ORGANIC CHEMISTRY CHM 207

CHAPTER 7:CHAPTER 7:CARBONYL COMPOUNDS CARBONYL COMPOUNDS

(ALDEHYDES AND (ALDEHYDES AND KETONES)KETONES)

NOR AKMALAZURA JANI

SUBTOPICS• Nomenclature• Physical properties:

- boiling points- water solubility

• Reactions:- Oxidation- Reduction- Condensation with 2,4-dinitrophenylhydrazine- Nucleophilic addition- Haloform reaction

• Uses of carbonyl compounds.

ALDEHYDES AND KETONES

• Functional group: carbonyl group

C O

C

O

R'RC

O

HR

ketone aldehyde

R, R' = substituents

Aldehyde: one hydrogen atom is bonded to the carbon in the carbonyl group.

Ketone: the carbon atom in the carbonyl group is bonded to two hydrocarbon groups.

Naming AldehydesNaming Aldehydes

The IUPAC names of aldehydes are obtained by dropping the –e and adding -al to the name of the parent hydrocarbon.

butane butanal al

• The parent hydrocarbon is the longest chain that carries the –CHO group.

4

3 2

1

• This chain has 4 carbon atoms.

• The parent hydrocarbon is the longest chain that carries the –CHO group.

4 3 2

1

• This chain has 5 carbon atoms.

5

4 3 25

3-methylpentanal

1

• The –CHO group is always at the beginning of the carbon chain. The carbonyl carbon is numbered as carbon 1.

1

• The common names of aldehydes are derived from the common names of the carboxylic acids.

butyric acid butyraldehyde

• The –ic acid or –oic acid ending of the acid name is dropped and is replaced with the suffix –aldehyde.

NOMENCLATURE OF CYCLIC ALDEHYDES

• Aliphatic aldehydes containing a ring as well as aromatic aldehydes in which the aldehyde (-CHO) group is attached directly to the benzene ring are named by adding suffix carbaldehyde to the name of the corresponding hydrocarbon.

CHOCHO

CH3

cyclohexanecarbaldehyde 2-methylcyclohexanecarbaldehyde

123

45 6

CHO

CHO

NO2benzaldehyde 4-nitrobenzaldehyde

• Naming aromatic compounds:

CH2CHO CH=CHCHO

phenylethanal 3-phenylpropenal

123

If the aldehyde group is not attached directly to the benzene ring, the aldehyde is named as an aryl derivatives of the corresponding aldehyde.

Naming KetonesNaming Ketones

• The IUPAC name of a ketone is derived from the name of the alkane corresponding to the longest carbon chain that contains the ketone-carbonyl group.

• The parent name is formed by changing the –e ending of the alkane to -one.

propane propanone one

• If the carbon chain is longer than 4 carbons, it’s numbered so that the carbonyl carbon has the smallest number possible, and this number is prefixed to the name of the ketone.

This end of the chain is closest to the C=O. Begin numbering here.

IUPAC name: 3-hexanone

New IUPAC name: hexan-3-one

4321 5 6

• The common names of ketones are derived by naming the alkyl or aryl groups attached to the carbonyl carbon followed by the word ketone.

ethyl propyl ketone

ethyl propyl

NOMENCLATURE OF CYCLIC KETONES AND AROMATIC COMPOUNDS

cyclohexanone 4-methylcyclohexanone

O

O

CH3

1 234

56

phenylethanone diphenylmethanone

C

O

CH3 C

O

Aromatic compound:

- phenyl is used as part of the name.

• The parent name is formed by changing the –e ending of the cycloalkane to -one.

• Carbonyl carbon is designated C1.

• A ketone or aldehyde group can also be named as a substituent on a molecule with another functional group as its root.

• The ketone carbonyl is designated by the prefix oxo-• The –CHO group is named as a formyl group.• Carboxylic acids frequently contain ketone or aldehyde groups

named as substituents.

CH2 C

O

CCH3CH2

O

HCOOH

C

O

H

CH2 C

O

CCH3

O

OH

3-oxopentanal12345

2-formylbenzoic acid

1

2

34

56

3-oxobutanoic acid

1234

BOILING POINTS- Polarization of the carbonyl group creates dipole-dipole attractions between the molecules of ketones and aldehydes.- this attractions resulting in higher boiling points for ketones and aldehydes than for hydrocarbons and ethers of similar molecular weights. - did not have O-H and N-H bonds → can not form hydrogen bonds with each other.- boiling points of ketones and aldehydes are lower than alcohols of similar molecular weight.

PHYSICAL PROPERTIES OF ALDEHYDES AND KETONES

CH3CH2CH2CH3 O CH2CH3CH3 CH3CH2 C

O

H CH3 C

O

CH3 CH3CH2CH2-OH

butane

bp 0oCmethoxyethane

bp 8oC

propanal

bp 49oC

acetone

bp 56oC1-propanol

bp 97oC

Boiling points of alkane, ether, aldehyde, ketone and alcohol of similar molecular weight.

bp alkane < bp ether < bp ketone, bp aldehyde < bp alcohol

bp ketone, bp aldehyde > bp alkyl halides

WATER SOLUBILITIES

- ketones and aldehydes have lone pairs of electrons and can act as hydogen bond acceptors with other compounds having O-H or N-H bonds.

- for example, the –OH hydrogen of water or an alcohol can form a hydrogen bond with unshared electrons on a carbonyl oxygen atom.

O

OH H

CR

R'

O

OH R

CR

Hδ+

δ-

δ+

δ-

δ-

δ- δ+

δ+δ+δ+

hydrogen bonding hydrogen bonding

• Because of the hydrogen bonding, ketones and aldehydes are good solvents for polar hydroxylic substances such as alcohols.

• Ketones and aldehydes are soluble in water.- ketones and aldehydes with up to 4 carbon atoms are fairly soluble in water.- the solubility of ketones and aldehydes in water decreases with the increasing length of the carbon chain.

• Oxidation• Reduction• Condensation with 2,4-dinitrophenylhydrazine• Nucleophilic addition• Haloform reaction

REACTIONS OF ALDEHYDES AND KETONES

OXIDATION OXIDATION OF ALDEHYDES WITH KMnO4 AND K2Cr2O7

aldehydes carboxylic acids

CH3CHO

CHO

CH3-C-OH

O

C-OH

O

KMnO4/H+

ethanal ethanoic acidheat

KMnO4/H+

heatbenzaldehyde benzoic acid

[O]

• When an aldehyde is heated with potassium dichromate (VI) solution acidified with dilute H2SO4, the solution changes colour from orange to green.

(orange) (green)

Cr2O2-7 Cr3+

R C H

O O

R-C-OHK2Cr2O7/H+

aldehyde carboxylic acidheat

Ketones are resistant to oxidation. Oxidation only occurs if the ketone is boiled with a strong

oxidising agent under reflux for a prolonged period of time. The oxidation of ketones involves breaking C-C bonds.

REACTIONS OF ALDEHYDES WITH TOLLENS’ REAGENT: SILVER MIRROR TEST

- Tollens’ reagent is called ‘ammoniacal silver nitrate’ solution.

- contains the silver amine complex ion, [Ag(NH3)2]+

- Tollens’ reagent is a mild oxidising agent.

- when aldehyde is warmed with Tollens’ reagent, the colourless complex ion, [Ag(NH3)2]+ is reduced by aldehyde to grey metallic silver.

- the precipitate forms a silver mirror on the walls of test tube.

• Equation:CH3CHO + 2 [Ag(NH3)2]+ + OH- → CH3COO- + 2Ag(s) + 2NH4

+ + 2NH3

aldehyde Tollen’s reagent grey metallic silver

• A simplified equation:CH3CHO + 2 Ag+ + H2O → CH3COOH + 2Ag(s) + 2H+

• General equation:RCHO + 2Ag+ + H2O → RCOOH + 2Ag(s) + 2H+

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

REACTIONS OF ALDEHYDES WITH FEHLING’S SOLUTION

- Fehling’s solution contains a copper (II) complex ion.

- Fehling’s solution: mixing copper (II) sulphate solution with a solution of sodium potassium tartrate in NaOH (aq).

- When Fehling’s solution is warmed with aldehydes, the deep blue colour of the Fehling’s solution dissapears and a brick-red (reddish-brown) precipitate of copper (I) oxide (Cu2O) is obtained.

R C H

O

aldehyde2Cu2+ + 5OH-

R C O-

O

Cu2O + 3H2O

Fehling's solution(blue colour)

copper (I) oxide(brick-red precipitate)

• Fehling’s solution can be used to distinguish between:

a) Aldehydes and ketones (ketones do not react with Fehling’s solution).

b) Aliphatic aldehydes and benzaldehyde (benzaldehyde does not react with Fehling’s solution).

• Aldehydes and ketones can be reduced to alcohols using:a) lithium aluminium hydride (LiAlH4)

b) sodium borohydride (NaBH4)

c) catalytic hydrogenation

H+ = diluted acid such as H2SO4

R C H

O

LiAlH4 or NaBH4 or H2, Ni R C H

O-

H

R C H

OH

H

H+

aldehyde

1o alcohol

R C R'

O-

H

R C R'

OH

H

H+

2o alcohol

R C R'

O

LiAlH4 or NaBH4 or H2, Niketone

REDUCTION

Examples:

CH3 C H

O

LiAlH4 CH3 C H

O-

H

CH3 C H

OH

H

H+

ethanal

ethanol

CH3 C CH3

O-

H

H+

2-propanol

CH3 C CH3

O

H2/Nipropanone

CH3 C CH3

OH

H

• Abbreviation for 2.4-dinitrophenylhydrazine is 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.

CONDENSATION WITH 2,4-DINITROPHENYLHYDRAZINE

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.

• 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

NUCLEOPHILIC ADDITION

δ-δ+

electrophilic attacknucleophilic attack

Nucleophilic addition of hydrogen cyanide

C

O

R R' HCN

C

O

CH3 CH3 HCN

CR R'

OH

CN

CCH3 CH3

OH

CN

ketone

cyanohydrin

example

propanone

2-hydroxy-2-methylpropanenitrile

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

• 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

HALOFORM REACTION

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

CCH3 H

OH

H

ethanol C

O

CH3 Hethanal

a secondary alcohol with the CHCH3

OH

group

a ketone with the CCH3

O

group

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:

• 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.

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)

• Formalin (40% aqueous solution of methanal):- as disinfectant- as a preservative for biological specimens

• Vanillin (C8H8O3):- a strong vanilla odour and used for food flavouring.

• Camphor (C10H16O):- used medically as an inhalant for colds.

• Cyclohexanone:- starting material for production of nylon.

• Used as solvents, starting materials and reagents. For example, propanone.

USES OF CARBONYL COMPOUNDS