Hydrolysis of Haloalkanes

Haloalkanes react with aqueous NaOH to form alcohols. the mechanism for which, is nucleophilic substitution:

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In this mechanism, the hydroxide ion acts as a nucleophile because it is donating an electron pair the the partially positive carbon atom. The hydroxide ion attacks the carbon from the opposite side to which the halogen is bonded because the bromine is very large in comparison, making it nigh on impossible for the nucleophile to attack from the front. 

The carbon atom is then making 5 bonds (10 bonding electrons in total), but it can only accommodate 8 electrons in its valence shell, therefore the bromine leaves as the C - Br bond breaks by heterolytic fission as the bromine comes away with both of the electrons from the bonding pair. 

The resulting solution contains alcohol, excess hydroxide and halide (in this case bromide) ions. 

From the perspective of organic synthesis, this reaction is a useful transition from haloalkane to alcohol, but from an organic analysis perspective, this reaction can also be used to identify the type of haloalkane when combined with another subsequent step. However, this is a destructive analytical technique and you will not get your sample back!

To test for the type of halogen in your haloalkane, you must firstly add nitric acid (HNO₃) to your reaction mixture. This will remove any other species that could give a false positive result in the next step. You then ad silver nitrate (AgNO₃). This will produce precipitates of various different colours:

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From left to right:

• Iodide produces a yellow precipitate ( I^-_aq) + Ag⁺_(aq) --> AgI_(s) )
• Iodide produces a yellow precipitate ( Br^-_aq) + Ag⁺_(aq) --> AgBr_(s) )
• Iodide produces a yellow precipitate ( Cl^-_aq) + Ag⁺_(aq) --> AgCl_(s) )

So, finally, an example to summarise the above: a sample of a haloalkane can be hydrolysed to substitute the halogen for an OH group. The resulting solution contains halides which can be tested from with nitric acid and silver nitrate. The colour of the precipitate formed can be used to identify the halogen present in the haloalkane. 

In a subsequent post, I will cover explaining the rate of hydrolysis of haloalkanes. 

The Significance of Optical Isomers

Firstly, sorry the blog has been a bit dead - I've been flat out with work and life commitments!

Optical isomerism is a discovery made by French Physicist Jean-Baptist Biot. It essentially means that any carbon atom in a molecule which is bended to four different groups may from two different versions of that molecule (enantiomers).

These two  which are non-superimposable mirror images of each other, as shown by the example below; the infamous thalidomide:

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Thalidomide was a drug which was originally licensed in 1958. Its primary use was to treat morning sickness in pregnant women. It was not known that thalidomide was optically active. The R enantiomer (shown above) was useful in treating morning sickness, whilst the S enantiomer led to the birth of babies with shortened limbs. For this reason, thalidomide's license was withdrawn in 1961.

Another, less extreme example of optical isomerism is carvone:

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R-carvone is used in chewing gum, because is has the taste and aroma of spearmint. S-carvone however, has a radically different aroma and taste - caraway (similar to dill and parsley). 

Imagine a batch of chewing gum made with a racemic mixture (50:50 of each enantiomer) of carvone.... eughhhhh!