Alkenes and Haloalkanes

Describe the structure of alkenes

Unsaturated hydrocarbons (C=C double bond)

Why are alkenes more reactive than alkanes?

C=C bond is an area of high electron density making it susceptible to attack from electrophiles

What are the two bonds in C=C?

A covalent bond and a pi bond

What is the test for alkenes?

Bromine water - orange-brown to colourless

What are electrophiles?

Electron acceptors

What area of molecules are electrophiles attracted to?

Areas of high electron density

Examples of electrophiles

HBr, Br2, H2SO4

What is formed when a double bond is broken during electrophilic addition?

A carbocation

What is a carbocation?

A carbon atom with only 3 bonds (+ charge)

Which carbocations are most to least stable?

Primary are least stable, tertiary are most stable

Which carbocations are more likely to form in reactions?

The most stable (tertiary)

Electrophilic addition of alkenes to haloalkanes

Electrophilic addition of alkenes to alkyl hydrogensulfates

What is an addition polymer?

A larger molecule produced from alkenes where double bond is broken to form a repeating unit

What type of polymers do high pressures and temperatures produce?

Branched chain polymers with weak intermolecular forces

What type of polymers do lower pressures and temperatures produce?

Straight chain polymers with stronger intermolecular forces

What is a disadvantage of polymers being unreactive?

Means that polymers are not biodegradable

Why do haloalkanes contain polar bonds?

The halogens are more electronegative than carbon atoms

Which area of the haloalkane is drawn to the halogen?

The area of high electron density

What are nucleophiles?

Positive liking molecules

what property of nucleophiles make them positive-liking?

They contain a lone electron pair that is attracted to slightly positive regions of molecules

Examples of nucleophiles

CN-, NH3, OH-

Nucleophilic substitution of haloalkanes to make alcohols

Nucleophilic substitution of haloalkanes to make amines

How does the Mr of the halogen in the polar bond of haloalkanes affect how easily the bond can be broken?

The greater the Mr, the lower the bond enthalpy meaning it can be broken more easily

What reaction conditions are required for elimination of haloalkanes?

High temperatures, ethanol solvent, strong base

What reaction conditions are required for nucleophilic substitution of haloalkanes?

Low temperatures, aqueous solvent, weak base

What does the nucleophile in elimination reactions do?

Acts as a base and accepts a proton, removing a H from the molecules

What does elimination of haloalkanes result in?

The removal of a halide to produce a C=C bond (alkene)

What types of haloalkanes does elimination reactions only occur in?

2° and 3°

What type of haloalkanes does nucleophilic substitution only occur in?

1° and 2°

Elimination of haloalkanes to form alkenes

Describe the test for halide ions

Add acidified silver nitrate

Br - cream pcp

Cl - white pcp

I - yellow pcp

Ozone equation