JEE Mains Chemistry · Organic Chemistry - Some Basic Principles and Techniques

Structure, Classification and IUPAC Nomenclature

A carbon's hybridisation follows from its bonds, a molecule's σ and π bonds can be counted from its formula, and an IUPAC name is built by picking the senior functional group, choosing and numbering the parent chain, and listing the other groups as prefixes in alphabetical order.

All Organic Chemistry - Some Basic Principles and Techniques notes JEE Mains Chemistry strategy

Why this matters

Thirty-four PYQs, twenty-nine of them multiple choice, and two from 2026. Thirteen rank functional groups by seniority or name a compound that carries two or more of them. Eleven turn on choosing and numbering the parent chain, or on drawing a structure from its name. Ten count sp³, sp² and sp carbons, σ and π bonds or π electrons, or place a compound in the acyclic, alicyclic, aromatic or heterocyclic class.

Concept 1 of 3: Hybridisation and counting sigma and pi bonds

Look at what each carbon is bonded to. Four single bonds make it sp³; one double bond makes it sp²; a triple bond, or two double bonds, makes it sp. Every bond between two atoms contains one σ bond. A double bond adds one π bond and a triple bond adds two. So once the structure is written out, every count in this block is simple bookkeeping.

Definition

Four single bonds: sp³, tetrahedral, 109.5°. One double bond (C=C or C=O): sp², trigonal planar, 120°. A triple bond (C≡C or C≡N), or two double bonds on one carbon: sp, linear, 180°.
Count every carbon, including each $\mathrm{CH_3}$ branch: a branch carbon has only single bonds, so it is sp³.
σ bonds in an open-chain molecule = number of atoms − 1. Each ring adds one more σ bond.
π bonds = number of double bonds + 2 × number of triple bonds. Each π bond holds 2 π electrons; a benzene ring holds 6.
Classification: acyclic (open chain); alicyclic (a ring that is not aromatic, such as cyclohexane or cyclohexene); aromatic benzenoid (a benzene ring); aromatic non-benzenoid (tropolone); heterocyclic (a ring containing N, O or S: furan, thiophene, pyridine).
Bond-line formula: every corner and every line end is a carbon, and the hydrogens on carbon are not drawn. Heteroatoms and the H on them are written: OH, $\mathrm{NH_2}$ , CN.
Homologous series: successive members differ by $\mathrm{CH_2}$ and share one general formula. Methanoic acid, HCOOH, is the first monocarboxylic acid and ethanoic acid, $\mathrm{CH_3COOH}$ , the second.

Sigma and pi bond count

n_\sigma = (\text{number of atoms}) - 1 + (\text{number of rings}) \qquad n_\pi = n_{\text{double}} + 2\,n_{\text{triple}}

Worked example

For 2-methylhex-1-en-3-yne,

\mathrm{CH_2{=}C(CH_3){-}C{\equiv}C{-}CH_2CH_3}

, find how many carbons are sp³, sp² and sp, and how many σ and π bonds the molecule has.

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The same idea in a real exam question:

JEE Mains · 2025 · 2 Apr 2025 · Q106Moderate

Example 1 · Organic Chemistry - Some Basic Principles and Techniques · Structure, Classification and IUPAC Nomenclature

In 3,3-dimethylhex-1-en-4-yne, there are ___

sp^{3}

, ___

sp^{2}

and ___

sp

hybridised carbon atoms respectively:

Branch carbons count too

A name such as 2,2-dimethylbutane carries two methyl carbons that the parent name hides. Each is a separate sp³ carbon. Write the full structure before you count.

A carbonyl or nitrile carbon is not sp³

The carbon of C=O, CHO and COOH is sp², and the carbon of C≡N is sp. Only a carbon with four single bonds is sp³.

One ring double bond does not make a ring aromatic

Cyclohexene has one C=C in a six-membered ring. It is alicyclic, not aromatic and not benzenoid.

Drill 9 more on hybridisation and counting sigma and pi bonds

Concept 2 of 3: Seniority of functional groups in IUPAC names

When a compound carries two or more functional groups, only one of them can give the suffix. It is the one highest in the seniority list; every other group becomes a prefix. Learn the list once and most naming questions reduce to reading it.

Definition

The senior group is the principal group: it gives the suffix and takes the lowest possible locant.
NCERT order: $\mathrm{-COOH > -SO_3H > -COOR > -COCl > -CONH_2 > -CN > -CHO > {>}C{=}O > -OH > -NH_2 > C{=}C,\ C{\equiv}C}$ .
Halo (–X), nitro ( $\mathrm{-NO_2}$ ) and alkoxy (–OR) groups are always prefixes.
The carbon of –COOH, –COOR, –COCl, $\mathrm{-CONH_2}$ , –CN and a chain-end –CHO is counted in the parent chain and numbered C-1.
Esters are named alkyl alkanoate, the alkyl on oxygen first as a separate word: $\mathrm{CH_3COOC_2H_5}$ is ethyl ethanoate.
Common names still appear: aniline and aminobenzene (benzenamine) are one compound.

Class	Group	Suffix when senior	Prefix when not senior
Carboxylic acid	$\mathrm{-COOH}$	-oic acid	carboxy
Sulphonic acid	$\mathrm{-SO_3H}$	-sulphonic acid	sulpho
Ester	$\mathrm{-COOR}$	alkyl …-oate	alkoxycarbonyl
Acid chloride	$\mathrm{-COCl}$	-oyl chloride	chlorocarbonyl
Amide	$\mathrm{-CONH_2}$	-amide	carbamoyl
Nitrile	$\mathrm{-C{\equiv}N}$	-nitrile	cyano
Aldehyde	$\mathrm{-CHO}$	-al	formyl, or oxo when its carbon is in the chain The aldehyde outranks the ketone: a compound with both is named as an -al with an oxo prefix.
Ketone	$\mathrm{{>}C{=}O}$	-one	oxo
Alcohol	$\mathrm{-OH}$	-ol	hydroxy
Amine	$\mathrm{-NH_2}$	-amine	amino
Alkene, alkyne	C=C, C≡C	-ene, -yne	Never a prefix; always part of the parent name
Halide, nitro, ether	–X, $\mathrm{-NO_2}$ , –OR	Never a suffix	halo, nitro, alkoxy

Seniority falls from the top row down. The last row never takes the suffix.

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The same idea in a real exam question:

JEE Mains · 2026 · 2 Apr 2026 Shift 1 · Q40Moderate

Example 2 · Organic Chemistry - Some Basic Principles and Techniques · Structure, Classification and IUPAC Nomenclature

In IUPAC nomenclature, the correct order of decreasing priority of functional group is :

The ketone does not outrank the aldehyde

–CHO comes before >C=O in the seniority list. An option that places the ketone first is wrong.

The nitrile sits between the amide and the aldehyde

The order runs

\mathrm{-CONH_2 > -CN > -CHO}

. An order that puts –CHO above –CN, or –CN above

\mathrm{-CONH_2}

, is wrong.

The junior group never takes the suffix

\mathrm{HOCH_2CH_2COCH_3}

is 4-hydroxybutan-2-one. A name such as 3-oxobutan-1-ol gives the suffix to the alcohol, which ranks below the ketone.

Drill 12 more on seniority of functional groups in iupac names

Concept 3 of 3: Choosing and numbering the parent chain

Once the principal group is fixed, naming is a short series of tie-breaks: choose the chain, number it, then list the prefixes. Each rule matters only when the one before it gives a tie. Most wrong options in these questions break one rule while obeying the rest.

Definition

Parent chain: the longest chain that contains the principal group and as many multiple bonds as possible. In a ring compound with a small side chain, the ring is the parent.
Numbering: the principal group (the suffix) gets the lowest locant first; then the multiple bonds, taken together; then all the prefixes as one set.
Lowest locant set: compare the sets term by term; the first point of difference decides. 2,4,4 is lower than 3,3,5 because 2 < 3.
ene and yne tie: when the double and triple bonds get the same locant set from either end, the double bond takes the lower number.
Alphabetical order of prefixes: ethyl before methyl. Multiplying prefixes (di-, tri-, tetra-) are ignored when alphabetising, as in 3-ethyl-2,2-dimethylpentane.
'ene' drops its final e before a vowel or 'y': hex-1-en-4-yne, pent-4-en-2-ol.
In a ring, the carbon carrying the principal group is C-1: cyclopent-2-en-1-ol.

Order of numbering decisions

\text{principal group} \;\to\; \text{multiple bonds} \;\to\; \text{all prefixes (lowest set)} \;\to\; \text{alphabetical order}

Worked example

Name

\mathrm{HC{\equiv}C{-}CH(OH){-}CH_2{-}CH{=}CH_2}

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The same idea in a real exam question:

JEE Mains · 2025 · 4 April 2025 · Q108Moderate

Example 3 · Organic Chemistry - Some Basic Principles and Techniques · Structure, Classification and IUPAC Nomenclature

The IUPAC name of the following compound is --

Prefixes go in alphabetical order, not locant order

1,1-Dimethyl-3-ethylcyclohexane lists the prefixes by number. The correct name is 3-ethyl-1,1-dimethylcyclohexane: e comes before m, and di is ignored.

Compare the whole locant set

2,3,6 beats 2,4,5 because the sets first differ at the second term, and 3 < 4. Adding up the locants, or looking only at the first one, gives the wrong numbering.

The OH carbon is C-1 in a ring

A cyclic alcohol with a ring double bond is named cyclohex-2-en-1-ol, never cyclohex-1-en-3-ol. The principal group takes C-1 before the double bond is numbered.

Drill 10 more on choosing and numbering the parent chain

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)

Hybridisation and counting sigma and pi bonds
Sigma and pi bond count
$n_\sigma = (\text{number of atoms}) - 1 + (\text{number of rings}) \qquad n_\pi = n_{\text{double}} + 2\,n_{\text{triple}}$
Choosing and numbering the parent chain
Order of numbering decisions
$\text{principal group} \;\to\; \text{multiple bonds} \;\to\; \text{all prefixes (lowest set)} \;\to\; \text{alphabetical order}$

Reference tables (1)

Seniority of functional groups in IUPAC names12 rows

Class	Group	Suffix when senior	Prefix when not senior
Carboxylic acid	$\mathrm{-COOH}$	-oic acid	carboxy
Sulphonic acid	$\mathrm{-SO_3H}$	-sulphonic acid	sulpho
Ester	$\mathrm{-COOR}$	alkyl …-oate	alkoxycarbonyl
Acid chloride	$\mathrm{-COCl}$	-oyl chloride	chlorocarbonyl
Amide	$\mathrm{-CONH_2}$	-amide	carbamoyl
Nitrile	$\mathrm{-C{\equiv}N}$	-nitrile	cyano
Aldehyde	$\mathrm{-CHO}$	-al	formyl, or oxo when its carbon is in the chain The aldehyde outranks the ketone: a compound with both is named as an -al with an oxo prefix.
Ketone	$\mathrm{{>}C{=}O}$	-one	oxo
Alcohol	$\mathrm{-OH}$	-ol	hydroxy
Amine	$\mathrm{-NH_2}$	-amine	amino
Alkene, alkyne	C=C, C≡C	-ene, -yne	Never a prefix; always part of the parent name
Halide, nitro, ether	–X, $\mathrm{-NO_2}$ , –OR	Never a suffix	halo, nitro, alkoxy

Seniority falls from the top row down. The last row never takes the suffix.

Watch out for (9)

Branch carbons count too
→ Hybridisation and counting sigma and pi bonds
A carbonyl or nitrile carbon is not sp³
→ Hybridisation and counting sigma and pi bonds
One ring double bond does not make a ring aromatic
→ Hybridisation and counting sigma and pi bonds
The ketone does not outrank the aldehyde
→ Seniority of functional groups in IUPAC names
The nitrile sits between the amide and the aldehyde
→ Seniority of functional groups in IUPAC names
The junior group never takes the suffix
→ Seniority of functional groups in IUPAC names
Prefixes go in alphabetical order, not locant order
→ Choosing and numbering the parent chain
Compare the whole locant set
→ Choosing and numbering the parent chain
The OH carbon is C-1 in a ring
→ Choosing and numbering the parent chain

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