oxidations and reductions

OXIDATION AND REDUCTION

 1. Reducing agent is oxidized; oxidizing agent is reduced. Redox always happens in a pair. 

2. Counting oxidation states on carbon: : of heteroatoms OR of pi bonds

3. Reduction reactions. OXIDATION AND REDUCTION

a. Catalytic hydrogenation – adds two H’s at once. 

i. Reduce alkenes: use Pd, Pt, or Ni as a catalyst. 

ii. Reduce alkyne to cis alkene: use Lindlar’s catalyst 

iii. Reduce aldehyde or ketone to alcohol: use H2 with Raney nickel 

iv. Reduce acid chloride to aldehyde: use a partially deactivated Pd catalyst (this is the Rosenmund reduction) 

v. Cannot use catalytic hydrogenation to reduce other acid derivatives.

b. “Dissolving metal” reductions – use metallic Na or Li. Electron transfer mechanism. 

i. Reduce alkyne to trans alkene: Na°/NH3 

ii. Reduce benzene to 1,4‐cyclohexadiene (this is the Birch reduction): Na°/NH3/EtOH

c. Hydride reductions. 

i. NaBH4 will reduce an aldehyde, ketone, or acid chloride to the corresponding alcohol. It will not reduce an acid or an ester. 

ii. LiAlH4 will reduce an aldehyde, ketone, acid, or ester to the corresponding alcohol. 

iii. LiAlH(OtBu)3 will reduce an acid chloride to an aldehyde. 

iv. DIBAL will reduce an ester to an aldehyde. 

v. DIBAL will reduce a nitrile to an aldehyde (via the imine). 

vi. LiAlH4 will reduce an amide to the corresponding amine (complete removal of the carbonyl; goes via an imine which is then further reduced). 

vii. Neither NaBH4 nor LiAlH4 will react with an alkene or an alkyne (there is no partial positive charge for the hydride to attack).

4. Oxidation of alcohols. 

a. By  Jones reagent is a solution of chromium trioxide in dilute sulfuric acid and acetone Chromic acid (H2CrO4)  in acetone / CrO3 + H2SO4 will oxidize a primary alcohol to a carboxylic acid and a secondary alcohol to a ketone. Note that tertiary alcohols cannot be oxidized. 

b. PCC (pyridinium chlorochromate) Primary alcohols become aldehydes; secondary alcohols become ketones. 

c.  Swern oxidation: uses DMSO, oxalyl chloride, triethylamine does the same thing as the PCC

5. Oxidation of aldehydes and ketones. 

a. Aldehydes to carboxylic acids. 

i. Any of the CrVI or MnVII reagents will work. 

ii. Ag2O

 iii. Tollens reagent: Ag(NH3)2 + 

iv. Baeyer‐Villiger oxidation: mCPBA or other peracids 

b. Ketones 

i. Haloform reaction: NaOH/excess I2 gives the carboxylate 

ii. Baeyer‐Villiger oxidation gives an ester. Note migratory aptitudes.

6. Sharpless epoxidation – an enantioselective way of converting allylic alcohols into epoxides using chiral diethyltartrate. Which enantiomer of the epoxide is formed depends on which enantiomer of diethyltartrate is used. We won’t cover this one. 

7. Hydroxylation of alkenes (adding two OHs) 

a. Cold, dilute KMnO4 with NaOH gives the vicinal diol. Syn addition.

b. OsO4/peroxide also gives the vicinal diol. Also a syn addition. 

c. Form the epoxide with mCPBA, then open it with either acid or base. Anti addition.

 

8. Oxidative cleavages 

a. Of vicinal diols – use periodic acid (HIO4). Produces aldehydes and ketones. 

b. Of alkenes – ozonolysis. This is a syn addition of ozone to the alkene, giving a molozonide. It then rearranges to form an ozonide (two O’s on one side, one O on the other, in a five‐membered ring).   

i. Oxidative workup (peroxide) gives ketones and carboxylic acids. 

ii. Reductive workup (Zn°/H2O or Me2S) gives aldehydes and ketones. 

c. Of alkenes – permanganate cleavage. Dilute, low temperature gives vicinal diols; concentrated, high temperature gives ketones and carboxylic acids. If it’s a terminal alkene, one end becomes carbon dioxide. 

d. Of alkynes 

i. Ozone, low temperature, aqueous workup gives carboxylic acids. If it’s a terminal alkyne, one end becomes carbon dioxide. 

ii. Permanganate, base, room temperature gives a vicinal diketone.