Haloalkanes

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  • Created by: chunks-42
  • Created on: 14-05-15 11:23

Haloalkanes

A haloalkanes is an alkane with at least one halogen atom in place of a hydrogen atom.

E.g. 

(http://scienceaid.co.uk/chemistry/organic/images/haloisomers.jpg)

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Polar bond

1. Halogens are much more electronegative than carbon. So, the carbon-halogen is polar.

2. The delta + doesn't have enough electrons. This means it can be attacked by a nucleophile. A nucleophile's an electron-pair donor. It donates an electron pair to somewhere without enough electrons.

3. Oh-, Cn- and NH3 are all nucleophiles which react with haloalkanes.

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Haloalkanes hydrolysed to alcohols

For example, bromoethane can be hydrolysed to ethanol. You have to use warm aqueous sodium or potassium hydroxide or it won't work. It's a nucleophilic substitution reaction. Here's how it works.

Here's the general equationfor this reaction: R-X + OH- ---> ROH + X-

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Iodoalkanes

1. The carbon-halogen bond strength (or enthalpy) decides reactivity. For any reaction to occur the carbon-halogen needs to break.

2. The C-F is the strongest - it has the highest bond enthalpy. So fluoroalkanes are hydrolysed more slowly than other haloalkanes.

3. The C-I bond has the lowest bond enthalpy, so it's easier to break. This means that iodoalkanes are hydrolysed more quickly.

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Reaction with Ammonia

If you warm a haloalkane with excess ethanolic ammonia, the ammonia swaps places with the halogen - yes, it's another one of those nucleophilic substitution reactions.

(http://www.scienceaid.co.uk/chemistry/organic/images/ammoniahaloalkane.jpg)

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Forming Nitriles

If you warm a haloalkane with ethanolic potassium cyanide, you get a nitrile.

It's yet another nucleophilic substitution reaction - the cyanide ion, CN-, is the nucleophile.

(http://www.chemtube3d.com/model/ALevel/sn2rxn.GIF)

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Elimination Reactions

If you warm a haloalkane with hydroxide ions dissolved in ethanolinstead of water, an elimination reaction happens, and you end up with an alkene. This is how you do it:

1.Heat the mixture under reflux or you'll lose volatile stuff: CH3CHBrCH3 + KOH -> CH2=CH3CH3 + H2O + KBr

2. Here's how the reaction works:

(http://www.chemguide.co.uk/mechanisms/elim/elimpropm.GIF)

3. This is an example of an elimination reaction. In an elimination reaction, a small group of atoms breaks away from a larger molecule. This small group is not replaced by anything else (whereas it would be in a substitution reaction). In the reaction above, H and Br have been eliminated from CH3CHBrCH3 to leave CH2=CHCH3.

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Changing Conditions

You can control what type of reaction happens by changing the conditions.

Water reflux - alcohol, nucleophilic substitution, OH- acts as a nucleophile

Ethanol reflux - alkene, elimination, Oh- acts as a base

Both of these reactions have their uses...

1) The elimination reaction is a good way of getting a double bond into a molecule. Loads of other organic synthesis reactions use alkenes, so the elimination reaction is a good starting point for making lots of different organic compounds.

2. The substitution reaction allows you to produce any alcohol molecule that you need. And alcohols can be the starting point for synthesis reactions that rpoduce aldehydes, ketones, esters and carboxylic acids.

So haloalkanes are very useful as a starting material for making other orgainc compounds.

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