Chemistry 3.3.4 - Alkenes

Why are alkenes more reactive than alkanes?

Double bond is more reactive than single bond

Why do alkenes exhibit E/Z isomerism?

• Restricted rotation around C=C double bond

• Each C atom of C=C bond must have 2 different groups attached

Why do alkenes have low boiling points?

• Simple molecular structures (only VdWs)

• Boiling point increases with chain length

How does alkene solubility vary with chain length?

Solubility in water will decrease with increasing chain length

Electrophile

A species that is electron deficient in some way (+ ion or δ+) so seeks out areas of electron density. Electron pair acceptor.

Why are alkenes susceptible to attack by electrophiles?

C=C double bond is an area of high electron density

Electrophilic addition reaction between alkenes and hydrogen halides forms:

Halogenoalkanes

Electrophilic addition reaction between alkenes and halogens forms:

Dihalogenoalkanes

Electrophilic addition reaction between alkenes and H₂SO₄ forms:

Alkyl hydrogen sulphates

What is LDPE?

Low-density poly(ethene) - more branched structure rather than straight chain, so less dense

What is HDPE?

High density poly(ethene) - less branching, more linear, so more densely packed

Synthesis of LDPE

Polymerisation of ethene at high pressure and high temperature via a free radical mechanism

Synthesis of HDPE

Temperature & pressure a little over room conditions, Ziegler-Natta catalyst

Properties of LDPE

Lighter, flexible, thin, more transparent than HDPE

Properties of HDPE

Heavier, stiffer, less flexible, more opaque than LDPE

Uses of LDPE

Plastic bags, sheeting, electrical cables

Uses of HDPE

Crates, buckets, bottles

Why are poly(alkenes) unreactive/inert?

Contain many strong non-polar C-C and C-H bonds. Also not broken down by enzymes, so non-biodegradable.

Plasticisers

Small molecules that fit between polymer chains, forcing them apart and making them more flexible