the topic of this lecture will be oxymercuration Deemer curation of alkenes oxymercuration Deemer curation is a method of hydrating an alkene adding water across the double bond however unlike acid catalyzed hydration oxymercuration de mer curation does not go through a carbo cation intermediate therefore there is no potential for rearrangements with this reaction let's look at the general reaction for the oxymercuration d recreation of alkenes in the first step water is added to the double bond using the mercury catalyst to produce the mercury cane containing intermediate in the next step sodium borohydride is used to reduce the mercury intermediate this gives us the final hydrated alkene product notice that I have drawn the addition of water adding anti to the double bond the hydroxyl group adds to one side of the double bond than the proton to the other this reaction is stereospecific we always get the anti product and to understand why we need to look at the mechanism first let's look at the mercury catalyst the structure of Mercer mercury diacetate looks like this I will continue to show the lone pair on Mercury because we're not used to seeing the structure of mercury in the first step of the mechanism this mercury diacetate loses an acetate group this produces the active form of the catalyst along with another acetate group the mercury cation that is formed then adds to the alkene the pi bond of the devil and first the pi bond attacks the mercury carbo cation i'm sorry the mercury cation then the mercury lone pair attacks back to form a cyclic intermediate it is this electrophilic cyclic intermediate that is attacked by water in order to hydrate the double bond water attacks one of the carbons of the cyclic intermediate kicking off the mercury this is this step the dirt determines the stereo specific nature of this mechanism because the water has to act attack opposite from the mercury in order to attack the carbon of the cyclic intermediate and this is why the mercury and the water end up opposite one another on the molecule finally another molecule of water will deprotonate to form the stable mercuric intermediate finally sodium borohydride is used to remove the mercury and replace the mercury carbon bond with a hydrogen carbon bond you don't need to understand the mechanism for this step now let's look at a specific example of this reaction let's consider this molecule if we treat this molecule with mercury diacetate and mother followed by sodium borohydride we will get a single product the markovnikov product we will not get the less substituted alcohol or the product that rivers result from a carbo-cation rearrangement again this will make more sense once we look at the mechanism in the first step the mercury diacetate ionizes to produce acetate ion and the mercury Kidani the mercury cation that is formed is then added to the alkyne to form the cyclic mercury intermediate it is a cyclic intermediate that is attacked by water is the nucleophile and is at this point that the regional chemistry of addition is determined the carbon which is attacked by the water as a nucleophile is the carbon that will later contain the hydroxyl group water attacks the more substituted carbon of the cyclic intermediate because this carbon can hold a larger present partial positive charge due to its additional substitution in other words it has more carbo cadion character because it is more substituted and can better handle the positive turk therefore the water attacks the more substituted carbon giving this protonated alcohol intermediate next another molecule of water d protonates the protonated alcohol in the final step sodium borohydride is used to remove the mercury and to produce the final alcohol product because we only get the markovnikov product and not the other possible alcohol products we say that this reaction is regiospecific a single video isomer is formed in this reaction additionally this reaction is stereospecific only certain stereo isomers of the product are formed because the hydroxyl group and the proton that are added across the double bond add in an anti-fashion let's look at an example let's consider the oxymercuration team recreation of this molecule oxymercuration of this molecule produces only two of the four possible stereoisomers of the product in both cases the water adds anti to the mercury producing a set of enantiomers following reduction and removal of the mercury we get two possible products again this product with a pair of enantiomers and in both cases the methyls are trans to one another we can understand why we get only the two enantiomers if we look at the mechanism in the first step of the mechanism we see the mercury add to the alkene double bond attacks the mercury can followed by back attack by the lone pair of electrons on the mercury to form the cyclic intermediate the cyclic intermediate can occur from the top of the ring or the bottom if attack of mercury occurs from the top we get this intermediate if the attack of mercury on the alkene occurs from the bottom we get this isomer attack of water on these intermediates must occur anti to the mercury attack of water on the great carbon of the top structure needs to this intermediate attack on the right carbon of the bottom structure leads to this intermediate deprotonation by water leads to the structures above you may find it helpful at this point to pause the video and make a model of each of these structures but as in this way that this reaction is stereospecific by hydroxyl group and the proton that are added across the alkyne always add anti