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
Watch this video that explains terpenes. This material will be on the next midterm!
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).
1. Alkenes have one sigma bond and one pi bond, The reactivity of alkenes is based on the pi bond. Click here for a picture that helps you remember pi bonds.
2. The most important consequences of the pi bond are that A) the double bond cannot rotate and B) electron density is above and below the bond axis.
3. Stereoisomers of alkenes with a single substituent on each carbon atom of the double bond are named as "cis" (same side) or "trans" (opposite side).
4. Use the "E-Z" nomenclature for naming complex alkenes. Remember that "Z" means "Zame Zide". When establishing priority for for E vs. Z, it is the same as for R and S. Higher atomic number wins and count multiple bonds as being equivalent to that same number of bonds to the atoms taking part.
5. Trans alkenes are more stable than cis alkenes because cis alkanes have some non-bonded interaction strain. In addition, more highly substituted alkenes are more stable than less substituted alkenes (we don't really tell you why, just learn it.)
6. Terpenes are complex natural molecules that are built from 5 carbon "isoprene" units in Nature, especially plants. Watch the video. You are responsible for this information for the next midterm!
7. 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 an haloalkane (alkyl halide). Click here to see the reaction movie of H-Br adding to propene.
8. Reactions are like crimes, they need motive and opportunity.
9. Heating up a reaction increases the rate by increasing the number of molecules with enough energy to get over an energy barrier.A. A reaction has a motive if the products are lower in energy than the starting materials. That is, stronger bonds are made than broken in the reaction, and/or a weaker acid/weaker base is produced, and/or entropy is increased through the creation of a small gaseous fragment. Motive (also called thermodynamic driving force) determines position of equilibrium.
B. A reaction has an opportunity to take place if the mechanism contains no species that are higher in energy than the energies of a resonable fraction of starting material molecules at a given temperature. Opportunity determines rate of a reaction (also called reaction kinetics).
10. In mechanisms, arrows are used to indicate movement of electrons.
11. Arrows never indicate movement of atoms directly.
12. 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.
13. Breaking a bond will occur to avoid overfilling valence at the electron sink atom.
14. 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.
15. 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..
16. 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
17. 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.
18. 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).
19. 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:
You must Watch the Terpene video. You are responsible for this information for the next midterm!
Read: Sections 5.1-5.4, 6.1-6.4A in the eBook.
Take the Daily Quiz 9 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.
Click here to go to the Aktive Homework Problem Set 5
Click here to get a copy of the Gradescope Homework Problem Set 5
