Chapter 13 Alkenes

• Unsaturated hydrocarbons (at least 1 C=C bond)

• CnH2n

• C has 4 electrons and 3 are used in sigma bonds (1 to the other C and the other 2 for other atoms) • The remaining electron is a pi bond

Formed by the sideways overlap of 2 electrons in the p-orbitals (one from each C of the C=C bond)

above and below the line joining the nuclei of the bonding atoms

Locks the C atoms in positions and prevents them from rotating around the C=C bond -> geometry of alkenes is different to that of alkanes

• Carotene = makes flamingos pink • Limonene = causes scent of oranges and lemon

Have the same structural formula but a different arrangement of the atoms in space

• E/Z isomerism • Optical isomerism

The rotation around the C=C bond is restricted so the groups attached to those Cs are therefore fixed relative to eachother

When the molecule has: * A C=C bond (restricted rotation) * 2 different groups attached to each C atom of C=C bond

• Special case of E/Z isomerism • Same requirements as E/Z isomerism but one of the attached groups on each C atom of the C=Cmust be the same

When there is a H on each C=C bond * Cis isomer = Z isomer * Trans isomer = E isomer

• Z isomer: If the groups of highest priority are on the same side of the C=C bond • E isomer: If the groups of highest priority are diagonally places across the C=C bond

• As atomic number increases, priority increases • If it is a large atom, find the first point of difference and use this to compare priority

pi bond -> the electrons in the pi bond are more exposed than the electrons in the sigma bond. The pi bond readily breaks and alkenes undergo addition reactions relatively easily

• hydrogen in the presence of a nickel catalyst • halogens • hydrogen halides • steam in the presence of an acid catalyst

Alkene + hydrogen -> alkane

Always saturated

Alkene + bromine -> dihaloalkane

Add bromine water orange -> colourless

Hydrogen + halogen e.g. HCl

haloalkane

Alkene + Water -> alcohol