Chemistry

UNIT 6: CARBONYL COMPOUNDS: ALDEHYDES AND KETONES

Key unit competency

To be able to compare the chemical nature of carbonyl compounds to their

reactivity and uses.

Learning objectives

• Describe the reactivity of carbonyl compounds

• State the physical properties of aldehydes and ketones

• Describe the preparation reactions of ketones and aldehydes

• Explain the mechanism of nucleophilic addition reactions of carbonyl

compounds

• Prepare ketones from secondary alcohols by oxidation reactions

• Compare aldehydes and ketones by using Fehling’s solution and Tollens’

reagent

• Write and name carbonyl compounds and isomers of ketones and aldehydes

• Write equations for the reactions of carbonyl compounds with other

substances

• Compare the physical properties of carbonyl compounds to those of alcohols

and alkenes

• Differentiate the methyl ketones from other ketones by using the iodoform

test

• Carry out an experiment to distinguish between carbonyl compounds and

other organic compounds

• Carry out an experiment to distinguish between ketones and aldehydes

• Carry out an experiment to prepare ethanol and propan-2-one.

6.1. Definition and nomenclature of carbonyl compounds

Introductory activity

Many fruits such as mangoes and honey contained sugar.The following images

represent mangoes, honey and some sugars such as fructose and glucose. 

6.1.1 Definition

Carbonyl compounds are compounds that contain carbon-oxygen double bond

(C=O). Carbonyl compounds are classified into two general categories based on the

kinds of chemistry they undergo. In one category there are aldehydes and ketones;

in the other category there are carboxylic acids and their derivatives. This unit looks

on category of aldehydes and ketones.

Aldehyde molecules

For aldehydes, the carbonyl group is attached to hydrogen atom and alkyl group as

shown in the molecule of propanal below. Methanal is the smallest aldehyde, it has

two hydrogen atoms attached to carbonyl group.

If you are going to write this in a condensed form, you write aldehyde as –CHO,

don’t write it as -COH, because that looks like an alcohol functional group.

Ketone molecules

6.1.2 Nomenclature

Aldehydes

The systematic name of an aldehyde is obtained by replacing the terminal “e” from

the name of the parent hydrocarbon with “al.”In numbering the carbon chain of an

aldehyde, the carbonyl carbon is numbered one.

Ketones

The systematic name of a ketone is obtained by removing the terminal“e” from the

name of the parent hydrocarbon and adding “one.”The chain is numbered in the

direction that gives the carbonyl carbon the smallest number.Ketone contains

a carbon-oxygen double bond just like aldehyde, but for ketone carbonyl groupis

bonded to two alkyl groups. 

6.2. Isomerism

6.2.1 Functional isomerism in aldehydes and ketones

Isomers are molecules that have the same molecular formula, but have a different

arrangement of the atoms in space. Functional group isomers are molecules that

have same molecular formula but contain different functional groups, and they

belong to different homologous series of compounds.

6.2.2. Position isomerism in ketones 

Position isomerism is isomerism where carbon skeleton remains constant, but the

functional group takes different positions on carbon skeleton.

6.2.3. Chain isomerism in aldehydes and ketones

In chain isomerism the same number of carbons forms different skeletons.

Aldehydes with 4 or more carbon atoms and ketones with five or more carbon

atoms show chain isomerism. 

6.3. Physical properties of aldehydes and ketones

6.3.1. Solubility in water aldehydes and ketones

The small molecules of aldehydes and ketones are soluble in water but solubility

decreases with increase of carbon chain. Methanal, ethanal and propanone - the

common small aldehydes and ketones are soluble in water at all proportions.

Even though aldehydes and ketones don’t form hydrogen bond with themselves,

they can form hydrogen bond with water molecules.

The slightly positive hydrogen atoms in a water molecule can be sufficiently

attracted to the lone pair on the oxygen atom of an aldehyde or ketone to form a

hydrogen bond.

Other intermolecular forces present between the molecules of aldehyde or ketone

and the water are dispersion forces and dipole-dipole attractions

Forming these attractions releases energy which helps to supply the energy

needed to separate the water molecules and aldehyde or ketone molecules from

each other before they can mix together.

Apart from the carbonyl group, hydrocarbon chains are non polar, they don’t

dissolve in water. By forcing hydrocarbon chain to mix with water molecules, they

break the relatively strong hydrogen bonds between water molecules without

replacing them by other attractions good like hydrogen bonds. This makes the

process energetically less profitable, and so solubility decrease.

6.3.2. Boiling points of aldehydes and the ketones

Methanal is a gas and has a boiling point of -21°C, and ethanal has a boiling point

of +21°C. The other aldehydes and ketones are liquids or solids, with boiling points

rising with rising of molecular mass hence rising of strength of Van der Waals force. 

Comparing the physical properties of carbonyl compounds to those of alcohols

and alkanes

Physical properties of covalent compounds depend on intermolecular forces.

Compounds that have similar molecular mass but different intermolecular forces

have different physical properties.

Alcohols have higher boiling point than aldehydes and ketones of similar lengths. In

the alcohol, there is hydrogen bonding, but the molecules of aldehydes and ketones

don’t form hydrogen bonds.Aldehydes and ketones are polar molecules but alkanes

are non polar molecules. 

6.4. Chemical properties of carbonyl compounds

6.4.1. Nucleophilic addition reactions

a. Polarity of carbonyl group

By comparing carbon-carbon double bond and carbon- oxygen double bond

the only difference between bonds C=C and C=O is distribution of electrons. The

distribution of electrons in the pi bond is heavily attracted towards the oxygen atom,

because oxygen atom is much more electronegative than carbon.

During chemical reactions nucleophiles will attack carbon of the carbonyl functional

group which bears apartial positive charge. While electrophile will attack oxygen of

the carbonyl functional group which bears a partial negative charge.

b. Reaction of HCN with aldehydes and ketones

Hydrogen cyanide adds to aldehydes or ketones to form cyanohydrins or

hydroxynitriles.The product has one more carbon atom than the reactant. For

example, ethanal reacts with HCN to form 2-hydroxypropanenitrile:

Because hydrogen cyanide is a toxic gas, the best way to carry out this reaction is to

generate hydrogen cyanide during the reaction by adding HCl to a mixture of the

aldehyde or ketone and excess sodium cyanide. Excess sodium cyanide is used in

order to make sure that some cyanide ion is available to act as a nucleophile. The

solution will contain hydrogen cyanide (from the reaction between the sodium or

potassium cyanide and the HCl)

The pH of the solution is maintained in range 4 - 5, because this gives the fastest

reaction. The reaction takes place at room temperature.

c. The mechanism of reaction between HCN and propanone

d. Application of the reaction

The product of the reaction above has two functional groups:

• The -OH group which behaves like ordinary alcohol and can be replaced by

other substituent like chlorine, which can in turn be replaced to give other

functional group, for example, an -NH₂

 group;

• The -CN group which can be hydrolysed into a carboxylic acid functional

group -COOH.

e.Reaction of NaHSO3 with aldehydes or ketones

The aldehyde or ketone is shaken with a saturated solution of sodium hydrogen

sulphite in water. Hydrogen sulphite with negative charge act as nucleophile,

where the product formed is separated as white crystals. Propanone react hydrogen

sulphite, as below:

Impure aldehyde and ketone can be purified by using this reaction. Impure

aldehyde or ketone is shaken with a saturated solution of sodium hydrogensulphite

to produce the crystals. Impurities don’t form crystals; these crystals formed are

filtered and washed to remove any impurities. Addition of dilute acid to filtered

crystals regenerates the original aldehyde. Dilute alkali also can be added instead

dilute acid. 

6.4.2. Condensation reactions

a. Experimental reaction

The procedure of the preparation of Brady’s reagent and carbonyl compounds

changesslightly depending on the nature of the aldehyde or ketone, and the solvent

in which 2,4-dinitrophenylhydrazine is dissolved in. The Brady’s reagent for activities

(6.4.1) is a solution of the 2,4-dinitrophenylhydrazine in methanol and sulphuric acid.

Add a few drops of Brady’s reagent to either aldehyde or ketone. A bright orange or

yellow precipitate indicates the presence of the carbonyl group in an aldehyde or

ketone. 

b. Structural formula of 2,4-dinitrophenylhydrazine.

The carbon of benzene attached to hydrazine is counted as number one.In

2,4-dinitrophenylhydrazine, there are two nitro groups, NO₂,attached to the phenyl

group in the 2- and 4- positions.

c. The reaction of carbonyl compounds with 2,4-dinitrophenylhydrazine

Brady’s reagent is a solution of the 2,4-dinitrophenylhydrazine in methanol

and sulphuric acid. The overall reaction of carbonyl compounds with

2,4-dinitrophenylhydrazine is:

Where R and R’ represent alkyl groups or hydrogen(s); if both or only one is hydrogens

the starting carbonyl compound is an aldehyde. If both R and R’ are alkyl groups

the carbonyl compound is a ketone. The following molecule shows clearly how the

product is formed.

The product formed is named”2,4-dinitrophenylhydrazone”. The simple difference

consists in replacing suffix “-ine” by “-one”.

The reaction of 2,4-dinitrophenylhydrazine with ethanal produces ethanal

2,4-dinitrophenylhydrazone; The reaction of 2,4-dinitrophenylhydrazine with

butanal produces butanal 2,4-dinitrophenylhydrazone. This is an example of

condensation reaction.

During the chemical reaction, the change takes place only on nitrogen (-NH₂) of

hydrazine in 2,4-dinitrophenylhydrazine. If the -NH₂ group is attached to other

groups a similar reaction as that of 2,4-dinitrophenylhydrazine will take place: 

6.4.3. Oxidation reactions using KMnO₄/H+ and K₂Cr₂O₇/H+

a. Difference in reactivity of ketones and aldehydes with K₂Cr₂O₇

By considering the structural formulae of aldehydes and ketones, the difference is

only the presence of a hydrogen atom attached to the carbonyl functional group in

the aldehyde whereas ketones have a alkyl group instead.

During chemical reaction aldehydes react with oxidizing agent; hydrogen on

carbonyl functional group is replaced by oxygen, look on figure below. The presence

of hydrogen atom makes aldehydes very easy to oxidize, in other words aldehydes

are strong reducing agents.

For ketone, absence of hydrogen on carbonyl functional group makes ketones to

resist oxidation. But very strong oxidising agents like potassium permanganate

solution oxidize ketones - and they do it in a destructive way, by breaking carboncarbon bonds.

Aldehyde oxidation can take place in acidic or alkaline solutions. Under acidic

solutions, the aldehyde is oxidized to a carboxylic acid. Under alkaline solutions, acid

formed react with base to form a salt of carboxylic acid. 

b. Oxidation of aldehyde by K₂Cr₂O₇/H+ solution

Add few drops of the aldehyde or ketone to a solution of potassium dichromate

(VI) acidified with dilute sulphuric acid. If the color doesn’t change in the cold, the

mixture is warmed gently in a beaker containing hot water.

6.4.4. Oxidation reactions using Tollens’ reagent

a. Difference in reactivity of Ketones and Aldehydes with Tollens’ reagent

Aldehydes can also be oxidized into carboxylic ions in basic medium.Tollens’ reagent

is a solution of diamminesilver (I) ion, [Ag(NH₃)₂]⁺ and OH-.In order to identify if a

substance is aldehyde or ketone, add few drops of Tollens reagent to test tubes

containing aldehyde or ketone and warm gently in a hot water bath for a few minutes. The formations of sliver mirror or grey precipitate is an indication of the presence of aldehyde.

6.4.5. Oxidation reactions using Fehling ;or Benedict; solution

a. Difference in reactivity of Ketones and Aldehydes with Fehling or Benedict

solution.

Fehling’s solution and Benedict’s solution react with aldehyde in the same way; both

solutions contain Cu²⁺ and OH^- . In Fehling’s solution Cu²⁺ is complexed with tartrate

ligand butin Benedict’s solution Cu²⁺ is complexed with citrate ligand.

Don’t worry about ligands, important reagents are Cu²⁺ and OH^- , ligands tartrate and

citrate are used to prevent formation of precipitate copper (II) hydroxide or copper

(II) carbonate.

A few drops of Fehling’s solution or Benedict’s solution is added to the aldehyde or

ketone and the mixture is warmed gently in a hot water bath for a few minutes.

6.4.6. Iodoform reaction with aldehydes and ketones

Activity 6.4.6

Materials:

a. Reagents for iodoform reaction

There are two different mixtures that can be used to do iodoform test, these

mixture are:

• Iodine and sodium hydroxide solution

• Potassium iodide and sodium chlorate (I) solutions

Don’t worry about Potassium iodide and sodium chlorate(I) solutions, Potassium

iodide and sodium chlorate(I) react to form final solution containI₂ and OH-. Both mixtures contain the same reagents.

Each of these mixtures contains important reagent I₂ and OHwhich react with

aldehyde or ketone. When I₂ and OH^-  is added to a carbonyl compound containing the group CH₃ CO (blue in the cycle) as shown below, pale yellow precipitate (triiodomethane) is formed.

a. Description of iodoform test

For iodine and sodium hydroxide solution

Iodine solution, I₃^- , is added to aldehyde or ketone, followed by just enough sodium

hydroxide solution to remove the colour of the iodine. If pale yellow precipitate

doesn’t form in the cold, it may be necessary to warm the mixture very gently. The positive result is pale yellow precipitate of CHI₃ 

For potassium iodide and sodium chlorate (I) solutions

Potassium iodide solution is added to a small amount of aldehyde or ketone, followed

by sodium chlorate (I) solution. If pale yellow precipitate doesn’t form in the cold,

warm the mixture very gently. The positive result is pale yellow precipitate of CHI₃.

Reaction of iodoform test

The reagents of iodoform test are I₂ and OHsolution. The reaction takes place into

two main steps:

• Three hydroxides, OH^- , remove three hydrogens from methyl group and the

place of hydrogen is taken by iodide.

6.5. Preparation methods of aldehydes and ketones

6.5.1. Oxidation of alcohols 

a. alcohol by K₂\Cr₂O₇/H+

Potassium dichromate (VI) acidified with dilute sulphuric acid is used as oxidizing agent during the preparation of aldehyde or ketone. Primary alcohol is oxidized to  aldehyde, oxygen atom from the oxidising agent removes two hydrogens; one from the -OH group of the alcohol and the other hydrogen comes from the carbon that is attached to hydroxide functional group .

b.Technique of stopping oxidation of aldehyde

The aldehyde produced by oxidation of alcohol could make further oxidation to a

carboxylic acid if the acidified potassium dichromate (VI) is still present in solution

where reaction takes place. In order to prevent this further oxidation of aldehyde to

carboxylic the following technique are used.

• Use an excess of the alcohol than potassium dichromate (VI). Potassium

dichromate (VI) is limiting reactant hence there isn’t enough oxidising agent

present to carry out the second stage of oxidizing the aldehyde formed to a

carboxylic acid.

• Distil off the aldehyde as soon as it forms. Removing the aldehyde as soon as it

is formed this means that aldehyde is removed from solution where oxidizing

agent is, to prevent further oxidation. Ethanol produces ethanal as shown by

the following reaction.

c. Oxidation of alkene by KMnO₄/H⁺

6.5.2. Preparation of ketone by distillation of calcium acetate

Procedure: Transfer 15g of calcium acetate in 50ml round bottom flask fixed on

a stand, and place it on a heating mantle fitted with a condenser and a receiver

flask. Adjust the temperature until the condensation starts. Use the aluminium

foil to insulate the flask. Heat the flask and collect the acetone in receiver flask. The

obtained product is a crude acetone and needs to be purified.Set up a distillation

apparatus and distil the crude product to obtain pure acetone (56^Oc). Do not forget

to use stirrer bar which must be placed in the round bottom flask containing the

acetone.

6.6. Uses of aldehydes and ketones

Aldehydes and ketones have many uses for example in industries such as

pharmaceutical industry and in medicine.

a. Formaldehyde:

Formaldehyde is a gas at room temperature but is sold as a 37 percent solution in water.

Formaldehyde is used as preservative and germicide, fungicide, and insecticide

for plants and vegetables. Formaldehyde is mainly used in production of certain

polymers like Bakelite (Figure 6.1). Bakelite and formaldehyde is used as

monomers in production of Bakelite

b. Acetone as solvent: 

Acetone  is soluble in water at all proportions and also dissolves in many organic

compounds. Boiling point of acetone is low, 56 °C, which makes it easier to be removed by

evaporation. Acetone is an industrial solvent that is used in products such as paints,

varnishes, resins, coatings, and nail polish removers.

c. Aldehydes and ketones

Organic molecules that contain ketones or aldehydes functional group are found

in different foods such as irish potatoes, yellow bananas.

Aldehydes and ketones perform essential functions in humans and other living organisms. For examples sugars, starch, and cellulose, which are formed from simple molecules that have aldehyde or ketone functional group

d. Aldehydes and ketones in human’s body

Aldehydes and ketones functional group are found in humans hormones like progesterone,

testosterone.