MHT-CET Chemistry · Basic Principles of Organic Chemistry

IUPAC Nomenclature, Functional Groups and Homologous Series

An IUPAC name is built from the longest chain containing the principal functional group, numbered to give that group the lowest locant; which group is principal follows a fixed priority order, and a homologous series steps by one CH₂ (14 g mol⁻¹) at a time.

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Why this matters

18 PYQs, 2 HARD — the biggest of the three pages. Five ask the priority order of functional groups (which is principal, which is lowest), five ask which heterocycle carries N, O or S, two ask the 14 g mol⁻¹ step of a homologous series, and four are a name or a bond-line formula from a drawn structure. Two tables and one naming routine.

Concept 1 of 4

Homologous Series: One CH₂ at a Time

Intuition

Members of a homologous series share a functional group and a general formula and differ by a CH₂ unit — 12 + 2 = 14 g mol⁻¹ between neighbours. Alkanes are ( ext{C}_n ext{H}_{2n+2}): undecane is ( ext{C}_{11} ext{H}_{24}), dodecane ( ext{C}_{12} ext{H}_{26}). The chapter also names a few real compounds by their functional groups; aspirin is an ester of salicylic acid, not an amide.

Definition

Homologues: same functional group, same general formula, successive members differ by $-\text{CH}_2-$ = 14 g mol⁻¹; physical properties grade smoothly, chemical properties are alike.
Alkanes $\text{C}_n\text{H}_{2n+2}$ , alkenes $\text{C}_n\text{H}_{2n}$ , alkynes $\text{C}_n\text{H}_{2n-2}$ , alcohols $\text{C}_n\text{H}_{2n+1}\text{OH}$ .
Aspirin (acetylsalicylic acid): made from salicylic acid, carries an ester and a carboxylic acid — NO amide linkage; fewer side effects than salicylic acid but still irritates the stomach.
Counting atoms from a formula: $\text{HO(CH}_2)_3\text{CH(CH}_3)\text{CH(CH}_3)_2$ has 8 carbons; but-2-ene $\text{C}_4\text{H}_8$ has 8 H per molecule, so n mol carries 8n mol of H.

Homologous step

M(\text{C}_{n+1}\text{H}_{2n+4}) - M(\text{C}_n\text{H}_{2n+2}) = 14\ \text{g mol}^{-1}

Worked example

Give the molecular formulas of the third and fourth alkanes and the difference between their molar masses.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Basic Principles of Organic ChemistryEASY

What is the numerical difference in molar masses of second and third member of a homologous series?

[Q65 · 15th May Shift 1 · 2023]

Taking the difference as 12 or 2

A CH₂ unit is one carbon AND two hydrogens: 12 + 2 = 14. Both 12 and 2 are offered.

Drill 2 more on homologous series: one ch₂ at a time

Concept 2 of 4

Heterocyclic Compounds by Heteroatom

Intuition

A heterocycle is a ring with an atom other than carbon in it. The exam's set is small: three five-membered aromatics — pyrrole (N), furan (O), thiophene (S) — and three six-membered rings — pyridine (N, aromatic), piperidine (N, saturated), pyran (O). Every question is 'which one has N / O / S' or 'which one has no N'.

Definition

Five-membered aromatic: pyrrole (N), furan (O), thiophene (S).
Six-membered: pyridine (N, aromatic), piperidine (N, fully saturated), pyran (O).
Nitrogen-free among the exam's list: furan, thiophene, pyran. Nitromethane is not a ring at all.

Compound	Ring size	Heteroatom	Aromatic?
Pyrrole	5	N	Yes
Furan	5	O	Yes
Thiophene	5	S	Yes The only sulphur ring in the list.
Pyridine	6	N	Yes
Piperidine	6	N	No — saturated Piperidine has N, not S; it is the reduced form of pyridine.
Pyran	6	O	No Pyran contains NO nitrogen.

Pyr- names carry nitrogen except pyran; fur- is oxygen; thio- is sulphur.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Basic Principles of Organic ChemistryEASY

Identify the correct pair of molecule and the heteroatom present in it respectively from following:

[Q70 · 11th May Shift 2 · 2023]

Reading 'pyran' as a nitrogen ring

Pyrrole, pyridine and piperidine all carry N, so the prefix feels like nitrogen — but pyran is the OXYGEN six-ring. It is the planted answer in every 'no nitrogen' question.

Drill 4 more on heterocyclic compounds by heteroatom

Concept 3 of 4

Priority Order of Functional Groups

Intuition

When a molecule has several functional groups, one becomes the suffix (principal group) and the rest become prefixes. The order runs from carboxylic acid down through its derivatives to nitrile, aldehyde, ketone, alcohol, amine — and the double and triple bonds are never principal over any of these.

Definition

Decreasing priority: $-\text{COOH} > -\text{SO}_3\text{H} > -\text{COOR} > -\text{COCl} > -\text{CONH}_2 > -\text{CN} > -\text{CHO} > \text{>C=O} > -\text{OH} > -\text{NH}_2 > \text{C=C} > \text{C≡C}$ .
Among $-\text{CONH}_2$ , $-\text{CN}$ , $-\text{OH}$ , $-\text{C≡C}-$ : the amide is principal. Among $-\text{COCl}$ , $-\text{SO}_3\text{H}$ , $-\text{CHO}$ , $-\text{OH}$ : the sulphonic acid.
Among $-\text{COOR}$ , $-\text{COCl}$ , $\text{>C=O}$ , $-\text{CONH}_2$ : the ketone is LOWEST.
Groups that are only ever prefixes: halo, nitro, alkoxy, alkyl.

Rank	Group	Suffix
1	−COOH	-oic acid
2	−SO₃H	-sulphonic acid Above esters and acid chlorides — the 2025 paper keys it over −COCl.
3	−COOR	-oate
4	−COCl	-oyl chloride
5	−CONH₂	-amide
6	−CN	-nitrile
7	−CHO	-al
8	>C=O	-one Lowest of the carbonyl family.
9	−OH	-ol
10	−NH₂	-amine
11	C=C, C≡C	-ene, -yne Never principal over a heteroatom group.

Acid first, then its derivatives in the order ester, chloride, amide, then nitrile, aldehyde, ketone, alcohol, amine.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Basic Principles of Organic ChemistryMODERATE

Identify the correct order of priority of functional groups in IUPAC nomenclature.

[Q96 · 9th May Shift 2 · 2023]

Putting the acid chloride above the ester

The derivative order is ester, THEN acid chloride, then amide: −COOR > −COCl > −CONH₂. The reversed pair is offered as an option.

Drill 4 more on priority order of functional groups

Concept 4 of 4

Naming From a Structure, and Reading Bond-Line Formulas

Intuition

Find the longest chain that contains the principal group or the double bond; number from the end that gives the principal group, then the multiple bond, then the substituents the lowest locants; list substituents alphabetically. In a bond-line drawing every vertex and line-end is a carbon with enough hydrogens to make four bonds.

Definition

Routine: longest chain containing the principal group → number for lowest locants (principal group first, then C=C/C≡C, then the substituent set) → prefixes alphabetically (iodo before methyl).
$\text{CH}_3\text{CH}_2\text{-C(CH}_3)\text{=C(Br)-CH}_2\text{CH}_3$ : hex-3-ene with Br on C3 and methyl on C4 → 3-bromo-4-methylhex-3-ene.
$\text{CH}_3\text{CH}_2\text{-CH(CH}_3)\text{-C(CH}_3)\text{=CH-CH(I)-CH}_3$ : seven-carbon chain through the double bond, numbered from the iodine end → 2-iodo-4,5-dimethylhept-3-ene.
Bond-line: neopentane $\text{C(CH}_3)_4$ is four lines from one point; 2-methylbutane is a zigzag with one branch; pentane a plain zigzag.
$\text{HO(CH}_2)_3\text{CH(CH}_3)\text{CH(CH}_3)_2$ = 4,5-dimethylhexan-1-ol: OH at a line-end, then three vertices, then two branched vertices.

Naming order

\text{locants: principal group} \to \text{C=C / C≡C} \to \text{substituents (lowest set)};\quad \text{prefixes alphabetical}

Worked example

Name

\text{CH}_3\text{-CH(Cl)-CH=CH-CH(CH}_3)\text{-CH}_3

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 4Basic Principles of Organic ChemistryMODERATE

What is IUPAC name of following compound?

[Q85 · 21 April Shift II · 2025]

Numbering from the substituent-rich end

The double bond outranks bromo and methyl for the lowest locant; only when the C=C locant ties do the substituents decide. 4-bromo-3-methylhex-3-ene is the planted misnumbering.

Drill 4 more on naming from a structure, and reading bond-line formulas

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Homologous Series: One CH₂ at a Time
Homologous step
$M(\text{C}_{n+1}\text{H}_{2n+4}) - M(\text{C}_n\text{H}_{2n+2}) = 14\ \text{g mol}^{-1}$
Naming From a Structure, and Reading Bond-Line Formulas
Naming order
$\text{locants: principal group} \to \text{C=C / C≡C} \to \text{substituents (lowest set)};\quad \text{prefixes alphabetical}$

Reference tables (2)

Heterocyclic Compounds by Heteroatom6 rows

Compound	Ring size	Heteroatom	Aromatic?
Pyrrole	5	N	Yes
Furan	5	O	Yes
Thiophene	5	S	Yes The only sulphur ring in the list.
Pyridine	6	N	Yes
Piperidine	6	N	No — saturated Piperidine has N, not S; it is the reduced form of pyridine.
Pyran	6	O	No Pyran contains NO nitrogen.

Pyr- names carry nitrogen except pyran; fur- is oxygen; thio- is sulphur.

Priority Order of Functional Groups11 rows

Rank	Group	Suffix
1	−COOH	-oic acid
2	−SO₃H	-sulphonic acid Above esters and acid chlorides — the 2025 paper keys it over −COCl.
3	−COOR	-oate
4	−COCl	-oyl chloride
5	−CONH₂	-amide
6	−CN	-nitrile
7	−CHO	-al
8	>C=O	-one Lowest of the carbonyl family.
9	−OH	-ol
10	−NH₂	-amine
11	C=C, C≡C	-ene, -yne Never principal over a heteroatom group.

Acid first, then its derivatives in the order ester, chloride, amide, then nitrile, aldehyde, ketone, alcohol, amine.

Watch out for (4)

Taking the difference as 12 or 2
→ Homologous Series: One CH₂ at a Time
Reading 'pyran' as a nitrogen ring
→ Heterocyclic Compounds by Heteroatom
Putting the acid chloride above the ester
→ Priority Order of Functional Groups
Numbering from the substituent-rich end
→ Naming From a Structure, and Reading Bond-Line Formulas

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