iupac-nomenclature MCQs for UPSC Prelims

25 practice questions on iupac-nomenclature from the Organic Chemistry - Some Basic Principles and Techniques section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 5 Hard

Sample questions

Q1
medium

A chemist accidentally mixed an alcohol and a ketone. Identify the principal functional group using IUPAC priority rules between an alcohol (-OH) and a ketone (>C=O) when they are both present in a compound.

  1. A Alcohol
  2. B Depends on the number of functional groups present
  3. C Both have equal priority
  4. D Ketone
Show answer and explanation

Correct answer: D - Ketone

IUPAC seniority places the ketone above the alcohol, so where both are present the carbonyl is the principal characteristic group and the hydroxyl is cited as a hydroxy prefix. The alcohol therefore cannot be the principal group here. The hierarchy is fixed, so the two do not share equal priority, and it does not depend on how many groups of each kind the molecule carries.

Q2
medium

A hydrocarbon has the IUPAC name 4-ethyl-1,4-dimethylcyclohexane. Which of the following statements is true about its structure?

  1. A It has a six-membered ring with one ethyl group on the fourth carbon and two methyl groups on different carbons.
  2. B The parent chain is hexane.
  3. C It has a six-membered ring with two methyl groups on the first carbon.
  4. D It has a six-membered ring with one ethyl and two methyl groups on the fourth carbon.
Show answer and explanation

Correct answer: A - It has a six-membered ring with one ethyl group on the fourth carbon and two methyl groups on different carbons.

The correct statement is that the compound has a six-membered cyclohexane ring with an ethyl group on the fourth carbon and two methyl groups on different carbons (one on the first carbon and one on the fourth). 'The parent chain is hexane' ignores the cyclo form, and the other options misplace substituents on the ring.

Q3
medium

How many carbon atoms are present in one molecule of 3-ethyl-2-methylpentane, and what is its molecular formula?

  1. A $8$ carbons, $\text{C}_8\text{H}_{18}$
  2. B $5$ carbons, $\text{C}_5\text{H}_{12}$
  3. C $7$ carbons, $\text{C}_7\text{H}_{16}$
  4. D $8$ carbons, $\text{C}_8\text{H}_{16}$
Show answer and explanation

Correct answer: A - $8$ carbons, $\text{C}_8\text{H}_{18}$

The name gives a five-carbon pentane chain plus an ethyl group of two carbons and a methyl group of one, so the total is $5 + 2 + 1 = 8$ carbons. Every bond is single, so the compound is a saturated alkane and fits $\text{C}_n\text{H}_{2n+2}$, giving $\text{C}_8\text{H}_{18}$. $\text{C}_8\text{H}_{16}$ is the formula of an alkene, and the smaller counts come from ignoring one or both substituents.

Q4
medium

An organic compound is named 2-bromo-3-nitrobutane. Which atoms/groups must be present in the structure?

  1. A A bromine atom on C-2 and a nitro group on C-4 of a 4-carbon chain
  2. B A nitro group on C-2 and a bromine atom on C-3 of a 4-carbon chain
  3. C A bromine atom on C-2 and a nitro group on C-3 of a 4-carbon chain
  4. D A bromine atom on C-3 and a nitro group on C-2 of a 3-carbon chain
Show answer and explanation

Correct answer: C - A bromine atom on C-2 and a nitro group on C-3 of a 4-carbon chain

In 2-bromo-3-nitrobutane, 'bromo' precedes 'nitro' alphabetically, indicating bromine on C-2 and nitro on C-3 of the butane chain. Incorrect options mix up position numbers or chain length.

Q5
easy

A hexane chain carries a bromo substituent and an ethyl substituent. When the IUPAC name is written, which of the two prefixes is cited first, and on what basis?

  1. A Bromo, because prefixes are cited in alphabetical order
  2. B Bromo, because halogen substituents outrank alkyl groups in every name
  3. C Ethyl, because the substituent with more carbon atoms is always cited first
  4. D Ethyl, because alkyl groups are always cited before halogens
Show answer and explanation

Correct answer: A - Bromo, because prefixes are cited in alphabetical order

Substituent prefixes in an IUPAC name are listed alphabetically, and 'bromo' comes before 'ethyl', so bromo is written first. The order has nothing to do with the type of group or its size, which rules out the other three explanations. Note that alphabetical order decides the order of citation only; the locants are still fixed by the lowest-locants rule applied to the whole set.

Q6
medium

What is the IUPAC name of $\text{CH}_2\text{=CH-CH}_2\text{-CH=CH}_2$?

  1. A Penta-1,4-diyne
  2. B Penta-1,3-diene
  3. C Penta-1,4-diene
  4. D Penta-2,4-diene
Show answer and explanation

Correct answer: C - Penta-1,4-diene

The chain holds five carbons and two double bonds, so the parent is pentadiene. The double bonds begin at carbons 1 and 4 whichever end the numbering starts from, giving penta-1,4-diene. Penta-1,3-diene would need the two double bonds conjugated, penta-2,4-diene misnumbers the chain, and the suffix -diyne would mean triple bonds.

Q7
medium

An organic compound has the IUPAC name 3-methylpent-2-ene. Which of the following correctly describes the structure of the compound?

  1. A A five-carbon chain with a double bond between C-3 and C-4 and a methyl group on C-3
  2. B A five-carbon chain with a triple bond between C-2 and C-3 and a methyl group on C-3
  3. C A six-carbon chain with a methyl group on C-3
  4. D A five-carbon chain with a double bond between C-2 and C-3 and a methyl group on C-3
Show answer and explanation

Correct answer: D - A five-carbon chain with a double bond between C-2 and C-3 and a methyl group on C-3

3-methylpent-2-ene has a five-carbon backbone (pent) with a double bond starting at C-2 (pent-2-ene) and a methyl group at C-3. Options with bonds or chains inconsistent with these features are incorrect.

Q8
medium

A student encounters a hydrocarbon that decolorizes bromine water and has three double bonds. What is the correct IUPAC name for a five-carbon chain variant of this compound?

  1. A Penta-1,3,5-triene
  2. B Penta-1,2,4-triene
  3. C Penta-2,3,4-triene
  4. D Penta-1,3,4-triene
Show answer and explanation

Correct answer: B - Penta-1,2,4-triene

A five-carbon chain has only four carbon-carbon bonds, so the highest locant a double bond can take is 4; penta-1,3,5-triene is impossible. The compound is CH2=C=CH-CH=CH2, and numbering from the end that gives the lowest set of locants makes it penta-1,2,4-triene. Penta-1,3,4-triene and penta-2,3,4-triene are the same molecules numbered from the wrong end.

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