Aldehydes and Ketones- Study Guide - IIT JEE

Preparation, properties and reactions

Characteristic reactions of
oxidation, reduction, oxime and hydrazone formation; aldol condensation, Perkin reaction; Cannizzaro reaction; haloform reaction and nucleophilic addition reactions (Grignard addition);


NCERT Text Book for Class XII Part II, Chapter 12

12.1 Nomenclature and Structure of Carbonyl Group
12.2 Preparation of Aldehydes and Ketones
12.3 Physical properties
12.4 Chemical Reactions
12.4.1 Nucleophilic addition reactions
12.4.2 Reduction
12.4.3 Oxidation
12.4.4 Reactions due to α-hydrogen
Aldol condensation
12.4.5 Other reactions
12.4.5(i) Cannizzaro reaction
12.4.5(ii) electrophilic substitution reaction
12.5 Uses of Aldehydes and Ketones

IIT JEE Revision - Ch.27 Aldehydes and Ketones - Core Points

syllabus

Aldehydes and Ketones:
Preparation, Physical properties and chemical properties
oxidation,
reduction,
oxime and
hydrazone formation;
aldol condensation,
Perkin reaction;
Cannizzaro reaction;
haloform reaction and
nucleophilic addition reactions (Grignard addition);
--------
Aldehydes contain carbonyl group C=O as functional group and the carbonyl atom carries at least one H atom.

Ketones

In ketones, also carbonyl group C=O is the functional group. But the carbonyl carbon does not contain any H atoms, but it is attached to two alkyl or aryl groups.


Getting an aldehyde from methylbenzene - by oxidation

Getting ketone from alcohols - By oxidation of secondary alcohols


Aldehydes and ketones are polar molecules because the C=O bond has a
dipole moment:

• Their polarity makes aldehydes and ketones have higher boiling points than
alkenes of similar molecular weight.

Oxidation

Carbonyl groups in aldehydes and ketones may be oxidized to form
compounds at the next “oxidation level”, that of carboxylic acids


Addition Using Grignard Reagents• Primary, secondary and tertiary alcohols may be formed in the reactions of
aldehydes or ketones with Grignard reagents.


http://www.cem.msu.edu/~reusch/VirtualText/aldket1.htm

Aldehydes - Ketones - Introduction and Nomenclature

Aldehydes contain carbonyl group C=O as functional group and the carbonyl atom carries at least one H atom.

Ketones

In ketones, also carbonyl group C=O is the functional group. But the carbonyl carbon does not contain any H atoms, but it is attached to two alkyl or aryl groups.


Getting an aldehyde from methylbenzene - by oxidation

Getting ketone from alcohols - By oxidation of secondary alcohols


Aldehydes and ketones are polar molecules because the C=O bond has a
dipole moment:

• Their polarity makes aldehydes and ketones have higher boiling points than
alkenes of similar molecular weight.



Aldehydes and Ketones – Nomenclature

Nomenclature of Aliphatic Aldehydes

Common system
Aldehydes are named according to the name of the corresponding acid which they form on oxidation.
The suffix –ic acid of the name of the acid is replaced by aldehyde.
Ex: The aldehyde that gives acetic acid is termed as acetaldehyde.

Braching in the aldehyde chain, is indicated by carbon atom positions α, β, γ, δ.

The carbon atom next to the carbonyl carbon is assigned the letter α. The carbon next to α-carbon is the β-carbon. The carbon next to β-carbon is the γ-carbon. The carbon next to γ-carbon is the δ Carbon.

δ - γ- β- α carbons
C-C-C-C-CHO
Ex: α-Methyl butyraldehyde

IUPAC system

Aldehydes are termed alkanals. The terminal ‘e’ of the name of corresponding alkane is replaced by ‘al’.
Ex: methanal, ethanal, propanal.
Nomenclature for Aldehydes with Branches
1. The longest chain containing –CHO group is considered as the parent chain and the name is derived as an alkanal.
2. To determine the number of the carbon where a substituent is attached to the aldehyde chain, the carbons in the chain are numbered in such a way that the aldehydic group carbon gets lowest number (i.e 1). In other words number of the aldehydic chain carbons is started from the carbon in the carbonyl group.

Nomenclature of Aromatic Aldehydes

The simplest aromatic aldehyde is benzaldehyde. In aromatic aldehydes, -CHO group is directly attached to the benzene ring.
In case of substituted aromatic aldehydes, the positions of the substituents in benzene ring with respect to –CHO group are indicated either by suffixes ortho, meta or para or by numbers 1,2,3… etc. with the carbon bearing the –CHO group as number 1.
Ex; 2-Hydroxybenaldehyde – OH is the substituent at the 2 carbon from CHO group.
The aldehydic group (CHO) can be a part of the side chain. In other words, an aldehydic group may be attached to benzene ring.
The name will be as an example 2-Phenylethanal. In this compound CH2CHO is attached to a benzene ring. The substituent is ethanal group, and the numbering of carbon atoms still starts from CHO group carbon and the terminology indicates that a benzyl group is attached to ethanal at 2nd carbon.

Nomenclature of Ketones

Common system
Ketones are named by using the names of alkyl groups present in the molecule.
Ex: Dimethyl ketone, Methyl isopropyl ketone

IUPAC system

Ketones are termed as alkanones.

Rules for arriving at names
1. The longest chain carrying the carbonyl groupis considered as the parent chain and the name is derived by replacing the terminal ‘e’ of the name of the corresponding alkane by letters ‘one’.
Ex: Propanone
2. In case of substituted ketones, theparent chain is numbered in such a way that the ketone group carbon gets the lowest number (but the numbering does not start from ketone group carbon – caron attached with a double bond to oxygen).
3. The position of carbonyl group and the substituents is indicated by numbers.
Ex: 3-methylbutan-2-one (indicated that there is methyl group at 3rd carbon and carbonyl group at 2nd carbon on butanone.

Compounds having both aldehyde and ketone groups.

For compounds having both aldehyde and ketone groups, the aldehyde group is considered as the principal functional group and ketonic group is regarded as substituent. It is named as prefix oxo- along with a number to indicate its position.
Ex: 2-Methyl-4-oxohexanal

Aromatic Ketones

Purely aromatic or mixed aromatics ketones are know by their common names.
Examples
Acetophenone – Methyl phenyl ketone
Propiophenone – Ethyl phenyl ketone
Benzophenone - Diphenyle ketone

IUPAC 1993 recommendation for 3 same functional groups

If an unbranched chain is directly bonded to more than two same functional groups, the organic compound is named as a derivative of parent alkane which does not include the carbon atoms of the functional groups. These compounds are named by use of suffix tricarboaldehyde (for three –CHO groups).
Ex: Butane-1,2,4-tricarbaldehyde

If three groups are not directly bonded to the unbranched carbon chain, the two like groups are considered in the parent chain and are named by using di before the name of the functional group. The third group forming the side chain is considered as a substituent group.

Ex: 3-Formylmethylhexane-1,6-dial (three –CHO groups are there. But at 1 and 6 they are directly bonded. At three there is a branching of the alkane chain and –CHO is attached to the methyl group of the branch).

Aldehydes and Ketones - physical Properties

Aldehydes and Ketones - Methods of Preparation

1. From alkenes

2. From alkynes

3 From alcohols

4. From alkyl halides

5. From Grignard reagent

6. From carboxylic acids

7.From Acid chlorides

8.From alkyl cyanides

1. From alkenes

Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc dust and water gives aldehydes and ketones.

2. From alkynes

Hydration of alkynes in the presence of dilute sulphuric acid and HgSO4 as catalyst gives aldehydes and ketones.

Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.

Hydration of acetylene gives acetaldehyde.

Hydration of alkynes other than acetylene gives ketones.


3. By oxidation of alcohols:

a) Primary alcohols on oxidation by potassium dichromate and dilute sulphuric acid give corresponding aldehyde

Potassium dichromate and sulphuric combine to give (0) nascent oxygen. Nascent oxygen oxidizes CH3OH to HCHO by removing H2 from CH3OH.

Nascent oxygen removes H2 from C2H5OH to give CH3CHO.

b)Ketones: Secondary alcohols on oxidation by potassium dichromate and dilute sulphuric acid mixture give ketones.

Isopropyl alcohol gives acetone.
2-butanol gives ethyl methyl ketone on oxidation.

Aldehydes and Ketones - Chemical Properties

Reactions

Can studied with the following grouping
A. Nucleophilic addition reactions.
B. Nucleophilic addition reactions that involve elimination of water molecule
C. Oxidation reactions
D. Reduction reactions
E. Miscellaneous reactions

A. Nucleophilic addition reactions
1. Hydrogen cyanide: addition product is cyanohydrin

2. Sodium bisulphite: addition proudct is bisulphite adduct.
3. Grignard reagent: addition intermediate product, when hydrolysed gives alcohol
4. Alcohol: product geminal dialkoxy compounds.

B. Nucleophilic addition reactions that involve elimination of water molecule

Aldehydes and ketones react with a number of ammonia derivatives in weakly acidic medium to form compound containing carbon-nitrogen double bonds with the elimination of water molecule.

1. Addition of various ammonia derivatives
i) Hydroxylamine - product oxime
ii) Hydrazine - product hydrazone
iii) Phenylhydrazine - product phenylhydrazone
iv) 2, 4 dinitrophenyl hydrazine - product 2,4 dinitrophenyl hydrazone
v) Semicarbazide - product semicarbazone

2. Addition of Ammonia
product aldehyde-ammonia ducts

3. Primary amines
product azomethines also known as Schiff bases.

C. Oxidation reactions
i) Tollen's reagent - silver mirror test
ii) Fehling's solution - aldehydes give a red precipitate of cuprous oxide
iii) Benedict's solutin - similar to Fehling's solution
iv) Oxidation with sodium hypohalite - iodoform is the product

D. Reduction of aldehydes and ketones

1. Reduction to alcohols: aldehydes give primary alcohols. Ketones give secondary alcohols.

2. Reduction to hydrocarbons:
i) Reduction with zinc amalgam and con HCL
ii) Reductin with basic solution of hydrazine
iii) Reductioin with HI in the presence of red phosphorus

3. Reduction to pinacols

E. Miscellaneous reactions

1. Aldol condensation
2. Cross aldol condensation
3. Cannizaro's reaction
4. Halogenation
5. Action with Schiff's reagent
6. Polymerisation
7. Sunstitution reactions of benzene nucleus in aldehydes and ketones.



For more detailed coverage of some reactions

http://www.chem.uic.edu/web1/OCOL-II/WIN/CH19/F3.HTM

Oxidation - Aldehydes - Ketones

Oxidation

Carbonyl groups in aldehydes and ketones may be oxidized to form
compounds at the next “oxidation level”, that of carboxylic acids.

• Alcohols are oxidized to aldehydes and ketones
(example: biological oxidation of ethanol to acetaldehyde)
• The carbonyl group may be further oxidized to carboxylic acids

Reduction - Aldehydes - Ketones

Oxidation of Aldehydes by Silver Oxide: Reaction of simple aldehydes with aqueous Ag2O in the presence of NH3 yields the corresponding carboxylic acid and metallic silver. The silver is generally deposited in a thin metallic layer which forms a reflective "mirror" on the inside surface of the reaction vessel. The formation of this mirror forms the basis of a qualitative test for aldehydes, called the Tollens Test.

Oxidation of Aldehydes to form Carboxylic Acids: Reaction of simple aldehydes with acidic MnO4-, or CrO3/H2SO4 yields the corresponding carboxylic acid. Aldehydes oxidize very easily and it is often difficult to prevent oxidation, even by atmospheric oxygen.

Oxidation of Ketones: Ketones are more resistant to oxidation, but can be cleaved with acidic MnO4- to yield carboxylic acids.

oxime - Aldehydes - Ketones

Oximes can be synthesized by condensation of an aldehyde or a ketone with hydroxylamine.

The condensation of aldehydes with hydroxylamine gives aldoxime.
Ketoximes are produced from ketones and hydroxylamine.

Generally, oximes exist as colorless crystals and do not easily dissolve in water. Oximes can be used for the identification of ketone or aldehyde.

hydrazone formation

Aldehydes and ketones also condense with other ammonia derivatives, such as hydroxylamine and hydrazines.

Generally these reactions are better than the analogous amine reactions (i.e. give superior yields).

Oximes are produced when hydroxylamines are reacted with aldehydes and ketones.

Hydrazones are produced through reaction of hydrazines with aldehydes and ketones.

Aldehydes - Ketones aldol condensation

Aldehydes and ketones containing α-hydrogen (H-atoms attached to the C-atom adjacent to the carbonyl group)undergo condensation in the presence of dilute alkali.

In the resulting compound both aldehyde group and alcohol group are present.

acetaldehyde and acetone undergo aldol condensation.
Formaldehyde, banzaldehyde do not undergo aldol condensation.



Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.

The reaction involves an enolate reacting with another molecule of the aldehyde.

Remember enolates are good nucleophiles and carbonyl C are electrophiles.

Since the pKa of an aldehyde is close to that of NaOH, both enolate and aldehyde are present.

The products of these reactions are β-hydroxyaldehydes or aldehyde-alcohols = aldols.

The simplest aldol reaction is the condensation of ethanal.








Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the reactive enolate.
Step 2:
The nucleophilic enolate attacks the aldehyde at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide.
Step 3:
An acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide and the β-hydroxyaldehydes or aldol product.

Aldehydes - Ketones Perkin reaction

The Perkin reaction is a type of aldol condensation of aromatic aldehydes and the anhydride of an aliphatic acid in the presence of sodium salt of the same acid, to give on heating an α,β-unsaturated acid.

Reaction mechanism

The carboxylate anion abstracts a proton from the a-carbon of the anhydride to form carbanion I.
This carbanion undergoes nucleophilic addition to carbonyl carbon of the aldehyde.
The anion II so formed takes up a proton to form a hydroxy compound III which first undergoes dehydration as an anhydride before it is hydrolyzed to the α,β-unsaturated acid.

Aldehydes - Ketones Cannizzaro reaction

Aldehydes which do not have α-hydrogen atom react with concentrated sodium hydroxide (NaOH) or potassium hydroxide (KOH) in such a way that one molecule get oxidized to acid and the second molecule gets reduced to alcohol.

Note two molecules of aldehyde participates in the reaction.

This self oxidation-reduction under the influence of a base is known as the Cannizzaro's reaction.

Formaldehyde does not possess α-hydrogen atom and therefore undergoes Cannizzaro's reaction. Acetaldehyde does not give this reaction.

Formaldehyde (HCHO) two molecules + warm NaOH give Methanol (CH3OH) and Sodium Formate (CHOONa)

CH3OH is the reduction product HCHO becomes CH3OH (two hydrogen atoms are getting in).
CHOONa is the oxidation product. one 'H' has gone out and One 'O' came in along with Na.

Aldehydes - Ketones -haloform reaction

When methyl ketones are treated with the halogen in basic solution, polyhalogenaton followed by cleavage of the methyl group occurs.

The products are the carboxylate and trihalomethane, otherwise known as haloform.
The reaction proceeds via successively faster halogenations at the α-position until the 3 H have been replaced.

The halogenations get faster since the halogen stablises the enolate negative charge and makes it easier to form.

Then a nucleophilic acyl substitution by hydroxide displaces the anion CX3(haloform) as a leaving group that rapidly protonates.

This reaction is often performed using iodine and as a chemical test for identifying methyl ketones. Iodoform(CI3) is yellow and precipitates under the reaction conditions.


Reaction mechanism

Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the enolate.

Step 2:
The nucleophilic enolate reacts with the iodine giving the halogenated ketone and an iodide ion.

Step 3:
Steps 1 and 2 repeat twice more yielding the trihalogenated ketone.

Step 4:
The hydroxide now reacts as a nucleophile at the electrophilic carbonyl carbon, with the C=O becoming a C-O single bond and the oxygen is now anionic.

Step 5:
Reform the favourable C=O and displace a leaving group, the trihalomethyl system which is stabilised by the 3 halogens. This gives the carboxylic acid.

Step 6:
An acid-base reaction. The trihalomethyl anion is protonated by the carboxylic acid, giving the carboxylate and the haloform (trihalomethane).

Aldehydes - Ketones-nucleophilic addition reactions

Addition Using Grignard Reagents•

Primary, secondary and tertiary alcohols may be formed in the reactions of
aldehydes or ketones with Grignard reagents.