Exam 4 Review

Why are we still here? Just to suffer?

What is required for SN₂ Reaction to Occur?

sp³ hybridized carbon (no pi bonds)

Me or 1^o carbon is ideal

2^o carbon can work

3^o carbon will NOT work

Can SN₂ Happen with Vinyl and Aryl Halides?

NO! They are not sp³

How to tell if cyclohexane ring can do SN₂

One of the 2 chair conformations must have the leaving group be AXIAL and opposite of new triangle

How to tell if reaction is radical by looking at the equation?

If the reaction has light (hv) on top of the arrow, then it is radical and needs fishhook arrows

What Makes a Good SN₂ Substrate?

The Size

Me - LG (Best)
1^o C - LG (Works)
2^o C - LG (Meh)
3^o C - LG (Never Works)

What Makes a Good SN₂ Nucleophile?

Look for Alkali Metals
- whatever the alkali metal is paired with, is the nucleophile
- alkali metal ions are spectators, you don’t even need to write them

Negative Formal Charge is Better than Neutral
O^- > O

Strong Bases

What Makes a Good SN₂ Leaving Group?

OTs

Weak Bases

Basically things that can Stabilize a Negative Charge

What Makes a Good SN₂ Solvent?

Polar Aprotic

DMF, DMSO, MeCN, HMPA

Draw the Transition State for SN₂ Reaction

1. Put Main Carbon in the Middle

2. Draw the Nucleophile on the Left with δ^-

3. Draw the Leaving Group on the Right with δ^-

4. Connect with Dashed Line

5. Draw Substituents (One with a Wedge, the other with a Dash)
if it is forming a ring, then the ring chain does not get a wedge or dash

6. Bracket the Whole Thing and put ‡ in the Top Right

What Happens to Stereochemistry in SN₂?

The Stereochemistry on the Main Carbon Flips

Stereochemistry for others does NOT Change

Rate Equations for SN₁ and SN₂

SN₁: Rate = k ⋅ [Substrate]

SN₂: Rate = k ⋅ [Substrate] ⋅ [Nucleophile]

What Makes a Good SN₁ Substrate?

The Size

Me - LG (Never Works)
1^o C - LG (Meh)
2^o C - LG (Works)
3^o C - LG (Best)

What Makes a Good SN₁ Nucleophile?

Does not matter for SN₁

What Makes a Good SN₁ Leaving Group?

OTs

Weak Bases

Basically things that can Stabilize a Negative Charge

What Makes a Good SN₁ Solvent?

Polar Protic

H₂O and Alcohols

What Happens to Stereochemistry in SN₁?

50% flipped, 50% same

When Doing E1 and E2 how do we tell which Alkene forms more often?

If E2: MUST make sure it is Anti Periplanar

Then do more substituted if possible → forms 80% of the time

Rate Equations for E1 and E2

E1: Rate = k ⋅ [Substrate]

E2: Rate = k ⋅ [Substrate] ⋅ [Base]

What is Alpha and Beta? When does it matter?

Matters for E1 and E2 Reactions

Alpha Carbon: Carbon attached to Leaving Group

Beta Carbon: Carbons attached to Alpha Carbon

What Makes a Good Reagent for E2?

Strong Bases

KOH, NaOH, NaOMe, NaOEt, KOtBu

What is Good Geometry for E2?

Periplanar: All Reacting Atoms in Same Plane

Anti Periplanar: Bonds are 180^o Apart

Syn Periplanar: Bonds are Eclipsed

What is Better for E2, Syn or Anti Periplanar?

Anti

What Hydrogen do we Remove on Cyclohexane in E2?

Removed Hydrogen must be AXIAL and TRANS to Leaving Group

What is Good Geometry for E1?

Does Not Matter

How to Tell Nucleophile vs Base?

Nucleophile (SN1 and SN2)
- attacks electrophile
- in protic solvent → down a column, Nu increases

Base (E1 and E2)
- attacks H+
- in protic solvent → down a column, Ba decreases

What Makes a Better Base?

Negative Charge

Left on Periodic Table

Protic and Aprotic Solvent —> Up a Column

What Makes a Better Nucleophile?

Negative Charge

Left on Perodic Table

Protic Solvent —> Down a Column

Aprotic Solvent —> Up a Column

How to Tell SN1, SN2, E1, and E2

First Look at Substrate

Steps for Methyl Substrate

Strong Nucleophile?

Yes → SN2

Steps for 1^o Substrate

Strong Nucleophile?

Yes → SN2

No → Strong Base?

Yes → E2

Steps for 2^o Substrate

Allylic / Benzylic?

Yes → SN1 or E1

No → Strong Base?

Yes → E2

No → Strong Nucleophile?

Yes → SN2

No → SN1 or E1

Steps for 3^o Substrate

Strong Base?

Yes → E2

No → Strong Nucleophile?

Yes → SN1

No → SN1 or E1

What Always Acts as a Base?

DBU, DBN, Metal Hydrides all act as Bases

How to Name Alkenes

1. Identify parent chain containing double bond

2. Number chain starting closest to double bond

3. Write name just like normal but note where double bond is
if bond is between carbon 2 and 3, it is pent-2-ene

4. Include cis or trans prefix if necessary

EZ Designation

1. Look at each carbon in the double bond

2. Compare highest atomic number on each and mark them

3. Split the bond hotdog

4. Compare
highest on same size → Z
highest on different side → E

Degree of Unsaturation

Number of Rings or Pi Bonds in a Structure

DoU = (2C + 2 + N - (Halogens) - H) / 2

Carbocation Stability

Methyl (Least) → primary → secondary → tertiary