CHEM MODULE 1

when does elimination occur?

when a substituent can be eliminated due to the presence of adjacent beta hydrogens

E1

LG leaves, weak base deprotonates carbon next to carbonation, forms a new pi bond between carbons

what orbital does the base donate electrons into?

C-H sigma star

why?

since carbocations' empty p orbitals are stabilized by hyperconjugation, the sigma bonds are weakened and more likely to accept e- than normal

initial e- acceptor in E1

C-H sigma star

final e- acceptor in E1

C+ P orbital

requirements for E1

- C-H adjacent to C+

- weak base

- sigma star is more accessible than P

is it possible to generate carbocations in strongly basic conditions?

no

E2

strong base deprotonates and removes LG simultaneously

E2 requirements

- strong base

- antiperiplanar geometry

- good LG

antiperiplanar geometry

same plane, 180 degrees apart

M.O explanation of E2

bonding C-H can simultaneously donate into antibonding LG-C to break and form new pi bond

SN2 versus E2

bulky bases favor e2, good nu- but poor bases favor SN2

examples of good nucleophiles but poor bases

N3-, CN-, R-S-

what does being a good nucleophile but a poor base mean?

The molecule is reactive towards electrophiles, but

it is not very reactive toward protons (H⁺)

regioselective

two or more possible structural isomers, but one is energetically favored over the other

stereoselctive

two or more possible stereoisomers, but one is energetically favored over the other

Zaitsev's rule

more substituted alkene is formed

di-halogens that are good electrophiles

Cl2, Br2, I2

why are Cl2, Br2, I2 good electrophiles?

large + low energy antibonding sigma orbitals and good LG

step 1 in mechanism of di-halogenation of alkenes

HOMO of alkene donates into LUMO of Ha-Ha and Ha will donate e- from LP into P of C simultaneously

what is the HOMO of the alkene

Pi C-C

what is the LUMO of the Ha-Ha?

sigma star Ha-Ha

what does step 1 form?

three membered ring (Ha-nium ion)

step 2 in mechanism of di-halogenation of alkenes

-Ha will donate e- into sigma star of C-Ha, breaking the bond on one side and forming sigma bond with new Ha-C

what kind of stereoisomer can be from?

trans only

what changes if a di-halogenation of an alkene occurs in a solvent such as water or alcohol?

a solvent molecule will attack the three membered ion and then get deprotonated by another solvent molecule

what side will the solvent attack?

the more substituted side

how can you tell if a product will be racemic?

if the reaction mechanism allows equal attack from both sides of a planar intermediate

why are peroxyacids reactive?

the low energy of the antibonding orbital

peroxyacid

a carboxylic acid with an extra oxygen, o-o bond

mechanism for peroxyacids step 1

pi bond of alkene (HOMO) donates into antibonding sigma of O-O (LUMO)

mechanism for peroxyacids step 2

e- move to carboxylic acid (stable enough)

mechanism for peroxyacids step 3

LP of O donates into antibonding O-H

mechanism for peroxyacids step 4

LP of O donates into p orbital of carbocation

how many steps does a peroxyacid reaction happen in?

1

what is it called when multiple steps of a mechanism happen at once?

butterfly transition

where will a Nu- addition happen in anionic/basic conditions?

less substituted side

why will Nu- attack the less substituted side?

less steric hinderance

where will a Nu- addition happen in catanionic/acidic conditions?

the more substituted side

why will the Nu- attack the more substituted side?

carbocation (+) stability dominates

what kind of reaction is a peryoxyacid reaction?

SN2

does SN2 cause an inversion of stereochemistry?

yes

what is the rate determining step?

when the LG leaves

linear

sp

trigonal planar

sp2

tetrahedral

sp3

why do orbitals hybridize?

To become more stable (hybridized orbitals overlap easier with their shape)

step 1 to draw a M.O diagram

count valence electrons

step 2 to draw a M.O diagram

draw the atomic orbitals for each atom

step 3 to draw a M.O diagram

combine orbitals of similar symmetry

step 4 to draw a M.O diagram

fill the M.Os with the total number of valence electrons

step 5 to draw a M.O diagram

determine bond order (e- in bonding - e- in antibonding / 2)

is sp2 or sp3 more basic?

sp2

acid

LUMO acceptor

base

HOMO donor

order of M.O from lowest to highest in energy

sigma, pi, n.b, pi star, sigma star

- charge impact on energy (within region)

electronegativity impact on energy (within region)

more EN, lower energy

why does more EN mean lower energy?

e- are held closer to nucleus, more stable

s character impact on energy (within region)

more s character = lower energy

can mechanisms be proven?

no, only disproven

why does more s character result in a lower energy?

sigma bonds are stronger (more stable) than pi bonds due to a higher overlap

common electron sources

lewis bases

common electron sinks

lewis acids

examples of lewis bases (electron sources)

nucleophiles, lone pairs of electrons, covalent bond (sigma or pi)

examples of lewis acids (electron sinks)

electrophile, partially positive atom, carbocation, H+ (proton)

bonding orbital

in phase overlap

non-bonding orbital or LP

no overlap with orbitals

antibonding orbital

out of phase overlap, has nodes

four main indicators to assess molecules reactivity

1. charge

2. surviving atomic orbitals

3. poorly overlapping A.O

4. M.O arising from A.Os w/ poor energy match

rules for mechanisms

1. only move electrons

2. negative to positive

3. conserve charge

4. electrons can't turn corners

only move electrons

identify most negative species of the available reactants/reagents (nucleophile, LPs, bases)

negative to positive

identify the most positive atom of the other available reagents/reactants (electrophiles, C-X, protons)

conserve charge

entire charge of the reactants/reagents must be equal to that of the products (if an anion nucleophile reacts, it needs to form an anionic product)

electrons can't turn corners

pay attention to orbital alignment + direction of attack

nucleophile

wants to donate electrons (arrows start here)

electrophile

want to accept electrons (arrows end here)

what kind of bonds do e- typically require?

polarized bonds, big EN difference

why do they require polarized bonds?

pull e- away from carbon, making it more reactive with a nucleophile

in the deprotonation step of forming a pi bond, stronger base means...

faster reactant (more basic, higher HOMO)

in the deprotonation step of forming a pi bond, a more acidic proton means...

faster reactant (weaker bond, lower LUMO)

if pi bonds is nucleophile when it is breaking a pi bond, more e- density means...

faster reactant (more - partial charge)

if pi bonds is nucleophile when it is breaking a pi bond, more substituents or e- donating groups means...

faster reactant (more - partial charge)

if pi bonds is an electrophile when it is breaking a pi bond, less e- density means...

faster reactant (more positive partial charge)

if pi bonds is nucleophile when it is breaking a pi bond, less e- withdrawing groups means...

faster reactant (more positive partial charge)

as you add e- density into nucleophile, you...

raise HOMO energy

as you lower e- density into electrophile, you...

lower LUMO energy

the smaller the gap between the HOMO-LUMO...

the faster the reaction proceeds

how to determine acidity

SERHI, evaluating conjugate base

S (size)

bigger atom = more acidic

E (electronegativity)

more EN = more acidic

R (resonance)

more resonance structures = more acid

H (hybridization)

more s character = more acidic

I (induction)

make EN elements near by more acidic

difference between EN and I

EN refers to a single atom, I is a molecule

why does having more resonance structures increase resonance?

makes the conjugate base more stable by delocalizing the negative charge

how to determine the nucleophile has the higher/lower HOMO

determine which one is more stable

how to determine which electrophile has the higher/lower LUMO?

determine which one is more stable

when determining acidity...

always look at conjugate base