Organic Stereochemistry and Reaction Mechanisms: Chirality, SN1/SN2, and Alkene Stability

Achiral

A molecule that is superimposable on its mirror image

Chiral

object that has non-superimposable mirror image

Plane of symmetry effect on chirality

The molecule is achiral if it has a plane of symmetry

How do you draw enantiomer?

Mirror or switch wedge and dash

how to name chiral centers

put chirality and C# in parentheses at start (ex. (1R, 3S)

Diastereomers

Stereoisomers w/ at least one matching chiral center and one differing chiral center

# of stereoisomers possible

2^n

n = # of chiral centers

meso compound

achiral molecule with chiral centers

Are meso compounds optically active?

no

Are racemic mixtures optically active?

no, the two components cancel out eachothers' effect on light

Are chiral molecules optically active?

yes they rotate plane polarized light

Horizontal line fischer

wedge

Vertical line fischer

dash

Chirality of Sn2

Inverts chiral center

SN2 rate law

rate = k[substrate][nucleophile]

SN1 rate law

rate = k[substrate]

SN1 rate determining step

loss of leaving group

SN2 solvent preference

polar aprotic

SN1 solvent preference

polar protic

Hammond’s postulate

the transition state is more similar in structure to the species to which it is more similar in energy

SN1 chirality

Produces racemic mix

How are carbocations stabilized?

inductive effect and hyperconjugation

hyperconjugation

spreading out charge by the overlap of an empty p orbital with an adjacent sigma bond

carbocation/radical stability high to low

3 benzyl > 3 allyl > 2 benzyl > 2 allyl > 3 > 1 benzyl > 1 allyl > 2 > 1 > methyl

β-Branching Sn2 Rate trend

Less beta branches -> faster rxn

Leaving group ranking good to bad

I- > Br- > Cl- > H2O >> F- > acetate- > OH- > CH3O- > NH2-

Convert the -OH into a better leaving group

Use tosycl chloride (Tscl) which replaces OH with OTs which is a good LG

Nucleophilicity trend across a period

Decreases left to right

Nucleophilicity trend down group in polar aprotic solvent

Decreases down

Nucleophilicity trend down group in polar protic

Increases down

Zaitsev

more substituted alkene

Hoffman

less substituted alkene

Alkene stability high to low

tetra > tri > trans > cis > mono

E1 substrate ranking

3 > 2 > 1 > Me

E2 substrate ranking

3 > 2 > 1 > Me

E2 base preference

strong base (negatively charged)

E2 LG and leaving H geometry

anit-periplanar

When do dehydration reactions take place?

Acid (Ex H2SO4), water and heat

If there is a bulky base reacting with a primary group, what rxn will proceed?

E2

Radical chemistry steps

Initiation, propagation, termination

What reagent for allylic bromination

NBS

H2O Pka

15.7

H3O+ Pka

-1.7

Carboxylic acid pka

5

R-OH Pka

16-18

Ph-OH Pka

10

R-NH2 Pka

36-40

R-NH3+ Pka

9-11

H-C≡N Pka

9

R-C≡C-H Pka

25

NH3 Pka

35

H2 Pka

35

E1 rate law

rate = k[substrate]

E2 rate law

rate = k[base][substrate]