Structural Isomerism in Alkanes – Alkanes (Senior 3 Chemistry) | Ganzaa

Alkanes

Lessons

Senior 3 · Chemistry

Alkanes

4 lessons

Lesson 2 of 4

Structural Isomerism in Alkanes

25 min

Same formula, different gas

Two gases are sold under the formula $\mathrm{C_4H_{10}}$ . One boils at −1 °C and the other at −12 °C, and the second is the refrigerant gas in many modern fridges, often printed on the label as "R600a". If the formula is the same, why are they different substances? Make models of $\mathrm{C_4H_{10}}$ with a molecular model kit, or with balls of clay and sticks, and you find there are exactly two ways to join four carbon atoms and ten hydrogen atoms, keeping four bonds on each carbon and one on each hydrogen.

Definition

Structural isomers: compounds with the same molecular formula but different structural formulas, because their atoms are joined in a different order. Example: butane and 2-methylpropane, both $\mathrm{C_4H_{10}}$ .

Structural isomerism: the existence of structural isomers. In alkanes it comes from branching of the carbon chain, so it is also called chain isomerism.

Definition

Alkyl group: an alkane with one hydrogen atom removed, so it can join to a chain as a branch. General formula $\mathrm{C_nH_{2n+1}-}$ ; the name ends in -yl. Examples: methyl, $\mathrm{CH_3-}$ ; ethyl, $\mathrm{C_2H_5-}$ ; propyl, $\mathrm{C_3H_7-}$ .

Branched alkane: an alkane in which at least one carbon atom is bonded to three or four other carbon atoms. Example: 2-methylpropane.

Naming a branched alkane

Find the longest continuous chain of carbon atoms, even if it bends. Its length gives the parent name: four carbons, butane.
Number the chain from the end nearer the branch, so the branch gets the lowest possible number.
Name each branch as an alkyl group with its position number in front, joined by a hyphen: 2-methyl. Write the branch and parent name as one word: 2-methylbutane.
Several identical branches: use di-, tri- or tetra-, and give every branch its own number, separated by commas: 2,2-dimethylpropane, 2,3-dimethylbutane.
Different branches are listed in alphabetical order: ethyl before methyl.

A bend is not a branch

Because the carbon chain is really a zig-zag and can twist freely about its single bonds, the same molecule can be drawn in many ways. To decide whether two drawings are isomers, trace the longest continuous chain in each and name both. Same name, same compound.

Worked example

Problem. Name: (a) $\mathrm{CH_3CH(CH_3)CH_2CH_3}$ ; (b) $\mathrm{CH_3C(CH_3)_2CH_3}$ ; (c) $\mathrm{CH_3CH(CH_3)CH(CH_3)CH_3}$ ; (d) $\mathrm{CH_3CH_2CH_2CH(CH_3)CH_3}$ .

Solution. A group in brackets is a branch on the carbon written just before it.

(a) Longest chain: 4 carbons. The $\mathrm{CH_3}$ branch is on carbon 2: 2-methylbutane.

(b) Longest chain: 3 carbons, with two $\mathrm{CH_3}$ branches on the middle carbon: 2,2-dimethylpropane.

(d) Longest chain: 5 carbons. Numbered from the left, the branch would be on carbon 4; from the right it is on carbon 2. Use the lower number: 2-methylpentane.

Worked example

Problem. Draw and name all the structural isomers of $\mathrm{C_5H_{12}}$ .

Solution. Work from the longest chain down, so that no isomer is missed or counted twice.

All 5 carbons in one chain: pentane.
A chain of 4 with one $\mathrm{CH_3}$ branch. On carbon 1 the branch would just extend the chain back to pentane. On carbon 2 it gives 2-methylbutane. On carbon 3 it is the same molecule numbered from the other end, so it is not new.
A chain of 3 with two $\mathrm{CH_3}$ branches. Both must go on carbon 2: 2,2-dimethylpropane.

A chain of 2 cannot hold three branches without making a longer chain, so $\mathrm{C_5H_{12}}$ has exactly three isomers (Figure 2.2). Each has 5 carbons and 12 hydrogens: check by counting.

Isomers have different properties

Isomers have the same $M_{r}$ but they are different compounds with different physical properties. Branching makes a molecule more compact, closer to a ball, so neighbouring molecules touch over a smaller area and attract each other less. Less energy is needed to separate them, so the boiling point is lower.

Table 2.1: Boiling points of the isomers of C₄H₁₀ and C₅H₁₂ (standard reference values, rounded)

Formula	Isomer	Shape of molecule	Boiling point / °C
C₄H₁₀	butane	unbranched	−1
C₄H₁₀	2-methylpropane	one branch	−12
C₅H₁₂	pentane	unbranched	36
	2-methylbutane	one branch	28
	2,2-dimethylpropane	two branches, nearly a ball	10

The number of possible isomers grows very quickly with the number of carbon atoms:

Table 2.2: How many structural isomers?

Alkane	C₁ to C₃	C₄H₁₀	C₅H₁₂	C₆H₁₄	C₇H₁₆	C₈H₁₈	C₁₀H₂₂
isomers	1 each	2	3	5	9	18	75

Warning

Common mistakes. (1) A name such as "1-methylbutane" or "2-ethylpropane" shows that the longest chain was missed: both molecules are really a longer chain. (2) Number from the end nearer the branch: $\mathrm{CH_3CH_2CH(CH_3)CH_3}$ is 2-methylbutane, not 3-methylbutane. (3) Write 2,2-dimethylpropane, not 2-dimethylpropane: every branch needs its own number. (4) Turning, bending or flipping a drawing does not make a new isomer.

Tip

Name both drawings. The quickest test of whether two structures are isomers or the same compound is to give each its IUPAC name. Different names with the same molecular formula mean isomers.

FuseSchool: What Are Structural Isomers. Watch how one molecular formula can be built into differently shaped molecules, and how to tell a real isomer from the same molecule drawn another way.

✦

Practice

Define structural isomers. Explain why propane, $\mathrm{C_3H_8}$ , has no isomers.
Draw the displayed formulas of the two isomers of $\mathrm{C_4H_{10}}$ and name them.
Name: (a) $\mathrm{CH_3CH_2CH(CH_3)CH_2CH_3}$ ; (b) $\mathrm{(CH_3)_3CCH_2CH_3}$ ; (c) $\mathrm{CH_3CH(CH_3)CH_2CH(CH_3)CH_3}$ .
Draw the displayed formula of 2,2-dimethylpropane and give its molecular formula.
Correct each name and explain the error: (a) 1-methylbutane; (b) 3-methylbutane; (c) 2-ethylpropane.
Hexane, $\mathrm{C_6H_{14}}$ , has five structural isomers. Draw a condensed formula for each and name it.
Use Table 2.1 to explain why, on a day when the temperature is 20 °C, pentane is a liquid but 2,2-dimethylpropane is a gas, although both are $\mathrm{C_5H_{12}}$ .
A student draws $\mathrm{C_4H_{10}}$ twice: once as a row of four carbons, once with the fourth carbon drawn below the third. She says these are two isomers. Is she right? Explain.

Show answersHide answers

Same molecular formula, different structural formula. Three carbons can only be joined in one way: a chain of three. Putting a carbon "below" the middle one is the same chain bent.
Butane, $\mathrm{CH_3CH_2CH_2CH_3}$ , and 2-methylpropane, $\mathrm{CH_3CH(CH_3)CH_3}$ (Figure 2.1).
(a) 3-methylpentane; (b) 2,2-dimethylbutane; (c) 2,4-dimethylpentane.
A central carbon bonded to four $\mathrm{CH_3}$ groups (Figure 2.2); $\mathrm{C_5H_{12}}$ .
(a) Pentane: the methyl on carbon 1 extends the chain to five carbons. (b) 2-methylbutane: number from the end nearer the branch. (c) 2-methylbutane: the ethyl branch is part of a four-carbon chain.
$\mathrm{CH_3(CH_2)_4CH_3}$ hexane; $\mathrm{CH_3CH(CH_3)CH_2CH_2CH_3}$ 2-methylpentane; $\mathrm{CH_3CH_2CH(CH_3)CH_2CH_3}$ 3-methylpentane; $\mathrm{(CH_3)_3CCH_2CH_3}$ 2,2-dimethylbutane; $\mathrm{CH_3CH(CH_3)CH(CH_3)CH_3}$ 2,3-dimethylbutane.
Pentane boils at 36 °C, above 20 °C, so it is liquid. 2,2-dimethylpropane boils at 10 °C, below 20 °C, so it is a gas. Its compact, branched molecules attract each other less.
No. Both drawings have one continuous chain of four carbons: both are butane. A bend in a drawing is not a branch.

Sources and credits

Video: What Are Structural Isomers, FuseSchool, embedded from YouTube.
Written by Ganzaa. The diagrams, tables, worked examples and Practice items were drawn and written by Ganzaa; the boiling points are standard reference values and the isomer counts are the standard counts of alkane structural isomers.

1Same formula, different gas

2Naming a branched alkane

3A bend is not a branch

4Isomers have different properties

Same formula, different gas

Naming a branched alkane

A bend is not a branch

Isomers have different properties