Pericyclics + Organoelement Chemistry

Question 1

properties of pericyclic reactions

Answer 1

involve cyclic movement of e-

cyclic transition state

concerted -> bonds broken and made in single step

Question 2

3 types of pericyclic reactions

Answer 2

1. electrocyclic - formation of a σ-bond between 2 ends of a linear conjugated π system
2. cycloadditions - formation of a σ-bond between ends of 2 (or more) conjugated π systems
3. sigmatropic rearrangements - migration of σ-bond from one end of a π system to another

Question 3

thermal vs photochemical process

Answer 3

thermal = ground state

photochemical = first excited state

Question 4

conrotatory

Answer 4

both rotating same way

Question 5

disrotatory

Answer 5

lobes rotating opposite ways

Question 6

dials-alder reaction - overlapping of lobes

Answer 6

involves HOMO of diene and LUMO of dienophile

to see if reaction will take place, you must see if the lobes are in the correct phase

Question 7

alkene metathesis

Answer 7

exchanging groups on the ends of 2 different alkenes

Question 8

when can intermolecular alkene metathesis occur?

Answer 8

when one is more reactive than the other (e.g. one is more sterically hindered or more extensively sub.)

avoid dimerisation (coupling with itself)

Question 9

which elements are involved in organoelement chemistry?

Answer 9

C, H, N, O and halogens

Question 10

sulfur

Answer 10

forms weaker bonds to C/H than O (easiest to break homolytically)

wide range of oxidation states (-2 to +6)

Question 11

thiols

Answer 11

R-SH

acidic compared to alcohols (S-H bond weaker than O-H)

deprotonated thiols = v. good nucleophiles (soft nucleophiles)
- due to low electronegativity of sulfur (high energy HOMO + polarisability of e- density on large sulfur atom)

easily oxidised to disulphides -> sulphide bridges have important role in folding and stability of proteins

Question 12

thioethers

Answer 12

R1-S-R2

more nucleophilic than oxygen-based ethers

Question 13

C-H acidity of compounds containing sulfur

Answer 13

C-S bond stabilises alpha-carbanion

due to donation of -ve charge into C-S σ* anti bonding orbital = lower in energy than C-C σ*

Question 14

why do phosphonium ylides behave differently?

Answer 14

P=O bond = much stronger than S=O

Question 15

sulfones

Answer 15

good at stabilising anions

can be deprotonated with strong base

Question 16

why are organosilicon compounds more widely used?

Answer 16

air and moisture sensitive

easy to prepare

no element-specific toxicity

rich and diverse chemistry

Question 17

HOMO and LUMO of C-Si bonds

Answer 17

HOMO = higher = excellent σ donors (conjugation)

LUMO = lower = moderately good σ acceptors

^ because Si is less electronegative than carbon + its orbitals are larger/more diffuse

Question 18

are silicon bonds weaker or stronger to oxygen/fluorine compared to carbon?

Answer 18

stronger

Question 19

can silicon form stable multiple bonds?

Answer 19

no

large 3p orbital on silicon doesn’t overlap well with 2p orbital on carbon/oxygen/nitrogen

Question 20

carbon vs silicon - expanding coordination number

Answer 20

silicon can (up to 5/6 ligands)

Question 21

organosilicon - charge stabilisation (alpha)

Answer 21

weak stabilisation

enough to allow deprotonation of weakly acidic C-H bonds alpha to silicon with alkyl lithium bases

stabilisation due to donation of -ve charge into C-Si σ* (lower in energy than C-C σ*)

Question 22

organosilicon - charge stabilisation (beta)

Answer 22

stabilise carbanion beta to silicon

due to e- donation of cation’s empty p-orbital from C-Si stabilise carbanion alpha to silicon σ (higher in energy than C-C σ)

Question 23

Peterson olefination - acidic vs basic conditions

Answer 23

acidic = anti elimination = E alkene

basic = syn elimination = Z alkene

Question 24

differences between selenium and sulfur

Answer 24

C-Se = weaker

selenoate anion (RSe-) = more powerful nucleophile than thiolates

selenoxides are thermally unstable at ambient temp.

higher oxn state are more readily accessible for selenium