ALDEHYDES AND KETONES

TOPICS TO BE COVERED
• A. Aldehydes and Ketones:
• 1.Nomenclature
• 2. nature of carbonyl group
• 3. methods of preparation
• 4. physical and chemical properties
• 5. mechanism of nucleophilic addition
• 6. reactivity of alpha hydrogen in aldehydes
• 7.uses.
• B.Carboxylic Acids:
• 1.Nomenclature
• 2.acidic nature
• 3. methods of preparation
• 4. physical and chemical properties
• 5.uses.
The carbonyl group
• The local geometry around the carbonyl group is trigonal planar.
The rest of the molecule doesn’t have to be planar:

Local trigonal
planar geometry
 Compounds containing the carbonyl group

• The following classes of organic compounds involve the carbonyl group:

• Aldehydes have a H-atom or a carbon substituent (alkyl, cycloalkyl,
aromatic) bound to a CHO group (carbonyl group bound to a H-atom):

General formula for aldehyde:

 Compounds containing the carbonyl group

• Ketones have two carbon substituents (akyl, cycloalkyl, aromatic

and not necessarily the same)

General formula for ketones:

 Compounds containing the carbonyl group

• Carboxylic acids have an OH (hydroxyl) group bound to the carbonyl carbon, in

addition to either a H-atom or a carbon group (alkyl, cycloalkyl, aromatic):

General formula for carboxylic acids:

 Compounds containing the carbonyl group

• Esters have a carbonyl group singly bound to an oxygen, which in turn is bound
to a carbon group (alkyl, cycloalkyl, or aromatic). The other bond to the
carbonyl is either to a H-atom or another carbon group:

General formula for an ester:

Compounds containing the carbonyl group
• Amides are the first nitrogen-containing organic compounds we’ve seen. In
these compounds, the carbonyl group is bound to a nitrogen (an amino
group), in addition to either a H-atom or a carbon group (alkyl, cycloalkyl,
aromatic). The R’ and R” groups of the amino group may either be H or
carbon groups:

General formula for an amide:

 Aldehyde and ketone functional group

• As we saw, alcohols can be used to create aldehydes and ketones. Oxidation of a

primary alcohol yields an aldehyde:

[O]

• And oxidation of a secondary alcohol yields a ketone:

[O]
Aldehyde and ketone functional group
• Cyclic aldehydes are not possible, because in order for the carbonyl group
to be part of the ring structure, two bonds to carbon groups would be
required.
• Aldehydes may incorporate ring structures, but not be part of the ring.
• Also, note that cyclic ketones aren’t heterocyclic compounds.

an aldehyde incorporating
a cyclic ketone a cyclic compound
a cyclic diketone
Name the compounds
Nomenclature
Name the compounds
Structure of The carbonyl group
• Aldehydes and ketones are among the first examples of compounds that possess
a C-O double bond that we’ve seen (oxidation of alcohols section, Ch-14).
• This group is called a carbonyl group, and it has very different chemical
properties than a C-C double bond in alkenes:

+ −

• Because oxygen is more electronegative than carbon, the bond is polar.

• Bond angles are about 120o around the carbon atom (see VSEPR theory).
 NCERT QUESTIONS
• Write the structures of the following compounds.
• (i) -Methoxypropionaldehyde
• (ii) 3-Hydroxybutanal
• (iii) 2-Hydroxycyclopentane carbaldehyde
• (iv) 4-Oxopentanal
• (v) Di-sec. butyl ketone
• (vi) 4-Fluoroacetophenone

i ii
Preparation of Aldehydes and Ketones

• 1. By oxidation of alcohols
• 2. By dehydrogenation of alcohols
• 3. From hydrocarbons
NOTE: Oxidation of 30 Alcohols is not possible
This method is suitable for volatile alcohols and is of industrial application. In this method alcohol
vapours are passed over heavy metal catalysts (Ag or Cu). Primary and secondary alcohols give
aldehydes and ketones, respectively. 30 alcohols give alkenes.
 3. A From hydrocarbons: Hydration of Alkynes

Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. All
other alkynes give ketones in this reaction
 3 A .Alkynes when treated with dil H2SO4 in presence of HgSO4 ,water add up to give enol form which
isomerises to form aldehydes and ketones

Enol form Keto form

Keto form
Enol form
 From hydrocarbons
Aromatic aldehydes (benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons by the
following methods:

• Strong oxidising agents oxidise toluene and its derivatives to benzoic acids. However, it is possible to
stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an
intermediate that is difficult to oxidise further. The following methods are used for this purpose:
• 3B) Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene
diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be
hydrolysed to corresponding benzaldehyde with aqueous acid.
• 3C From Hydrocarbons
• (i) By ozonolysis of alkenes: As we know, ozonolysis of alkenes
followed by reaction with zinc dust and water gives aldehydes ,ketones or a
mixture of both depending on the substitution pattern of the alkene
Preparation of Aldehydes:
• 1. From acyl chloride (acid chloride) Rosenmund reduction
• 2. From nitriles and esters(Stephen reaction.)
• 3. From nitriles by selective reduction by diisobutylaluminium hydride, (DIBAL-H) to imines followed by
hydrolysis to aldehydes.
• 4. From hydrocarbons
3. From nitriles and esters
 4.A

By oxidation of methylbenzene: a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride

oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding
benzaldehyde.
4.B

By side chain chlorination of toluene followed by hydrolysis

4.C

When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in
the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or
substituted benzaldehyde. This reaction is known as Gatterman-Koch reaction.
Treatment of acyl chlorides with dialkylcadmium, prepared by the
reaction of cadmium chloride with Grignard reagent, gives ketones.
 From nitriles
• Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.
 From benzene or substituted benzenes
• When benzene or substituted benzene is treated with acid chloride in
the presence of anhydrous aluminium chloride, it affords the
corresponding ketone. This reaction is known as Friedel-Crafts
acylation reaction.
 Boiling point
• The boiling point of methanal is -19o C and for ethanal it is +21o C. From this
we can say that the boiling point of ethanal is close to room temperature.
Generally the boiling point of aldehydes and ketones increases with increase in
molecular weight. Boiling point depends upon the strength of the
intermolecular forces.
• Vander Waals dispersion forces: As the molecules get longer and the number
of electrons increases, the attraction between them also increases. For both
aldehydes and ketones the boiling point increases with the increase in number
of carbon atoms.
• Vander Waals dipole-dipole attraction: Because of the presence of carbon-
oxygen double bond both aldehydes and ketones are polar in nature. There
will be attraction between permanent dipoles as well as the molecules which
are near to it. This is the reason for aldehydes and ketones having boiling point
higher than the similar sized hydrocarbons.
 Solubility:
• Aldehydes and ketones are soluble in water but their solubility
decreases with increase in the length of the chain. Methanal, ethanal
and propanone are those aldehydes and ketones which are of small
size and are miscible with water in almost all proportions.
• Aldehydes and ketones cannot form hydrogen bonds with
themselves, but they can have hydrogen bonds with water molecules
and this forms the basis for good solubility of aldehydes and ketones
in water. This is also because of dispersion forces and dipole-dipole
interactions. Aldehydes and ketones find many applications in
different industries. They are extensively used in the manufacturing
of polymers, blending of perfumes and also as flavouring agents.
A nucleophile attacks the electrophilic carbon atom of the polar
carbonyl group from a direction approximately perpendicular to the
plane of sp2 hybridised orbitals of carbonyl carbon .The
hybridisation of carbon changes from sp2 to sp3 in this process, and
a tetrahedral alkoxide intermediate is produced. This intermediate
captures a proton from the reaction medium to give the electrically
neutral product. The net result is addition of Nu– and H+ across the
carbon oxygen double bond
FAQ: For Nucleophilic Addition Why Aldehydes are more
reactive than ketones?

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric
and electronic reasons.
Sterically, the presence of two relatively large substituents in ketones hinders the approach of
nucleophile to carbonyl carbon than in aldehydes having only one such substituent.
Electronically, two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively
in ketones than in aldehyde.Hence Aldehydes are more reactive toward nucleophilic addition
reactions than Ketones.
 Question :Would you expect benzaldehyde to be more reactive or less reactive in
nucleophilic addition reactions than propanal? Explain your answer

Answer:The carbon atom of the carbonyl group of benzaldehyde is less

electrophilic than carbon atom of the carbonyl group present in propanal. The
polarity of the carbonyl group is reduced in benzaldehyde due to resonance as
shown below and hence it is less reactive than propanal.
Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs
veryslowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN-) being a
stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.
Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products.
The position of the equilibrium lies largely to the right hand side for most aldehydes and to
the left for most ketones due to steric reasons. The hydrogensulphite addition compound is water soluble and can
be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore,
these are useful for separation and purification of aldehydes.
Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of
aldehydes and ketones. The reaction is reversible and catalysed by acid.
The equilibrium favours the product formation due to rapid dehydration of the
intermediate to form >C=N-Z.
 20 ALCOHOL
10 ALCOHOL

Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium
borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) aswell as by catalytic hydrogenation
 Reduction to hydrocarbons CLEMMENSEN REDUCTION

The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and
concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium
or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).
Aldehydes and ketones can be converted to a hydrazone derivative by reaction with
hydrazine. These "hydrazones" can be further converted to the corresponding alkane by
reaction with base and heat.
• Reaction of Aldehydes or Ketones with Hydrazine
Produces a Hydrazone
•

Reaction with a Base and Heat Converts a Hydrazone to an Alkane

Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to
carboxylic acids on treatment with common oxidising agents like nitric acid, potassium
permanganate, potassiumdichromate, etc.
Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.
Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated
temperatures. Their oxidation involves carbon-carbon bond cleavage to afford a mixture of carboxylic
acids having lesser number of carbon atoms than the parent ketone.
Tollens Test is given only by Aliphatic and Aromatic Aldehydes and not by ketones as it is a mild oxidising agent.
Tollens Reagent is a mixture of ammonium hydroxide and silver nitrate.
AgNO3 + NH4OH→[Ag(NH3)2]+
Tollens Reagent

Note: Tollens test is also given by formic acid, alpha hydroxy ketones and hemiacetals
Tollens Reagent is a better oxidising agent than Fehlings reagent
 Fehling Test is given only by Aldehydes and not by ketones

Fehlings reagent is a combination of Fehlings Solution A and Fehlings solution B

Alkaline potassium tartarate

On heating an aldehyde with Fehling’s reagent,

a reddish brown precipitate is obtained.
Aldehydes are oxidised to corresponding
carboxylate anion.
Aromatic aldehydes do not respond to this test.
 This test is shown by Aldehydes and ketones having at least
one methyl group linked to the carbonyl carbon atom
Iodoform reaction with sodium hypoiodite is also used for
detectionof CH3CO group or CH3CH(OH) group which
produces CH3CO groupon oxidation.

haloform reaction is the reaction of a methyl ketone with chlorine, bromine, or iodine in the presence of
hydroxide ions to give a carboxylate ion and a haloform. There is one aldehyde that undergoes the haloform
reaction, which is acetaldehyde.
Question:An organic compound (A) with molecular formula C8H8O forms an orange-red precipitate with 2,4-DNP reagent
and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollens’ or
Fehlings’ reagent, nor does it decolourise bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it
gives a carboxylic acid (B) having molecular formula C7H6O2. Identify the compounds (A) and (B) and explain the
reactions involved.

Answer:(A) forms 2,4-DNP derivative. Therefore, it is an aldehyde or a ketone. Since it does not
reduce Tollens’ or Fehling reagent, (A) must be a ketone. (A) responds to iodoform test. Therefore, it
should be a methyl ketone. The molecular formula of (A) indicates high degree of unsaturation, yet it
does not decolourise bromine water or Baeyer’s reagent. This indicates the presence of unsaturation
due to an aromatic ring. Compound (B), being an oxidation product of a ketone should be a carboxylic
acid. The molecular formula of (B) indicates that it should be benzoic acid and compound (A) should,
therefore, be a monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that
it should be phenyl methyl ketone (acetophenone).
• Reactions are as follows:

Orange red ppt

Reactions due to a-hydrogen :Acidity of a-hydrogens of aldehydes and ketones
• The aldehydes and ketones undergo a number of reactions due to the acidic nature
of -hydrogen.
Question: Why is alpha (α) hydrogen of carbonyl compounds acidic in nature?
• Answer :In a carbonyl group, the oxygen is extremely electronegative and it attracts the electron
cloud towards itself developing a partial positive charge on the α-carbon. To reduce the positive
charge, α-carbon looses its hydrogen readily and makes it acidic in nature Or we can say that The
acidity of -hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect
of the carbonyl group and resonance stabilisation of the conjugate base.
•
 Reactions due to a-hydrogen-ALDOL CONDENSATION

Aldehydes and ketones having at least one -hydrogen undergo a reaction in the presence of dilute alkali as
catalyst to form -hydroxy aldehydes (aldol) or -hydroxy ketones (ketol), respectively. This is known as
Aldol reaction.
Methanal and Benzaldehyde does not give Alsdol condensation due to absence of -hydrogen atoms
 Cross aldol condensation
• When aldol condensation is carried out between two different aldehydes and / or ketones, it is called
cross aldol condensation. If both of them contain -hydrogen atoms, it gives a mixture of four
products. This is illustrated below by aldol reaction of a mixture of ethanal and propanal
Cross aldol reactions
Ketones can also be used as one component in the cross aldol reactions.
IMPORTANT POINTS
1. Cannizzaro reaction is shown by those aldehydes
which do not have alpha hydrogen atom.
2. It takes place in strongly basic Medium.
3. In this reaction, one molecule of the aldehyde is
reduced to alcohol while another is oxidised to
carboxylic acid salt. (disproportionation reaction).