4.1.1 (e) Structural Isomers

Syllabus

(e) explanation of structural isomers (compounds with the same molecular but different structural formulae) and determination of possible structural formulae of an organic molecule, given its molecular formula

What does this mean?

Alkanes

When molecules are longer than 4 or 5 Carbon atoms then it is generally possible to rearrange the atoms and bonds in a substance but keep the same number of atoms - these are structural isomers.

The basic rules for naming structural isomers of alkanes are explained in 4.1.1 (a) Organic nomenclature

Examiners are keen on asking students to draw all the structural isomers of a compound - generally this compound will contain 6 or fewer Carbon atoms or the number of isomers can get out of hand.

If you are happy with skeletal formulas then these are usually the easiest way to work out all the structural isomers.

Start with the straight chain.

Then remove one Carbon atom and then draw all the forms with 1 methyl group that are not the same.

Then remove another Carbon atom and draw all the di-methyl compounds.

If the number of Carbon atoms is over 7 or 8 there may be isomers with ethyl groups but be careful that you are not drawing a different isomer twice - (ethyl propane is really methyl butane).

Video

Alkenes

One question that comes up repeatedly is isomers of alkenes.

If we take Pentene (C₅H₁₀)as an example we can have structural isomers such as the various methyl butenes

We can have position isomers where we simply move the double bond - note we number of the lowest Carbon atom that the double bond touches.

But not all isomers of alkenes have to be alkenes - forming 5 Carbons into a ring also makes substances with the fromula C₅H₁₀.

So all these are isomers but not all are alkenes.

Exam-style Questions

1. (a) Define the term structural isomerism.

.............................................................................................................................................................................

.........................................................................................................................................................................(2)

(b) Draw graphical formulae for the structural isomers of C4H10.

Isomer 1 Isomer 2

(2)

(c) Give the name and draw the graphical formula of an alkene that is an isomer of but-1-ene and that has a different Carbon skeleton.

Name ....................................................................................................................................................................

Graphical formula

(2)

2. There are four structural isomers of molecular formula C4H9Br. The structural formulae of two of these isomers are given below.

CH3CH2CH2CH2Br

Isomer 1 Isomer 2

(i) Draw the structural formulae of the remaining two isomers.

Isomer 3 Isomer 4

(ii) Name isomer 1.

..............................................................................................................................................................................................................................(3)

Answers

1

(a) same molecular formula / same number of each type of atom (1)

different arrangements of atoms (in the molecule) (1)

(not just same structural formula)

(b) (i)

(1) (1)

c) Name methylpropene (1)

Diagram

2.

Isomer 3 (1) Isomer 4 (1)

(ii) 2-bromomethylpropane (1)

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