Orgo CHM II Memorize

Strong nucleophiles

• Halogens (I, Br, Cl)

• Hydroxide (OH-)

• Alkoxides (RO-, OCH3-, OEt-)

• Cyanide (CN-)

• Sulfur (HS-, RS-)

• Amines/Amides (NH3, NH2-)

• Azide (N3-)

Characteristics of strong nucleophiles

Negative, smallish, electron rich

Strong bases and nucleophiles

• OH-

• OCH3-

• OC2H5-

• NH2-

Strong bases, weak nucleophiles

• t-BuO-

• LDA

• DBU/DBN

Characteristic of strong bases that are weak nucleophiles

Bulky

Strong, bulky bases favour

Elimination (E2)

Strong leaving groups

• halogens: I- > Br- > Cl-

• OTs, OMs

• H2O

Bad leaving groups

• OH-

• OR-

• NH2-

What is a characteristic of good leaving groups

Weak bases

Strong nucleophiles that aren’t bulky favour

SN2

Weak nucleophiles with good leaving groups that can form a stable carbocation

SN1/E1

Primary substrate

carbon attached to one carbon and 2 hydrogens

i.e. CH3CH2Br

Secondary substrate

carbon attached to 2 carbons and a hydrogen

Tertiary substrate

carbon attached to 3 carbons and no hydrogens

I.e. (CH3)3CBr

Primary substrates favour ___ because ___

SN2 because they allow for easy backside attack

UNLESS: bulky strong base, then E2

Secondary substrates favour ___

Nothing, so you need to analyze base strength, nucleophile strength, solvent, and temperature

Heat involved indicates

elimination

Tertiary substrates favour ___ because ___

SN1 with weak nucleophiles

E2 with strong base

Characteristics of SN2

• one step, backside attack

• inversion

• strong nucleophile

• prefers primary substrate in polar aprotic solvent

Characteristics of SN1

• two steps, carbocation intermediate

• possible rearrangements and subsequent racemization

• tertiary substrate favoured in polar protic solvent

• can have weak nucleophiles

Characteristics of E2

• one step

• strong base

• anti-periplanar with no rearrangements

• usually Zaitsev but Hofmann with bulky base

Characteristics of E1

• two steps, carbocation intermediate

• rearrangements possible

• weak base

• heat favours elimination

• Zaitsev product

Polar protic solvent (examples) favour ___, because ___

• water

• methanol

• ethanol

favour SN1 and E1 because they stabilize ions and carbocations

Polar aprotic solvent (examples) favour ___, because ___

• DMSO

• DMF

• acetone

favour SN2 because they enhance nucleophilicity

Rearrangements happen with

SN1, E2, never SN2, E2 —> specifically hydride and alkyl shifts

Zaitsev product

more substituted alkene major

Hofmann product

less substituted alkene major, usually caused by bulky bases, especially t-BuOK, LDA