Naming of Alkanes

Q: What is the general formula for naming alkanes?

Open-chain alkanes follow CnH2n+2 and always end in the suffix -ane. The root word comes from the number of carbons in the longest continuous chain: meth, eth, prop, but, pent, hex, hept, oct, non, dec for one to ten carbons.

Q: Why is 2,3-dimethylbutane sometimes mislabelled in textbooks?

Check the structure, not the caption. Two methyl groups on adjacent carbons of a four-carbon chain is 2,3-dimethylbutane. If both methyls sit on the same carbon it becomes 2,2-dimethylbutane, a different compound.

Q: What is the difference between a primary and a quaternary carbon?

A primary carbon is bonded to exactly one other carbon and carries three hydrogens in an alkane. A quaternary carbon is bonded to four other carbons and carries no hydrogen at all, so it cannot take part in free-radical substitution reactions.

Q: How many isomers does C5H12 have?

Three chain isomers: pentane, 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane). Alkanes show only chain isomerism, and the count for C4 to C9 runs 2, 3, 5, 9, 18, 35.

Q: How are cycloalkanes named differently from open-chain alkanes?

Add the prefix cyclo- before the root word and use the formula CnH2n. A single substituent on the ring needs no locant. With two or more substituents, begin numbering at a substituted carbon and travel round the ring in the direction giving the lowest locant set.

ChemistryHydrocarbonsFor JEE aspirants

The naming of alkanes follows IUPAC rules built on four parts: locants, substituent prefixes, a root word for the longest carbon chain, and the suffix -ane. Alkanes are saturated hydrocarbons with general formula $C_{n} H_{2 n + 2}$ , so every carbon is $s p^{3}$ hybridised and singly bonded. Correct naming of alkanes means finding the longest continuous chain first, numbering it so branches get the lowest locants, then listing substituents alphabetically. Master this and every later organic chapter becomes easier.

General formula

C_{n} H_{2 n + 2}

open chain, saturated

Every carbon

s p^{3}

109. 5^{\circ}

tetrahedral,

σ

bonds only

IUPAC suffix-aneroot word from chain length

IsomerismChain onlystarts at

C_{4}

: 2 butanes

Key Formulas - Quick Reference

Open-chain alkane: $C_{n} H_{2 n + 2}$ , degree of unsaturation $= 0$
Cycloalkane: $C_{n} H_{2 n}$ , degree of unsaturation $= 1$
Alkyl group: $C_{n} H_{2 n + 1}$ , written as $R -$
Degree of unsaturation $= \frac{2 n _{C} + 2 - n _{H}}{2}$
IUPAC name $=$ locants $+$ prefix $+$ root $+$ suffix
Number of $σ$ bonds in $C_{n} H_{2 n + 2} = 3 n + 1$

1Where Alkanes Sit Among Hydrocarbons

Hydrocarbons are compounds made of carbon and hydrogen only. They come mainly from petroleum, natural gas and coal. The family splits first into aliphatic (open chain and ring) and aromatic (benzene-based) compounds, and the aliphatic branch splits again by how much unsaturation it carries.

Fig 1Where alkanes sit in the hydrocarbon family. Alkanes are the saturated open-chain hydrocarbons, general formula

C_{n} H_{2 n + 2}

Alkanes (also called paraffins) are saturated hydrocarbons containing only carbon-carbon and carbon-hydrogen single bonds, with general molecular formula

C_{n} H_{2 n + 2}

. The name paraffin comes from Latin parum affinis, meaning little affinity, a nod to how unreactive they are.

Fig 2Every carbon in an alkane is

s p^{3}

hybridised, so the four bonds point at the corners of a tetrahedron with an

H - C - H

angle of

109. 5^{\circ}

. The cube picture on the right shows where that angle comes from: put the carbon at the cube centre and the four hydrogens on alternate corners.

Fig 3The same compound,

C H_{3} C H (C H_{3}) C H_{2} C H_{3}

, in the three notations you will meet. Bond-line drawings are the quickest to sketch under time pressure: every vertex and every free end is a carbon, and enough hydrogens are assumed to complete four bonds.

2The Homologous Series

A series of compounds in which each member differs from the next by a $C H_{2}$ group is called a homologous series, and the individual members are homologues. Alkanes are the textbook example.

Fig 4The alkane homologous series. Each step adds one

C H_{2}

unit, so every member fits

C_{n} H_{2 n + 2}

All members share one general formula, here $C_{n} H_{2 n + 2}$
Consecutive members differ by $C H_{2}$ , that is by $14$ u in molar mass
They can all be made by similar methods and show similar chemical behaviour
Physical properties change gradually down the series

Root words you must know cold

Carbons	Root	Alkane	Formula	Alkyl group
1	meth	Methane	$C H_{4}$	Methyl
2	eth	Ethane	$C_{2} H_{6}$	Ethyl
3	prop	Propane	$C_{3} H_{8}$	Propyl
4	but	Butane	$C_{4} H_{10}$	Butyl
5	pent	Pentane	$C_{5} H_{12}$	Pentyl
6	hex	Hexane	$C_{6} H_{14}$	Hexyl
7	hept	Heptane	$C_{7} H_{16}$	Heptyl
8	oct	Octane	$C_{8} H_{18}$	Octyl
9	non	Nonane	$C_{9} H_{20}$	Nonyl
10	dec	Decane	$C_{10} H_{22}$	Decyl

3Classifying Carbon and Hydrogen Atoms

Before naming, you need the language of carbon classes. A carbon is labelled by how many other carbon atoms it is bonded to. Hydrogens take the label of the carbon they sit on.

Fig 5Classifying carbons in

2, 2, 3

-trimethylpentane. A carbon is

1^{\circ}

2^{\circ}

3^{\circ}

4^{\circ}

according to how many other carbons it touches.

Class	Bonded to	Example carbon
Primary $(1^{\circ})$	1 carbon	$C H_{3}$ of propane
Secondary $(2^{\circ})$	2 carbons	middle $C H_{2}$ of propane
Tertiary $(3^{\circ})$	3 carbons	$C H$ of isobutane
Quaternary $(4^{\circ})$	4 carbons	central $C$ of neopentane

A quaternary carbon carries no hydrogen at all, so it can never be attacked in a free-radical substitution. That single fact explains a lot of the product ratios you will meet in the next concept.

4Alkyl Groups: The Substituent Vocabulary

Remove one hydrogen from an alkane and you get an alkyl group, formula $C_{n} H_{2 n + 1}$ , generally written $R -$ . Substituent names are simply the root word plus -yl.

Fig 6The first alkyl groups. Removing one H from

C_{n} H_{2 n + 2}

gives an alkyl group

C_{n} H_{2 n + 1}

From four carbons onwards the removal site matters, and $C_{4} H_{9}$ gives four different groups. These turn up constantly in JEE questions, so learn all four shapes.

Fig 7All four butyl groups share the formula

C_{4} H_{9}

. Only the attachment carbon differs, and that is enough to make them four different substituents.

Group	Structure	Attachment carbon	IUPAC (substitutive) name
n-Butyl	$C H_{3} C H_{2} C H_{2} C H_{2} -$	$1^{\circ}$	butyl
sec-Butyl	$C H_{3} C H_{2} C H (C H_{3}) -$	$2^{\circ}$	butan-2-yl
Isobutyl	$(C H_{3})_{2} C H C H_{2} -$	$1^{\circ}$	2-methylpropyl
tert-Butyl	$(C H_{3})_{3} C -$	$3^{\circ}$	2-methylpropan-2-yl

5The IUPAC Naming Rules, Step by Step

Find the longest continuous chain. This is the parent chain and it fixes the root word. The chain does not have to be drawn in a straight line.
If two chains tie in length, pick the one with more substituents. More branch points means a more informative name.
Number the chain from the end nearer the first branch point. This gives the lowest locant to the first substituent.
If both ends tie, use the lowest set of locants. Compare the two locant sets term by term and stop at the first point of difference.
If locants still tie, give the lowest number to the substituent that comes first alphabetically.
Name each substituent and place it with its locant. Repeat substituents use di, tri, tetra.
List substituents alphabetically, ignoring the multiplying prefixes. Ethyl comes before methyl; di and tri do not count for alphabetising, but iso, neo and cyclo do.
Assemble the name as one word, with commas between numbers and hyphens between numbers and letters.

Fig 8The naming algorithm in five steps, with each step applied to the same molecule. Work top to bottom and the answer falls out:

2, 3

-dimethylpentane. Steps 2 and 3 are where most marks are lost.

Rule 1 in practice: the longest chain can turn a corner

Fig 9Rule 1 in action. The parent chain is the longest continuous chain, not the one drawn straight across the page. Same molecule, two names, only

3

-ethylhexane is correct.

Rule 3 in practice: number towards the nearer branch

Fig 10Rule 3. Number from the end that gives the branch the lower locant, so this is

2

-methylpentane and never

4

-methylpentane.

Putting the whole name together

Fig 11Assembling a full name. Parent chain of 4 carbons gives the root but-, two methyl branches at C-2 and C-3 give

2, 3

-dimethylbutane.

Rule 4 in practice: the first point of difference

Fig 12When two directions tie at the first locant, compare term by term. Here

{2, 3, 5}

beats

{2, 4, 5}

at the first point of difference.

The lowest-set-of-locants rule is a term-by-term comparison, not a comparison of sums. Both

{2, 3, 5}

and

{2, 4, 5}

add to

10

and

11

respectively, but even when the sums are equal you still stop at the first position where the two sets differ.

Fig 13Alphabetising trips students up because only some prefixes count. Structural prefixes set in italics (

n -

sec -

t er t -

) and the multiplying prefixes (

d i -

t r i -

t e t r a -

) are skipped, while

i so -

n eo -

and

cy c l o -

are part of the substituent name and do count.

6Complex Substituents

When a branch is itself branched, it gets its own internal numbering. The carbon that attaches to the parent chain is always numbered $1$ within the substituent, and the complete substituent name goes inside brackets.

Fig 14Complex substituents get their own numbering, starting at the attachment carbon, and the whole name goes in brackets:

4

-(1-methylethyl)heptane.

Both styles are accepted in Indian exams: the older common style (isopropyl, tert-butyl) and the systematic style (1-methylethyl, 1,1-dimethylethyl). Recent NCERT editions use the systematic style in tables but keep the common names in worked examples, so recognise both.

7Naming Cycloalkanes

A ring of carbons is named with the prefix cyclo- in front of the root for the number of ring carbons. Rings hold two hydrogens fewer than the open-chain alkane, so the formula becomes $C_{n} H_{2 n}$ .

Fig 15Rings take the prefix cyclo-. A single substituent needs no number, two or more are numbered from a substituted carbon. Formula

C_{n} H_{2 n}

One substituent on the ring needs no locant, because all ring positions are equivalent
Two or more substituents: number from a substituted carbon and go round the ring in the direction giving the lowest locant set
If the ring has fewer carbons than the attached chain, the ring becomes the substituent instead, for example cyclopropylmethane is written as methylcyclopropane only when the ring is larger

8Common Names and Chain Isomerism

Before IUPAC, alkanes were named with the prefixes n-, iso- and neo-. These survive in exam papers and in industry, so you should be able to translate both ways.

Fig 16

C_{5} H_{12}

has exactly three chain isomers. The prefixes n-, iso- and neo- are trivial names, still common in exam questions.

Molecular formula	Number of chain isomers	Names
$C_{4} H_{10}$	2	butane, 2-methylpropane
$C_{5} H_{12}$	3	pentane, 2-methylbutane, 2,2-dimethylpropane
$C_{6} H_{14}$	5	hexane and four branched isomers
$C_{7} H_{16}$	9	heptane and eight branched isomers

Methane, ethane and propane have no chain isomers. The count

2, 3, 5, 9, 18, 35

for

C_{4}

through

C_{9}

is worth memorising, because JEE has asked for it directly.

Fig 17Chain isomer counts for

C_{4}

C_{10}

. The jump from

C_{8}

C_{10}

is the reason exam questions stop at hexane or heptane: beyond that, systematic counting is the only safe method.

9Solved Examples

Solved Example 1

Give the IUPAC name of

(C H_{3})_{3} C - C H_{2} - C (C H_{3})_{3}

Solution:

Write the skeleton out: two $t er t$ -butyl units joined by a $C H_{2}$ . The longest continuous chain runs from a methyl of the first $C (C H_{3})_{3}$ , through that carbon, through the $C H_{2}$ , through the second quaternary carbon, and out to one of its methyls. That is $5$ carbons, so the root is pent.

Numbering from either end is symmetric. The quaternary carbons are at positions $2$ and $4$ , each carrying two extra methyls.

Substituents: methyl at $2$ , methyl at $2$ , methyl at $4$ , methyl at $4$ .

Name: 2,2,4,4-tetramethylpentane. Molecular formula $C_{9} H_{20}$ .

Solved Example 2

Give the IUPAC name of

C (C H_{3})_{4}

Solution:

One central carbon bonded to four methyls. The longest continuous chain is $C H_{3} - C - C H_{3}$ , that is $3$ carbons, so the root is prop.

The central carbon is $C - 2$ and it carries the two remaining methyl groups.

Name: 2,2-dimethylpropane, common name neopentane. Molecular formula $C_{5} H_{12}$ .

Solved Example 3

Give the IUPAC name of

C H_{3} C (C l)_{2} C H (C H_{3})_{2}

Solution:

Expand: $C H_{3} - C C l_{2} - C H (C H_{3}) - C H_{3}$ . The longest chain is $4$ carbons, root but.

Number from the end nearer the first substituent. From the left, the two chlorines sit on $C - 2$ and the methyl on $C - 3$ , giving the locant set ${2, 2, 3}$ . From the right you would get ${2, 3, 3}$ . Term by term, ${2, 2, 3}$ wins at the second position.

Alphabetical order: chloro before methyl.

Name: 2,2-dichloro-3-methylbutane.

Solved Example 4

Draw and name the alkane with a

4^{\circ}

carbon, a

3^{\circ}

carbon and the molecular formula

C_{7} H_{16}

Solution:

A quaternary carbon needs four carbon neighbours and a tertiary carbon needs three. Put them adjacent: $(C H_{3})_{3} C - C H (C H_{3}) - C H_{3}$ .

Count the atoms: $3 + 1 + 1 + 1 + 1 = 7$ carbons, and the hydrogen count is $9 + 1 + 3 + 3 = 16$ . Formula checks out as $C_{7} H_{16}$ .

Longest chain is $4$ carbons (but), with three methyls: two on $C - 2$ and one on $C - 3$ .

Name: 2,2,3-trimethylbutane.

Solved Example 5

A hydrocarbon has molecular formula

C_{6} H_{14}

. How many

σ

bonds does it contain, and how many chain isomers exist?

Solution:

For an alkane $C_{n} H_{2 n + 2}$ , the number of $σ$ bonds is $(n - 1)$ C-C bonds plus $(2 n + 2)$ C-H bonds, which is $3 n + 1$ .

$Number of σ bonds = 3 (6) + 1 = 19$

The number of $σ$ bonds is the same for every isomer, because isomers differ only in connectivity, not in bond count.

Chain isomers of $C_{6} H_{14}$ : hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane, so five in all.

Common Mistakes to Avoid

Watch out

Reading the chain straight across the page. The longest chain often bends. Always trace every possible path before fixing the root word.
Alphabetising with the multiplying prefix. In 4-ethyl-2,3-dimethylhexane, you compare ethyl with methyl, not with dimethyl. But iso, neo and cyclo are part of the name and do count.
Adding locants instead of comparing them. The lowest-set rule is term by term at the first point of difference, never a comparison of sums.
Putting a locant on a lone ring substituent. Methylcyclohexane is correct; 1-methylcyclohexane is redundant.
Confusing isobutyl with sec-butyl. Isobutyl attaches through a $1^{\circ}$ carbon, sec-butyl through a $2^{\circ}$ carbon. Both are $C_{4} H_{9}$ .
Counting a quaternary carbon as having a hydrogen. It has none, which matters immediately in halogenation problems.
Forgetting that hyphens and commas are marked. Write $2, 3$ -dimethylbutane, not 2 3 dimethyl butane. Examiners do cut marks for this.

Frequently Asked Questions

What is the general formula for naming alkanes?

Open-chain alkanes follow $C_{n} H_{2 n + 2}$ and always end in the suffix -ane. The root word comes from the number of carbons in the longest continuous chain: meth, eth, prop, but, pent, hex, hept, oct, non, dec for one to ten carbons.

How do you decide which end of the chain to number from?

Number from the end that reaches the first branch point sooner, so the first substituent gets the lowest possible locant. If both ends tie, compare the full locant sets term by term and pick the one that is lower at the first point of difference. If they still tie, the substituent that is first alphabetically gets the lower number.

Why is 2,3-dimethylbutane not called 2,3-dimethylbutane in some books?

It is, but older sources sometimes print the structure $(C H_{3})_{2} C H C H (C H_{3})_{2}$ under the wrong label. Check the structure, not the caption: two methyls on adjacent carbons of a four-carbon chain is 2,3-dimethylbutane. If both methyls sit on the same carbon it becomes 2,2-dimethylbutane, a different compound.

Do di, tri and tetra count when alphabetising substituents?

No. Multiplying prefixes such as di, tri and tetra are ignored when you alphabetise. You alphabetise on the substituent name itself. However, iso, neo, cyclo and spiro are part of the substituent name and are counted, which is why isopropyl files under $i$ and not under $p$ .

What is the difference between a primary and a quaternary carbon?

A primary $(1^{\circ})$ carbon is bonded to exactly one other carbon and carries three hydrogens in an alkane. A quaternary $(4^{\circ})$ carbon is bonded to four other carbons and carries no hydrogen at all. Because it has no hydrogen, a quaternary carbon cannot take part in free-radical substitution reactions.

How many isomers does $C_{5} H_{12}$ have?

Three chain isomers: pentane (straight chain), 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane). Alkanes show only chain isomerism until a functional group or a ring is introduced, so the count for $C_{4}$ to $C_{9}$ runs $2, 3, 5, 9, 18, 35$ .

How are cycloalkanes named differently from open-chain alkanes?

Add the prefix cyclo- before the root word and use the formula $C_{n} H_{2 n}$ instead of $C_{n} H_{2 n + 2}$ . A single substituent on the ring needs no locant since all ring carbons are equivalent. With two or more substituents, begin numbering at a substituted carbon and travel round the ring in the direction that gives the lowest locant set.

Is the common name or the IUPAC name accepted in JEE and NEET?

Questions are set using both. IUPAC names are expected in answers, but the paper itself may present a structure as isobutane, neopentane or tert-butyl chloride. Learn to read n-, iso-, neo-, sec- and tert- fluently and convert them to systematic names on sight.

Previous year questions on Naming of Alkanes

5 questions from past papers, each with a step-by-step solution.

Ready to master Hydrocarbons?

Take a full mock test, practice concept-by-concept, and get an AI-powered rank prediction — all on Fundamenthol.

Practice questions →Take a mock test

1Where Alkanes Sit Among Hydrocarbons

2The Homologous Series

Root words you must know cold

3Classifying Carbon and Hydrogen Atoms

4Alkyl Groups: The Substituent Vocabulary

5The IUPAC Naming Rules, Step by Step

Rule 1 in practice: the longest chain can turn a corner

Rule 3 in practice: number towards the nearer branch

Putting the whole name together

Rule 4 in practice: the first point of difference

6Complex Substituents

7Naming Cycloalkanes

8Common Names and Chain Isomerism

9Solved Examples

Common Mistakes to Avoid

Frequently Asked Questions

What is the general formula for naming alkanes?

How do you decide which end of the chain to number from?

Why is 2,3-dimethylbutane not called 2,3-dimethylbutane in some books?

Do di, tri and tetra count when alphabetising substituents?

What is the difference between a primary and a quaternary carbon?

How many isomers does C5​H12​ have?

How are cycloalkanes named differently from open-chain alkanes?

Is the common name or the IUPAC name accepted in JEE and NEET?

Previous year questions on Naming of Alkanes

Ready to master Hydrocarbons?

How many isomers does $C_{5} H_{12}$ have?