Structural isomers
Same molecular formula different structures (or structural formulae)
What can structural isomers arise from
• chain isomerism
• positions isomerism
•functional group isomerism
Functional group isomers
Compounds with the same molecular formula but with atoms arranged to give different functional groups
Examples of fcuntional group isomers
- Alcohols and esters
- Aldehydes and ketones
- Carboxylic acids and esters
- Alkenes and cylcoalkanes
Stereoisomerism
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms
The 2 types of stereoisomerism
• geometrical (E-Z isomerism)
• optical isomerism
What is a chiral carbon atom
A carbon atom that has 4 different groups attached to it
Optical isomerism
Occurs in carbon compounds with 4 different groups of atoms attached to a carbon
What is a mixture containing 50/50 mixture of the 2 isomers called
A racemate or a racemic mixture
What are 2 compounds that are optical isomers of each other called
Enantiomers
When one enantiomer rotates it in one direction..
The other enantiomer rotates it by the same amount in the opposite direction
Optical isomers have the same..
Physical and chemical properties but they rotate plane polarised light in different directions
A racemic mixture will not..
Rotate plane-polarised light
Why do structural isomers have different boiling points?
Answer:
Different structures = different intermolecular forces (e.g. more branching = weaker van der Waals = lower bp).
What’s the difference between E and Z isomers?
Answer:
• Z (zusammen) = high-priority groups on the same side
• E (entgegen) = high-priority groups on opposite sides
Why don’t alkanes show E/Z isomerism?
Answer:
Because they only have single bonds, so atoms can rotate freely — no fixed spatial arrangement.
Why do E and Z isomers have different boiling points or reactivity?
Answer:
• Different dipole moments and intermolecular forces
• E-isomers often pack better, so slightly higher bp
Ethene does not show E/Z isomerism. Why?
Answer:
Because both carbons in the C=C bond are bonded to identical atoms (hydrogens only), so there’s no difference in groups = no isomerism.
Suggest why Z-isomers can have higher boiling points than E-isomers.
Answer:
Z-isomers may have polar groups closer together, creating a greater net dipole, so stronger permanent dipole–dipole forces.
Why is your answer using bond enthalpies not 100% accurate?
Answer:
Mean bond enthalpies are averaged values from different compounds, not specific to the exact molecules involved.
What are stereoisomers
Same structure but have a different arrangement of atoms in space
How does E-Z isomerism occur
- Lack of rotation around C=C
- Each C atom of C=C has 2 different groups attached
Explain why step 4 produces a racemic mixture
- Equal chance attack from above/below
- Giving equal amounts of both optical isomers/enantiomers
