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!

A Level - Exam Technique: Shapes of Molecules

Shapes of molecules can be difficult, but there are several key criteria to hit in an exam question that will help you maximise your marks:

• number of bonding pairs
• number of lone pairs
• electron pairs repel
• strength of bond pair - bond pair repulsion vs. bond pair - lone pair repulsion
• bond angle and/or shape

For example, consider this question about ammonia:

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Q11. a) State and explain the bond angle and shape of an ammonia molecule

(4 marks)

In the following answer, I have broken each marking point down by alternating colours, and each relates to the points listed above. 

Ammonia contains 3 bonding pair and 1 lone pair. The electron pairs repel, but the bond pair - bond pair repulsion is weaker than the bond pair - lone pair repulsion, therefore constricting the bond angle to 107° and making the shape pyramidal. 

So, if you follow the logic of addressing those 5 points above then you won't go far wrong in an exam situation - good luck!

GCSE - Determining Ionic Formulae

Ionic compounds have a set formula, based on the charges of the ions they are made from. For example, calcium chloride is CaCl<sub>2

This is the case because the calcium ion has a charge of 2+ and each fo the chloride ions have a charge of 1-. Overall, this means that the two positive charges and the two negative charges cancel each other out.</sub> 

A good way of laying this out on paper is by using an ionic equation to show the formation of the ionic compound from its ions:

Ca²⁺ + Cl^- --> ?

So if we left the above equation as it is, we would get CaCl⁺ as only one of the positive charges on the calcium ion is cancelled by the negative charge on the chloride. 

However, if we were to introduce a second chloride ion in the formula, then the negative charge it has would cancel the second negative charge on the calcium ion:

Ca²⁺ + 2Cl^- --> CaCl₂

_{And that's it - that's why ionic substances have the formula that they do. I would strongly recommend that you get familiar with the list of ions below, as in the new GCSE's you don't get them given to you on a data sheet as previous years did.} 

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