Module 1 CHEM 203

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