Week 1: Oxidation, reduction, hydrogenation

Question 1

Describe LiALH4

Answer 1

More reactive than LiBH4 as Al-H is weaker than B-H.

can also reduce aldehydes, ketones, esters plus amides.

Reduces carboxylic acids slowly

Can reduce amides to aldehydes as well

Question 2

What is chemoselectivity, regioselectivity and stereoselectivity

Answer 2

Chemoselectivity: Which functional group will react

Regioselectivity: Where it will react

Stereoselectivity: How it will react (stereochemistry of products)

Question 3

Describe NaBH4

Answer 3

Mild reagent, easier to handle

Mainly reduces only aldehydes and ketones

Needs protic solvent or electrophilic cations

Question 4

Explain TPAP

Answer 4

Mild oxidising agent for 1º to aldehydes

Question 5

Explain Swern oxidation

Answer 5

1º to aldehyde by using oxalyl chloride and DMSO.

Makes biproducts Me2S, CO, CO2 and HCl.

Question 6

Define reactivity regarding steric and electronics

Answer 6

• Sterics: Space and hinderance
  e. g. Aldehyde is more reactive than ketone as it is on the end of a molecule.
• Electronics: Due to electrons and electronegativity
  e. g. Ester is less reactive than ketone due to having a resonance structure and thus spread out electrons.

Question 7

Explain KMnO4

Answer 7

Used for over-oxidation for 1º or aldehydes to carboxylic acids.

Can also use Cr(VI) or Mn(VII).

Will not stop at aldehyde.

Question 8

Explain dess martin periodinane

Answer 8

Mild oxidising agent for 1º to aldehyde. Also good for sensitive alcohols.

Question 9

Name the reagents of carbonyl reduction

Answer 9

NaCNBH3
NaBH4
LiBH4
LiAlH4
BH3
DIBAL

Question 10

List the reagents of carbonyl oxidation

Answer 10

Jones oxidation
Swern oxidation
PDC
TPAP
Dess-Martin periodinane
KMnO4
PCC

Question 11

What is the hierarchy of carbonyl reactivity as electrophiles

Answer 11

Aldehyde (most reactive)
Ketone
Ester
Amide
Carboxylate (least reactive)

Question 12

Explain lindlers catalyst

Answer 12

Hydrogenation of an alkyne to an alkene.

H2, Pd, CaCO3 and Pb(OAc)2. Does not hydrogenate alkenes to alkanes due to the Pb deactivating the catalyst to make reduction of the alkene slow.

Question 13

Describe BH3

Answer 13

Reduces amides and carboxylic acids

aldehydes, ketones and esters very slowly

Question 14

Describe LiBH4

Answer 14

More reactive than NaBH4 so can also reduce aldehydes, ketones + esters.

Question 15

Describe the process of catalytic hydrogenation

Answer 15

Always needs a H2 source which is bubbled onto a catalyst. Then the alkene coordinates to it and Hs delivered to same face (syn)

Chemoselective for C=C because weaker than C=O so more reactive

Question 16

Describe NaCNBH3

Answer 16

Reduces imine to amine

Question 17

Explain the rosenmund reaction

Answer 17

Hydrogenation of acyl chlorides to aldehydes.

Use H2, Pd and BaSO4 (the support stops reduction to alcohol by allowing products to escape catalyst, CaCO3 also).

Question 18

Describe DIBAL

Answer 18

Converts esters to an aldehyde

Could also do this in 2 steps by reducing to alcohol with LiAlH4 and then Cr(VI) to convert to aldehyde

Question 19

Explain PCC:

Answer 19

Used if alcohol is acid sensitive (means it would eliminate to form an alkene) to retain the alcohol.

Does not work in H2O as hydrate acts as alcohol and oxidises to carboxylic acid.

Must be an organic solvent like dichloromethane.

Question 20

List the catalysts of hydrogenation for an alkene and an aromatic

Answer 20

Alkene: Pd, Pt or Ni

Aromatic: Pt, Rh or Ni (under high pressure)

Question 21

Explain PDC

Answer 21

1º to aldehyde

Question 22

Explain jones oxidation

Answer 22

(Na2Cr2O7, H2SO4, acetone

Oxidises 2º to a ketone

Question 23

Explain what a protic and lewis acid is

Answer 23

Protic acid: Bronsted acids, thus, electrophiles that can donate protons

Lewis acid: Electrophiles that accept a pair of electrons