AU26 CHEM 2310 - Lecture 15 - October 13 - Quiz 7 - Sub vs Elim (needs updated transcript)

Overview

Dr. Jennifer Walters reviews quiz 5 aromaticity, then introduces quiz 7’s substitution-versus-elimination unit, focusing on nucleophile, base, and leaving-group trends before explaining SN2 and SN1 mechanisms. The lecture contrasts one-step SN2, with backside attack and inversion, against two-step SN1, with carbocation formation, racemization, and possible rearrangements; elimination is introduced as formation of an alkene but its mechanisms are deferred to the next lecture.

Key takeaways

SN2 is a one-step, bimolecular process whose rate depends on both substrate and nucleophile; backside attack causes inversion, and tertiary substrates are too crowded for this pathway.
SN1 is a two-step, unimolecular process whose rate-determining step forms a carbocation; it favors tertiary substrates, can racemize stereocenters, and requires a good leaving group.
Protic solvents hydrogen-bond to anionic nucleophiles and weaken them, while stabilizing ions and carbocations; this generally favors SN1 over SN2.
Nucleophilicity differs from basicity because steric bulk affects access to carbon, and solvent can reverse halide nucleophile rankings: small halides tend to be favored in aprotic solvent, large halides in protic solvent.
A carbocation mechanism requires an immediate stability and rearrangement check; a hydride shift may occur before nucleophilic attack if it produces a more stable carbocation.
Quiz 5 aromaticity classification yielded 5 aromatic, 4 non-aromatic, and 0 anti-aromatic structures; lone pairs count only when positioned in a p orbital and their participation avoids unfavorable anti-aromaticity.

Chapters

0:00 Class Opening and Quiz 5 Performance

Dr. Jennifer Walters checks microphone audibility while noting that she is still sick and losing her voice.
Quiz 5 scores remained historically high, though slightly below quiz 4, prompting students to compare how they prepared.

7:30 Quiz 5 Aromaticity: Counting Electrons and Testing Conjugation

Aromatic molecules must be cyclic and fully conjugated; structures A and E are non-aromatic because they contain sp3 carbons that interrupt conjugation.
Structure B avoids anti-aromaticity because its lone pair does not participate in resonance; structure C is aromatic with 6 pi electrons, and benzene D has 6.
For the bicyclic structure F, count the outermost loop and include nitrogen’s lone pair to reach 10 pi electrons; acyclic G is non-aromatic.
Thiophene H contributes one sulfur lone pair to a 6-pi-electron system; in I, only the bottom nitrogen’s lone pair enters the aromatic count, also giving 6.

18:20 Quiz 5 Answers and Fall Break Deadlines

The reviewed set contains 0 anti-aromatic molecules, 4 non-aromatic molecules, and 5 aromatic molecules.
Four structures are aromatic with 6 pi electrons, while one is aromatic with 10 pi electrons.
Homework is due Thursday; quiz 6 opens Friday at 5 p.m. and closes Sunday at 11:59 p.m., with quizzes 1 through 6 available for retakes October 23 to 25.

21:30 Quiz 7 Introduces Substitution and Elimination

Quiz 7 is the final unit on new reaction classes, with homework due October 29 and the quiz open October 30 through November 1.
Substitution replaces one functional group with another; elimination converts a substrate into an alkene by forming a pi bond.
The central challenge is predicting both the product and whether substitution proceeds by SN1 or SN2, or elimination by E1 or E2.

24:00 Reviewing Basicity to Predict Nucleophile Strength

Walters reviews five acid-base factors: charge, atom size or electronegativity, resonance, induction, and hybridization.
Strong bases favor negative charge, smaller and less electronegative atoms, less resonance and electron withdrawal, and lower s character.
The general rule is that a strong base is often a strong nucleophile, but nucleophilicity has additional steric and solvent effects.

26:00 Steric Hindrance and Solvent Effects on Nucleophiles

Nucleophile size matters because it must reach a carbon: a primary alkoxide is generally more effective than a secondary one, which is more effective than a tertiary one.
For nucleophilicity, steric access is distinct from basicity: bases abstract exposed hydrogen atoms, while nucleophiles must approach a crowded electrophilic carbon.
In protic solvents such as water, hydrogen bonding forms a solvent shell around anions and weakens their attack; larger halides such as iodide are less tightly solvated.
Halide nucleophilicity reverses with solvent: smaller halides are generally more nucleophilic in aprotic solvent, while larger halides are favored in protic solvent.

29:00 Common Nucleophiles and Leaving-Group Rankings

Common nucleophilic atoms include halogens, oxygen, nitrogen, sulfur, and negatively charged carbon; sulfur is described as a particularly effective nucleophile.
Negative charge without resonance often indicates a strong nucleophile; resonance-stabilized anions and amines are moderate, while neutral oxygen nucleophiles are weak.
Good leaving groups are stable after departure: examples include water, amines, iodide, bromide, chloride, and sulfonates.
Mesylate (OMs), tosylate (OTs), and triflate (OTf) are strong sulfonate leaving groups; triflate is especially effective because of resonance and the electron-withdrawing CF3 group.

36:00 Poor Leaving Groups and the SN1/SN2 Reaction Framework

Fluoride is treated as a poor leaving group in this course; hydroxide is also poor unless the starting material already has an O− group or conditions are strongly basic.
Substitution always exchanges a leaving group on an electrophilic substrate for a nucleophile.
SN2 means two molecules participate in the rate-determining event, not two steps: bond formation and leaving-group departure occur together.
SN1 has one molecule in its rate-determining step and proceeds in two stages: leaving group departure, then nucleophile attack.

43:00 SN2 Backside Attack, Rate, and Stereochemical Inversion

SN2 is concerted and has a rate proportional to both substrate and nucleophile concentrations.
The nucleophile attacks from the backside to access the carbon-leaving-group antibonding orbital, causing inversion of configuration, commonly described as R to S or S to R.
Substrate crowding slows SN2: methyl is most favorable, followed by primary and secondary substrates; tertiary substrates do not undergo SN2 in this course.
Additional acid-base steps can surround the SN2 event, but the substitution step itself remains a single concerted step.

47:30 SN2 Substrate Sterics and the Mid-Lecture Break

Walters illustrates electrophile crowding with an attempted hug: groups surrounding the reactive carbon obstruct nucleophilic access, just as steric bulk obstructs SN2.
A methyl or primary substrate offers the least obstruction, while a tertiary carbon is too crowded for the required simultaneous attack and bond cleavage.
The class pauses for a five-minute break; students share fall-break plans before the lecture resumes.

52:00 Break Conversation and Campus Wi-Fi Reminder

During the break, students mention plans including travel, work, hiking, rock climbing, and visiting family or pets.
Walters reminds students to reconnect phones, laptops, and tablets to eduroam through the university wireless instructions before a campus Wi-Fi update.
The lecture resumes by restating that SN2 has one concerted step, whereas SN1 has two steps: leaving group departure followed by nucleophile attack.

1:03:30 Acid-Base Steps Around SN2 Reactions

A poor OH leaving group can be activated by acid: protonation converts an alcohol into an oxonium ion that can depart as water.
With a neutral nucleophile such as water, SN2 attack displaces bromide in one step, then a separate deprotonation gives the neutral product.
In the water-and-bromide example, water is the better base than bromide because bromide is the conjugate base of the strong acid HBr.
A reaction coordinate diagram places reaction progress on the x-axis and energy on the y-axis; the SN2 transition state shows partial bonds forming and breaking.

1:11:00 SN1 Carbocations, Racemization, and Substrate Stability

SN1’s rate-determining step involves only the substrate: the leaving group departs first to produce a carbocation.
The trigonal-planar carbocation can be attacked from either face, so a chiral center can yield both enantiomers and lose its original stereochemical specificity.
SN1 favors tertiary substrates over secondary ones because tertiary carbocations are more stable; primary and methyl substrates do not form sufficiently stable carbocations for this pathway.
SN1 commonly accommodates weak nucleophiles because nucleophile strength does not affect the rate-determining carbocation-forming step.

1:17:00 SN1 Activation, Deprotonation, and Carbocation Rearrangements

As with SN2, acid can protonate an alcohol before SN1 so that water becomes a good leaving group.
When water attacks the SN1 carbocation, it initially forms an oxonium ion; a later acid-base step produces a neutral alcohol.
Every carbocation requires a rearrangement check before nucleophilic attack; a hydride shift can move the positive charge to a more stable carbon.
The first SN1 step, leaving-group departure and carbocation formation, is the rate-determining step; later attack and proton transfer are faster.

1:21:00 Choosing SN1 or SN2 by Substrate and Nucleophile

SN1 depends on carbocation stability and a good leaving group; SN2 depends on steric access and a strong nucleophile.
Substrate patterns differ: methyl and primary favor SN2, tertiary favors SN1, and secondary substrates can follow either route depending on conditions.
SN2 produces inversion of configuration, while SN1 proceeds through a planar carbocation and can produce racemization.
Both mechanisms require a good leaving group, but SN1 is especially dependent on departure because that step must form the carbocation.

1:23:00 Solvent Effects and the SN1/SN2 Comparison

Polar protic solvents such as water, alcohols, and thiols favor SN1 by stabilizing ions and carbocations, but their solvent shells weaken nucleophiles and disfavor SN2.
Aprotic solvents favor SN2 by leaving nucleophiles more available; examples include DMSO, acetone, acetonitrile, ethyl acetate, dichloromethane, carbon tetrachloride, and ethers such as THF.
Walters ranks solvent as the final factor when choosing a mechanism, after substrate structure, nucleophile strength, and leaving-group ability.
The lecture ends with a mechanism checkpoint: SN1 has two steps and accepts weak nucleophiles; SN2 has one step and requires a strong nucleophile.

Transcript excerpt

1:59 Mhm. >> All right. How is everyone doing today?

5:21 Good? All right. Can everyone in the back hear me okay? Cuz I know I'm losing my voice. Move that a little bit closer. Is that Is that better in the back? Is it doing anything at all? I'll try turning it up. Is that Is that better for everyone in the back? Can you hear me okay? No? Still nothing? Is that better? Okay. All right. How is everyone doing today? All right. Got a couple goods, and okay.

6:08 All right. I am, as you can probably hear, uh still sick. Uh although it sounds worse, I actually feel better, which I feel like is how all illnesses go. Um so, that's good. >> [snorts] >> Um but, I do apologize that my voice is this right now. Uh hopefully you can still hear me okay, and I will try to uh turn the microphone off before coughing um as I will periodically cough uh to make sure that um I'm not coughing into all of your ears. All right. So, quiz five uh was still really good. Not quite as good good as quiz four. Um so, hopefully again, we want to be reflecting on then what worked well in

6:57 preparing for the quiz this time versus last time. What is different to make sure that you know how you are learning best with the content. Um but, still amazing average. Like, still like, groundbreaking historical highs. Um the most missed question was some variation of this. It might not This particular version was the one that had the lowest score. You may not have had these structures. You may not have been asked this question. But, it was something along the lines of trying to identify if out of this large group of

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Related notes

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.