How is a pi bond formed?

By side-on overlap of parallel p orbitals above and below the axis

Why can a long conjugated molecule appear coloured?

Extended conjugation lowers the energy gap so it absorbs visible light

Restricted rotation about a carbon-to-carbon double bond, with two different groups on each carbon

A carbon atom bonded to four different groups

It contains equal amounts of the two enantiomers, whose opposite rotations cancel

Nucleophilic substitution with aqueous alkali

The actual product obtained compared with the theoretical maximum

The molecular mass of the compound

The number of different hydrogen environments

A drug that binds and produces the same response as the body's natural compound

So it can fit and bind to the receptor or enzyme binding site

14questions. Pick an answer and you'll see why right away.