Posted by: Mark Foreman | January 28, 2010

Organic Chemistry is Simple

Since coming to Chalmers I have had to teach the chem eng students about organic chemistry. Some of these students seem to hate the subject or find it super hard. Some of them both hate the subject and find it hard.

As part of my quest to understand what is going on and to improve my teaching I contacted a series of leading organic chemists and asked them “what is the most important thing in organic chemistry for a student to know, understand or be able to do ?”

I then waited for the replies. Many of the organic chemists who responded suggested that mechanism and the ability to picture molecules were very important

Then over the christmas break I was thinking  about a range of topics which included the turkey, homogenous cat teaching, an example for teaching organic chemistry (soap) and about epoxy resins. One thing came to mind, the same thread can be seen through a series of reactions.

In some modern soaps in place of the carboxylate salts of yesteryear, some soaps are using esters of a hydroxysulphonic acid which is formed by the reaction of sodium hydrogen sulphite and ethylene oxide. As I pictured the reaction in my mind it was clear that the attack of the sulphur atom lone pair on the epoxide carbon is likely to be a concerted process which looks to me like a SN2 reaction dressed up in fancy clothing.

The next reaction which came to my mind was the first step in the reaction of hydrogen, carbon monoxide and ethylene oxide with a cobalt hydride tetracarbonyl “cat” to form 3-hydroxypropanal. While the catalytic cycle might have  a few more  steps, the ring opening reaction does look like the action of a nucleophile on the ethylene oxide.

I then cast my mind to epoxy resin glues and epoxy paint, in this case again it is a SN2 like reaction of a nucleophile at a sp3 carbon. While the three reactions might look different as the nucleophile in each case is different, all three ‘reactions’ have the same mechanism and are thus related. So in place of three reactions we now have one which looks like it is a SN2 reaction which has been dressed up by using a fancy looking leaving group and not the simple halide in the classic teaching example of methyl bromide and iodide. These reductionist thoughts on organic chemistry prompted me to ask of “how few reactions are there in organic chemistry ?”. I considered this question in my office, at home and on the tram and I am now ready to share my list with the world. I reason that if a person has a full and deep understanding of these reactions then they will be able to go a long way in organic chemistry. More reactions do exist but the law of diminishing returns means that learning the more reactions will have a smaller effect on the student.

Mark’s top eleven reactions in organic chemistry

1. SN2 (substitution)

2. SN1 (substitution)

3 E2

4 E1

5 Nucleophilic attack on a carbonyl resulting in addition

6 Nucleophilic attack on a carbonyl resulting in substitution

7 Formation of enolates

8 Electrophilic attack on enolates

9 Electrophilic attack on alkenes or other non aromatic pi systems

10 Aromatic substitution

11 Diels-Alder and the pericyclic reactions

Now let us put these ideas into use, if we start with an aldol reaction which is a classic of organic chemistry.

The first step in the reaction is the formation of the enolate by the action of a base on a ketone or aldehyde, the next stage is the enolate acting as a nucleophile on a carbonyl group (1,2 addition) which is the electrophile. During my time as a postdoc at Reading I read a book on Catastrophe Theory which prompted me to think long and hard about how people learn chemistry. I think that the way people learn organic and inorganic chemistry is by a slow hard slog, but once in a while there will be those eureka moments where a “happy catastrophe” occurs. The happy catastrophe is the moment where suddenly the person’s ability to understand and make sense of the subject will increase greatly.

So my advice at this point to any student who is lost in the jungle of organic chemistry, is do not be dismayed you are likely to be going through the chemical version of ‘the dark night of the soul’ where at the other side you will be rewarded with a new and deeper insight into chemistry.

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