Quick questions on Structural and geometric isomerism (QCE Chemistry Unit 4)

Structural isomers have the same molecular formula but different atom connectivity. There are three sub-types in the Unit 4 syllabus.

Stereoisomers have identical connectivity but different spatial arrangement. The Unit 4 syllabus covers cis-trans isomerism in alkenes.

Isomerism is not just a naming exercise. QCAA past papers use isomerism to test:

For a question asking for "all isomers of C4H10O":

Drawing rotated versions as new isomers. Rotating a structural formula in 2D does not produce a new isomer. Check connectivity, not appearance.

Same functional group and substituents, different carbon skeleton (branching).

Same carbon skeleton and same functional group, but the functional group sits at a different position.

Same molecular formula, different functional group (and different homologous series).

When the higher-priority group on each sp2 carbon sits on the same side of the double bond, the isomer is "cis"; opposite sides is "trans".

Cis isomers usually have a small net dipole; trans isomers are usually symmetrical and so non-polar. Cis tends to have slightly higher boiling point (dipole-dipole adds to dispersion) but lower melting point (less efficient crystal packing). The cis vs trans melting and boiling difference is a common QCAA stimulus.

Rotating a structural formula in 2D does not produce a new isomer. Check connectivity, not appearance.

"It has a C=C double bond so it must have cis-trans isomers" is wrong. Both ends of the double bond need two different substituents.

All three subtypes are structural isomers; QCAA marking guides usually want the specific subtype named.

The Unit 4 syllabus uses cis/trans. E/Z is more rigorous (uses Cahn-Ingold-Prelog priority) and is the only valid notation when each sp2 carbon has three different substituents. For Unit 4, the simpler cis/trans suffices.