Rules of the Day CH320M/328M

Featured Golden Rules of Chemistry: 5. Delocalization of charge over a larger area is stabilizing. 8. Reactions will occur if the products are more stable than the reactants (motive) and the energy barrier is low enough (opportunity).

1. The biochemical literature uses the classic designations of "D" and "L" for key molecules such as amino acids. The common 19 chiral amino acids are all "L". Rather than a reference to rotation of plane polarized light by these structures, this designation refers to a structural connection to L-glyceraldehyde.

2. Acid strength is described by the quantities Ka and pKa. The stronger the acid, the lower pKa.

3. In an acid-base reaction, equilibrium favors formation of the weaker acid-weaker base (side with higher acid pKa value).

4. For acids that deprotonate to give an anion conjugate base, the stronger acid will produce the more stable anion conjugate base. In other words, when analyzing relative acid strength, compare the relative stabilities of the anion conjugate bases.

5. Use five rules when determining relative stabilities of anions (i.e. when you are predicting relative acid strength) These five rules are actually application of the following two principles: 1) negative charge is neutralized by nuclear positive charge and 2) delocalizing negative charge over a larger area is stabilizing.

a) Across a row of the Periodic Table, negative charge on a more electronegative atom is more stable. (Principle 1)

b) Down a column of the Periodic Table, larger anions are more stable than smaller ones. (This is more confusing than you think, so make sure you understand it). (Principle 2 dominates here)

c) Hybridization: The more S orbital character of the hybridization on the atom that has the negative charge, the more stable the anion (stability of anions is in the order sp>sp2>sp3) Principle 1)

d) Charges distributed over more atoms are better. (Nature hates isolated charges, so delocalizing a charge around by resonance is very stabilizing) (Pinciple 2)

e) Inductive Effect: Electronegative atoms such as F on atoms adjacent to the atom(s) with the negative charge will pull some of the charge away, thus spreading it out and leading to stabilization. (Principle 1 and 2)

6. To say it in a new way, in an acid-base reaction, equilibrium favors the side opposite that with the stronger acid, meaning the side with the more stable anion is favored.

HOMEWORK:

Read: Sections 4.1-4.7 in the eBook.

Take the Daily Quiz 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 3% of your final grade.

Start working on the Homework Problem Set 5, due at 10 PM on Tuesday, October 8. Click here to access the Homework Problem Set 5. Note there are Aktiv Learning and Gradescope Questions, and you MUST DO BOTH. Collectively, homeworks count for 10% of your final course grade. The Aktiv Learning homework provides multiple attempts and provides feedback. It is intended to help you prepare for the Gradescope Questions, so we recommend you do the Aktiv Learning questions first.