PART 14 ORGANIC ISOMERISM and Stereochemistry Doc Brown's Organic Chemistry Revision Notes

Part 14.1 Types of structural Isomerism based on constitutional isomers

What are constitutional isomers? What are the types of structural isomerism?

Index for pages on constitutional structural isomerism

14.1.1 Introduction to isomerism in general (this page)

14.1.2 Introduction to structural isomerism (this page)

Appendix The numbers of isomers from a molecular formula - just out of academic interest! (this page)

On separate pages with lots more detailed examples

14.1.2(1a) Examples of chain isomerism

14.1.2(1b) Examples of positional isomerism

14.1.2(1c) Examples of functional group isomerism

14.1.2(1d) Examples of tautomerism

Part 14.1.1 ISOMERISM in organic compounds - Introduction

Isomerism is introduced and explained with numerous examples-case studies.

14.1 defines and describes examples of four types of structural isomerism, namely (a) chain isomerism, (b) positional isomerism, (c) functional group isomerism and (d) tautomerism - see sub-index.

PLEASE NOTE some of the constitutional isomers can exhibit other forms of isomerism

Both abbreviated structural, displayed formula and skeletal formulae are used to portray the examples described.

Case studies of structural isomerism are discussed in detail concerning structure, naming, formation, consequences of the isomeric forms on their physical and chemical properties.

Below is a diagram illustrating all the types of isomerism that you will encounter in pre-university organic chemistry courses.

Isomerism occurs when two or more compounds have the SAME MOLECULAR FORMULA but exist in at least two different forms due to some structural or 2D/3D spatial arrangement difference in their molecular structure (i.e. different unique molecules in some way).

The different molecular forms for the same molecular formula are known as isomers, and the phenomena is called isomerism, and very common with many organic molecules.

They may be quite similar, or significantly different, in terms of their physical and chemical properties.

The existence of isomers is one of the reasons why such a huge array of different organic compounds exist.

Other reasons for the diversity of organic compounds include (i) carbon chain catenation i.e. the ability of carbon chains to link together and (ii) the formation of stable bonds with elements such as oxygen, nitrogen, hydrogen etc.

In order to analyse a molecular formula for its possible isomers, you need to know what forms of isomerism are possible and then 'take the molecular formula apart' and reconstruct the molecule's atoms in as many different ways that fit according to

(i) the valency of the atoms e.g. carbon 4 >C<, hydrogen 1 -H, oxygen 2 -O- or O=. and nitrogen is often 3 >n- (but 5 in nitro compounds - this is the basis of structural isomerism (this page 1).

(ii) different spatial arrangement cannot be superimposed on each other - this is the basis of stereoisomerism (see page 2)

So, ensure the 'reconstructs' are genuinely different in some way and obey the isomerism criteria described on these pages,

AND, beware!, molecular structures that seems different on 2D paper might not be in 3D reality, especially as there is free rotation of groups of atoms about single bonds e.g. C-CH₃, so using models for simpler molecules is extremely useful at the start of studying isomerism.

TOP OF PAGE and sub-index

Part 14.1.2 Constitutional isomerism or structural isomerism

Structural constitutional isomerism occurs when two or more compounds have the same molecular formula but have a different structural formula because of different arrangements of how the atoms are connected.

So constitutional isomers differ in their connectivity of the atoms in the molecule for the same molecular formula.

The terms 'structural isomers' and 'constitutional isomers', 'structural isomerism' and 'constitutional isomerism' seem to be interchangeable, i.e. these terms are used to mean the same thing as defined above!

They are best appreciated in terms of their graphic or full displayed structural formula - but I've chosen to display in a variety of different styles that you need to be able to read including skeletal formulae.

At least one atom is bonded to a different atom in each molecule when isomers are compared.

There are four main sub-divisions of structural isomerism, (a) to (d), are considered below.

(1a) Carbon chain isomerism (browse through alkanes, aromatic hydrocarbons (arenes), alkanes and octane number)

(1b) Positional isomerism (browse through haloalkanes, alkenes, aromatic sulfonic acids, products of alkene addition reactions, alcohols, amines)

(1c) Functional group isomerism (browse through alcohols/ethers, carboxylic acids/esters/other isomers, aldehydes/ketones, alkenes/cycloalkanes, aldehydes/cycloalcohols, phenols/alcohols, aromatic amines/nitro compounds)

(1d) Tautomerism (e.g. hydroxy-ketones/enols)

Two series may overlap in terms of their general molecular formula e.g. non-cyclic mono-alkenes and cycloalkanes

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