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 For this reason, thalidomide's license was withdrawn in 1961.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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