Rules of the Day CH320M/328M

Featured Golden Rules of Chemistry: 8. Reactions will occur if the products are more stable than the reactants (motive) and the energy barrier is low enough (opportunity).10. A reaction mechanism describes the sequence of steps occurring during a reaction. 11. Most bond-making steps in reaction mechanisms involve nucleophiles reacting with electrophiles.

1. Carbocations (carbon atoms with a positive charge) are sp2 hybridized, with an empty 2p orbital. They are stablilized by hyperconjugation (adjacent sigma bonds from attached alkyl groups overlap and share electron density with empty 2p orbital) and inductive effect (a carbocation is more electronegative than other carbon atoms [it wants electron density to 'quench' its charge], so attached carbon atoms will share some electron density through sigma bonding network.) Bottom line: carbocations with more alkyl groups attached are more stable (3°>2°>1°>methyl.

2. Because of the electron density of the pi bond, alkenes react as nucleophiles to make new bonds with molecules containing high partial positive charge (electrophiles) or protons (add a proton).

3. Regiochemistry is the analysis of which of the possible constitutional isomes (also called regioisomers) are made in a reaction

A. In the H-X addition reaction, the regiochemistry of the predominant product has the H ending up on the C atom of the alkene with more H atoms, and the X ends up on the C atom of the alkene that has fewer H atoms (i.e. more C atoms attached), a prediction that is referred to as Markovinikov's rule.

B. Markovnikov's rule is another way of saying the reaction predominantly follows the path that involves the more stable (more highly substituted) carbocation.

4. Alkenes that are not chiral can sometimes react with H-X to create a chiral center. In those cases a racemic mixture is formed because the nucleophiles (i.e. X-) can react with the carbocation intermediate from either face with equal probability. Stereochemistry is the analysis of which of the possible stereoisomers are made in a reaction.

5. (Time capsule, you will understand this after the next lecture) Stereochemistry is the analysis of which of the possible stereoisomers are made in a reaction.

A. In the H-X addition reaction, the stereochemistry is mixed (i.e. the H and X can end up on either the same or opposite sides of the original double bonds) because the X- can add to either face of the flat carbocation intermediate. If a new chiral center is created, a racemic mixture of enantiomers is produce.

6. Alkenes can undergo acid-catalyzed hydration (i.e. addition of H2O) involving a mechanism similar to that for H-X addition.(Markovnikov regiochemistry, stereochemistry mixed due to flat carbocation intermediate).

7. Carbocation intermediates can rearrange if migration of an H atom ,or less often a methyl group, will create a more stable carbocation. This is relatively rare, but can occur in certain situations. This is not a new effect, it is simply hyperconjugation taken to the extreme.

HOMEWORK:

If you have not already, you must Watch the Terpene video. You are responsible for this information for the next midterm!

Read: Section 6.4B,C in the eBook.

Take the Daily Quiz 10 before 10 PM tomorrow. Click here to access the quiz .These quizzes are designed to review the important material from today's lecture.

Start Working on Homework Problem Set 5, due at 10 PM on Wednesday, October 7.

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