3.3.3 Halgenoalkanes

Halogenoalkanes and the halogen carbon bond

General formula CnH2n+1

The C-X bond is polar as the halogen atom is more electronegative than the carbon atom 

-The halogen atom withdraws the shared pair of electrons in the covalent bond more than the carbon atom 

-This results in an uneven distribution of electrons, making the carbon partially positive and the hydrogen partially negative 

-This makes the carbon atom target nucleophiles- electron pair donors.

Why halogenoalkanes have a higher boiling point than halogens

Haloalkenes have higher boiling points than alkanes because alkanes only have van der waals forces between molecules, while haloalkane have dipole dipole forces between molecules and van der waals forces between molecules. Dipole dipole forces are stronger than VDW so more energy is required to overcome force of attraction.

-The reactivity of the C-X bond is determined by the strength of the bond and the bond enthalpy. The C-F bond is very strong as it has the largest bond enthalpy, so fluoroalkanes are the least reactive. As you go up the down, from fluorine at the top to iodine at the mid way through, the reactivity of halogenoalkanes increases.

• Another reason is because fluorine is the smallest molecule, so it has a smaller atomic radius, meaning that there is a stronger attraction between the nucleus and the outer shell electron so it is harder to lose and electron 

Nucleophilic substitution

Nucleophiles are:

• Hydroxide ions

• Cyanide ions

• Ammonia

Hydroxide ions

Reagents: NaOH of KOH

Products: alcohols and potassium halide / sodium halide

Conditions: aqueous and heat under reflux

Cyanide ions

Reagents: KCN (potassium cyanide)

Product: alkanenitriles

Conditions: ethanol and heat under reflux

Ammonia

Reagents: NH3 (excess ammonia)

Products: alkylamine and ammonium halide

Conditions: ethanol, excess and heat under reflux

The second ammonia acts as a base by accepting a proton from the alkyl ammonium intermediate which is deprotonated to form an amine

Elimination reaction

Reagents: KOH or NaOH

Conditions: ethanol and heat under reflux

Hydroxide ion acts as a base which accepts a proton from halogens alkane

Primary, secondary and tertiary halogenoalkanes

• Primary have one R group attached to the carbon bonded to the halogen - these usually react by nucleophilic substitution 

• Secondary have two R group attached to the carbon bonded to the halogen- these usually react by elimination react

• Tertiary have three R group attached to the carbon bonded to the halogen- these usually react by both elimination and nucleophilic substitution 

-This is the case because the alkyl groups are electron releasing so push electrons towards the central carbon, having a positive inductive effect 

-Positive inductive effect by the alkyl groups makes the carbon less electron deficient so less likely to undergo nucleophilic substitution 

Tertiary are more stable than secondary which is more stable than primary, and the more stable compound is more likely to be formes,

Ozone depletion

The ozone layer is found in the stratosphere

ozone absorbs UV, acting as a chemical sunscreen

UV can cause skin cancer or sun burn

Chlorofluorocarbons and hydrochlorocarbons are organohalogens

Chlorofluorocarbons are attacked by UV, which breaks the CFCs carbon chlorine bond

The UV radiation breaks the carbon halogen bond by Homolytic fission to form radicals, this is called photodissociation

The choline radical behaves as a catalyst because it is regenerated at the end of the reaction

CFCs uses

Used as

• Refrigerants: transfer thermal energy during cooling cycle

• solvents: dissolve oils and other substances

Atmospheric avoidance for CFCs damaging atmosphere

Chlorine free alternative

Hydrofluorocarbons

Do not contain chlorine, so do not harm ozone layer, but are powerful greenhouse gasses

The CF bond is stronger than the CCl bond, therefore UV does not provide enough energy to break it