Rules of the Day
Click here for a copy of my lecture notes from today's lecture
Click here for a copy of the handout used in class today
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. In mechanisms, arrows are used to indicate movement of electrons.
2. Arrows never indicate movement of atoms directly.
3. Arrows always start at an electron source and end at an electron sink. An electron source is a pi bond, lone pair, or bond that can break to avoid overfilling a valence, and an electron sink is an atom that can accept a new bond, often with a full or partial positive charge.
4. Breaking a bond will occur to avoid overfilling valence at the electron sink atom.
5. A nucleophile contains an electron rich pi bond or lone pair that is the electron source for a bond forming process. Analogous to a Lewis base.
6. An electrophile contains an electron deficient atom that serves as the electron sink in a bond forming process (Analogous to a Lewis acid) or possesses a weak bond that breaks to make a stable ion or fragment..
7. Most organic mechanisms are composed of only four different mechanistic elements, so that predicing mechanisms often comes down to a fourway multiple choice for each step. (Note these statements are written from the point of view of the carbon containing reactant, i.e. the alkene)A. Make a bond between a nucleophile (electron rich species with an electron source like a pi bond or lone pair) and an electrophile (electron deficient species that has an electron sink atom).
B. Break a bond to create stable molecules or ions
C. Add a proton
D. Take a proton away
8. Alkenes react with H-X (X = Cl, Br, I) in a mechanism that involves initial protonation of the pi bond (add a proton) to give a carbocation intermediate, that then adds X- (make a bond between a nucleophile and electrophile) to give a haloalkane. Click here to see the reaction movie of H-Br adding to propene.
9. 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.
10. 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).
11. (Time capsule, you will understand this after next Tuesday's 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.
12. 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.
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. Together, they will count as 5% of your final grade.