halo alkanes

Are halogenoalkanes soluble in water?

Insoluble as C-H bonds are non-polar, not compensated for enough by C-X bond polarity

Do halogenoalkanes have a polar bond? why?

polar bonds as the halogens are more electronegative than carbon atoms. This means electron density is drawn towards the halogen forming ∂+ and ∂- regions

Which intermolecular forces do they have? why?

Permanent dipole-dipole and van der Waals forces of attraction

C-X bond polarity creates permanent dipoles

When would they have higher boiling points?

Increase Carbon chain length Halogen further down group 7

How would the mass of a haloalkane compare with the mass of an alkane of the same chain length?

Greater as mass of halogen > mass of H

What is the most important factor in determining their reactivity?

Carbon-halogen bond enthalpy

What is the order of reactivity of halogenoalkanes?

Although C-F is the most polar bond, the bond enthalpy of C-X decreases down the group, so reactivity increases down the group

What is a nucleophile?

A negatively charged ion/δ− atom with a lone pair of electrons which can be donated to an electron deficient atom

Give 3 examples of nucleophiles

:OH :CN :NH3

What is nucleophilic substitution?

A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ− atom leaves molecule (replaced by nucleophiles)

Draw the mechanism for the reaction of bromoethane with NaOH warm (aq).

Draw the mechanism for the reaction of bromoethane with KCN

Draw the mechanism for the reaction of bromoethane with NH3

The intermediate formed has an N+ atom, so electrons are transferred to it causing a hydrogen to be lost too.

Draw a mechanism for the reaction of bromoethane with NaOH in ethanol

What are CFCs?

Chlorine-fluoro-carbons - haloalkanes containing C, F and Cl only (no H)

What is the problem with CFCs?

Although unreactive under normal conditions, they catalyse the breakdown of ozone in the atmosphere via free radical substitution

What are CFCs being replaced with?

HCFCs (hydrogen, chlorine, fluorine, carbon) HFCs (hydrogen, fluorine, carbon)

What are the conditions/reactants needed for the elimination reaction of haloalkanes?

NaOH or KOH dissolved in ethanolic (no water present) Heated under reflux

What is formed in the elimination reaction of haloalkanes?

An alkene, water and halogen ion

when elimination occurs.

, the nucleophile acts as a base and accepts a proton, removing a hydrogen atom from the molecule. This results in the elimination of the halide too producing a carbon-carbon double bond, an alkene.

Elimination reactions can only occur

from 2 o and 3o (tertiary) halogenoalkanes.

Ozone in the atmosphere absorbs

UV radiation

CFC-free solvents are now being produced to

prevent them entering the atmosphere. This helps minimise ozone depletion and global warming.

equation for ozone depletion

The greater the Mr of the halogen…

in the polar bond, the lower the bond enthalpy meaning it can be broken more easily

• Therefore the rate of reaction for these halogenoalkanes is faster.

• Nucleophilic substitution reactions can only occur for 1 o (primary) and 2o (secondary) halogenoalkanes.