Topic 5: Reduction

Question 1

definition of oxidation state

Answer 1

charge remaining on an atom when all ligands are removed heterolytically with the electrons being transferred to the more electronegative partner

Question 2

definition of formal charge

Answer 2

the charge remaining on the atom when all ligands are removed homolytically

Question 3

definition of coordination number

Answer 3

the number of atoms bonded to the atom of interest

Question 4

adding H2 to alkene with transition-metal catalyst like Pt

Answer 4

reduction > formation of alkane

Question 5

how to reduce azides or nitro compounds to amines

Answer 5

via dissolving metal reductions, involving electron transfer

use Zn/AcOH or Fe/aq. HCl

Question 6

what is Birch reduction

Answer 6

dissolving metal reduction to partially reduce aromatic rings to form 1,4-cyclohexadienes

use Li, aq NH3 with t-BuOH

Question 7

general reactivity trend for carbonyl compounds

Answer 7

aldehydes > ketones > acid chlorides > esters > COOH > amides

based on steric, resonance and inductive effects

Question 8

reactivity order of the common reducing agents

Answer 8

LiAlH4 > LiBEt3H > LiBH4 > NaBH4 > NaBH3CN

Question 9

comparison between using LiAlH4 and NaBH4 to reduce ketone

Answer 9

both lead to formation of secondary alcohol

LiAlH4: must use in aprotic solvents like THF or Et2O (cannot use H2O or will generate H2 gas)

NaBH4: use in presence of protic solvents (R-OH or H2O)

Question 10

difference in reduction of enones using LiAlH4 and NaBH4

Answer 10

enones contain both a carbonyl group and C=C double bond

LiAlH4: mostly reduce only ketone to alcohol, but sometimes reduce double bond together with it

NaBH4: will 100% reduce both ketone and double bond

Question 11

difference between soft and hard bases

Answer 11

hard: small, high electronegativity, highly charged, low polarisability and hard to oxidise > hold valance electrons tightly > prefer to bind to hard acids

soft: large, low electronegativity, high polarisability and easy to oxidise > hold valence electrons loosely > prefer to bind to soft acids

Question 12

difference between soft and hard acids

Answer 12

soft: acceptor atoms are large, have low positive charge, and contain unshared pairs of electrons (p or d) in their valance shells; have low electronegativity and high polarisability

hard: acceptor atoms are small, have high positive charge, and do not contain unshared pairs in their valence shells; have low polarisability and high electronegativity

Question 13

properties of LiBEt3H and NaBH3CN reducing agents