Even More Additions to C=C !!!!

We can't get enough of this addition of various species to alkenes. Yesterday was pretty much the final installment -- addition of H2, ozonolysis and di-hydroxylation were examples of reactions that we just have to know, without mechanistic details. One reaction in which the mechanism is approachable was the radical synthesis of some classes of polymers (we used polystyrene as an example).

A new podcast has been posted. It is a short-form of generic additions of various electrophiles to alkenes.

The image is that of an ozone generator. It is similar to the one that I used to destroy the rubber tubing many years ago.

More Alkenes as Nucleophiles

We've continued to look at alkenes as nucleophiles but we have made the electrophiles more complicated than simple HX. Our examples of electrophiles include X2, Hg(OAc)2 and BH3. In those cases, we use available electrons to swing back down to the developing carbocation and eliminate the positive charge on the carbon. In the case of BH3, this is relatively straight forward and takes us to a product in which we have added H-BH2 across a C=C. In the other two cases we make a three-membered ring with a positive charge and we must open the ring by reaction with a nucleophile. Note that the nucleophile must attack the three-membered ring from exactly the opposite side of the leaving group (Br or Hg), so there can be stereochemical implications.

Alkenes as Lewis Bases/Nucleophiles

Yesterday we looked at how alkenes can react as nucleophiles. The reasoning behind this activity is due to the fact that the pi electrons are not trapped between the nuclei, as are electrons in a sigma bond, but are out in a more open space (remember--side to side overlap of the fat parts of p-orbitals). As a result, the pi electrons can act somewhat like a lone pair of electrons.

The reaction that we looked at was the reaction of alkenes with HBr. It is a two-step process, leading to an intermediate cation which is then attacked by the generated bromide ion. The entire process is just the reverse of an E1 reaction.

The regiochemistry of the reaction is governed by Markovnikov's Rule -- the cation formed is the more stable of the two possibilities. Note that the intermediate cation has an unhybridized p-orbital so the bromide can attack from either side; this can have implications for stereochemistry.

Problem Set 7 and its key have been added to the list of useful links. At this point, we have only covered how to do reaction series (a).

The picture is of Vladimir Markovnikov.

Alkenes

Yesterday was a day to start talking about alkenes. We spent most of the time discussing the physical chemistry of the C=C bond -- how it consists of an end-to-end bond and a side-to-side pi bond. I used the example of a pair of forearms and showed how they had to be parallel in order for the bond to exist; as a result it is nearly impossible to rotate around a C=C bond. We also saw the basis for Zaitsev's rule. Essentially, it is hyperconjugation that is responsible for the increased stability of more highly substituted alkenes. At the end, we saw a foreshadowing of the chemistry that we will discuss, the ability of the alkene to act as a Lewis base.

Modeling #7 has been posted; molecule slips will be available tomorrow.

New Postings

I've added the key to exam 2, along with Need to Know links for Chapters 6, 7, 8, and 12. I hope your spring break is off to a great start!

See you next Monday.

Work, work, work!

Today we worked pretty hard on being able to determine which reactions are possible for a given set of conditions. All of this is based on looking at the Lewis acid (the compound with the leaving group) and the Lewis base (the nucleophile or base). Once the determination is made which reaction(s) is/are possible, all that remains is to draw the arrows.

We will continue to practice drawing reactions on Wednesday.

Some Additions to the Links......

To the left there are some new items -- Problem Set 6 (Eliminations) and its key, along with a key from last year's exam #2. If you want some more practice with substitution reactions, go to this site for a problem set that Professor Crouch has used in the past. The key is here.

Elimination Reactions

Today we covered both the E1 and E2 reactions. In both cases, we use a Lewis base to abstract a proton from the substrate (at a carbon next to the carbon with the leaving group) and form an alkene. There are differences, however: The E2 is a single-step reaction and the stereochemistry of the product is determined at the beginning of the reaction. The E1 reaction is two steps and the presence of the intermediate allows for C-C bond rotation, therefore the stereochemistry is not set until the second step of the reaction.

We started to look at how to determine which of the four possible reactions (SN1,SN2, E1, E2) would take place under a particular set of reaction conditions, with the promise of more to come next Monday and Wednesday.

Over the weekend a copy of last year's exam will be posted, along with a podcast of the material from today's lecture and an elimination problem set. I am also hoping to get some more substitution problems posted to give you more practice with drawing arrows.

The picture shows what it is all about -- a pair of electrons is moved according to the direction of an arrow, from Lewis base to Lewis acid.