Haloalkanes

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Haloalkanes

Haloalkanes are alkanes with halogen functional groups

The general formula is CnH2n+1X

They are often used in the production of synthetic products such as PVC, as a solvent in dry cleaning and

When reacting, it is nearly always the C-X bond that breaks

The C-X bond is polar as the carbon is partially positive and the halogen partially negative due to the nature of the halogens electro-negativities.

The C-X bond is however not polar enough to dissolve in water so it is insoluble.

It is the bond polarity as well as the bond enthalpy that determines the reactivity of a haloalkane.

The most reactive of the haloalkanes are those that contain a C-I (iodine) bond as the bond enthalphy of C-I is the lowest, so is most easily broken releasing the iodine.

The C-F bond would be the most polar due to the high electronegativity of the fluorine atom (the most electronegative atom)

The main IMF are dipole-dipole forces and Van der Waals

The mpt's and bpt's of haloalkanes increase with chain length, and as you go down halogen group. However, with increased branching bpt/mpt decrease.

Nucleophilic Substitution - Is a good way of introducing new functional groups into compounds.

Nucleophile: An electron pair donor e.g hydroxide ion (:OH-), cyandide ion (:CN-), ammonia (:NH3)

Substitution: The nucleophile replaces an atom/group of atoms, in this case the halogen atom

Leaving Group: In the case of haloalkanes this is the halide ions

Curly Arrows: Show the movement of electrons in organic mechanisms.

When hydroxide group is introduced alcohols are formed

When cyandide groups are introduced nitriles are formed

When ammonia groups are introduced amines are formed

Elimination

Elimination = OH acting as base, in warm ethanolic conditions

Results in an alkene

Substitution= OH acting as nucleophile, in cool aqueous conditions

Primary haloalkanes favour substitution

Tertiary haloalkanes favour elimination

Secondary will do both

Free Radical Substitution - Synthesis of Haloalkanes

3 stage process:

Initiation - UV light is necessary to break covalent bond homolytically (one electron goes to each atom in the bond). This results in the formation of free radicals (highly reactive species with an unpaired electron)

Propogation - This is the chain part of the reaction, the free radical is constantly regenerated

Termination - Two free radicals react together to form a stable compound ending the reaction

CFC's cause the depletion of the ozone layer because of the Chlorine free radicals reacting and removing O3 in the upper atmosphere.

CF2Cl2 ->CF2Cl. + Cl.

Cl. + O3 -> ClO. + O2

ClO.

+ O3 ->2O2 + Cl.

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