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Made to make your eyes water

OK it is this time of year again and time to teach organic chemistry again. Now before we get going you might want to consider the question of “why are the basic reactions in organic chemistry important to me ?” 

The SN2, SN1, E2, E1, interconversion of carboxylic acid derivatives and aromatic substitution reactions are key reactions which are in the top 11 all time reactions of organic chemistry, they also explains a lot of things. Now I feel that I should continue with the theme of “things which make your eyes water”. We have already had CS “gas” the riot control agent, now we need to consider capsaicin which is in chilli peppers. Time to put on your difractospecs and look at a  picture of capsaicin in the solid state rendered with POVray. 

The molecular strcuture of capsaicin

 

I looked at a synthesis for capsaicin which is the eye watering neuro toxin in chilli peppers. This synthesis used some simple first year organic chemistry. One route starts with a Friedel Crafts reaction of 2-methoxyphenol, I have looked at this and I do not like such a reaction as the aromatic substrate has two activating groups, I suspect that selectivity might be hard with this reaction. 

A better starting point is vanillin, the aldehyde which gives vanilla ice cream its flavour. I liked the reaction of vanillin with a nitrogen nucleophile followed by reduction. The reason I liked this route is that it starts from chemical which is made on a 10000 ton scale. Much of the vanillin which is needed for ice cream is made by the reaction of 2-methoxyphenol under basic conditions with glyoxylic acid, under basic conditions the phenol group deprotonates and becomes a phenolate which is much more activating than a methoxy group. If you attend my organic chemistry lessons you would have discovered that the phenolate is more activating than the phenol or methoxy group. Here we are using some first year organic chemistry to explain and predict how a reaction will give us one product rather than an isomer. 

After an oxidation reaction (converting an alcohol into a ketone) which is something which we deal with in the first year, a reaction not on the first year course occurs. This is a decarboxylation, it is a loss of carbon dioxide. I view the decarboxylation as an important reaction which pops up a lot in chemistry. 

Then we can react our vanillin with hydroxylamine, this reaction is closely linked to many of the reactions of the carboxylic acid group which are taught in the first year. The reaction will start with an attack on the carbonyl carbon by the nucleophile to make a tetrahedral intermediate. This intermediate will then lose water to form a nitrogen version of a aldehyde which is an oxime. An oxime is an imine with a OH group on the nitrogen. 

After reduction (another thing we deal with in year one) an amine can be obtained. A whole host of reducing agents exist which can do the reduction. 

Finally the reaction of an acid chloride with the amine will give us the amide which we wanted. Hence it is possible to make capsaicin using first year chemistry. 

Making the carboxylic acid is a little harder, we might deal with that in another blog post. 

I would like to point out that capsaicin is a nasty so and so of a chemical, I have experienced it first hand. It made my skin hurt and once when I rubbed an eye hours after making a curry using it I felt great pain ! I think in some ways that a glove box should be installed in a kitchen to make the use of chillis somewhat safer, the idea is that it would become impossible to contaminate your hands or other body parts with capsaicin if the chillis were cut up and used inside a plastic box which had two ports for gloves. I might draw a diagram of Dr Foreman’s design for a glove box for chilli pepper work for you. As an alternative to a glove box I have a few designs for a smaller and cheaper chilli pepper processing device which would allow a chilli to be cut up without a direct contact with a human. This device is based on the thing which a dentist uses to inject your gum before what would otherwise be painful dental work.

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2 Responses

  1. Hey Mark, fascinating post 🙂

    First year chemistry eh? Perhaps for European unis, this would be a second/third year topic for American Universities.. I’m teaching AP chem this year which is supposed to be the equivalent to first year college chem (taken in high school) and reaction mechanisms is not even covered (infact, only naming and recognising functional groups is).

    Vanillin = common chemical, easily bought in a supermarket, by any chance?? If I could synthesize it in the lab, would it smell strongly of vanilla? The kids love reactions that change colour or smell…

    I usually cut chillis up in the blender after de-seeding them.. minimal contact 🙂

  2. I heartily agree with the warning about peppers. We grow Jalepino peppers and have learned to use lab gloves when preparing them for cooking. The seeds and membranes contain the highest content of capsaicin. If we are preparing several of them we have noticed our eyes burning also.

    Capsaicin is being sold in an ointment as a topical analgesic. After the burning comes a numbness.

    The natural source of vanillin is the seed pod of an orchid. The pods are sold in supermarkets here in So Cal.

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