|Lecture Notes: 14 June||
Hydration of Alkenes (addition of water to alkenes). We looked at this reaction last time:
Recall that in aqueous situations, that the proton is considered to be a hdrodium ion as per the mechanism shown in your text.
Addition of Halogens to Alkenes (halonium ion intermediates): Halogen molecules can also add across a double bond. In fact the reaction with bromine or iodine is often used to determine the presence of double bonds - disappearance of the red or violet color indicates the presence of double bonds. At first glance we might expect the same reaction mechanism we have seen before, and in fact there would be no reason to suspect a difference when looking at the reaction with linear alkenes - a dihaloakane results as expected. But if we try the reaction with a cyclic alkane, something weird happens - only the trans product results (anti stereochemistry)!
So what's going on? We must not be forming a carbocation intermediate, which should give little or no stereospecificity since a carbocation with sp2 hybridization should be open to attack on either side. Instead we believe the intermediate is a Halonium ion, as seen in Figure 4.5 of your text on pg 111. This intermediate is cyclic, and blocks one side so the product must be trans.
Hydrogenation of Alkenes: The catalytic reaction gives syn stereochemistry (same side attack). Hydrogen will attack a double bond in the presence of a catalyst. In this case the two hydrogens attack on the same side (syn stereochemistry). (This can be determined by using deuterated alkenes and hydrogen or vice versa.) Commonly this process is catalyzed by metals, such as palladium on charcoal. Apparently both the alkene and the hydrogen molecule adsorb onto the catalyst next to each other. The adsorption weakens the pi bond of the alkene and the sigma bond of the hydrogen so that an attack can occur.
Most of the reactions we've looked at so far produce racemic mixtures when they produce chiral compounds. This is because they go through achiral intermediates, such a carbocations, which have three substituents and are flat, so they may be attacked from either the top or the bottom. Thus for the bromination of 1-Butene we see the formation of a "single" product 2-Bromobutane (by Markovnikov's Rule), until we look at the stereoisomerism, when we see it is a racemic mixture of two chiral compounds:
As a general rule, we can't make chiral products from achiral starting materials. Note that chiral molecules are frequently made, they are just part of a racemic mixture. Note also that chiral catalysts can cause a selection of a single isomer, giving a chiral product - this is what happens in biological systems.
Oxidation of Alkenes:
Alkene Polymerization by Radical Reactions. Use a radical forming catalyst such as Benzoyl peroxide to initiate radical formation from the alkene monomer:
The radical (In .) will now react with an alkene double bond to form another radical etc.:
This reaction may now propagate for up to thousands of times to give the polymer. Finally the reaction can terminate by reactions which consume the radical, such as two radicals reacting with each other.
Last modified 15 June 2004