Carbonyl Compounds

Transcript Carbonyl Compounds

ORGANIC CHEMISTRY CHM 207

CHAPTER 7: CARBONYL COMPOUNDS (ALDEHYDES AND 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



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.

O O R C R' ketone C R H aldehyde R, R' = substituents

Naming Aldehydes

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

butane butan

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

• This chain has 4 carbon atoms.

3 2 1 4

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

• This chain has 5 carbon atoms.

5 4 3 2 1

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

5 4 3 2

3-methylpentanal

•

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

•

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

butyric acid butyraldehyde

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.

CHO cyclohexane carbaldehyde CHO 5 6 4 3 1 2 CH 3 2-methylcyclohexane carbaldehyde

•

Naming aromatic compounds: CHO CHO benzaldehyde NO 2 4-nitrobenzaldehyde



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

CH 2 CHO 3 2 1 CH=CHCHO phenylethanal 3-phenylpropenal

Naming 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 propan

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.

1 2 3 4 5 6

IUPAC name: 3-hexanone New IUPAC name: hexan-3-one

•

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

ethyl propyl ketone

• •

NOMENCLATURE OF CYCLIC KETONES AND AROMATIC COMPOUNDS –e ending of the The parent name is formed by changing the cycloalkane to -one. Carbonyl carbon is designated C1.

O O 6 5 1 2 4 3 cyclohexanone CH 3 4-methylcyclohexanone



Aromatic compound: - phenyl is used as part of the name.

O C CH 3 O C phenylethanone diphenylmethanone

• • • •

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.

O O CH 3 CH 2

5 4

2 C

3-oxopentanal H O

4 3 2

C H

5 6 1

COOH 2-formylbenzoic acid O O CH 3

CH 2

OH 3-oxobutanoic acid

PHYSICAL PROPERTIES OF ALDEHYDES AND KETONES



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.

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

CH 3 CH 2 CH 2 CH 3 butane bp 0 o C CH 3 O CH 2 CH 3 methoxyethane bp 8 o C O CH 3 CH 2 C propanal bp 49 o C H O CH 3 C CH 3 acetone bp 56 o C CH 3 CH 2 CH 2 -OH 1-propanol bp 97 o C 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.

R R' C O H δ+ O δ H R H C O δ+ H O δ+ R δ+ δ δ+ δ+ hydrogen bonding δ hydrogen bonding δ-

•

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.

REACTIONS OF ALDEHYDES AND KETONES

• • • • •

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



OXIDATION

OXIDATION OF ALDEHYDES WITH KMnO 4 aldehydes [O] AND K carboxylic acids 2 Cr 2 O 7 CH 3 CHO ethanal CHO benzaldehyde KMnO 4 /H + heat KMnO 4 /H + heat O CH 3 -C-OH ethanoic acid O C-OH benzoic acid

•

When an aldehyde is heated with potassium dichromate (VI) solution acidified with dilute H 2 SO 4 , the solution changes colour from orange to green.

R O C H aldehyde K 2 Cr 2 O 7 /H + heat O R-C OH carboxylic acid Cr 2 O 2 7 (orange) Cr 3+ (green)

 

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(NH 3 ) 2 ] + Tollens’ reagent is a mild oxidising agent.

when aldehyde is warmed with Tollens’ reagent, the colourless complex ion, [Ag(NH 3 ) 2 ] + is reduced by aldehyde to grey metallic silver.

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

•

Equation: CH 3 CHO + 2 [Ag(NH 3 ) 2 ] + + OH → CH 3 COO + 2Ag(s) + 2NH 4 + + 2NH 3 aldehyde Tollen’s reagent grey metallic silver

•

A simplified equation: CH 3 CHO + 2 Ag + + H 2 O → CH 3 COOH + 2Ag(s) + 2H +

•

General equation: RCHO + 2Ag + + H 2 O → 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 (Cu 2 O) is obtained.

O R C H aldehyde 2Cu 2+ + 5OH Fehling's solution (blue colour) R O C O Cu 2 O + 3H 2 O 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).

REDUCTION

•

Aldehydes and ketones can be reduced to alcohols using: a) lithium aluminium hydride (LiAlH 4 ) b) sodium borohydride (NaBH 4 ) c) catalytic hydrogenation O R C H aldehyde LiAlH 4 or NaBH 4 or H 2 , Ni O R C ketone R' LiAlH 4 or NaBH H + = diluted acid such as H 2 SO 4 4 or H 2 , Ni R O C H H R O C H R' H + H + OH R C H H 1 o alcohol R OH C R' H 2 o alcohol

Examples: O CH 3 C ethanal H LiAlH 4 O CH 3 C CH 3 propanone H 2 /Ni CH 3 O C H H H + CH 3 O C CH 3 H H + OH CH 3 C H ethanol H OH CH 3 C CH 3 H 2-propanol

CONDENSATION WITH 2,4 DINITROPHENYLHYDRAZINE

• • •

Abbreviation for 2.4-dinitrophenylhydrazine is 2,4-DNP.

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

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

REAGENT POSITIVE TEST

H C O benzaldehyde R R' C O NO 2 H NO 2 H 2 N N H NO 2 2,4-dinitrophenylhydrazine room C N N NO 2 temperature H benzaldehyde 2,4-dinitrophenylhydrazone (yellow-orange precipitate) H 2 O NO 2 H 2 N N NO 2 room H 2,4-dinitrophenylhydrazine temperature R' R C NO N N H 2 NO 2 H 2 O

• •

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.

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.

δ+ C nucleophilic attack δ O electrophilic attack

Nucleophilic addition of hydrogen cyanide

R O C ketone R' HCN R OH C R' CN cyanohydrin

example

O CH 3 C CH 3 propanone HCN OH CH 3 C CH 3 CN 2-hydroxy-2-methylpropanenitrile

R O C H aldehyde HCN

example

O CH 3 C ethanal H HCN R OH C CN H cyanohydrin H 2 O/H + R OH C COOH H carboxylic acid NH 4 + OH CH 3 C CN H 2 O/H + H 2-hydroxypanenitrile OH CH 3 C COOH H 2-hydroxypropanoic acid (lactic acid) NH 4 +

MECHANISM

O C CN O C CN H + OH C CN

HALOFORM REACTION

•

IODOFORM TEST - a solution of I 2 oxidising agent.

in an alkaline medium such as NaOH or KOH is a - 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).

CH 3 CHO + 3I 2 CI 3 CHO + 3HI triidoethanal Cl 3 CHO + OH CHI 3 + HCOO iodoform -

•

Iodoform test is useful for the methyl ketone group (CH 3 C=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: ethanol CH 3 OH C H H ethanal CH 3 O C H a secondary alcohol with the CH 3 OH CH group a ketone with the CH 3 O C group

•

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 CH 3 CH(OH) group give a positive iodoform test.

iii) methyl ketones (ketones that contain CH 3 CO- group) give positive iodoform test.

For example, propanone and phenylethanone give a yellow precipitate, but 3-pentanone and diphenylmethanone give negative iodoform tests.

O O C CH 3 phenylethanone O I C C I I The overall reaction is C CH 3 phenylethanone 3I 2 3I 2 NaOH warm NaOH heat O C I C I I O C O Na + 3HI CHI 3 O C O Na + sodium benzoate CHI 3 3HI iodoform (yellow precipitate)

USES OF CARBONYL COMPOUNDS