frontier orbital theory

Question 1

what does higher HOMO energy mean

Answer 1

its more reactive towards electrophiles, the reactant is electron rich

Question 2

what does lower LUMO energy mean

Answer 2

more reactive towards nucleophile, reactant is electron poor

Question 3

explain what an unfavourable reaction is in filled-filled interaction

Answer 3

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Question 4

explain filled-unfilled interaction and explain the favourable reaction

Answer 4

the HOMO-LUMO is the strongest when orbitals close in energy

Question 5

what are hard nucleophiles

Answer 5

low energy HOMO, often small and negative

Question 6

what are soft nucleophiles

Answer 6

high energy HOMO, often neutral species

Question 7

what are hard electrophiles

Answer 7

high energy LUMO, small and positive

Question 8

what are soft electrophiles

Answer 8

low energy LUMO, neutral

Question 9

why is hard-hard interaction favoured

Answer 9

due to electrostatic interactions

Question 10

why is soft-soft interactions favoured

Answer 10

the frontier orbitals are close together in energy and this allows for stronger interactions

Question 11

why is axial conformation best with oxygen lone pair

Answer 11

oxygen lone pair in plane with sigma* c-x, so favourable filled-unfilled interaction

Question 12

in [2+2] cycloaddition, explain why two alkenes are not allowed by thermal suprafacial reaction

Answer 12

the HOMO and LUMO do not match up in the same plane as one interaction would be antibonding

Question 13

woodward-hoffman rule, what does odd and even mean

Answer 13

odd = thermally
even = photochemically

Question 14

how to make diene react faster

Answer 14

make it electron rich with a EDG as they donate electron density into the diene
(NR2, OR, allkyl)

Question 15

how to make dienephile react faster

Answer 15

make them electron-poor with EWG, this lowers the LUMO
(NO2, COR, CN)

Question 16

how to make the diels-alder reaction to occur if its at room temp

Answer 16

add lewis acid to dienephile, lower LUMO as there is more allyl cation character on dieneophile

Question 17

what does the trans or cis dienephile form in diels-alder reaction

Answer 17

trans dienophile => trans product
cis dienophile => cis product
because they react suprafacially

Question 18

why is the endo product preferred kinetically

Answer 18

1. there is secondary orbital interaction
2. electrostatic effects
3. regioselectivity

Question 19

what is the best regioselectivity for dipoles and dipolarphiles

Answer 19

for electron-poor dipolarophiles = HOMO of dipolar and LUMO of dipolarophile

opposite for electron-rich dipolarophile

Question 20

explain cycloaddition of allyl cation with butadiene

Answer 20

allyl cation is 2pi unit spread over 3 atoms, its electron deficient so we use the LUMO for cation and HOMO of diene

Question 21

Why is the endo product preferred kinetically?

Answer 21

1. Electrostatic effects
   - avoid electrostatic repulsion (atom in axial position)
   - avoid steric clash