2.4 Alkenes

What are alkenes

Unsaturated hydrocarbons

General formula

CnH2n

Double bond features

Centre of high electron density, so alkanes more reactive

What is a diene

diene is an alkene with two C=C bonds in the main carbon chain

Sigma (σ) Bond

A covalent bond formed by the linear overlap of atomic orbitals resulting in free rotation along the bond axis.

Рі (п) Bond

Рі (п) Bond

A covalent bond formed by the sideways overlap of p-orbitals, which restricts rotation along the bond

Alkene structure diagram

Pi bond freely rotate

Cannot freely rotate as there is restricted rotation about the carbon carbon double bond,

What bond C=C or C-C is stronger?

C=C stronger as contains both sigma and pi bonds

C-C only sigma

What bond C=C or C-C is longer

C=C shorter bond length than C-C

Why are alkenes more reactive than alkanes

The C=C bond in alkenes is a region of high electron density which makes it more susceptible to attack band.

by electron deficient species (Electrophiles). This explains why alkenes are more reactive than alkanes and the pi bond is broken in addition reactions.

Bromination with bromine water use and results

Additional reaction, test for saturation as unsaturated hydrocarbons decolourise yellow bromine water

What is addition reaction

Addition reaction involves pi bond of a double covalent bond being broken and two species adding on across the double bond.

Hydrogenation reaction conditions

Finely divided solid nickel, 180oC, 4atm

Catalyst increases surface area

hydrogenation reaction use

Manufacture of margarine, oils unsaturated runny, hydrogen increases molecular mass,

Hydrogenation formula

C2H4 + H2 → C2H6

Hydration formula and meaning

Addition of water as steam produce alcohol

C2H4 + H2O(g) → C2H5OH

Conditions for hydration

concentrated phosphoric acid (H3PO4), 300°C, 60 atm.

Electrophile definition

An ion or molecule which attacks regions of high electron density

Addition of a hydrogen halide to an Alkene mechanism ethene and hydrogen bromide

Heterolytic fission

Bond breaking in which the shared electrons go to one atom

Carbo cation definition

A positive ion with the charge centred on a carbon atom

Carbocation reactivity

Highly reactive, reacts instantly, bromoethane formed instantly

Electrophilic addition fro non symmetrical alkenes produces

Major and minor product

What is a primary carbocation

One alkyl group attached to the carbon with the positive charge

What is secondary carbocation

2 alkyl group attached to the carbon atom with the postive charge

Reactivity of carbo cation primary/sepcmdary and reasons

3o most stable> 2o> 1o least stable

Alkyl groups tend to donate electrons carbon atom attach to.

Markovnikov's Rule:

When we add an HX molecule to an alkene, the H atom will join on to the carbon that already has more Hatoms.

What are polymers

(Polymers are long chain molecules made up from joining together small molecules, called monomers. which are linked together.

What is polymerisation

Polymerisation is the joining together of small molecules to form a large molecule, addition reaction

Ethene polymerisation

chloroethane: Polymerisation

polyvinyl chloride (PVC)

Excess reagent of bromine with penta-1,4-diene