AU26 CHEM 2310 - Lecture 17 - October 22 - Quiz 7 - Sub vs Elim (needs updated transcript)

Overview

Dr. Jennifer Walters reviews CHEM 2310 Quiz 7 concepts by contrasting substitution and elimination, then develops E2 and E1 mechanisms, selectivity, and practical reaction-picking rules. The class applies strong or weak base character, substrate structure, bulky-base effects, temperature, and anti-periplanar geometry to predict products, including Hofmann and Zaitsev alkenes and possible carbocation rearrangements.

Key takeaways

For E2 versus E1, base strength is the fastest discriminator: strong bases favor one-step E2, while weak bases can accompany two-step E1 if the substrate forms a stable carbocation.
E2 is less restricted by substrate crowding than SN2 because the base removes a beta hydrogen rather than attacking the alpha carbon; more substituted substrates also offer more beta hydrogens.
Bulky bases such as tert-butoxide, diisopropylamide, and triethylamine tend to form the less substituted Hofmann alkene, while non-bulky bases tend to form the more substituted Zaitsev alkene.
E2 requires anti-periplanar alignment between the beta hydrogen and leaving group; in cyclohexanes, this generally means the leaving group must be axial.
Every E1 carbocation intermediate should be checked for rearrangement before product formation, because an alkyl shift can produce a more stable carbocation and change the resulting alkene.
When substitution and elimination compete, classify the reagent first, inspect the substrate next, and use temperature or solvent only as secondary clues.

Chapters

0:00 Class Check-In and Course Feedback

Jennifer Walters opens with a class check-in before discussing student feedback.
About 53% of students submitted feedback, which was generally positive.
Walters emphasizes that students should feel welcome, safe, and able to ask for support.

2:39 Practice Requests, Office Hours, and Course Resources

Students requested more worked practice problems, so Walters plans to include more recitation-style problem solving in lecture.
Walters lists Tuesday and Thursday office hours from 10:30 to 11:30 a.m. in Evans Lab, room 10:45, and encourages students to attend TA hours.
The Pearson access link provides the e-text, homework, extra practice, and Study Prep tailored to the course syllabus.
Students can use the quiz-retake period to catch up on Quizzes 1 through 6 and strengthen first-half material.

11:34 Reviewing SN1 and SN2 with Substrate and Reagent Clues

A tertiary leaving group rules out SN2 because the nucleophile cannot reach the crowded carbon in a concerted backside attack.
SN1 depends primarily on carbocation stability, while SN2 depends on nucleophile attack and steric accessibility.
For substitution problems, Walters recommends checking the electrophile first: primary typically favors SN2, tertiary favors SN1, and secondary requires additional clues.

17:22 Solvent Clues and the Shift from Substitution to Elimination

Neutral ethanol acts as a weak nucleophile and can also serve as a protic solvent; an O− reagent is a strong nucleophile.
Carbon tetrachloride is aprotic because it has no hydrogen attached to an electronegative atom.
A reagent such as sodium hydroxide can act as either a nucleophile, producing substitution, or a base, producing an alkene by elimination.

20:25 E2 and E1 Mechanisms: Concerted Versus Stepwise

E2 is a one-step reaction in which the base removes a beta hydrogen as the C-H electrons form a C=C bond and the leaving group departs.
E1 is a two-step reaction: the leaving group leaves first to form a carbocation, then a base removes a beta hydrogen.
The numeral in E2 describes the two reacting species involved in the rate-determining step, not two mechanism steps.

26:05 Why E2 Can React Across Primary, Secondary, and Tertiary Substrates

E2 can react with primary, secondary, or tertiary substrates, unlike SN2, because the base removes a beta hydrogen rather than attacking the crowded alpha carbon.
More substituted substrates generally provide more beta hydrogens, increasing the number of available elimination pathways.
E2 is the elimination mechanism that can act on an sp2 carbon substrate to form a triple bond.

30:13 Bulky Bases Select Hofmann Alkenes, Smaller Bases Select Zaitsev Alkenes

A bulky base preferentially removes a more accessible beta hydrogen on the less crowded side, producing the less substituted Hofmann alkene.
A non-bulky base can reach the more crowded side and generally produces the more substituted, more stable Zaitsev alkene.
Walters identifies tert-butoxide, diisopropylamide, and tertiary amines such as triethylamine as bulky-base examples.

34:26 E2 Stereochemistry Requires Anti-Periplanar Alignment

E2 requires the beta hydrogen and leaving group to be anti-periplanar, meaning they are 180 degrees apart.
With multiple beta hydrogens, rotation can often provide the geometry that leads to the more stable E alkene.
If only one beta hydrogen is available on the reacting side, its alignment determines the alkene stereochemistry.

39:36 Newman Projection and E2 Reaction-Coordinate Example

In the worked example, Walters views the leaving-group carbon as the front carbon in a Newman projection and rotates the back carbon to align the bromine and beta hydrogen.
Removing that hydrogen and bromine while forming the double bond produces a Z alkene because its highest-priority groups are on the same side.
The E2 transition state marks four simultaneous bond changes: forming O-H and C=C bonds, and breaking C-H and C-Br bonds.

43:07 Cyclohexane E2 Requires an Axial Leaving Group

In a cyclohexane chair, the leaving group must be axial to achieve the required anti-periplanar relationship with a beta hydrogen.
An equatorial bromine is aligned with a carbon bond rather than the needed hydrogen, so reaction may be very slow or not occur.
A chair flip can place the leaving group axial, but may also force other substituents, such as tert-butyl, into unfavorable axial positions.

45:50 E1 Selectivity and Carbocation Rearrangements

E1 favors substrates that form stable carbocations: tertiary is better than secondary, while primary and methyl substrates generally do not react by E1.
After a carbocation forms, check for rearrangements such as an alkyl shift that converts a secondary carbocation into a tertiary one.
E1 typically gives the most substituted Zaitsev alkene and favors the E alkene, whose larger groups are farther apart.

50:09 Choosing E1 or E2 from Base Strength and Substrate Structure

E2 requires a strong base because base removal occurs in the concerted rate-determining step.
E1 does not depend on base strength; its key requirement is a good leaving group and a substrate capable of forming a stable carbocation.
Both E1 and E2 can occur with secondary or tertiary substrates, while primary substrates are unsuitable for E1 but can react by E2.
Solvent is usually a weak discriminator because elimination is not selected by nucleophilicity, though protic solvents commonly accompany E1 and aprotic solvents commonly accompany E2.

55:24 Temperature and a First E1-versus-E2 Classification

A triangle or delta symbol indicates heating; when no temperature is shown, Walters assumes room temperature.
Higher temperature favors elimination, but base strength is the more reliable initial clue for distinguishing E1 from E2.
A negatively charged ethoxide reagent is a strong, non-bulky base, supporting E2; if it removes a proton, it can form ethanol, a protic solvent.

57:50 Small-Group Practice: Predicting Elimination Products

Students work through examples B, C, and D while Walters and David circulate to help identify base strength, bulk, substrate class, and product geometry.
The extended group exercise applies the distinction between strong-base E2 and weak-base E1 to several substrates.
For stereochemical cases, students must check whether a beta hydrogen and leaving group can adopt anti-periplanar alignment before drawing an alkene.

1:15:08 Reviewing Bulky-Base, Weak-Base, and Ring-Geometry Examples

Example B uses a bulky tert-butoxide base with a secondary leaving group, so E2 removes a more accessible hydrogen and forms an alkene at the end of the chain.
Example C combines a weak neutral oxygen base with a tertiary leaving group and heat, leading Walters to classify it as E1.
Example D uses a strong, non-bulky base and a secondary leaving group; rotation to anti-periplanar geometry yields an E alkene, though the adjacent methyl groups make it less favorable.

1:19:56 Combining Substitution and Elimination Decisions

When substitution and elimination compete, Walters recommends checking the reagent first, then the substrate, and finally using solvent or temperature as a tiebreaker.
The reagent framework separates strong nucleophile-only species, strong base-only bulky bases, strong nucleophile/strong base anions, and weak neutral oxygen reagents.
Nucleophile-only reagents indicate substitution, base-only reagents indicate elimination, strong/strong reagents favor concerted mechanisms, and weak/weak reagents favor stepwise mechanisms.
Walters plans to revisit the combined framework in the next class and let students vote on whether to keep or postpone the quiz.

Transcript excerpt

2:39 All Right. How is everyone doing today? Good. Good. Does anyone have any questions before we get started? All right. So, I wanted to summarize the feedback. So, I want to thank you all for uh filling it in or 53% uh filling it in. Um, and I just wanted to highlight a couple key parts of this. So, uh, overall I was glad that the general vibe was positive. So, that that made me really happy. Um, what I do want to point out though is that even though

3:27 there were only a couple people that were saying that they uh did not feel welcome or uh felt like neutral on being welcome. Um, this is something that's really important to me because like I feel like you need to feel comfortable and like that you belong and you are safe otherwise you can't learn and if you can't learn then I can't teach you and if you can't if I can't teach you then I can't do my job. So like it's really important to me that you know that you do belong here. Um so if there is anything that I can do to change this even for anybody or improve any part of anybody uh to make the class feel more like welcoming and uh safe, please do let me know. Even if it's like you tell David and David tells me or you write it

4:13 on a note and slip it under my door like or you can tell me directly. I'm also okay with that. But like I I really do want to make sure that everyone does feel safe and comfortable in class. Um so I just wanted to highlight that. Uh again, everything was overall positive. The big thing that uh kind of repeatedly

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Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Dr. Jennifer Walters.