SN1/SN2, Sn1 and Sn2, E1 and E2, Organic Chemistry Sn1 vs Sn2

SN2 reaction order

second order reaction

SN1 reaction order

first order reaction

SN2 minimum # of steps

1 or more steps

SN1 minimum # of steps

2 or more steps

SN2 intermediates

no

SN1 intermediates

carbocation

SN2 Stereochemical consequences

stereospecific inversion of configuration at the electrophile site.

SN1 Stereochemical consequences

racemization (full or partial) at electrophile site.

SN2 Importance of nucleophile strength

very important; no reaction for weak nucleophiles like water.

SN1 importance of nucleophile strength

unimportant

SN2 importance of leaving group

very important; no reaction for weak leaving groups like HO- .

SN1 importance of leaving group

very important; no reaction for weak leaving groups like HO- .

SN2 substrate structure dependence

avoid steric hinderance;

CH3 > 1 degree > 2 degrees (slow) > 3 degree (no); no reaction for aryl/vinyl.

SN1 substrate structure dependence

need carbocation stabilization;

3 degree > 2 degree (slow) > 1 degree (no); resonance stabilization helps; no reaction for aryl or vinyl.

SN 2 solvent

polar aprotic (its lacking , no protons).

SN1 solvent

polar protic (has acidic protons)

nucleophile strength: I

VERY GOOD

nucleophile strength: HS

VERY GOOD

nucleophile strength: RS

Very Good

I>Br>Cl>F

best leaving group to worst leaving group

ACN>DMF>DMSO>H2O>MeOH

best to worst aprotic solvents

rate=k[R-LG][Nuc]

Sn2 rate of reaction formula

rate=k[R-LG]

Sn1 rate of reaction formula

the sn1 reaction at a chiral center will produce ______ _______

2 steroisomers

H20>HCO2H>MeOH>CH2CO2H

Best to worst protic solvents

E1 structure substrate?

3>2>>1 substrate

E1 what about the leaving Grup?

usually needs to make an LG

doesn't need a strong base

E1

faster in polar protic solvent

E1

Zaitsev is the major product

E1

like high temperatures

E1 and E2

elimination 3>2>1 substrate

E1 and E2

good LG needed

E2

needs strong base

E2

no solvent preference

E2

H and Leaving Grp must be anti & periplanar to each other

E2

small bases (E2)

fits into tight spots. Removes proton from more substituted C to give more substituted alkene

bulky bases(E2)

go for most accessible proton. Removes proton from less substituted C to give a less substituted alkene

Weak Nucleophile

SN1

Strong nucleophile

Sn2

Sn1

which reaction is this?

Sn2

which reaction is this?

Tertiary carbocations yield what kind of stereochemistry? (Sn1)

Racemic mixture, 50% S and 50% R

Secondary carbocations yield what kind of stereochemistry? (Sn1)

It is a net inversion, therefore if it is originally S the major product will have a stereochemistry of R

Sn1

Tertiary carbocations and sometimes secondary

Sn2

Primary carbocations and sometimes secondary

The side the nucleophile attack for the Sn2 mechanism?

The backside (180 degrees)

Why can't a tertiary carbocations be apart of the Sn2 mechanism?

Because of the steric bulk, the tertiary R-X blocks the backside from attack

What is the stereochemistry after the SN2 reaction? (Secondary or primary)

Complete inversion

how does the leaving group effect the reaction rate?

stability of leaving group permits the reaction to occur faster

Why is Polar Protic Solvent better for Sn1?

Because The rate determining step involves charge separation (when the leaving group leaves) which the polar protic stabilizes by creating a shell, spreading out the positive charge. (Making it hard for any compound/molecule to come in until then nucleophile) it has lots of electrons and protons to give.

Why is polar aprotic more stable for Sn2?

Because the polar protic would solvate the nucleophile, therefore weakening it before the nucleophile could attack. The aprotic doesn't weaken the nucleophile/form the full shell around the nucleophile. But it can stabilize the charged species to some extent

Sn2

concerted process (nucleophilic attack occurs the same time as the leaving group leaves)

Sn1 Reaction Coordinate Diagram

Sn2 Reaction Coordinate Diagram

Why is R-I better for Sn1?

the rate determine step involves C+ formation and the breaking of the R-X bond, the weaker the R-X (C-X) bond the easier it is to break. R-I is the most reactive because it is the weakest R-X bond.

The conjugate base of strong acids

Is a better leaving group (I, Cl, Br, H2O, OTs)

Sn1

Stable Leaving group

Sn2

Polar protic solvents hinder the mechanism by solvating the nucleophile

why does SN1 favor a weak nucleophile?

Because a weak nucleophile permits the leaving group dissociation before the Nucleophilic attack

do relative reaction rates differ (RI>RBr>RCl>>RF) for SN1 and SN2?

no, but SN1 is better when the reaction rates are higher, and SN2 is better with lower reaction rates

SN2 transition state

identify the middle step

Sn1

Requires heat and stable leaving group

Sn2

requires stable leaving group, often occurs at room temperature or below

why do secondary carbocations result in net inversion for an Sn1 reaction?

they are less stable and the leaving group is in close proximity to the carbocation during nucleophilic attack, partially protecting one side from the attack

good leaving groups

weak (conjugate) bases

bad leaving groups

strong bases

hydride shift needs to be on neighboring carbons

1,2 hydride shifts

Sn2

Doubling the Alkyl Halides or nucleophile doubles the rate of reaction

Sn1

Rate is only dependent on the concentration of the Alkyl Halides