chemical properties of aldehydes and...

Chemical Properties of Aldehydes and Ketones


Oxidation of Aldehydes

Aldehydes are very easily oxidized to carboxylic acids.

R C O HH

HR C O H

OR C

OH

primary alcohol aldehyde carboxylic acid

[O][O]

From the standpoint of the organic component, an oxidation can involve the removal of hydrogen atoms OR the addition of oxygen to the structure.

Oxidation reagents: MnO4-, Cr2O72-, Ag+, Cu2+ Usually, the metal ion is “reduced” in the process.


Oxidation of AldehydesOxidation of Aldehydes forms the basis of two useful identification tests

Tollen’s Test:

Benedict’s Test:

RC

H

O+ Ag+ + 3 OH-

RC

O-

O+ Ag0 + 2 H2O

RC

H

O+ 2 Cu2+ + 5 OH-

RC

O-

O+ + 3 H2OCu2O


Benedict’s Test for Aldehydes

Cu2+ Cu+

Oxidation of Aldehydes

+ ++ +++

The degree of reduction varies with time and the concentration of the aldehyde.


Tollen’s Test for Aldehydes

Ag+ Ago

Ketones DO NOT undergo the same

transformations with these oxidizing agents !!


Addition Reactions to Carbon-Oxygen Double Bonds

REVIEW:Reactions of Carbon-Carbon Double Bonds

“A” and “B” can be the same or different atoms or groups of atoms.

In some cases, the identity of A and B and the groups attached to the double bond influence the orientation of the addition.

C

C

A

B

C

C

A

B C

C

A

B



Reactions of the Carbon-Oxygen double bond can be viewed in a similar manner.

“A” and “B” can be the same or different atoms or groups of atoms.

The identity of A and B and the polar nature of the carbon-oxygen double bond influence the orientation of the addition.

O

C

δ−

δ+

A

B δ−

δ+O

CA

B

O

C

A

B

δ−

δ++

δ−

δ+


Reduction (Hydrogenation) of Aldehydes and Ketones

O

C

H

H O

C

H

H

O

C

H

H

Break Bonds

Form Bonds

Aldehyde or Ketone Alcohol


Reduction (Hydrogenation) of Aldehydes and Ketones

2,3-dimethylbutanal 2,3-dimethyl-1-butanol

H2 Ni (catalyst)

H3CCH

CHC

O

H

H3C

CH3H3C

CHCH

CH2

OHH3C

CH3

C6H12O C6H14O

H3CCH

CH2C

O

CH3

F

H3CCH

CH2 HC

OH

CH3

F

H2 Ni (catalyst)C5H9OF C5H11OF

4-fluoro-2-pentanone 4-fluoro-2-pentanol


Hydrates, Hemiacetals, Hemiketals, Acetals, and Ketals

O

C

H

O X

O

C

δ−

δ+ δ−

δ+ H

O X

X = H, R

If X=H, H-O-X is water;

If X=R, H-O-R is an alcohol.

aldehyde hydrate ketone hydrate

hemiacetal hemiacetal (hemiketal)


Hydrates, Hemiacetals and Hemiketals

OC

H

O H R H

OC

H

O H R Rʼ

OC

H

O Rʼ R H

OC

H

O Rʼʼ R Rʼ

O

C?

O

?

O

C? ?

O

CCH3 H

H

O H + O

C

H

O H

HCH3

O

CCH3 CH3

H

O H + O

C

H

O H

CH3 CH3

acetaldehyde acetaldehyde hydrate

acetone acetone hydrate


Addition of Water - Hydrate Formation


Addition of an Alcohol - Hemiacetal Formation

O

CCH3CH2 H

H

O CH3 + O

C

H

O CH3

H

O

CCH3

H

O CH3 + O

C

H

O CH3

CH3

propionaldehyde propionaldehyde methyl hemiacetal

butanone butanone methyl hemiacetal

CH3CH2

CH3CH2

CH3CH2

Addition of an Alcohol - Hemiacetal Formation

Further Reactions of Hemiacetals

Reaction with an Alcohol - Acetal Formation

O

C

H

O CH3

HCH3CH2

O

C

CH3

O CH3

HCH3CH2

H O CH3

propionaldehyde propionaldehyde methyl hemiacetal methyl acetal

++ H2O?

O

C

H

O CH3

CH3CH3CH2

O

CO CH3

CH3CH3CH2

CH3H O CH3

butanone butanone methyl hemiacetal methyl acetal

+ ? + H2O

Note: Acetal formation from a hemiacetal is another example of an

INTERMOLECULAR DEHYDRATION or CONDENSATION reaction.

OC

H

O Rʼ R H

OC

H

O Rʼʼ R Rʼ

OC

Rʼʼ

O Rʼ R H

OC

Rʼʼʼ

O Rʼʼ R Rʼ

Further Reactions of Hemiacetals

Reaction of a Hemiacetal with an Alcohol - Acetal Formation

O RʼʼH O RʼʼʼH

CH3 CH2 CO

H

CH3 CH2 COH

OCH3

H CH3 CH2 COCH3

OCH3

H

CH3OH, H+ CH3OH, H+

H2O

CH3OH

H2O, H+

H+

CH3OH

Aldehyde Hemiacetal Acetal

Most hemiacetals are unstable and cannot be isolated.

The equilibrium forming a hemiacetal lies far to the left, or to the reactant side of the reaction.

Hemiacetals and Acetals

The equilibrium forming an acetal from a hemiacetal lies far to the right, if excess alcohol is present.

Hemiacetals and Acetals

CH3 CH2 CO

H

CH3 CH2 COH

OCH3

H CH3 CH2 COCH3

OCH3

H

Aldehyde Hemiacetal Acetal

Addition Condensation

Elimination Hydrolysis

In the presence of excess alcohol and acid, an aldehyde or ketone is converted into its corresponding acetal.

In the presence of excess water and acid an acetal is converted into its parent aldehyde or ketone.

Hemiacetal and acetal formations are reversible reactions:

Amylose Maltose Glucose

Acetal linkages

Hemiacetal linkage

chemical properties of aldehydes and...

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