ChemistryISC Class 11 · Organic Chemistry: Some Basic Principles and Techniques

IUPAC Nomenclature and Isomerism: Structural, Geometrical and Optical

Updated 17 October 2026 · Fact-checked

IUPAC names are built from the longest carbon chain (parent), the principal functional group (suffix, numbered lowest), and substituents (prefixes, in alphabetical order). Isomers share a molecular formula; structural isomers differ in connectivity (chain, position, functional group, metamerism) and stereoisomers differ in arrangement in space (geometrical and optical).

Understand IUPAC Nomenclature and Types of Isomerism

Steps for naming. (1) Find the longest chain containing the principal functional group; that gives the root (meth-, eth-, prop-, but-, pent-, hex-). (2) Number from the end that gives the lowest locant to the principal functional group (then multiple bonds, then substituents). (3) Name the substituents (alkyl groups and halo, nitro, alkoxy) as prefixes in alphabetical order, with di-, tri- ignored in alphabetising. (4) Write: prefixes + root + saturation (-an-, -en-, -yn-) + suffix. Example: CH₃CH(CH₃)CH₂CH₃ is 2-methylbutane; CH₃CH₂CH(OH)CH₃ is butan-2-ol; CH₃COCH₂CH₃ is butan-2-one; (CH₃)₂CHCHO is 2-methylpropanal; HOOCCH₂CH(Cl)CH₃ is 3-chlorobutanoic acid.

Priority of groups. Carboxylic acid > ester > acid chloride > amide > nitrile > aldehyde > ketone > alcohol > amine; halo, nitro and alkoxy are only prefixes.

Structural isomerism. *Chain*: butane and 2-methylpropane. *Position*: butan-1-ol and butan-2-ol. *Functional group*: ethanol and methoxymethane. *Metamerism*: diethyl ether and methyl propyl ether. C₄H₁₀ has 2 isomers, C₅H₁₂ has 3, and C₄H₉Br has 4.

Geometrical (cis-trans) isomerism. Needs a double bond (or ring) that prevents rotation and two different groups on each doubly bonded carbon: but-2-ene and 1,2-dichloroethene show it; but-1-ene does not. The cis isomer has identical groups on the same side.

Optical isomerism. A molecule with a chiral (asymmetric) carbon, attached to four different groups, is non-superimposable on its mirror image and exists as a pair of enantiomers, which rotate the plane of polarised light in opposite directions. For n different stereocentres there are up to 2ⁿ stereoisomers; a meso compound with internal symmetry is optically inactive and reduces the count.

IUPAC Nomenclature and Types of Isomerism: formulas and key facts

Naming pattern: $p r e f i x (es) + r oo t + an / e n / y n + s u f f i x$; Lowest locants to the principal group, then double bonds, then substituents.
Stereoisomers: $M a x im u mn u mb er = 2^{n}$; n = number of different stereocentres; fewer if meso forms exist.
Chiral carbon: Attached to four different groups; Gives enantiomers.
Isomers of C₄H₉Br: 4 structural isomers; 1-, 2-bromobutane and two bromomethylpropanes.

IUPAC Nomenclature and Types of 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.

Which of the butanols can rotate polarised light?

Suppose a chemist has samples of the four isomeric alcohols C₄H₉OH.

Name them and find which one shows optical isomerism.

Try it first, then show the solutionSolution

Butan-1-ol CH₃CH₂CH₂CH₂OH; butan-2-ol CH₃CH₂CH(OH)CH₃; 2-methylpropan-1-ol (CH₃)₂CHCH₂OH; 2-methylpropan-2-ol (CH₃)₃COH.
In butan-2-ol, C2 is attached to H, OH, CH₃ and C₂H₅, which are four different groups.
Only butan-2-ol has a chiral carbon, so it exists as two enantiomers.

Answer: Butan-2-ol is optically active.

Memory hook: Look for a carbon with four different substituents.

Tricks to remember IUPAC Nomenclature and Types of Isomerism

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

Longest chain with the group
The parent chain must contain the principal functional group even if it is not the longest.
Use it when: Naming branched alcohols and acids.
Four different, so chiral
A carbon with two identical groups cannot be a stereocentre.
Use it when: Optical isomerism questions.

IUPAC Nomenclature and Types of Isomerism: solved examples

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

Example 1: Name of an alkane · 1 mark · MCQBoard level

The IUPAC name of CH₃CH(CH₃)CH₂CH₃ is:
(a) isopentane
(b) 2-methylbutane
(c) 3-methylbutane
(d) 2-ethylpropane

Try it first, then show the solutionSolution

The longest chain has four carbons (butane) and a methyl on C2 (lowest locant).

Answer: (b) 2-methylbutane

Example 2: Naming compounds · 3 marks · Short answerBoard level

Write the IUPAC names of (i) CH₃CH₂CH(OH)CH₃, (ii) CH₃CH=CHCH₂CH₃ and (iii) (CH₃)₂CHCHO.

Try it first, then show the solutionSolution

(i) Four carbons with OH on C2: butan-2-ol. (1 mark)
(ii) Five carbons, double bond between C2 and C3 (lowest locant): pent-2-ene. (1 mark)
(iii) Three-carbon chain with an aldehyde at C1 and a methyl at C2: 2-methylpropanal. (1 mark)

Answer: (i) butan-2-ol; (ii) pent-2-ene; (iii) 2-methylpropanal.

Example 3: Isomers of pentane · 4 marks · Short answerHarder board level

Draw the structural formulae and give the IUPAC names of all the isomers of C₅H₁₂.

Try it first, then show the solutionSolution

Straight chain: CH₃CH₂CH₂CH₂CH₃, pentane. (1 mark)
One methyl branch: CH₃CH(CH₃)CH₂CH₃, 2-methylbutane. (1.5 marks)
Two methyls on one carbon: (CH₃)₄C, 2,2-dimethylpropane. (1.5 marks)

Answer: Pentane, 2-methylbutane and 2,2-dimethylpropane (three isomers).

Example 4: Types of structural isomerism · 4 marks · Short answerHarder board level

Identify the type of isomerism in each pair: (a) butane and 2-methylpropane; (b) butan-1-ol and butan-2-ol; (c) ethanol and methoxymethane; (d) diethyl ether and methyl propyl ether. Which pair of C₄H₈ shows cis-trans isomerism?

Try it first, then show the solutionSolution

(a) Chain isomers. (0.75 mark)
(b) Position isomers (position of OH). (0.75 mark)
(c) Functional group isomers (alcohol and ether). (0.75 mark)
(d) Metamers (different alkyl groups on the same functional group). (0.75 mark)
But-2-ene (cis and trans); but-1-ene does not show it. (1 mark)

Answer: (a) chain; (b) position; (c) functional; (d) metamerism; but-2-ene shows cis-trans.

Example 5: Counting stereoisomers · 5 marks · Long answerHOTS / competency

Find the number of stereoisomers of (i) lactic acid CH₃CH(OH)COOH, (ii) 2,3-dibromopentane CH₃CHBrCHBrCH₂CH₃ and (iii) butane-2,3-diol CH₃CH(OH)CH(OH)CH₃. Explain the difference in (iii).

Try it first, then show the solutionSolution

(i) One chiral carbon (C2): 2¹ = 2 enantiomers. (1 mark)
(ii) Two different chiral carbons (C2 and C3): 2² = 4 stereoisomers (two pairs of enantiomers). (1.5 marks)
(iii) Two chiral carbons, but the two ends of the molecule are identical, so one stereoisomer is a meso compound with an internal plane of symmetry. (1.5 marks)
The total is 3 stereoisomers: one pair of enantiomers and one optically inactive meso form. (1 mark)

Answer: (i) 2; (ii) 4; (iii) 3 (two enantiomers and one meso form).

Where students lose marks in IUPAC Nomenclature and Types of Isomerism

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

Mistake: Wrong chain choice
Picking the longest chain without the principal functional group.
Do this: The chain must contain the functional group.
Usually costs: 2 marks.
Mistake: Wrong numbering
Numbering from the end that gives a substituent the lowest number.
Do this: The principal group gets the lowest locant first.
Usually costs: 1 to 2 marks.
Mistake: Calling any carbon with a substituent chiral
Naming CH₃CH₂Cl carbons chiral.
Do this: Chirality needs four different groups.
Usually costs: 1 mark.

Exam tips

Write the structure with all carbons numbered before naming.
List substituents alphabetically.
Draw all isomers systematically (chain first, then position).
Mark the chiral carbon with an asterisk.

Practice questions on IUPAC Nomenclature and Types of 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 functional group present in an aldehyde is:
(a) –OH
(b) –CO–
(c) –CHO
(d) –COOH
Hint
An aldehyde has the carbonyl group at the end of the chain.
Answer
(c) –CHO
Q2 · 2 marks · Short answerBoard level
Give the IUPAC names of CH₃CH₂CH₂COOH and CH₃CHBrCH₃.
Hint
Identify the principal chain and the substituent.
Answer
CH₃CH₂CH₂COOH: butanoic acid. CH₃CHBrCH₃: 2-bromopropane.
Q3 · 3 marks · Short answerHarder board level
Write the four structural isomers of C₄H₉Br with IUPAC names, and say which is chiral.
Hint
Two chain skeletons, with the bromine in different positions.
Answer
1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane, 2-bromo-2-methylpropane. 2-Bromobutane is chiral (C2 has H, Br, CH₃, C₂H₅).
Q4 · 3 marks · Short answerHarder board level
Which of the following are chiral? (a) CH₃CH(OH)CH₂CH₃ (b) (CH₃)₂CHOH (c) CH₃CH(Cl)COOH. Give reasons.
Hint
Look for four different groups on a carbon.
Answer
(a) Chiral: C2 has H, OH, CH₃, C₂H₅. (b) Not chiral: the carbon with OH carries two CH₃ groups. (c) Chiral: C2 has H, Cl, CH₃, COOH.
Q5 · 3 marks · Short answerHarder board level
Which of the following show geometrical isomerism: but-1-ene, but-2-ene, 2-methylbut-2-ene and 1,2-dichloroethene? Explain the condition.
Hint
Each doubly bonded carbon needs two different groups.
Answer
But-2-ene and 1,2-dichloroethene show it. But-1-ene (=CH₂ has two H) and 2-methylbut-2-ene (one carbon has two CH₃) do not.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student writes the isomers of C₄H₈O₂.
(i) Name butanoic acid and 2-methylpropanoic acid and their type of isomerism. (1 mark)
(ii) Name ethyl ethanoate and methyl propanoate and their type of isomerism. (1 mark)
(iii) What is the isomerism between butanoic acid and ethyl ethanoate? (1 mark)
(iv) Which of these would turn blue litmus red? (1 mark)
Hint
Acids and esters are functional group isomers.
Answer
(i) CH₃CH₂CH₂COOH and (CH₃)₂CHCOOH: chain isomers. (ii) CH₃COOC₂H₅ and C₂H₅COOCH₃: metamers. (iii) Functional group isomers (acid and ester). (iv) The two carboxylic acids.

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.How many isomers does C₄H₁₀ have?Show answer
2.
2.What is a chiral carbon?Show answer
A carbon attached to four different groups.
3.Condition for cis-trans isomerism?Show answer
Restricted rotation and two different groups on each doubly bonded carbon.
4.What is a meso compound?Show answer
A compound with stereocentres but internal symmetry, hence optically inactive.

IUPAC Nomenclature and Types of Isomerism: frequently asked questions

Why do we number from the end nearest the functional group?

IUPAC rules give the lowest locant to the principal functional group so that every compound has one unique name.

Do enantiomers have the same physical properties?

They have the same melting and boiling points but rotate plane-polarised light in opposite directions by equal amounts.

Is a molecule with two chiral centres always optically active?

No. If the molecule has an internal plane of symmetry (a meso form), it is optically inactive.