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 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 Aldehydes

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

The parent hydrocarbon is the longest chain that carries the –CHO group This chain has 4 carbon atoms.

The parent hydrocarbon is the longest chain that carries the –CHO group This chain has 5 carbon atoms. 5

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 acidbutyraldehyde 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.

Naming aromatic compounds: 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 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. propanepropanone 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

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  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.

 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

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. δ+δ+ δ-δ- δ+δ+ δ-δ- δ-δ- δ-δ- δ+δ+ δ+δ+ δ+δ+ δ+δ+ 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 KMnO 4 AND K 2 Cr 2 O 7 aldehydes carboxylic acids [O]

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. 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 NH 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.

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 (LiAlH 4 ) b) sodium borohydride (NaBH 4 ) c) catalytic hydrogenation REDUCTION

Examples:

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. CONDENSATION WITH 2,4- DINITROPHENYLHYDRAZINE POSITIVE TEST REAGENT

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. NUCLEOPHILIC ADDITION δ-δ-δ+δ+ electrophilic attack nucleophilic attack

Nucleophilic addition of hydrogen cyanide

IODOFORM TEST - a solution of I 2 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). CH 3 CHO + 3I 2 CI 3 CHO + 3HI triidoethanal Cl 3 CHO + - OH CHI 3 + HCOO - iodoform HALOFORM REACTION

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:

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.

Formalin (40% aqueous solution of methanal): - as disinfectant - as a preservative for biological specimens Vanillin (C 8 H 8 O 3 ): - a strong vanilla odour and used for food flavouring. Camphor (C 10 H 16 O): - 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