Structural Isomerism

Introduction to Organic Chemistry

What are structural isomers?

Compounds with the same molecular formula but different structural formulae.

What is structural isomerism?

The existence of compounds with the same molecular formula but different structural formulae.

Why are butane and methylpropane structural isomers?

They both have the molecular formula C₄H₁₀ but have different structural formulae.

What happens to the number of possible structural isomers as the number of carbon atoms increases?

The number of possible structural isomers increases.

What are the two important types of structural isomerism?

Chain isomerism and position isomerism.

What is chain isomerism?

Isomerism where molecules have the same molecular formula but different carbon chains.

Give an example of chain isomers.

Butane and methylpropane.

Why are butane and methylpropane chain isomers?

They have the same molecular formula, C₄H₁₀, but different carbon chains.

What is position isomerism?

Isomerism where molecules have the same carbon chain and functional group, but the functional group is attached at different positions on the chain.

Give an example of position isomers.

Propan-1-ol and propan-2-ol.

Why are propan-1-ol and propan-2-ol position isomers?

They have the same carbon chain, but their –OH groups are attached to different carbon atoms.

Can structural isomers show both chain and position isomerism?

Yes. They can have different carbon chains as well as functional groups in different positions.

What does a molecular formula show?

The actual numbers of atoms of each element in a molecule, but not how the atoms are arranged.

Why can some molecular formulae have several possible structures?

The atoms can be arranged and connected in different ways.

What is the first step when drawing structural isomers from a molecular formula?

Draw all the carbon atoms separately in a straight chain with bonds between them.

What should you do after drawing the straight carbon chain?

Add atoms other than hydrogen in as many different positions as possible.

When should hydrogen atoms be added when drawing structural isomers?

Add them last to ensure every carbon atom has four bonds.

How many bonds must each carbon atom have?

Four.

How can you find more structural isomers after using a straight carbon chain?

Draw a shorter carbon chain with one or more branches and then add the other atoms in different possible positions.

How can you check whether two structures that look different are actually the same compound?

Give both structures their IUPAC names. If they have the same name, they are the same compound.

Why is IUPAC naming useful when finding structural isomers?

It helps identify structures that look different when drawn but actually represent the same compound.

How many structural isomers does C₅H₁₂ have?

Three.

What are the three structural isomers of C₅H₁₂?

Pentane, 2-methylbutane and 2,2-dimethylpropane.

What must you check when drawing structural isomers?

Every carbon atom must have four bonds and the total number of hydrogen atoms must match the molecular formula.