Aldehydes & Ketones

Question 1

Preparation of aldehydes:

Answer 1

• primary alcohols oxidation, PCC
• ozonolysis (oxidative cleavage) of alkenes w/ at least 1 vinylic H
• partial reduction of an ester w/ diisobutylaluminium hydride (DIBAH) in toluene solution in presence of dry ice (CO₂(s)) to prevent further reduction
• hydroboration/oxidation of terminal alkynes

Question 2

Preparation of ketones:

Answer 2

• secondary alcohols oxidation (various agents)
• ozonolysis of alkenes, if one of dbl bond alkenes is disubstituted
• Friedel-Crafts acylation of benzene ring w/ acid chloride in the presence of AlCl₃ catalyst => aromatic ketones
• hydroboration/oxidation of internal alkynes => two ketones
• mercury(II)-catalyzed hydration of alkynes => enol => methyl ketones

Question 3

Reactions of Carbonyl Compounds

Answer 3

• Oxidation
• Nucleophilic Addition Reactions
• Nucleophilic Addition of H2O: Hydration
• Nucleophilic Addition of HCN: Cyanohydrin Formation
• Nucleophilic Addition of Grignard and Hydride Reagents: Alcohol Formation
• Nucleophilic Addition of Hydrazine: The Wolff-Kishner Reaction

Question 4

Oxidation of Aldehydes and Ketones:

Answer 4

aldehydes: easily oxidized to COOH w/ various agents, ketones: generally resistant to oxidation (in hot alkaline a slow cleavage may take place).

Question 5

Better yields at room temperature are obtained when oxidising aldehyde to COOH with:

Answer 5

Cro3 in aqueous acid, as opposed to KMnO4

Question 6

Tollens test:

Answer 6

alternative to CrO3 oxidation, if there are sensitive molecules involved (to prevent side reactions)

oxidation of an aldehyde by a solution of silver oxide, Ag2o, in aqueous ammonia, the so-called Tollens reagent => COOH + Ag as a thin mirror coating deposited on the inner walls of the glass

Question 7

Basis of Nucleophilic Addition Reactions of Aldehydes and Ketones

Answer 7

carbinyl grp C is delta + => it can be attacked by a nucleophile (neg: -OH^-, -H^-; OR neutral: HOH, ROH) => carbonyl group undergoes rehybridization (sp2 to sp3) & pi-bond is broken => 2 electrons go to O atom as a lone pair => formation of intermediate alkoxide ion

Question 8

Nucleophilic Addition of H2O

Answer 8

Aldehydes and ketones + HOH => 1,1-diols aka geminal diols

reaction is reversible, slow under neutral conditions but is catalyzed by both acid and base

Question 9

Nucleophilic Addition of HCN

Answer 9

Aldehydes and ketones + HCN => cyanohydrins

reaction is normally slow, may be catalyzed by adding a little bit of base, or by adding KCN or NaCN salt to the HCN

-CN => -CH₂NH₂ (LAH)
-CN => hydrolysis to -COOH (hot acid)

Question 10

Nucleophilic Addition of Grignard and Hydride Reagents

Answer 10

Formaldehyde => Primary alcohol
Any aldehyde other than formaldehyde => Secondary alcohol
Ketone => Tertiary alcohol
Ester => Tertiary alcohol

Question 11

If the addition of hydride ions is followed by acidification…

Answer 11

alcohol will result

Question 12

Wolff-Kishner Reaction

Answer 12

aldehyde or ketone + hydrazine (H₂NNH₂) in the
presence of KOH => carbonyl group converts to methylene grp (-CH₂-))

this reaction is used to convert aldehydes & ketones to alkanes