CHEM 2 - Alkenes

• Alkenes are unsaturated hydrocarbons
• General formula CnH2n
• They are the starting material for many different products mainly plastics
• There is no rotation about the double bond
• pi bond made from 2 p orbitals overlap: half above half below. This causes restricted rotation

Alkenes have structural isomers and geometrical isomers.

Structural

If the two groups with the higher priorities are on the same side of the double bond, that is described as the (Z)- isomer. If not it is the (E)- isomer.

• The double bond doesn't really affect melting point and boiling point
• However the double bond does affect reactivity
• The double bond is an electron-rich area so it attracts electrophiles
• Alkenes undergo electrophilic addition

Electrophilic addition

• The electrophile is attracted to the C-C double bond 
• Electrophiles are positively charged and accept a pair of electrons
• A carbocation is formed

When bromine reacts with propene bromopropane is produced. However this can be 1-bromopropane or 2-bromopropane

• Due to the inductive effect, it is almost all 2-bromopropane
• Alkyl groups have a tendency to release electrons which stabilizes the intermediate carbocation
• Therefore the more alkyl groups, the more stable

When a mixture of a halogen and alkane are exposed to sunlight, a haloalkane forms

Free-radical substitution

• Initiation - Breaking of the diatomic halogen molecule into two free radicals
• Propagation - The free-radical takes a hydrogen from the alkane to form a hydrogen halide and another free radical
• Termination - Two free radicals join to form a stable molecule