Reading: Finish reading Ch. 9 if you haven't already HW - finish smartwork above and begin working on competition problems from the Ch. 9 Handout.

Transcription

1 Ch. 8 Smartwork available - due 10 AM - don't forget to check for carbocation rearrangements! Ch. 9 Smartwork available - due Saturday 11:59 PM Final Exam (Monday 8 AM here) will be comprehensive/cumulative - slightly heavier on the Ch. 8/9 material than on other material. The exam is written by me and is up to 50 multiple choice questions (MC only). Some Ch. 8/9 old test questions available on Exam website. Be sure to utilize the Study Guides available on the website. Model kit allowed. No calculator needed. Last SI Sessions tonight & tomorrow night. Final Exam Cram is Wed 12/6 from 7-9 PM in Wiggins library 3rd floor My office hours this week: Today 12-1 PM, Tuesday 12-1 PM, Wednesday 1:30-2:30 PM, Friday 11 am - 12 pm, or by appointment. Reading: Finish reading Ch. 9 if you haven't already HW - finish smartwork above and begin working on competition problems from the Ch. 9 Handout.

2 The First Part of the Frontier Orbital Interactions of an E2 Step Ref: Karty, Interchapter 1, p.394

3 The Second Part of the Frontier Orbital Interactions of an E2 Step Ref: Karty, Interchapter 1, p.394

4 Basic Conditions

5 Reasonable Mech. In Basic Cond.

6 Acidic Conditions

7 Reasonable Mechanism for Acidic Cond.

8 Favor E2, Hoffman product Strong, Bulky Bases

9 Water Leaving Group Mechanism The leaving group leaves as H 2 O, not HO. Being uncharged, H 2 O is much more stable than HO, and is an excellent leaving group.

10 S N 1 Mechanisms/Rate Determining Steps What favors each mechanism? S N 2 E1 E2

11 Chapter 9 Cheat Sheet Nucelophile vs. Base: Great Leaving Groups: Rule of thumb - the weaker (more stable) the base, the better the LG ALSO WATER AND ALCOHOLS OR OTHER NEUTRAL MOLECULES ARE GREAT LEAVING GROUPS Solvents:

12 Chapter 9: Competition between S N 1, S N 2, E1, and E2 A decision pathway 1. First consider the substrate. If the carbon with the leaving group is: a. Methyl (0 ), the reaction will be S N 2. b. Primary (1 ), most likely S N 2. Exceptions: a) if a strong, bulky base, E2 would be favored. b) if allylic or benzylic primary, then could be S N 1 or E1. c. Secondary (2 ) can t rule any mechanism out, but usually S N 2 or E2 unless the base or nucleophile is a weak one, though allylic or benzylic may encourage more S N 1/E1. d. Tertiary (3 ), rule S N2 out (too sterically hindered for S N2), but all others are fair game. Consider base/nuc strength and temperature. Other substrate-related issues: S N2 is fastest with the LEAST substituted substrates fastest with 0 or 1 (sterics) E2 is fastest with 3 substrate because of + charge buildup at the carbon during the transition state. S N 1 and E1 are fastest w/ tertiary, benzylic or allylic substrates. 2. Second, consider the quality & strength of the other reagent (the nucleophile or base the reagent that is not the substrate w/leaving group). Note that some reagents can act as nucleophiles or bases. a. General Nucleophile Strength: Stronger Nucleophiles favor S N 2 (PUSH off the leaving group). Weaker nucleophiles favor S N 1. Nucleophile strength is solvent-dependent b. Base Strength: We are familiar with base strength from Ch. 6 - review the pka Chart as necessary. Strong bases favor E2 (attack the substrate with the leaving group still attached) over E1. Weak bases favor E1. The bases below, or any other very strong, bulky bases tend to favor E2 and the Hofmann (less substituted) product over the Zaitsev (more substituted) product (regiochemistry effect)

13 3. If there is a reagent that can act as both a strong base and a strong nucleophile, and the substrate is secondary (2 ), the reaction will likely favor the E2 mechanism, unless a low or moderate temperature is indicated (more on temp next). 4. Consider temperature. If all else is equal, heat favors elimination over substitution. ( S rxn is more positive for an elimination reaction than for a substitution reaction.) 5. Consider the Solvent. 6. Consider the Leaving Group (LG). The best leaving groups are neutral molecules or are stable with a negative charge. S N 1 and E1 reactions need good leaving groups. S N 2 and E2 reactions can work with good or moderate leaving groups. All reactions work better with better leaving groups, though. 7. Consider the concentration. A high concentration of the base/nucleophile favors the bimolecular reactions (S N 2 and E2) because the base or nucleophile are part of the rate law for these reactions. Low concentration of the base/nuc favors the unimolecular reactions (S N 1 and E1), because the base or nucleophile are not part of the rate law for these reactions. Concentration is not very often indicated, so it is a little unusual to use it as a determining factor.

14 Organic Chemistry I - Ch. 9 S N 2/S N 1/E2/E1 Competition Problems Draw the Major Product(s) for each reaction, given correct stereo- and regio- chemistry, as appropriate 1) 2) 3) 4) 5) 6) 7)

15 8) 9) 10) 11) 12) 13)