Halogenoalkanes

-dont tend to occur naturally but are the basis of many synthetic compounds 

-general formula: CnH2n+1X (where X is the halogen) 

- the C-X bond is polar ( C+ve and X-ve). As you descend the group from F to I, polarity decreases

- The polar C-X bond isnt polar enough to make halogenoalkanes soluble in water 

-Main IM forces are van der waals and dipole-dipole attractions

-mix with hydrocarbons to be used as dry cleaning fluids and remove oily stains

-BP increases with increased chain length and going down halogen group (Inc VdW's) 

- Halogenoalkanes have higher BP's than alkanes as they have higher Mr's and are more polar 

-when halogenoalkanes react, it is almost always the C-X bond that breaks. 2 factors determine how readily the C-X bond breaks:

Bond polarity:

• because halogens are more electronegative than C, C ends up with a partially +ve charge and electron deficient 
• they can be attacked by reagents that are electron rich (called nucleophiles)
• Nucleophiles are electron pair donors
• the polarity of the C-X bond predicts C-F is the most reactive as its the most polar and so C can be most easily attacked by nucleophiles 

Bond enthalpy:

• C-X bonds get weaker going down the group as the shared electrons get further away from the halogen nucleus (as atom size increases and polarity gets less) 
• this predicts that flouro-compounds would be least reactive as C-F bond is strongest 
• experiments confirm reactivity increases down group- so enthalpy is more important than polarity

-Nucleophiles are reagents that attack and form bonds with +vely or partially +vely charged C atoms 

-they are either -vely charged ions or has an atom with a partially -ve charge (delta-ve) 

-A nucleophile has a lone pair of electrons which it uses to form covalent bonds. The lone pair is situated on an electronegative atom

- so a nucleophile is a species which has a lone pair of electrons with which it can form a bond by donating its electrons to an electron deficient carbon 

- :OH- ion, :NH3, :CN- ion are common nucleophiles

- they will replace the halogen in the halogenoalkane 

General equation: 

Reaction mechanism:

the lone pair in the nucleophile is attracted towards a partially +vely charged C atom. 

The electron pair in the C-X bond moves towards the halogen atom, making it a halide ion 

Halogenoalkanes with aqueous sodium/ potassium hydroxide 

- the nucleophile is the :OH- ion 

-reaction occurs very slowly at room temp. speed it up by warming the mixture.

-ethanol is used as a solvent in which the halogenoalkane and the aqueous hydroxide solution dissolve in 

-hydrolysis reaction 

-rate of reaction depends on the strength of the C-X bond (C-F is the strongest, therefore least reactive) 

Halogenoalkanes with cyanide ions: :CN- ion

-when warmed with aqueous alcoholic solution of potassium cyanide 

-forms a nitrile 

-useful if you want to make a product that has one extra C atom 

Halogenoalkanes with ammonia: :NH3

-excess conc solution of ammonia in ethanol carried out under pressure 

- R-X + 2NH3 ---> RNH2 + NH4X 

- is a nucleophile although not -vely charged because it has a lone pair and N is partially -vely charged 

-under different conditions halogenoalkanes react by elimination 

-a hydrogen halide is eliminated from the molecule, leaving a double bond in its place so an alkene is formed 

-:OH- ion under different conditions acts as a base, removing a H+ ion from the molecule to form water. 

- eg potassium hydroxide + bromoethane 

Conditions: 

- sodium or potassium hydroxide is dissolved in ethanol (instead of water for sub) then the mixture is heated to produce and alkene 

Mechanism:

• OH- ion uses its lone pair to form a bond with one of the H atoms on the C next to the C-X bond 
• the electron pair from the C-H bond now becomes part of a C=C double bond 
• halogen takes pair of electrons in C-X bond and leaves as a halide ion 

• OH- ions at room temp dissolved in water favour substitution 
• OH- ions at high temp dissolved in ethanol favour elimination
• Primary halogenoalkanes (X at end of the chain) favours substitution 
• Secondary halogenoalkanes (X in body of chain) will do both
• Tertiary halogenoalkanes (X in a branch of chain) favours elimination 

CFC's

• very unreactive under normal conditions 
• short chains are gases and used as aerosol propellants, refigerants, blowing agents 
• long chains used as dry cleaning and de-greasing solvents 

CFC's eventually end up in atmosphere where they decompose to give chlorine atoms (free radicals) and destroy the ozone 

so CFC's are being phased out and replaced by eg hydroflourochlorocarbons