Substitution and Elimination Reactions

SN2 description

• single step

• nucleophile approaches (180 away) + backside attack of electrophile

• bond breaks between electrophile and leaving group

• stereochemistry is inverted due to  backside attack 

SN2 mechanism

1. nucleophile uses lone pair electrons to attack the backside of the alkyl halide (electrophile)

2. transitions state w/partially formed C-nu bond and partially broken C-X bond (high energy bond that doesn’t last long)

3. C-nu bond forms fully and X ion departs w/electron pair from former bond 

SN2 favors

• methyl and primary alkyl halides (3 are too bulky for nu to get to back of carbon)

• strong nu (OH-, OR-, CN-, SH-)

• less steric hindrance

• polar aprotic solvents (stabilize electrophile b/c form weaker interactions)

• weak bases make better leaving groups (TosO-, I, Br-, Cl-)

SN1 reaction

• two-steps

• carbocation formation + nu bonds to cc

• racemic mixture (ish)

• carbocation rearrangements possible

SN1 mechanism 

1. rate-limiting step: dissociation of alkyl halide generates carbocation intermediate and halogen ion

2. fast step: carbocation reacts w/water as nucleophile to yield protonated product

3. loss of proton from protonated intermediate gives neutral product 

SN1 favors

• tertiary alkyl halides (stable carbocation) 

• weaker bases and large halide ions as leaving groups

• polar protic solvents (promote ionization, H2O, OH-)

• carbocation is biased to react on opposite side of LG (slightly higher steric inversion)  

E2 reaction

• one step

• base attacks beta-H, alkene C=C bond forms, LG leaves (halogen)

• zaitsev’s rule

• requires anti-periplanar geometry (allows orbital overlap + minimizes steric hindrance)

E2 mechanism

1. base attacks a neighboring hydrogen and begins to remove the H while alkene C=C bond starts to form and the X group starts to leave

2. neutral alkene is produced when the C-H bond is fully broken and the LG has departed w/C-LG pair 

E2 favors 

• strong base (-OH)

• tertiary, secondary, primary (steric bulk pushes elimination)

• high temperatures

zaitsev’s rule

the more stable alkene is more stable product = more substituted alkene 

E1 reaction

• two-steps

• intermediate carbocation and neutral alkene product formation 

• rearrangements possible

• competes w/SN1

• forms Zaitsev major product

E1 mechanism

1. rate-determining step: dissociation of tertiary alkyl halide yields an intermediate carbocation

2. fast step: loss of neighboring H+ yields the neutral alkene product —> electron pair from the C-H bond forms alkene pi bond 

E1 favors

• 3 carbocations 

• weak base

• polar protic solvent

• high heat