CHEM 109B - Quiz 1

Zaitsev’s Rule

A regioselectivity rule stating that the more substituted alkene is the major product, obtained by removing a hydrogen from the B-carbon bonded to the fewest hydrogens.

Anti-periplanar Orientation

The specific staggered conformation required for E2 reactions, where the leaving group and the B-hydrogen are on opposite sides (180º relative to each other)

Cyclohexane E2 Requirement

The leaving group must be in the axial position to achieve the necessary anti-periplanar alignment with a neighboring hydrogen.

Temperature Effect on E2

Higher temperatures favor elimination over substitution because elimination results in more product molecules, leading to a greater positive change in entropy (ΔS)

SN2 vs. E2 for Primary (1º) Alkyl Halides

These substrates primarily undergo SN2 substitution; E2 elimination is only favored if a strong, bulky base is used.

SN2 vs. E2 for Secondary (2º) Alkyl Halides

Competition depends on the base: weak bases favor SN2 while strong, bulky bases (like tert-butoxide or DBU) favor E2.

PBr₃ (Phosphorus Tribromide) Reaction

Converts 1º and 2º alcohols into alkyl bromides via an SN2 mechanism, resulting in inversion of stereochemistry at the reaction center.

TsCl (p-toluenesulfonyl chloride) and Pyridine

Converts an alcohol into a tosylate (OTs), an excellent leaving group, while retaining the original configuration of the oxygen-carbon bond.

POCl₃ Phosphorus Oxychloride and Pyridine

Dehydrates alcohols into alkenes via an E2 mechanism; this method is preferred for avoiding carbocation rearrangements.

Epoxide Opening (Basic/Nucleophilic)

An SN2 reaction where the nucleophile attacks the less substituted carbon of the epoxide from the back side, resulting in an anti-addition relationship.

Cis-1,2-diol Synthesis

The reaction of an alkene with osmium tetroxide (OsO₄) followed by hydrolysis, which adds two hydroxyl groups to the same side (syn addition) of the double bond.

SN1/E1 favoring conditions

Poor or weak nucleophiles and bases.

SN2/E2 favoring conditions

Good or strong nucleophiles and bases.

Typical strong nucleophile/base

Molecules or ions typically carrying a negative charge.

Amine nucleophile/base classification

Neutral molecules that are still considered good nucleophiles or strong bases.

SN1/E1 reagent examples

CH₃OH, H₂O, CH₃SH.

SN2/E2 reagent examples

HO^-, CH₃O^-, CN^-, I^-, NH₂^-, DBU, CH₃S^-.

Most stable carbocation

3º allylic

Least stable carbocation

vinyl

Minimum SN1/E1 stability

Substrate must be capable of forming a carbocation at least as stable as a localized 3º alkyl carbocation.

SN2 relative rates

1º > 2º > 3º because the reaction is slowed by steric hindrance

E2 relative rates

3º > 2º > 1º because the reaction is favored by the stability of the alkene formed.

The weaker the base…

the better the leaving group.

Most basic halide

F^-

2nd most basic halide

Cl^-

3rd most basic halide

Br^-

Least basic halide

I^-

Best halide LG

I^-

Worst halide LG

F^-

1º halide major product

SN2 due to low steric hindrance and high speed compared to E2.

3º halide major product

E2 because SN2 is blocked by steric hindrance.

SN2 is the preferred reaction when…

there is a 2º halide and weakly basic.

Both SN2 and E2 reaction products can occur when…

there is a 2º halide and basic.

Primarily or exclusively E2 reaction products occur when…

there is a 2º halide and extremely basic.

Factors favoring E2

Using large, bulky bases (like tert-BuO or DBU) and higher temperatures.