Chem #3

Sn2

Bimolecular nucleophilic substitution (substitute Nu for LG)

• not in transition state long (exists for 1 molecular vibration) not able to be isolated long

inverts stereochemistry at chiral center it attacks

SN2 Rate

k[Nu][R-LG]

k

proportionality constant, closer to 1 means more likely

• found at rate determining step

[Nu]/[Base]

conc nucleophile or conc base

SN2 Stereochemisty

thing eith e- comes 180 from behind

-neg comes from opp side of neg thing: backside attack

Never in transition state (spectators)

Na, Mg, etc

Amount of e- in SN2 central carbon

always have the same amount

Transition states

occur while arrows are acting, between attatched adn unattatched (form intermediates)

SN1 RDS

heterolysis, rate = [R-LG]

E2

base wants H, not nucleus

looks for H’s on carbon adjactent to LG

if solvent matches the base, it favors E2

E2 rate

[B][R-LG]

showing locations of Hs

beta, alpha, epso

E1

2 steps, rate = [R-LG]

why is first step of Sn1 RDS

makes intermediate, which is high in energy so energy barrier is high

Solvent amount

always assume its in extreme excess

Sn1 Stereochemistry

crappy Nu, so reaction is slow

need proton transfer when alcohol is starting material

• if at chiral center, 50/50 racemic is produced

SN1 carbocation

trigonal planar with open p-orbital, so it doesn’t care what side it’s attacked from (racemic)

ways to indicate racemic

molecule + E, molecule with no dash/wedges, dashed molecule + wedged molecule, molecule with squiggle on dashed/wedged group

Ion pair

pair of ions

Polar protic

H2O, HOR, amines, thiols (favor 1’s)

Polar aprotic

DMF, THF, ET2O, acetone, ETOAc, CH2Cl2, CHCL3 (favor 2’s)

protic

has proton (H)

Stereochem of all 4 reactiosn

product may look similar but stereochem differs

walden inversion

3 groups of substrate flip to the other side with a LG, like umbrella inverting

Polar Aprotic solvents

partial pos is inside bulky groups, cant get very close to partial pos

If new chiral center produced in an elementary step, R+S configuration..

—equal amounts if reactants and environ achiral

—unequal amounts if reactants/environ are chiral

chiral environment: set of surrounding not superimposable on its mirror image

Catalytic

small amount, not in excess

Proton Transfer

fastest reaction in organic chemistry, if it can happen, it will

Nu only examples

HS-, RS-, RC=C-

Primary Nu

sn2

Secondary Nu

Sn1/sn2

Tertiary Nu

SN1

Substitution

substrate must being as sp3

Base only examples

H-, NAH,NaNH2, LDA, DBU/DBN, KOtBu

primary strong base/strong nu

minor E2, major sn2

secondary strong base/strong nu

major e2, minor sn2

tertiary strong base/strong nu

e2

Strong base/strong Nu examples

RO-, NAOH, LiOPr