reagent I. C shows oxidation of cyclohexanol to cyclohexanone using CrO₃, hence reagent II. D shows oxidation of ethylbenzene side chain to potassium benzoate using alkaline KMnO₄, hence reagent III. Thus: A-IV, B-I, C-II, D-III Hence, option (A) is correct.

9. Question 9 (NEET 2024)

   NEET 2024MediumElectronic Effects & Reaction Mechanisms

   The most stable carbocation among the following is:

   1. AOption A: Compound (1)
   2. BOption B: Compound (2)
   3. COption C: Compound (3)
   4. DOption D: Compound (4)

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   Correct answer: (B) Compound (2)

   Explanation

   Carbocation stability increases with degree of alkyl substitution due to hyperconjugation and inductive effects. Compound (2) forms a tertiary carbocation which is more stable than the other given carbocations. Hence, option (B) is correct.

10. Question 10 (NEET 2023)

    NEET 2023HardAromatic Hydrocarbons

    Consider the following compounds/species: The number of compounds/species which obey Huckel's rule is ____.Consider the following compounds/species: The number of compounds/species which obey Huckel's rule is ____.

    1. AOption A: 4
    2. BOption B: 6
    3. COption C: 2
    4. DOption D: 5

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    Correct answer: (A) 4

    Explanation

    Compounds/species obeying Huckel's rule must contain (4n + 2) π electrons and be cyclic, planar, and conjugated. The aromatic species among the given compounds are i, iv, v, and vii. Hence, the total number of species obeying Huckel's rule is 4.