ChemistryCBSE Class 11 · Organic Chemistry – Some Basic Principles and Techniques

IUPAC Nomenclature and Structural Isomerism Class 11

Updated 17 October 2026 · Fact-checked

To name an organic compound by IUPAC rules, choose the longest chain containing the principal functional group, number it so that the principal group gets the lowest locant (then multiple bonds, then substituents), and write substituents alphabetically before the parent name. Structural isomers have the same molecular formula but different connectivity: chain isomers differ in the carbon skeleton, position isomers in the place of a substituent or functional group, functional isomers in the functional group, and metamers in the alkyl groups on either side of a functional group such as an ether or ester.

Number the longest chain so the OH group gets the lowest locant, then write the substituent first: 3-methyl + pentan + 2-ol.

Illustration drawn for QuestionBuddy.

Understand IUPAC Nomenclature and Structural Isomerism

Parts of a name. word root (number of carbons in the main chain: meth, eth, prop, but, pent, hex, hept, oct), suffix for the principal group, and prefixes for the other groups.

Steps for naming.
1. Choose the principal functional group by priority: carboxylic acid > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > halo and other substituents. Halogens, nitro, alkoxy and alkyl groups are only prefixes.
2. Select the longest chain that carries the principal group (and, as far as possible, the multiple bonds).
3. Number from the end that gives the principal group the lowest locant; if there is still a choice, the multiple bonds (together), then the substituents (first point of difference).
4. Name each substituent with its locant, arrange them in alphabetical order (ignoring di-, tri-; counting complex-substituent names as written), and attach the parent name and suffix, for example *3-hydroxy-5-oxopentanoic acid*.
5. A $- C H O$ group as a substituent is *oxo* (or *formyl*), a $C = O$ inside a chain is *oxo*, and $- C O O H$ on a ring is *carboxylic acid*.

Structural isomerism.

*Chain isomerism*: butane and 2-methylpropane.
*Position isomerism*: propan-1-ol and propan-2-ol; 1-chloropropane and 2-chloropropane.
*Functional isomerism*: ethanol and methoxymethane (C₂H₆O); propanal and propanone (C₃H₆O).
*Metamerism*: diethyl ether and methoxypropane (C₄H₁₀O).
*Ring-chain isomerism*: propene and cyclopropane (C₃H₆).

Counting. Pentane family: C₅H₁₂ has 3 isomers, C₆H₁₄ has 5, C₄H₈ has 6 including cyclic and geometrical forms. Branching lowers the boiling point because the surface area, and with it the van der Waals attraction, is smaller. Stereoisomerism (geometrical and optical) is a separate category and is developed further in higher classes.

IUPAC Nomenclature and Structural Isomerism: formulas and key facts

Alkane family: $C_{n} H_{2 n + 2}$; Alkene C_nH_{2n}; alkyne C_nH_{2n-2}.
Degree of unsaturation: $DBE = \frac{2 C + 2 + N - H - X}{2}$; Counts rings plus π bonds.
Functional isomer pair examples: $C_{n} H_{2 n + 2} O : alcohol \leftrightarrow ether$; For C₂H₆O: ethanol and methoxymethane.

How to solve IUPAC Nomenclature and Structural Isomerism questions

For a name, identify the principal group first. For isomers, fix the molecular formula and change one feature at a time: skeleton, position, then functional group.

1Mark the principal functional group using the priority list.
2Pick the longest chain containing it and number to give it the lowest locant.
3List substituents alphabetically with locants and hyphens/commas in the right places.
4For isomers, calculate the degrees of unsaturation first.
5Enumerate the carbon skeletons, then place substituents or groups on each, avoiding duplicates by checking the names.
6Classify each pair (chain, position, functional, metamers) and name every structure.

IUPAC Nomenclature and Structural Isomerism in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Cooking gas cylinder

Suppose a cylinder contains butane and its branched isomer 2-methylpropane, both C₄H₁₀.

Which is of lower boiling point and why? What kind of isomers are they?

Try it first, then show the solutionSolution

2-Methylpropane is more compact, with less surface area, so weaker van der Waals forces and a lower boiling point.
They differ in the carbon skeleton: chain isomers.

Answer: 2-Methylpropane boils lower; chain isomers.

Memory hook: More branching, lower boiling point.

Rubbing alcohol and an anaesthetic ether

Suppose two bottles each hold a liquid of molecular formula C₃H₈O: one is propan-2-ol and the other methoxyethane.

Name the kind of isomerism and say which has hydrogen bonding.

Try it first, then show the solutionSolution

The functional groups differ (alcohol and ether), so they are functional isomers.
Propan-2-ol has an O–H bond and forms hydrogen bonds, so its boiling point is higher.

Answer: Functional isomers; propan-2-ol shows hydrogen bonding.

Memory hook: O–H means hydrogen bonds, which means a higher boiling point.

Tricks to remember IUPAC Nomenclature and Structural Isomerism

Short memory aids. Say each one aloud once, then use it on a question the same day.

Priority ladder
Acid, ester, acid chloride, amide, nitrile, aldehyde, ketone, alcohol, amine: the earliest in the list is the suffix.
Use it when: Naming polyfunctional compounds.
Alphabetical, not numerical
Substituents are listed in alphabetical order; di, tri and tetra are ignored.
Use it when: Prefix order.
Same formula, new connection
Chain changes the skeleton, position moves the group, function swaps the group.
Use it when: Types of isomerism.
Branching lowers boiling point
A compact molecule has less contact area.
Use it when: Comparing isomers.

IUPAC Nomenclature and Structural Isomerism: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Naming a substituted alkane · 2 marks · Very short answerBoard level

Write the IUPAC name of CH₃–CH(CH₃)–CH₂–CHCl–CH₃.

Try it first, then show the solutionSolution

The longest chain has five carbons (pentane); a methyl group is on one carbon and chlorine on another, with locants 2 and 4 from either end. (1)
With a tie in locants the substituent first in alphabetical order (chloro) gets the lower number: 2-chloro-4-methylpentane. (1)

Answer: 2-chloro-4-methylpentane.

Example 2: Polyfunctional names · 3 marks · Short answerHarder board level

Give the IUPAC names of (a) HOOC–CH₂–CH(OH)–CH₂–CHO and (b) CH₃–CH=CH–CH₂–C≡CH.

Try it first, then show the solutionSolution

(a) The acid is the principal group (C1 = COOH); the aldehyde is named as oxo, the hydroxyl as hydroxy: 3-hydroxy-5-oxopentanoic acid. (1)
(b) The chain has six carbons with a double and a triple bond. Numbering from the alkyne end gives locants {1, 4}, from the other end {2, 5}; {1, 4} is lower. (1)
Name: hex-4-en-1-yne (the ending is yne, with the ene locant given before it). (1)

Answer: (a) 3-hydroxy-5-oxopentanoic acid; (b) hex-4-en-1-yne.

Example 3: Chain isomers of C₆H₁₄ · 3 marks · Short answerHarder board level

Write the condensed formulas and the IUPAC names of all the isomers of C₆H₁₄ and say which has the lowest boiling point, with the reason.

Try it first, then show the solutionSolution

Hexane CH₃(CH₂)₄CH₃; 2-methylpentane (CH₃)₂CHCH₂CH₂CH₃; 3-methylpentane CH₃CH₂CH(CH₃)CH₂CH₃. (1)
2,2-Dimethylbutane (CH₃)₃CCH₂CH₃; 2,3-dimethylbutane (CH₃)₂CHCH(CH₃)₂. Total five isomers. (1)
2,2-Dimethylbutane is the most branched and nearly spherical, with the smallest surface area and weakest van der Waals forces, so it has the lowest boiling point. (1)

Answer: Five isomers; 2,2-dimethylbutane boils lowest.

Example 4: All isomers of C₄H₈ · 4 marks · Long answerHarder board level

Write all the isomers of C₄H₈ (alkenes and cycloalkanes), name them and classify each pair as chain, position, geometrical or ring-chain isomers.

Try it first, then show the solutionSolution

Alkenes: but-1-ene CH₂=CHCH₂CH₃, but-2-ene CH₃CH=CHCH₃ (cis and trans) and 2-methylpropene (CH₃)₂C=CH₂. (1)
Cycloalkanes: cyclobutane and methylcyclopropane. Total six isomers counting cis and trans separately. (1)
But-1-ene and but-2-ene are position isomers; either with 2-methylpropene are chain isomers; cis and trans but-2-ene are geometrical isomers. (1)
Each alkene and the cycloalkanes are ring-chain isomers of each other (the degree of unsaturation is 1 in both). (1)

Answer: Six isomers: but-1-ene, cis- and trans-but-2-ene, 2-methylpropene, cyclobutane, methylcyclopropane.

Example 5: Isomers of C₄H₈O₂ · 4 marks · Long answerHOTS / competency

Write the carboxylic acids and esters of formula C₄H₈O₂ with their IUPAC names, and classify the relationships: (a) between the two acids, (b) between methyl propanoate and ethyl ethanoate, (c) between an acid and an ester.

Try it first, then show the solutionSolution

Acids: butanoic acid CH₃CH₂CH₂COOH and 2-methylpropanoic acid (CH₃)₂CHCOOH. (1)
Esters: methyl propanoate CH₃CH₂COOCH₃, ethyl ethanoate CH₃COOCH₂CH₃, propyl methanoate HCOOCH₂CH₂CH₃ and propan-2-yl methanoate HCOOCH(CH₃)₂. Total six isomers. (1)
(a) The acids are chain isomers; (b) the two esters differ in the alkyl groups on either side of the –COO– group: metamers. (1)
(c) An acid and an ester are functional isomers (same formula C₄H₈O₂, different functional groups). (1)

Answer: Two acids and four esters; acids are chain isomers, esters like methyl propanoate and ethyl ethanoate are metamers, acid and ester are functional isomers.

Where students lose marks in IUPAC Nomenclature and Structural Isomerism

The slips examiners see most often, and what to write instead.

Mistake: Numbering from the end that is nearer to a substituent when a principal functional group is present.
Alkane-style numbering applied to a polyfunctional compound.
Do this: The principal group gets the lowest locant before any substituent.
Usually costs: The name.
Mistake: Arranging prefixes by locant instead of alphabetically.
Ignoring the alphabetical rule.
Do this: Alphabetical order, ignoring di, tri; use the first letter of the substituent name.
Usually costs: A mark on the name.
Mistake: Choosing the longest chain that does not contain the principal functional group.
Alkane rule applied to a functionalised compound.
Do this: The chain must contain the principal group.
Usually costs: The parent name.
Mistake: Counting the same structure twice among the isomers.
Drawing mirror or rotated forms of the same chain.
Do this: Name every structure and compare the names.
Usually costs: Isomer count.

Exam tips

Write the principal functional group and its priority before naming.
Show the numbering on the structure to justify the locants.
Use a hyphen between a number and a letter and a comma between numbers.
Name every isomer you draw so duplicates are easy to spot.
State the type of isomerism, not just the pair.

Practice questions on IUPAC Nomenclature and Structural Isomerism

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The IUPAC name of (CH₃)₃C–CH₂–CH(CH₃)₂ is
(a) 2,4,4-trimethylpentane (b) 2,2,4-trimethylpentane (c) 2,2-dimethyl-4-methylpentane (d) 1,1,1,3,3-pentamethylpropane
Hint
Choose the numbering that gives the lowest set of locants.
Answer
(b): the locants {2, 2, 4} are lower than {2, 4, 4}.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Ethanol and methoxymethane are functional group isomers.
Reason (R): They have the same molecular formula, C₂H₆O, but contain different functional groups.
(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 true.
Hint
Alcohol and ether.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Define chain isomerism and position isomerism and give one example of each using the formulae C₄H₁₀ and C₃H₇Cl.
Hint
Skeleton versus place of the substituent.
Answer
Chain isomers differ in the carbon skeleton: butane and 2-methylpropane. Position isomers have the same skeleton but the substituent in a different place: 1-chloropropane and 2-chloropropane.
Q4 · 3 marks · Short answerHarder board level
Name the following by IUPAC rules: (a) CH₃CH₂CH(NH₂)CH₃, (b) ClCH₂CH₂COOH and (c) CH₂=CH–CH(CH₃)–CH₂OH.
Hint
Alcohol outranks alkene; acid outranks halogen.
Answer
(a) Butan-2-amine. (b) 3-Chloropropanoic acid. (c) 2-Methylbut-3-en-1-ol (the –OH carbon is C1).
Q5 · 4 marks · Long answerHOTS / competency
(a) Write the three chain isomers of C₅H₁₂ with their IUPAC names. (b) Find how many monochloro structural isomers each can form. (c) Give the total number of monochloro derivatives of C₅H₁₂ (ignore optical isomers) and name the isomer that gives only one.
Hint
Count the sets of equivalent hydrogens.
Answer
(a) Pentane, 2-methylbutane, 2,2-dimethylpropane. (b) Pentane: 3 (1-, 2-, 3-chloropentane); 2-methylbutane: 4 (1-chloro-2-methylbutane, 2-chloro-2-methylbutane, 2-chloro-3-methylbutane, 1-chloro-3-methylbutane); 2,2-dimethylpropane: 1. (c) Total 8 structural monochloro derivatives; 2,2-dimethylpropane gives only 1-chloro-2,2-dimethylpropane because all twelve hydrogens are equivalent.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which is higher in priority: –OH or –COOH?Show answer
–COOH.
2.Number of isomers of C₅H₁₂?Show answer
Three.
3.Metamerism is shown by?Show answer
Compounds with different alkyl groups around the same functional group, such as ethers and esters.
4.Prefix for –CHO in a higher-priority compound?Show answer
Oxo.
5.General formula of alkynes?Show answer
CₙH₂ₙ₋₂.
6.Which isomer of an alkane has the lowest boiling point?Show answer
The most branched.

IUPAC Nomenclature and Structural Isomerism: frequently asked questions

Why are IUPAC names used?

Each structure has a unique systematic name that can be translated back into a structure without ambiguity.

How is the lowest set of locants decided?

Compare the sets term by term; the first point of difference decides.

Do propanal and propanone show functional isomerism?

Yes: both are C₃H₆O, one an aldehyde and one a ketone.

Do cis-trans isomers count as structural isomers?

No. They have the same connectivity and are stereoisomers.

What is ring-chain isomerism?

Isomerism between an open-chain compound and a cyclic compound of the same formula, such as propene and cyclopropane.