Chem 1152: Ch. 14

Aldehydes and Ketones

IntroductionIntroduction

Aldehyde: Carbonyl group with one or two H attached.•Named by replacing –e with –al (IUPAC).•The aldehyde C is always numbered as 1.

O

R H

O

R R

O

H H

Ketone: Carbonyl group with two C attached.•Named by replacing –e with –one (IUPAC).•Numbered from end closest to carbonyl group

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Common Aldehydes and KetonesCommon Aldehydes and Ketones

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Naming AldehydesNaming Aldehydes

O

HCH3

pentanal

Cl

O

H

5-chloropentanal

H O

CH3CH3

2-ethylbutanal

CH3

O

H

CH3

CH3

3,4-dimethylhexanal

Cl

CH3 O

H

5-chloro-2-methylbenzaldehyde

Cl

O

H

m-chloro-benzaldehyde

CH3

O

CH3 O

H

5-methoxy-2-methylbenzaldehyde

Drawing AldehydesDrawing Aldehydes

2-bromo-2-methylpropanal

2-hydroxy-5-methoxybenzaldehyde

CH3

O

OH O

H

Br H

O

3-bromo-4-phenylbutanal

BrCH3

H

O

CH3

3-chloropentanal

CH3

Cl

H

O

Naming KetonesNaming Ketones

2-pentanone 5-chloro-2-pentanone 1-chloro-3-pentanone

4-ethylcyclohexanone 2-fluoro-5-methyl-4-octanone

O

CH3 CH3

Ketone: Carbonyl group with two C attached.•Named by replacing –e with –one (IUPAC).•Numbered from end closest to carbonyl group

ClO

CH3

O

Cl

CH3

O

CH3CH3

F O

CH3

CH3

Physical PropertiesPhysical Properties

CH3

OH

CH3 CH3

CH3

OCH3

O

CH3 H

Carbonyl O cannot form H-bonds between molecules, but can form weaker dipolar bonds. Carbonyl O can form H-bonds with water (polar solvent) for low MW ketones and aldehydes.

BP values lower than similar alcohols, but higher than those of alkanes.

O

R R

O

R R

δ-

δ-

δ+

δ+

Intermoleculardipolar

H HO

Hydrogen bond δ+

δ-

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Hydrogenation Rxn – AldehydesHydrogenation Rxn – AldehydesAldehyde hydrogenation rxnAldehyde hydrogenation rxn

Alcohol oxidation rxnAlcohol oxidation rxn

H-OH+RC

O

HH

H

(O)+ R C

O

H

(O)R C

O

OH

Carboxylic acid

Primary alcoholaldehyde

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Hydrogenation Rxn – AldehydesHydrogenation Rxn – AldehydesAldehyde hydrogenation rxnAldehyde hydrogenation rxn

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

butanal

CH3

O

H

CH3

CH3

3,4-dimethylhexanal

+ H HPt

O

HCH3

+ H HPt

O

HCH3

HH

O

HCH3

1-butanol

CH3 HH

O

CH3

CH3 H

3,4-dimethyl-1-hexanol

Hydrogenation Rxns – KetonesHydrogenation Rxns – KetonesKetone hydrogenation rxnKetone hydrogenation rxn

H-OH+RC

O

HR

H

(O)+ R C

O

R

Secondary alcohol ketone

Alcohol oxidation rxnAlcohol oxidation rxn

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

Hydrogenation Rxns – KetonesHydrogenation Rxns – Ketones

Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

+ H HPt

O

CH3CH3

HH

O

CH3CH3

2-butanone 2-butanol

Ketone hydrogenation rxnKetone hydrogenation rxn

+ H H

Pt

CH3CH3

CH3

OCH3

H CH3

CH3

CH3

OH

CH3

4,4-dimethyl-3-hexanone

4,4-dimethyl-3-hexanol

Addition of alcohol to aldehydes and ketonesAddition of alcohol to aldehydes and ketones

Hemiacetal is unstable, hard to isolate. With excess alcohol and an acid catalyst, a stable acetal is formed.

Note the bidirectional arrows

O

R H

+ H

R

O

RO

HO

R H

H+, RO-H

RO

RO

R H + H-O-H

Hemiacetal Hemiacetal intermediateintermediate

acetalacetal

O

R R

+ H

R

O

RO

HO

R R

H+, RO-H

RO

RO

R R + H-O-H

Hemiketal Hemiketal intermediateintermediate

ketalketal

Hydrolysis of acetals and ketalsHydrolysis of acetals and ketals

“Cutting by water” is essentially the reverse of the addition of alcohol to either aldehyde or ketone.

aldehydealdehydeacetalacetal

RO

RO

R H +H

H

O H+ O

R H

2 R-OH+

alcoholalcohol

ketoneketoneketalketal alcoholalcohol

RO

RO

R R +H

H

O H+ O

R R

2 R-OH+

Intramolecular addition of alcohols to aldehydeIntramolecular addition of alcohols to aldehyde

C1 is hemiacetal carbon. Attached to it you will find: H, OH, OR and R, just like non-cyclical compounds.

Intramolecular Intramolecular hemiacetalhemiacetalglucoseglucose

C

C

C

C

CH2

C

OH

OH

OH

OH

HO H

H

H

H

H O1

2

3

4

5

6

*CH2OH

C

C

C C

O

C

6

5

4

3 2

1

OH

H

OH

OH

OH

H

H

H

HR

Aldehyde Rxns – Testing for AldehydesAldehyde Rxns – Testing for Aldehydes

Benedict’s Reagent reacts with ketones that have an alcohol on the adjacent carbon (e.g., glucose)

Products include a red precipitate of copper (I) oxide

Tollens’ Reagent (reacts with all aldehydes, but not ketones)Tollens’ Reagent (reacts with all aldehydes, but not ketones)

Benedict’s Reagent (reacts with aldehydes, alpha-hydroxy ketones)Benedict’s Reagent (reacts with aldehydes, alpha-hydroxy ketones)