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Alkenes

Dear Reader,

Some thing happened on monday which relates to alkene chemistry, I had an interesting discussion about food. One of my coworkers was shocked to hear about the chip butty (Chip sandwich), this brings us onto lipids.

First the chip butty, this is a Scottish / Northern England item of food which is a bit like the chip pizza which I have seen in Sweden. It is a bread roll which is buttered and then stuff with chips, or you can use sliced bread to make the chip sandwich. To be blunt it may not be healthy but I have heard of worse, the deep fried mars bar. This was invented in a place called Stonehaven which is just south of Aberdeen, I recall the days of my postdoc in Aberdeen. I used to drive south from Aberdeen to Stonehaven every now and then for a session of folk dancing, I used to attend something called the “Crazy Ceilidh Club”. It was a club for people who liked an evening of vigorous folk dancing. But back to science.

The thing which links alkenes to this delightful traditional chip butty is the fact that the alkenes in the long whip like tails of the glycerol triester can make the lipid more or less harmful. Current opinion is that lipids which are made of polyunsaturated carboxylic acids are less of a threat to good cardiac health than those fats made from saturated carboxylic acids.

The carbon backbones of the saturated fats are made up of only sp3 carbons while the unsaturated fats have some sp2 carbons which are used to make pi systems.

It is possible to react the unsaturated fats with hydrogen gas (using a nickel catalyst) to form hydrogenated fats which either have fewer or no alkene groups in them. One of the problems with this technology is that the cis unsaturated fats can be converted by the nickel surface into the more thermodynamically stable trans fats.

Debate currently rages over the possible threat which trans fats pose to human health, I would like to be able to either tell you to stop worrying about trans fat or to be able to tell you that trans fat is a evil killer and a bigger threat to the public than Typhoid Marry but if I was to give you an answer like that today then it would be just me guessing and bluffing. Frankly I do not want to insult your intelligence by bluffing to you good people, so I will have the intellectual courage to admit “I do not know”.

One of the key reactions of the alkenes is with electrophiles to form addition products, this is a reaction which only the alkenes can do. Alkanes and the aromatic rings such as benzene can not do this reaction. What happens is that an electrophile such as bromine or iodine monochloride comes close to the alkene.

The electrophile is either polarised already or the filled pi bonding orbital of the alkene will polarize it. The next step is that the alkene will bond to one of the atoms in the electrophile after sending away one of the atoms as a leaving group. This will typically form a cyclic cationic intermediate which has three man canonical forms. Please see the ring opening of an epoxide under acidic conditions to help you understand this step.

The more stable carbocation is typically formed and it is then attacked by the anion which was formed earlier when it was the leaving group. An alternative is for the carbocation to be attacked by any nucleophile which happens to be present in the mixture. This could be things like water or ethanol.

Now back to lipids

Some time ago I saw a British TV show on the subject of plastic surgery, it had a big jolly surgeon who explained how everything worked. He showed examples of good plastic surgery and also some very bad examples. He also each week showed the viewer how a different operation was done, not for the faint hearted !

I recall some years ago being shown some silicone by a chemist from AWE (Atomic Weapons Establishment, the British military nuclear organisation), he explained how it was the same grade of silicone as was used in breast implants. I know as a result of a series of law suits that Dow Corning are no longer making silicones for medical purposes, one of the alternatives which has been considered for breast implants is vegetable oil. I suspect that this is quite a nasty substance to be walking around with two bags of it in your body.

While the silicone gel is water insoluble, not very mobile and quite inert, the vegetable oil is not. I suspect that if oxygen can diffuse into the implant then the oil will slowly become rancid with time. If the bag bursts then it could suddenly dump a large volume of rancid oil into the body. I imagine that this would be very unhealthy !

I was reading recently about the degradation of FAME biodiesel recently, the paper explained how oxygen and light is very bad for biodiesel as it forms singlet oxygen which reacts with the sp3 carbon centres which are next to the alkene in an ene reaction. We will leave the ene reaction for another day, I suspect that inside a woman’s breast that it will be very dark which will retard the ene reaction even if oxygen diffuses into the implant. But I know that the oil will oxidise with time, some workers have found some horrible substances which are formed by reactions which involve the alkene groups.

The great problem is that a small trace of a free radical initator can start the process going, what happens is that a free radical attacks the CH2 unit next to an alkene unit. This is the allylic site. This then forms a radical which can react with oxygen to form a peroxyl radical. The peroxyl radical can react further in chain reactions which make more radicals and in cleavage reactions which form aldehydes. The oil inside implants which were never placed inside people but were stored at the factory under ideal conditions also showed signs of similar degradation suggesting that the container holding the oil was not totally impervious to oxygen. As this free radical chemistry is not part of the first year chemistry course at Chalmers I will not go into it in detail but if you want to read it, then I suggest that you look at the following site.

I think that the oil in the implants and biodiesel would be greatly improved by the addition of a small amount of BHT, this is food additive which prevents lipids going rancid by scavenging free radicals. Maybe another day I will tell you about BHT.

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