Chapter 2 : ALDEHYDE Norfazrin Mohd Hanif

Faculty of Applied ScienceUiTM Negeri Sembilan

SUBTOPICS

Nomenclature – common and IUPAC names for aldehyde

Physical properties of aldehyde : Boiling points and solubility

Preparation of aldehydeOxidation of alcoholReduction of acid chlorides

Reactions of aldehydeOxidationReductionAddition (with HCN, NaHSO3, H2O, Grignard reagentsCondensation (with ammonia, hydrazine and their derivatives)Iodoform reaction

SUBTOPICS

INTRODUCTION

Aldehyde contain the carbonyl group – a group in which a carbon atom has a double bond to oxygen :

Carbonyl group

Oxygen Carbonyl

Carbon Carbonyl

The carbonyl group in aldehyde is bonded to at least one hydrogen atom.Using R, we can designate the general formula as:

C

O

R Hor RCHO ( R = alkyl or aryl or H)

C

O

R H

2.0 NOMENCLATURE 2.1 IUPAC 2.2 Common name

2.1 IUPAC Names

Aldehydes are named by replacing the final -e of the name of the corresponding alkane with –al.

~ The aldehyde functional group is always carbon 1 & need not be numbered.

IUPAC :COMMON:

IUPAC :COMMON:

2.1 IUPAC Names

Aldehyde functional groups bonded to a ring are named using the suffix carbaldehyde.

~ Benzaldehyde is used more commonly than the name benzenecarbaldehyde.

IUPAC :COMMON:

2.2 Common Names

Aldehydes are named from the common names of the corresponding carboxylic acid.

The ‘ic acid’ ending is replaced with ‘aldehyde’.

Structure IUPAC name Common name Structure IUPAC Common name

HCO2H methanoic acid HCHO methanal

CH3CO2H ethanoic acid CH3CHO ethanal

CH3CH2CO2H propanoic acid CH3CH2CHO propanal

CH3(CH2)2CO2H butanoic acid CH3(CH2)2CHO butanal

CH3(CH2)3CO2H pentanoic acid CH3(CH2)3CHO pentanal

CH3(CH2)4CO2H hexanoic acid CH3(CH2)4CHO hexanal

formic acid

acetic acid

propionic acid

butyric acid

valeric acid

caproic acid

formaldehyde

acetaldehyde

propionaldehyde

butyraldehyde

valeraldehyde

caproaldehyde

2.2 Common Names

Substituents locations are given using Greek letters (, , , ,….) beginning with the carbon next to the carbonyl carbon, the -carbon.

-bromobutyraldehyde

-hydroxyvaleraldehyde

-phenylacetaldehyde

CH3CHBrCH2C

O

H CH3CHCH2CH2C

O

H

OH

CH2C

O

H

2.0 PHYSICAL PROPERTIES 2.1 Boiling Point 2.2 Solubility

2.2 Physical Properties

Oxygen is more electronegative than carbon (3.5 vs 2.5) and, therefore, a C=O group is polar

aldehydes and ketones are polar compounds and interact in the pure state by dipole-dipole interactions

they have higher boiling points and are more soluble in water than nonpolar compounds of comparable molecular weight

C O C O –

Polarity of acarbonyl group

-+C O

+

More importantcontributing

structure

::: : :

2.2 Physical Properties

PROPERTY OBSERVATION

Boiling Point

Solubility

RCHO having ≤ 5 C’s are H2O soluble because they can hydrogen bond with H2O.RCHO having > 5 C’s are slightly soluble in H2O.

δ+

δ-…………

HO

Hδ+ Hydrogen bond with water.

2.2 Physical Properties

Solubility of Aldehydes :

2.0 PREPARATION OFALDEHYDE

2.1 Oxidation of 1° alcohol 2.2 Reduction of 2.2.1 Acyl Chlorides, 2.2.2 Esters 2.2.3 Nitriles

A) Oxidation of 1o Alcohols

General formula:

Using PCC as oxidizing agent :

PCC: Pyridinium chlorochromate

A) Oxidation of 1o Alcohols

Using strong oxidizing agent:

CH3CH2OH CH3 C OH

O

Ethanol Ethanoic Acid

H2CrO4

acetone35oC

CH3CH2OH CH3 C OH

O

Ethanol Ethanoic Acid

KMnO4/ H+

B) Reduction of Acyl Chlorides

R C

O

Clacid chloride lithium aluminium tri(t-butoxy)hydride

Li+ -AlH(O-t-Bu)3R C

O

Haldehyde

CH3CHCH2C

CH3 O

Cl lithium aluminium tri(t-butoxy)hydride

Li+ -AlH(O-t-Bu)3

CH3CHCH2C

CH3 O

H

Example:

* Lithium aluminium tri(t-butoxy)hydride is a milder reducing agent that reacts faster with acid chlorides than with aldehydes.

CO

Cl

LiAlH(O-t-Bu)3C

O

H

LiAlH(O-t-Bu)3

benzoyl chloride benzaldehyde

CH3CHCH2CO

Cl

CH3

CH3CHCH2CO

H

CH3

isovaleryl chloride isovaleraldehyde

B) Reduction of Acyl Chlorides

3.0 REACTIONS3.1 Oxidation3.2 Reduction3.3 Nucleophilic Addition3.4 Aldol Condensation &

Cannizaro Reaction3.5 Iodoform reaction

1) Oxidation of Aldehydes

Aldehydes are easily oxidized to carboxylic acid by: strong oxidizing agent such as potassium permanganate,KMnO4

mild oxidizing agent such as silver oxide, Ag2O in aqueous ammonia (Tollen’s Test : differentiate between aldehyde & ketone)

General Reaction

RC

H

O

[o]

R OH

O

Aldehyde Carboxylic Acid

CH3─CH2─CH2─CH2─C─OH

[O] :KMnO4, OH-

K2Cr2O7/H2SO4

Ag(NH3)2+OH- (Tollen’s reagent)

K2Cr2O7

H2SO4

=

O

CH3─CH2─CH2─CH2─C─H

=

O

Pentanal Pentanoic acid

Examples

1) Oxidation of Aldehydes

In the laboratory, Tollens’ test may be used to distinguish between an aldehyde and ketone. Tollens’ reagent, a solution of Ag+ (AgNO3) and ammonia, oxidizes aldehyde, but not ketones. The silver ions is reduced to metallic silver, which forms a layer called a “silver mirror” on the inside of the container

* Tollens’ test is used to distinguish aldehydes from ketones. Ketones DO NOT react with Tollens’s reagent.

Tollens’ Test (Silver Mirror Test)

2) Reduction of Aldehydes

Hydride ionLithium aluminum hydride (LAH)

Sodium borohydride

H

H H

H

H-B-H H-Al-HLi +Na+

H:

Reduction of an aldehyde gives a primary alcohol .

Aldehydes can be reduced to alcohol by

• H2/Ni or H2/Pd

• LiAlH4

• NaBH4

(most often used)

2) Reduction of Aldehydes

Examples:

CH3 C H

O

CH3 C H

O-

H

CH3 C H

OH

H

H+

ethanal

ethanol

LiAlH4

3.3 Nucleophilic Addition Reaction Of

3.3.1 HCN: Cyanohydrin Formation

3.3.2 Ammonia & Its Derivatives

3.3.3 Grignard Reagent : Formation of Alcohol

3) Nucleophilic Addition

The carbonyl groups in aldehydes and ketones are polarised because of the difference in the electronegativity of carbon and oxygen.

The carbon atom carries a partial positive charge while oxygen atom carries a partial negative charge.

Aldehydes and ketones are susceptible to attack both by nucleophiles at the carbonyl carbon atom and by electrophiles at the oxygen atom.

C O

electrophilic attacknucleophilic attack

δ-δ+

3) Nucleophilic Addition

Nucleophilic Addition Reaction of :

a. HCN: Cyanohydrin Formation

b. NaHSO3

c. Grignard Reagent : Formation of Alcohol

a) Nucleophilic addition of hydrogen cyanide

C

O

R R' HCN CR R'

OH

CNketone or aldehyde

cyanohydrin

example

C

O

CH3 H HCN CCH3 H

OH

CNethanal

1-hydroxy-1-methylpropanenitrile

* Cyanohydrin may be formed using liquid HCN with a catalytic amount of sodium cyanide or potassium cyanide.

C

O

R H HCN CR CN

OH

H

C

O

CH3 H HCN CCH3 CN

OH

H

CR COOH

OH

H

CCH3 COOH

OH

H

NH4+

NH4+

aldehyde

cyanohydrin

example

ethanal

2-hydroxypanenitrile

H2O/H+

carboxylic acid

H2O/H+

2-hydroxypropanoic acid(lactic acid)

C

O

CN C CN

OH+

C CN

OH

MECHANISM

a) Nucleophilic addition of hydrogen cyanide

• When shaken with an aqueous of sodium bisulphite, most aldehydes and ketones formed carbonyl bisulphite (a colourless crystal).

• The reaction takes place more readily with aldehydes than with ketones.

• The nucleophile is the hydrogensulphite ion, HSO3-

• Example:

b) Nucleophilic addition of sodium bisulphite (NaHSO3)

NaHSO3 H C

O

CH3 H C

OH

CH3

OSO2- Na+

Bisulphite salts

ethanal

3) Condensation with Hydrazines, Hydroxlamine and Phenylhydrazine

• Aldehydes and ketones condense with ammonia derivatives such as hydroxylamine and substituted hydrazines to give imine derivatives.

i) Reaction with hydrazine:Hydrazines derivatives reacts with aldehydes or ketones to form hydrazones.

R C

O

R' H2N-NH2 R C

N

R'

NH2H+

H2Oaldehyde or ketone hydrazine hydrazone derivative

Example:

CH

O

H2N-NH2H+

hydrazineC

H

N NH2

H2O

benzaldehydebenzaldehyde hydrazone

imine

R N

C

3) Condensation with Hydrazines, Hydroxlamine and Phenylhydrazine

ii) Reaction with hydroxylamine:Hydroxylamine reacts with ketones and aldehydes to form oximes.

R C

O

R' H2N-OH R C

N

R'

OHH+

H2Oaldehyde or ketone hydroxylamine oxime

Example:

H2N-OH H+

hydroxylamineH2O

butanal

butanal oxime

H

O

H

N

OH

3) Condensation with Hydrazines, Hydroxlamine and Phenylhydrazine

ii) Reaction with phenylhydrazine :

R C

O

R' R C

N

R'

NH-PhH+

H2Oaldehyde or ketone phenylhydrazine

phenylhydrazone

Example:

H+

H2O

butanal butanal phenylhydrazone

H N NH

H

Ph

phenylhydrazine

H N NH

H

Ph

H

O

H

N-NH-Ph

3a) Condensation with 2,4-dinitrophenylhydrazine (2,4-dnp)

A solution of 2,4-DNP in methanol and H2SO4: Brady’s reagent.

Aldehydes reacts with 2,4-DNP at room temperature to give a yellow-orange precipitate of 2,4-dinitrophenylhydrazone.

Positive TestReagent

C O

HNO2

NO2NH2N

H

NO2

NO2NN

H

C

H

H2Oroom

temperature

benzaldehyde 2,4-dinitrophenylhydrazone(yellow-orange precipitate)

benzaldehyde 2,4-dinitrophenylhydrazine

C O

NO2

NO2NH2N

H

NO2

NO2NN

H

CR'

R

H2Oroom

temperature

2,4-dinitrophenylhydrazine

R

R'

• 2,4-Dinitrohydrazones have characteristic sharp melting points.

• The formation of a yellow or orange precipitate when 2,4-DNP reacts with an organic compound at room temperature is used

a) As chemical test for aldehydes or ketones,

b) To identify an aldehyde or a ketone by measuring the melting point of the 2,4-dinitrophenylhydrazone formed.

3a) Condensation with 2,4-dinitrophenylhydrazine (2,4-dnp)

• Condensation : combination of two or more molecules with the loss of a small molecule such as water or an alcohol.

• Aldol condensation : involves the nucleophilic addition of an enolate ion to another carbonyl group.

• The product, a β-hydroxy ketone or aldehyde, is called an aldol because it contains both an aldehyde group and the hydroxy group of an alcohol.

• This reaction is for aldehyde or ketone that have α-hydrogen atom.

R C

O

CH2 R C CH2

O

R' R' R C

O

CH

R'

CR

OH

CH2 R'H+ or -OH

aldehyde or ketonealdol product

4) Aldol Condensation

Cannizaro reaction: Chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position.

Disproportionation: oxidation-reduction reaction in which the same element is both oxidized and reduced.

Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and benzoic acid from the treatment of benzaldehyde with potash (potassium carbonate).

In this disproportionation reaction, one molecule of the aldehyde acts as an oxidant and converts a second molecule of aldehyde into a carboxylic acid while consequently being reduced to an alcohol itself.

4) Cannizaro Reaction

(CH3)3CCHO2NaOH

(CH3)3CCOONa + (CH3)3CCH2OH

alcoholcarboxylate salt

Examples:

aldehyde with no α-hydrogen atom

CH3CH2CH2NaOH

CH3CHC

C

OH

CH3CH2 HO

O

H

aldol product

aldehyde with α-hydrogen atom

4) Cannizaro Reaction

5) Reaction With Grignard Reagent

A Grignard reagent (a strong nucleophile resembling a carbanion, R:- attacks the electrophilic carbonyl carbon atom to give an alkoxide intermediate.

Subsequent protonation gives an alcohol.

MgBrCH3CH2C O

H3C

HC O- +MgBr

CH3

H

CH3CH2

C OH

CH3

H

CH3CH2

H3O+

2-butanol

alkoxideethanalethylmagnesium bromide

6) Haloform Reaction

IODOFORM TEST

- a solution of I2 in an alkaline medium such as NaOH or KOH is a oxidising agent.- when ethanal warmed with this solution, triiodoethanal will be formed as the intermediate product.- triiodoethanal then reacts with the base to form a yellow precipitate of triiodomethane (iodoform).

CH3CHO + 3I2 → CI3CHO + 3HI triidoethanal

Cl3CHO + -OH → CHI3 + HCOO-

iodoform

6) Haloform Reaction

• Iodoform test is useful for the methyl ketone group (CH3C=O) in ethanal and methyl ketones.

• If an alkaline solution of iodine is warmed with an organic compound and a yellow precipitate of triiodomethane is produced, the organic compound is likely to be one of the following:

CCH3 H

OH

H

ethanol C

O

CH3 Hethanal

a secondary alcohol with the CHCH3

OH

group

a ketone with the CCH3

O

group

6) Haloform Reaction

Iodoform test can be used to distinguish:

i) ethanal from other aldehydes, because ethanal is the only aldehydes that gives a positive iodoform test.ii) ethanol and secondary alcohols that contains the CH3CH(OH)- group give a positive iodoform test.

iii) methyl ketones (ketones that contain CH3CO- group) give positive iodoform test.For example, propanone and phenylethanone give a yellow precipitate, but 3-pentanone and diphenylmethanone give negative iodoform tests.

6) Haloform Reaction

C

O

CH3 3I2

C

O

C

I

I

I NaOH

C

O

CH3 3I2 NaOH

C

O

C

I

I

I

C

O

O- Na+

C

O

O- Na+

3HI

CHI3

CHI3 3HI

warmphenylethanone

The overall reaction is

heat

phenylethanone sodium benzoate iodoform(yellow precipitate)

Thank you!