A LevelChemistryEdexcelRevision Notes3. Organic ChemistryIntroduction to Organic ChemistryStructural Isomerism

Structural Isomerism (Edexcel A Level Chemistry): Revision Note

Exam code: 9CHO

Stewart Hird

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on

Structural Isomerism

  • Structural isomers are compounds that have the same molecular formula but different structural formulae

    • E.g. propene and cyclopropane

Displayed formulae of propene and cyclopropane, both C3H6

Both propene and cyclopropane are made up of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

  • There are three types of structural isomerism: chain, positional and functional group isomerism. Functional group isomers have the same molecular formula but different functional groups, e.g. propanal and propanone (both C3H6O). The two types described in detail below are:

    • Chain isomerism

    • Positional isomerism

Chain isomerism

  • Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same

  • This is caused by branching

    • E.g. pentane and 2,2-dimethylpropane 

Displayed formulae of pentane and 2,2-dimethylpropane, both C5H12

Both compounds are made up of the same atoms, however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Positional isomerism

  • Positional isomers arise from differences in the position of a functional group in each isomer

    • The functional group can be located on different carbons

    • For example, butan-1-ol and butan-2-ol 

Displayed formulae of butan-1-ol and butan-2-ol, both C4H10O

Both compounds have an alcohol group and are made up of 4 carbons, 10 hydrogens and one oxygen, however in butan-1-ol the functional group is located on the first carbon and in butan-2-ol on the second carbon

Worked Example

Isomers of dibromopropane

How many isomers are there of dibromopropane, C3H6Br2?

Answer

Step 1: Draw the structural formula of the compound

Displayed formula of dibromopropane, C3H6Br2, as the starting structure

Step 2: Determine whether it is a stereo or structural isomer

There is no C=C bond, so there is no E/Z isomerism; this question counts structural isomers only (1,2-dibromopropane also exists as a pair of optical isomers)

Step 3: Determine whether it is a functional group, chain or positional isomerism

  • Functional group? No, as Br is the only functional group possible

    • Chain? No, as the longest chain can only be 3

    • Positional? Yes, as the two bromine atoms can be bonded to different carbon atoms

Carbon skeletons of three-carbon chains showing that the longest chain can only be three carbons
Displayed formulae of 1,1-, 2,2-, 1,2- and 1,3-dibromopropane, showing C3H6Br2 has 4 structural isomers

The four structural isomers of C3H6Br2 are 1,1-dibromopropane (CH3CH2CHBr2), 2,2-dibromopropane (CH3CBr2CH3), 1,2-dibromopropane (CH3CHBrCH2Br) and 1,3-dibromopropane (BrCH2CH2CH2Br), so C3H6Br2 has 4 structural isomers.

Worked Example

Deducing isomers of C4H10

How many isomers are there of the compound with molecular formula C4H10?

Answer

Step 1: Draw the structural formula of the compound

Displayed formula of butane, C4H10, as the starting structure

Step 2: Determine whether it is a stereo or structural isomer.

There is no restricted bond rotation around the C-C bond so it is structural isomerism

Step 3: Determine whether it is a functional group, chain or positional isomerism

  • Functional group? No, as there are no functional groups

    • Positional? No, as there are no functional groups which can be positioned on different carbon atoms

    • Chain? Yes!

Displayed formulae of butane and 2-methylpropane, showing C4H10 has 2 structural isomers

The two structural isomers of C4H10 are butane (CH3CH2CH2CH3) and 2-methylpropane ((CH3)2CHCH3), so C4H10 has 2 structural isomers.

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

I would just like to say a massive thank you for putting together such a brilliant, easy to use website.I really think using this site helped me secure my top gradesin science and maths. You really did save my exams! Thank you.

Beth
IGCSE Student

This website is soooo useful and I can’t ever thank you enough for organising questions by topic like this. Furthermore, the name of the website could not have been more appropriate as it literally did SAVE MY EXAMS!

Fathima
A Level Student

Incredible! SO worth my money, the revision notes have everything I need to know and are so easy to understand. I actually enjoy revising! It makes me feel a lot more confident for my GCSEs in a few months.

Kate
GCSE Student

Absolutely brilliant, both my girls used it for A levels and GCSE. It's saves on paper copies, also beneficial exam questions ranked from easy to hard. It's removed a lot of stress from the exams.

Sameera
Parent

Just to say that your resources are the best I have seen and I have been teaching chemistry at different levels for about 40 years

Mark
Chemistry Teacher

Excellent

Our rating represented in Trustpilot stars
Read more reviews

Build on this topic

Check your understandingTry an Exam Question on this topicTest your recallReinforce key facts and definitions with Flashcards
Previous:Nomenclature & ClassificationNext:Stereoisomerism
Author
Reviewer
Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.