AS Chemistry - Haloalkanes

notes on the haloalkanes group for AQA AS level chemistry Unit 2 (CHEM2). May also be relevant to other specs/exam boards.

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haloalkanes are just alkanes with at least one halogen atom in place of a hydrogen atom

they can be primary, secondary or tertiary (have one, two or three alkyl groups)

They can be produced by adding bromine to an alkene (it breaks the C=C double bond). Consequently, this is also a test for a double bond - add bromine water and if the solution goes clear, then there is (was) a double bond - a haloalkane is produced, removing the bromine that gives the orange-brown colour.

the boiling points of HA's increase down the group, due to the increase in atomic radius and no. of electron shells, so stronger VDW's forces - more energy req'd to overcome them.

Fluoroalkanes are the least reactive, iodoalkanes are the most reactive (as their bond enthalpy is the smallest)

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Reactions of haloalkanes

HA's will react will silver nitrate (AgNO3) solution to give characteristic precipitates.

Fluoroalkanes - no reaction

Chloroalkanes - white ppt, dissolves in aqueous ammonia (NH3)

Bromoalkanes - cream ppt, dissolves in concentrated NH3

Iodoalkanes - yellow ppt, does not dissolve in aq. or conc. NH3

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Mechanisms of HA's

the Halogen-carbon bond is polar as there is a large difference in electronegativity between the atoms

the slightly positively charged Carbon can be attacked by a nucleophile. Nucleophiles are electron pair donors, and are therefore Lewis bases.

Some nucleophiles are NH3, CN-, OH-, and H2O

Nucleophilic Substitution:

with OH- ions (conditions: warm aqueous NaOH)

the OH- replaces the halogen aton, and a halide ion with a lone pair is released.

warming a HA with H2O can also produce this reaction (a hydrolysis reaction)

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Nucl. subs. continued

HA's can also undergo Nucl. subs. with ammonia, NH3, to form amines (and NH3Br, if using a bromoalkane, for example)

conditions: warm (reflux) with excess ethanolic NH3

forming Nitriles:

warm haloalkane with ethanolic KCN will produce a nitrile (and KBr)


CFC's are haloalkanes - they have had all the H atoms replaced by Cl and F atoms.

CFC's used to be used in lots of things eg solvents in drycleaning and degreasing as the're quite unreactive. However in the 1970's scientists discovered that they were causing damage to the ozone layer, so they were banned.

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Elimination reactions

If you warm (reflux) a haloalkane with anhydrous OH- ions dissolved in ethanol, you end up with an alkene, water and a hydrogen halide eg HBr

Anhydrous conditions, with the nucleophile dissolved in ethanol (and reactants refluxed) will give an elimination reaction.

Aqueous conditions, with the nucleophile dissolved in H2O (and reactants refluxed) will give a nucleophilic substitution reaction.

Uses of elimination reactions: producing a molecule with a double bond, as loads of organic synthesis reactions use alkenes. Starting point for making lots of organic chemicals.

Uses of nucleophilic substitution reactions: producing any alcohol you need. Alcohols are the starting point for synthesis reactions to produce aldehydes, ketones, carboxylic acids and esters.

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if you dont mind could you explain how a nucleophilic substitution rwaction takes place with ammonia where it acts as a nucleophile and a base?

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