What are the types of isomerism?

Structural isomerism (chain, position, functional, metamerism) and stereoisomerism (geometric/optical).

How do you name an organic compound by IUPAC?

Find the longest carbon chain (parent), number from the end nearest the functional group, identify substituents, and write the name as: substituent(s)-prefix + parent + suffix.

Organic Chemistry Fundamentals | KalviGavani

Q: What is a homologous series?

A series of organic compounds where each successive member differs by a –CH₂– unit, sharing the same functional group and similar chemical properties.

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How to Use This Page

Welcome to the Fundamentals of Organic Chemistry exercise solutions for Class 11 Chemistry (Tamil Nadu State Board). Here is how to get the most from this page:

1. Attempt every question on your own first — do not jump straight to the answers. 2. Compare your answer with the solution provided. If they differ, trace through the explanation step by step. 3. Re-work mistakes — write the corrected answer in your own words; this builds deeper understanding. 4. Use the Key Concepts section for a quick recap before exams.

Key Concepts (Quick Recap)

IUPAC Nomenclature — Step-by-Step Rules

Step	Action	Example
1	Find the longest continuous carbon chain (parent chain)	In CH₃CH(CH₃)CH₂CH₃, the longest chain has 4 carbons
2	Number the chain from the end nearest the principal functional group	Give –CHO the lowest possible number
3	Identify substituents and assign position numbers	Methyl on C-2
4	Arrange substituents in alphabetical order (ignoring prefixes like di-, tri-)	Ethyl before methyl
5	Write name: prefix(es) + parent + suffix	2-Methylbutane

Functional Groups — Suffixes and Prefixes

Functional Group	Formula	IUPAC Suffix	IUPAC Prefix	Priority
Carboxylic acid	–COOH	-oic acid	carboxy-	1 (highest)
Sulphonic acid	–SO₃H	-sulphonic acid	sulpho-	2
Ester	–COOR	-oate	alkoxycarbonyl-	3
Acid chloride	–COCl	-oyl chloride	chlorocarbonyl-	4
Aldehyde	–CHO	-al	formyl-	5
Ketone	>C=O	-one	oxo-	6
Alcohol	–OH	-ol	hydroxy-	7
Amine	–NH₂	-amine	amino-	8
Alkene	C=C	-ene	—	9
Alkyne	C≡C	-yne	—	10

Types of Isomerism

Type	Sub-type	Description	Example
Structural	Chain	Different carbon skeleton arrangements	Butane vs Isobutane (2-methylpropane)
	Position	Same functional group at different positions	1-Propanol vs 2-Propanol
	Functional	Different functional groups	Ethanal (aldehyde) vs Ethenol (enol)
	Metamerism	Different alkyl groups on either side of a divalent group	Diethyl ether vs Methyl propyl ether
Stereo	Geometric (cis-trans)	Restricted rotation around double bond	cis-2-Butene vs trans-2-Butene
	Optical	Non-superimposable mirror images	Lactic acid enantiomers

Homologous Series — Properties

Property	Description
General formula	Each series has a formula CₙH₂ₙ₊₂ (alkanes), CₙH₂ₙ (alkenes), etc.
Gradation in physical properties	Melting point, boiling point, and density increase gradually
Similar chemical properties	All members undergo the same types of reactions
Common difference	Each successive member differs by –CH₂– (14 amu)
Same functional group	Determines the chemical behaviour of all members

IUPAC Naming of Common Parent Chains

Carbon atoms	Alkane name	Carbon atoms	Alkane name
1	Methane	6	Hexane
2	Ethane	7	Heptane
3	Propane	8	Octane
4	Butane	9	Nonane
5	Pentane	10	Decane

I. Choose the Best Answer (MCQs)

1. The IUPAC name of CH₃–CH(CH₃)–CH₂–CH₃ is:

Answer

(a) Isopentane (b) 2-Methylbutane (c) 3-Methylbutane (d) 2-Methylpentane

Answer(b) 2-Methylbutane

ExplanationThe longest continuous carbon chain has 4 carbons (butane). Number from the end nearest the methyl branch, which falls on carbon-2. Hence the name is 2-Methylbutane. Option (c) is wrong because numbering from the other end gives a higher locant (3). Option (a) is a common name, not IUPAC. Option (d) incorrectly counts 5 carbons in the parent chain.

2. Which of the following pairs are functional isomers?

Answer

(a) Ethanol and Dimethyl ether (b) Propane and Cyclopropane (c) 1-Butene and 2-Butene (d) Butane and Isobutane

Answer(a) Ethanol and Dimethyl ether

ExplanationEthanol (C₂H₅OH) has the –OH functional group, while dimethyl ether (CH₃OCH₃) has the –O– ether linkage. Both share the molecular formula C₂H₆O but differ in their functional groups — this is functional isomerism. Options (b) and (d) are chain isomers. Option (c) represents positional isomers.

3. The number of structural isomers possible for C₄H₁₀ is:

Answer

(a) 2 (b) 3 (c) 4 (d) 5

Answer(a) 2

ExplanationC₄H₁₀ (butane) has only two structural isomers: n-butane (CH₃CH₂CH₂CH₃) and isobutane/2-methylpropane (CH₃CH(CH₃)CH₃). The carbon skeleton can only be a straight chain of 4 or a branched chain with 3 in the main chain.

4. Which functional group has the highest priority in IUPAC nomenclature?

Answer

(a) –OH (alcohol) (b) –CHO (aldehyde) (c) –COOH (carboxylic acid) (d) >C=O (ketone)

Answer(c) –COOH (carboxylic acid)

ExplanationThe carboxylic acid group has the highest priority among common functional groups. The priority order is: carboxylic acid > sulphonic acid > ester > acid chloride > aldehyde > ketone > alcohol > amine. Hence, when multiple functional groups are present, –COOH determines the suffix.

5. In a homologous series, each successive member differs by:

Answer

(a) CH (b) CH₂ (c) CH₃ (d) CH₄

Answer(b) CH₂

ExplanationA homologous series is defined by the addition of a –CH₂– (methylene) unit between successive members. For example, methane (CH₄) to ethane (C₂H₆) to propane (C₃H₈), each differing by CH₂ (14 amu).

6. Which of the following is NOT a characteristic of a homologous series?

Answer

(a) Members have the same functional group (b) Members have similar chemical properties (c) Members have the same molecular formula (d) Members show a regular gradation in physical properties

Answer(c) Members have the same molecular formula

ExplanationMembers of a homologous series differ in molecular formula by CH₂ — they do NOT share the same formula. They do share the same functional group, similar chemical properties, and a regular gradation in physical properties like boiling point.

7. The IUPAC name of CH₃CH₂COCH₂CH₃ is:

Answer

(a) Diethyl ketone (b) 3-Pentanone (c) Pentan-3-one (d) Ethyl propanone

Answer(c) Pentan-3-one

ExplanationThe parent chain has 5 carbons (pentane). The ketone group (>C=O) is on carbon-3 when numbered from either end (both directions give the same locant). The suffix for ketone is "-one", giving Pentan-3-one.

II. Fill in the Blanks

1. The IUPAC name of HCHO is methanal.

2. The general formula of alkynes is CₙH₂ₙ₋₂.

3. Isomers that differ in the position of the functional group on the carbon chain are called position isomers.

4. The prefix used for the –OH group when it is not the principal functional group is hydroxy.

5. The molecular formula C₃H₈O has three structural isomers (1-propanol, 2-propanol, and methoxyethane).

6. The suffix for a compound with –COOH as the principal functional group is -oic acid.

7. In IUPAC nomenclature, substituents are arranged in alphabetical order.

8. The homologous series of alkenes has the general formula CₙH₂ₙ.

9. Isomers with the same molecular formula but different carbon skeletons are called chain isomers.

10. The IUPAC name of CH₃CH₂CH₂OH is propan-1-ol.

11. The functional group –NH₂ is called the amino group.

12. Metamerism is a type of isomerism seen in compounds containing a divalent functional group like –O– or –S–.

13. The IUPAC name of CH₃COCH₃ is propan-2-one.

14. The carbon atom bonded to four different groups is called a chiral centre.

15. The series where each member differs from the next by –CH₂– is called a homologous series.

III. True or False (with Corrections)

1. Statement: All isomers have the same molecular formula.

AnswerTrue. By definition, isomers are compounds with the same molecular formula but different structural arrangements or spatial orientations.

2. Statement: Chain isomers have the same carbon skeleton.

AnswerFalse. Chain isomers differ in the arrangement of the carbon skeleton. For example, n-butane (straight chain) and 2-methylpropane (branched) have different carbon skeletons.

3. Statement: The –COOH group is named as the prefix "carboxy" when it is the principal functional group.

AnswerFalse. When –COOH is the principal functional group, it is named as the suffix "-oic acid". The prefix "carboxy-" is used only when it is NOT the principal functional group.

4. Statement: Optical isomerism arises when a molecule has a plane of symmetry.

AnswerFalse. Optical isomerism arises when a molecule has a chiral centre (carbon bonded to four different groups) and lacks a plane of symmetry. A plane of symmetry would mean the molecule is achiral and NOT optically active.

5. Statement: In a homologous series, the boiling points increase with increasing molecular mass.

AnswerTrue. As the number of carbon atoms increases in a homologous series, van der Waals forces increase, raising the boiling point.

IV. Match the Following

Column A	Column B
1. –COOH	(a) -amine
2. –CHO	(b) -oic acid
3. >C=O	(c) -al
4. –NH₂	(d) -ol
5. –OH	(e) -one

Answer

1. –COOH → (b) -oic acid 2. –CHO → (c) -al 3. >C=O → (e) -one 4. –NH₂ → (a) -amine 5. –OH → (d) -ol

Additional Match the Following:

Column A	Column B
1. Chain isomerism	(a) Different functional groups
2. Position isomerism	(b) Different carbon skeletons
3. Functional isomerism	(c) Non-superimposable mirror images
4. Geometric isomerism	(d) Same skeleton, different group position
5. Optical isomerism	(e) Restricted rotation around double bond

Answer

1. Chain isomerism → (b) Different carbon skeletons 2. Position isomerism → (d) Same skeleton, different group position 3. Functional isomerism → (a) Different functional groups 4. Geometric isomerism → (e) Restricted rotation around double bond 5. Optical isomerism → (c) Non-superimposable mirror images

V. Answer Briefly (2-mark)

1. Define functional group with two examples.

Answer

A functional group is an atom or a group of atoms in an organic molecule that is responsible for its characteristic chemical reactions. The rest of the molecule (the carbon chain) mainly affects physical properties.

Example 1: –OH (hydroxyl group) in alcohols like ethanol (C₂H₅OH)
Example 2: –COOH (carboxyl group) in carboxylic acids like ethanoic acid (CH₃COOH)

2. What is a homologous series? Give one example.

Answer

A homologous series is a group of organic compounds that:

Have the same general formula
Contain the same functional group
Show similar chemical properties
Differ from each other by a –CH₂– unit
Show a regular gradation in physical properties

Example: The alkane series — CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane) — where each successive member differs by CH₂.

3. Differentiate between chain isomerism and position isomerism.

Answer

Feature	Chain Isomerism	Position Isomerism
Definition	Different arrangement of the carbon skeleton	Same carbon skeleton but functional group at a different position
Carbon skeleton	Different	Same
Example	n-Pentane vs Neopentane	1-Butanol vs 2-Butanol
Functional group	Same, at same position	Same, at different position

4. Write the IUPAC name of the following compound:

   CH₃
    |
CH₃–CH–CH₂–CH₂–CH₃

AnswerThe longest chain has 5 carbons (pentane). The methyl group is on carbon-2. Hence, the IUPAC name is 2-Methylpentane.

VI. Answer in Detail (5-mark)

1. Explain the rules of IUPAC nomenclature for naming organic compounds with suitable examples.

Answer

The IUPAC system provides a systematic method for naming organic compounds. The rules are:

Rule 1 — Select the longest carbon chain (parent chain): The longest continuous chain of carbon atoms is selected as the parent chain. This determines the root name.

CH₃–CH₂–CH₂–CH₃  →  4 carbons → Butane

Rule 2 — Number the carbon atoms: Number the parent chain from the end nearest the principal functional group (or substituent, if no functional group is present). The lowest set of locants (numbers) should be used.

CH₃–CH(Cl)–CH₂–CH₃  →  Number from left: Cl on C-2 (not C-3)

Rule 3 — Identify and name substituents: Each substituent (branch or group attached to the main chain) is named as a prefix.

Substituent	Prefix
–CH₃	Methyl
–CH₂CH₃	Ethyl
–Cl	Chloro
–Br	Bromo
–NO₂	Nitro

Rule 4 — Arrange substituents in alphabetical order: When multiple substituents are present, they are listed in alphabetical order (ignoring di-, tri-, etc.). Each gets its own locant.

CH₃–CH(Br)–CH(Cl)–CH₃  →  2-Bromo-3-chlorobutane

Rule 5 — Use appropriate suffix for the principal functional group: The functional group with the highest priority determines the suffix.

CH₃–CH₂–COOH  →  Propanoic acid (–COOH → -oic acid)
CH₃–CH₂–CHO   →  Propanal (–CHO → -al)
CH₃–CO–CH₃    →  Propan-2-one (>C=O → -one)
CH₃–CH₂–OH    →  Propan-1-ol (–OH → -ol)

Rule 6 — Indicate multiple bonds: Double bonds are indicated by "-ene" and triple bonds by "-yne", with position numbers.

CH₂=CH–CH₃       →  Prop-1-ene
CH₃–C≡C–CH₃     →  But-2-yne

Worked Example: Name the compound: CH₃CH(CH₃)CH(OH)CH₂CH₃

Longest chain: 5 carbons → pentane
–OH is the principal functional group → suffix "-ol"
Number from the end nearest –OH: –OH on C-3
Methyl substituent on C-4
Name: 4-Methylpentan-3-ol

2. Explain the different types of isomerism with examples.

Answer

Isomerism is the phenomenon where two or more compounds have the same molecular formula but different structural arrangements or spatial orientations.

A. Structural Isomerism (Constitutional Isomerism)

1. Chain Isomerism: Different arrangements of the carbon skeleton.

n-Butane:  CH₃–CH₂–CH₂–CH₃
Isobutane: CH₃–CH(CH₃)–CH₃  (2-methylpropane)

Both have the formula C₄H₁₀ but differ in carbon chain branching.

2. Position Isomerism: Same carbon skeleton but the functional group is at a different position.

Propan-1-ol:  CH₃–CH₂–CH₂–OH
Propan-2-ol:  CH₃–CH(OH)–CH₃

Both have the formula C₃H₈O, but –OH is on C-1 vs C-2.

3. Functional Isomerism: Different functional groups.

Ethanal (aldehyde):  CH₃–CHO
Ethenol (enol):      CH₂=CH–OH

Both have the formula C₂H₄O but different functional groups.

Ethanol (alcohol):      C₂H₅–OH
Dimethyl ether (ether): CH₃–O–CH₃

Both have the formula C₂H₆O.

4. Metamerism: Different alkyl groups on either side of a divalent functional group (ethers, ketones, esters).

Diethyl ether:         CH₃CH₂–O–CH₂CH₃
Methyl propyl ether:   CH₃–O–CH₂CH₂CH₃

B. Stereoisomerism

1. Geometric (cis-trans) Isomerism: Occurs due to restricted rotation around a double bond or ring.

cis-2-Butene:   CH₃ groups on the same side of C=C
trans-2-Butene: CH₃ groups on opposite sides of C=C

2. Optical Isomerism: Occurs when a molecule is non-superimposable on its mirror image (has a chiral centre).

Lactic acid: CH₃–CH(OH)–COOH
The carbon bonded to –H, –OH, –CH₃, and –COOH is chiral.
Two mirror-image forms exist: d-lactic acid and l-lactic acid.

3. Describe the characteristics of a homologous series with examples. What is its significance?

Answer

A homologous series is a series of organic compounds in which each successive member differs from the previous one by a –CH₂– unit (methylene group), while retaining the same functional group.

Characteristics

1. Same General Formula: Every member can be represented by a single general formula.

Alkanes:    CₙH₂ₙ₊₂  (n = 1, 2, 3, ...)
Alkenes:    CₙH₂ₙ    (n = 2, 3, 4, ...)
Alkynes:    CₙH₂ₙ₋₂  (n = 2, 3, 4, ...)
Alcohols:   CₙH₂ₙ₊₁OH (n = 1, 2, 3, ...)

2. Gradation in Physical Properties: There is a regular increase in boiling point, melting point, and density as the molecular mass increases. This is because larger molecules have stronger van der Waals forces.

Methane (CH₄):  bp = -161.5 °C
Ethane (C₂H₆):  bp = -88.6 °C
Propane (C₃H₈): bp = -42.1 °C
Butane (C₄H₁₀): bp = -0.5 °C

3. Similar Chemical Properties: All members undergo the same types of chemical reactions because they have the same functional group.

All alcohols (R–OH) undergo:
- Esterification: R–OH + R'–COOH → R'–COOR + H₂O
- Oxidation: R–OH → R–CHO → R–COOH
- Dehydration: R–OH → Alkene + H₂O

4. Common Difference of CH₂: Each successive member has one more –CH₂– unit and is 14 amu heavier.

CH₄ (16 amu) → C₂H₆ (30 amu) → C₃H₈ (44 amu) → C₄H₁₀ (58 amu)
Differences: 14, 14, 14

5. Same Functional Group: The functional group remains identical throughout the series, so the nature of chemical reactions does not change.

Significance of Homologous Series

Predictability: Knowing the properties of one member allows prediction of others.
Systematic Study: Organises millions of organic compounds into manageable groups.
Synthesis Planning: Helps in predicting products and selecting reagents.
Physical Property Trends: Boiling points, densities follow predictable patterns, aiding in separation techniques like fractional distillation.

Common Mistakes to Avoid

1. Numbering from the wrong end: Always number from the end that gives the lowest locants to the principal functional group. Do not number from whichever end is shorter — prioritise functional groups.

2. Confusing suffix and prefix for functional groups: When –COOH is present, it ALWAYS takes the suffix "-oic acid"; it never becomes a prefix on the main chain. Learn the priority order by heart.

3. Forgetting alphabetical order of substituents: In IUPAC names, substituents are listed alphabetically, not by their position numbers. "Ethyl" comes before "methyl" regardless of their locants.

4. Counting the parent chain incorrectly: Always find the longest continuous chain — even if it is not drawn as a straight line. Branches do not count towards the parent chain.

5. Mixing up position and chain isomerism: Position isomers have the SAME carbon skeleton but different functional group positions. Chain isomers have DIFFERENT carbon skeletons.

6. Omitting locants for multiple bonds: In alkenes and alkynes, you must specify the position of the double/triple bond (e.g., but-2-ene, not just butene).

7. Writing common names instead of IUPAC: "Isobutane", "iso-propyl alcohol", and "acetylene" are common names. The IUPAC equivalents are 2-methylpropane, propan-2-ol, and ethyne.

8. Confusing metamerism with position isomerism in ethers: Metamerism involves different alkyl groups on either side of a divalent atom (–O–, –S–), not just different positions of the same groups.

Exam Checklist

[ ] I can identify the longest carbon chain in a branched compound
[ ] I can assign correct locants by numbering from the appropriate end
[ ] I know the priority order of functional groups for IUPAC naming
[ ] I can convert between common names and IUPAC names
[ ] I can distinguish between chain, position, functional, and metameric isomerism
[ ] I can identify geometric (cis-trans) isomers and explain why they arise
[ ] I can explain optical isomerism and identify a chiral centre
[ ] I can list the characteristics of a homologous series with examples
[ ] I can write the general formulas for alkanes, alkenes, alkynes, alcohols, carboxylic acids
[ ] I can name compounds with multiple substituents and multiple functional groups correctly

Frequently Asked Questions

A functional group is an atom or group of atoms that determines the characteristic chemical properties of an organic compound (e.g., –OH in alcohols, –COOH in carboxylic acids).