Alkenes

Question 1

Characteristics

Answer 1

Terminal alkene: C=C bond located at end of carbon chain
Alkenes with 2 C=C bonds are dienes, 3 bonds are trienes

sp2 hybridised, shape about each C atom is trigonal planar, ethene is a planar molecule: 2 sigma hydrogen-carbon bond, 1 sigma carbon-carbon bond, 1 pi carbon-carbon bond

Question 2

Isomerism conditions

Answer 2

1. Restricted rotation about C-C bond
2. Each doubly bonded C atom is joined to 2 different atoms or groups

Question 3

Diff in stability boiling point melting point between cis and trans alkene

Answer 3

Cis alkene less stable because of steric strain arising from crowding between 2 alkyl groups in cis isomer

Cis alkene has higher boiling point than trans alkene
- Cis alkene slightly polar, id-id and pd-pd dipole attractive forces
- Trans alkene non-polar, only id-id
- More energy needed to overcome stronger IMF present

Cis alkene has lower melting point
- Molecules in cis alkene pack poorly in solid lattice, 2 bulky methyl groups located on same side in molecule
- Larger distances between molecules caused by poor packing, leading to weaker intermolecular forces