Pericyclic reactions

Question 1

What are syn and anti isomers?

Answer 1

Syn = cis, anti = trans

Question 2

What is conrotatory?

Answer 2

rotation of the lobes in the same direction

Question 3

What is disrotatory?

Answer 3

rotation of the lobes in the opposite direction

Question 4

For 2, 6, and 10e- complexes, what component is used for Woodward-Hoffmann?

Answer 4

(4q+2)x

Question 5

For 4, 8, and 12e- complexes, what component is used for Woodward-Hoffmann?

Answer 5

(4r)x

Question 6

What do a and s refer to for Woodward-Hoffmann?

Answer 6

antarafacial and suprafacial (NOT ANTI AND SYN)

Question 7

How are a and s decided for Woodward-Hoffmann?

Answer 7

if the lobes are on opposite side (a), if the lobes are on the same sides (s)

Question 8

How many e- per double bond?

Answer 8

2e-

Question 9

What is the Woodward-Hoffmann rule?

Answer 9

To be symmetry allowed under thermal conditions, the sum of (4q+2)s and (4r)a must be an odd number (zero+1)=1

Question 10

How to work out HOMOs and LUMO drawings?

Answer 10

no. of atoms in the reaction = no. of MOs

MO number -1= no. of inversion centres to decide orientation of lobes

Question 11

Effect of EDG on HOMO and LUMO?

Answer 11

EDG raises energy of HOMO and LUMO, makes the lobe where the EDG is larger in the LUMO, and the lobe next to it larger in the HOMO

Question 12

Examples of EDG?

Answer 12

OMe, OSiMe3, NMe2

Question 13

Effect of EWG on HOMO and LUMO?

Answer 13

EWG lowers energy of HOMO and LUMO, makes the lobe next to where the EWG is larger in both the LUMO and HOMO

Question 14

Examples of EWG?

Answer 14

Ketone, ester, amide, nitrile, NO2

Question 15

Effect of conjugated group on HOMO and LUMO?

Answer 15

CONJ raises energy of HOMO and lowers the LUMO, bringing them closer together, makes the lobe next to where the CONJ is only very slightly larger in both the HOMO and LUMO

Question 16

Example of CONJ group?

Answer 16

phenyl, aryl

Question 17

How do you know if something reacts as its HOMO or LUMO?

Answer 17

Electron rich reacts as homo (EDG), electron poor reactions as LUMO (EWG)

Question 18

What can catch you out with HOMO LUMO?

Answer 18

double bonds from ketones etc will have pi orbitals too so must be considered

Question 19

What are endo and exo diastereoisomers?

Answer 19

Exo = the R group and bridge are on the same side

Endo = the R group and bridge are on opposite sides (more favoured TS)

Question 20

Why isn’t 2-2 cycloaddition possible with heat (unless ketene)?

Answer 20

light promotes one e- from the HOMO to the LUMO forming pseudo ‘HOMO’ and ‘LUMO’

Question 21

How do HOMOs and LUMOs react with pseudo ones?

Answer 21

e.g. LUMO reacts with ‘LUMO’, ‘LUMO’ is the same as the LUMO, just flipped around so the lobes line up for the reaction

Question 22

What is the exception for 2-2 cycloaddition with heat?

Answer 22

ketenes