General Organic Chemistry - Hanoi University of Agriculture - 12

1.22A 0



C O

120 0

Link In the C=O group, it is always polarized towards oxygen, because oxygen has a greater electronegativity than oxygen.

electricity of carbon. We can describe:



CO

This polarization creates a C  + center for nucleophilic reactions.

Different hydrocarbon radicals have different effects on the positive charge value of the functional carbon.

- The alkyl group causes a positive inductive effect +I, reducing the positive charge value of carbon in the C=O group. The number of alkyl groups increases, the weight of the alkyl group increases, the +I effect increases, so the  + of carbon in the C=O group decreases.



R CH O

R

 '

 '

=

C R

O  '

- Phenyl group causes +C effect, reducing the positive charge value of functional carbon. For example:



GIVE

+C

- In conjugated unsaturated aldehyde and ketone molecules, the vinyl group causes a positive conjugation effect.

+C. This effect reduces the positive charge value of functional carbon.

Example:

CH 2



+C CH

GIVE

With polycarbonyls, especially 1,2-dicarbonyls, the C=O groups exert the –I effect and interact with each other. This interaction increases the positive charge of the functional carbon.

Example:

-I

O CH



GIVE

Besides the influence of hydrocarbon radicals on carbonyl groups, carbonyl groups also have certain influence on hydrocarbon radicals and C- hydrogen bonds .

- With saturated radicals, the C=O group causes a negative inductive effect (-I). This effect increases the polarity of the C  -H bond.

Example:

H

H C -IH

CH=O

- With aromatic radicals, the C=O group causes electron-withdrawing effects –I and -C. These effects reduce the electron density in the aromatic ring, and mainly reduce it at the octo and para positions. Thus, an electrophilic substitution reaction center is created at the meta position.

Example:

-C

GIVE

-I

In conjugated unsaturated carbonyl compounds, the C=O group causes the negative conjugation effect (-C). This effect polarizes the conjugated system.

Example:



CH 2

-C 

CH

GIVE

The bonds in the C=O group are always polarized towards oxygen, causing the electron density at carbon to decrease, carbon carries a partial positive charge, leading to increased polarization of the C-H bond in the aldehyde and creating the center of the oxidation reaction at this bond.



C

O

δ

H

3.6. Chemical properties

a. Addition reaction to the C=O group

C

    OH

CO + HY

or XY Y

HY is H-OH, H-OR, H-SO 3 Na, …XY is R-MgX, CH  CNa,…

The addition reaction to the C=O group occurs by a two-step nucleophilic addition mechanism.

Step one: The nucleophile Y - combines with the positively charged carbon atom of the C=O group and forms an intermediate product, an anion.

Step two: The protonation stage of the intermediate product occurs rapidly.



CO + Y

slow

O fast OH

C

Y

C Y H

- Specific reactions

+ Alcohol addition: The reaction of aldehydes and ketones with alcohol forms a product called acetal compound.

R CH = O + R OH

R - CH - OH OR

When heating acetal with excess alcohol, acetal is obtained.

R - CH - OH

R OH

R- CH - OR'

OH 2

OR OR

Acetal sale

+ Hydrogen cyanide addition (HCN)



CO + HCN

Acetal

C

OH CN

The reaction occurs in the presence of a base as a catalyst and produces cyanhydrin as a product. For example:

CH 3

CH 3 C = O + HCN

CH 3 CH 3 C OH

CN

When hydrolyzed, cyanhydrin compounds will form α - hydroxy acid.

CH 3 CH 3 C OH

CN

+ Add sodiumbisulfite (NaHSO 3 )



2 H 2 O

CH 3

CH 3 C COOH OH

OH

NH3​

CO + NaHSO 3

C

SO 3 Na

The product of the reaction is in crystalline state and is called bisulfite addition compound. This reaction

used to separate aldehydes or ketones from mixtures with other substances.

Example:

OH C 6 H 5 CH =O + HSO 3 Na C 6 H 5 -CH -SO 3 Na

OH

=

CH 3 -C -CH 3 + HSO 3 Na CH 3 -C -SO 3 Na O CH 3

The general mechanism is as follows:



O O

=

=

C H

C = O +

S - O Na

OH

SO​

ONa OH

C OH

SO 3 Na

+ Addition of organometallic compounds

From organomagnesium compounds and aldehydes or ketones, alcohols of different degrees can be synthesized. In these reactions, organomagnesium compounds act as polar substances:



R MgX (R: alkyl group)



C = O +



R MgX

C OMgX R

HO H

-MgXOH

CO H R

The initial carbonyl compound is HCHO, we get a primary alcohol. If it is a homologue of formic aldehyde, we get a secondary alcohol. If it is a ketone, we get a tertiary alcohol.

+ Reacts with ammonia and its derivatives (NH 3 and B-NH 2 . B can be: Hydrocarbon radical;

-NH 2 ; -OH, etc…)

3 2

General diagram of the reaction:

 

H 2 O

C ON -B

CNH2 - BO

CNH-BOH

C = N -B

Yellow.

Phenyl hydrazine (C 6 H 5 NHNH 2 ) reacts with aldehydes or ketones to form a colored precipitate.

CO​

Example:

H 2 N -NH -C 6 H 5

C = N -NH -C 6 H 5 + H 2 O

CH =O

H 2 N -NH -C 6 H 5

CH =N -NH -C 6 H 5

H 2 N -NH -C 6 H 5

CH =N -NH -C 6 H 5

CH =O

- H 2 O

CH =O - H 2 O

CH =N -NH -C 6 H 5

bisphenylhydrazone

b. Oxidation reaction

Oxidation reaction forms carboxylic acid.

Aldehydes are easily oxidized by various oxidizing agents, even by weak oxidizing agents such as Tolens reagent, Phelin reagent, etc.

General equation:

R -CH=O + [O] R -C =O OH

carboxylic acid aldehyde