Ketones and aldehydes are both simple chemical molecules with a carbonyl group. The carbonyl group has a double bond between carbon and oxygen. Because the carbon atom in the carbonyl group lacks reactive groups like OH or Cl, these chemical molecules are simple.
Table of Contents
Preparation of Aldehydes and Ketones
Some of the general methods of preparation of aldehydes and ketones are discussed below:
From Alcohols:
By oxidation – Aldehydes can be prepared by the oxidation of primary alcohols with normal oxidizing agents such as acidified potassium dichromate, potassium permanganate, chromium oxide, and sulphuric acid.
Aldehydes undergo readily oxidation to carboxylic acids. therefore, to prevent further oxidation of aldehydes, these are distilled off as soon as they are formed. Oxidation of aldehydes is carried out under controlled conditions.
Ketones can be prepared by the oxidation of secondary alcohols with similar oxidizing agents.
By the catalytic dehydrogenation of alcohols – Aldehydes and Ketones can be prepared by the dehydrogenation of alcohols. It is carried out by passing the vapour of alcohol over reduced copper at 573 K.
Primary alcohols give aldehydes while secondary alcohols give ketones.
By Catalytic Decomposition – Aldehydes and ketones can be prepared by catalytic decomposition of carboxylic acids. This can be done by heating a mixture of methanoic acid or other acids to 573 K in the presence of manganous oxide which acts as a catalyst.
This method is, however not very suitable for the preparation of aldehydes because the yield is very low. This is because when a mixture of two calcium salts is heated three products are formed. For example, dry distillation of a mixture of calcium formate gives a mixture of formaldehyde. Similarly, this method cannot be used for the preparation of unsymmetrical ketone because it gives a mixture of three ketones.
For example, dry distillation of a mixture of calcium acetate will give a mixture of acetone.
Cyclic ketones are formed when calcium salts of dicarboxylic acids are heated.
From Hydrocarbons
By hydration of alkynes – Aldehydes and ketones can be prepared by the hydration of alkynes in the presence of dil. H2SO4 and HgSO4 as catalysts. Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.
Hydration of alkynes other than acetylene gives ketones.
By Hydroboration-oxidation reaction: The alkynes can be converted into aldehydes and ketones by hydroboration- oxidation reaction. Borane adds to an alkyne forming vinylic borane, which on oxidation with H2O2 gives aldehydes and ketones. The symmetrical non-terminal alkynes give a single ketone while unsymmetrical non-terminal alkynes give a mixture of both possible ketones in which the methyl ketones predominate.
Terminal alkynes give aldehydes. However, to avoid complications due to double bond addition of diborane, bulky sterically hindered boranes such as bis (1,2-dimethyl propyl) borane commonly known as disiamylborane is used in place of diborane.
By ozonolysis of alkenes: Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc dust and water gives aldehydes and ketones. It is clear that if the carbon forming the double bond carries an H-atom attached to it, aldehydes are formed otherwise ketones are formed.
By Wacker’s process: Alkenes can be converted to aldehydes and ketones by treating with an acidified aqueous solution of palladium chloride containing a catalytic amount of cupric chloride in the presence of air or oxygen. This method is known as Wacker’s process.
Acid chlorides are converted into aldehydes by catalytic hydrogenation in the presence of palladium catalyst supported over barium sulfate. The catalytic mixture is poisoned by the addition of a small amount of sulphur or quinoline. This reaction is known as Rosenmund reduction.
From Reduction of alkyl cyanides and esters
Alkyl cyanides when reduced with stannous chloride and hydrochloric acid in absolute ether followed by hydrolysis give aldehydes. This reaction is known as Stephen reduction.
From aromatic hydrocarbons
Aromatic aldehydes are prepared from aromatic hydrocarbons by the following methods:
By oxidation of methylbenzene: Strong oxidizing agents such as acidified or alkaline KMnO4, acidified K2Cr2O7, conc. HNO3, etc. oxidize toluene and its derivatives to benzoic acid through the oxidation of intermediate aldehydes.
However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents which convert the methyl group to an intermediate that is difficult to oxidize further. The following methods are used for this purpose:
Use of chromic oxide in acetic anhydride: Aromatic aldehydes are prepared when the alkyl side chain aromatic ring is oxidized using chromium trioxide and acetic anhydride. The aldehyde formed immediately gets acetylated with acetic anhydride forming gem diacetate and this does not get further oxidized. Therefore, the function of acetic anhydride is to prevent further oxidation of the aldehyde to acid. The gem diacetate formed is isolated and hydrolyzed with aqueous acid to give corresponding aromatic aldehyde.
It may be noted that the further oxidation of benzaldehyde to benzoic acid is prevented by trapping the aldehyde as gem-diacetate with acetic anhydride. The gem diacetate cannot be further oxidized.
Use of chromyl chloride: Alkyl benzenes can also be oxidized to benzaldehyde with a solution of chromyl chloride in CCl4. A brown chromium complex is formed which is decomposed by water to give corresponding benzaldehyde. This reaction is called the Etard reaction.
In case, the side chain contains a group higher than the —CH3 group, the end carbon atom of the chain is oxidized by CrO2Cl2 to the —CHO group.
By Gattermann Koch reaction: Benzene or its derivatives are converted to benzaldehyde or substituted benzaldehyde by treatment with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl3 or cuprous chloride.
This reaction is known as the Gattermann Koch reaction.
From Hydrocyanic acid (HCN)
Aldehydes can be prepared from hydrocyanic acid on treatment with Grignard reagent and subsequently followed by hydrolysis gives an aldehyde.
Phenolic aldehydes are obtained by treating the phenol with chloroform in an aqueous sodium hydroxide solution at about 343 K. This reaction is called the Reimer-Tiemann reaction.
Preparation of Ketones
From acyl chlorides
Ketones can be prepared from acyl chlorides by treating with dialkyl cadmium. The dialkyl cadmium needed for this purpose is prepared by the reaction between the Grignard reagent and anhydrous cadmium chloride.
Both aliphatic and aromatic ketones can be prepared by treating an alkyl or aryl nitrile with a suitable Grignard reagent followed by acid hydrolysis.
1-Phenylpropanone can also be prepared by the action of ethyl magnesium bromide on benzonitrile followed by acid hydrolysis.
From benzene or substituted benzene by Friedel Crafts reaction
Aromatic ketones can be prepared by Friedel Crafts acylation or benzoylation by treating aromatic hydrocarbons with acid chlorides in the presence of a Lewis acid like anhydrous Aluminium chloride.
Friedel Crafts reaction is a typical example of an electrophilic substitution reaction. Benzophenone can also be prepared by Friedel Crafts’ reaction of carbonyl chloride with an excess benzene.
From phenyl esters by Fries rearrangement
Phenolic esters or phenyl esters on heating with anhydrous AlCl3 in the presence of CS2 as solvent undergo a rearrangement in which the acyl group migrates from the phenolic oxygen atom to the o- and p-positions of the benzene ring to give a mixture of o- and p-phenolic ketones.
This reaction is called Fries rearrangement.
Sample Questions(FAQs)
Question 1: In the preparation of acetaldehyde from ethyl alcohol, it is distilled out as soon as it is formed. Explain.
Answer:
Aldehyde is easily oxidizable to acetic acid. Therefore, to prevent its oxidation, it is distilled out as soon as it is formed.
Question 2: Aliphatic aldehydes do not show position isomers. Why?
Answer:
In the case of aliphatic aldehydes, the —CHO group is always present at the end. Therefore, they do not show position isomerism.
Question 3: What is formalin?
Answer:
Formalin is a 40% aqueous solution of formaldehyde.
Question 4: What reacts with aldehydes and ketones?
Answer:
Aldehydes and Ketones react with primary amines to form a class of compounds called imines. An unshared pair of electrons on the nitrogen of the amine is attracted to the partial-positive carbon of the carbonyl group.
Question 5: What products are formed when aldehydes and ketones?
Answer:
Most aldehydes and ketones react with 2°- amines to give products known as enamines. These are acid-catalyzed reversible reactions in which water is lost. enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis.
Question 6: Are Aldehydes and Ketones acidic or basic?
Answer:
Aldehydes and Ketones are hydrogen bond acceptors; this makes them have considerable solubilities in water. Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds. Acetone is a polar, aprotic solvent. Reactions with acids:- The carbonyl oxygen is weakly basic.
Question 7: Are aldehydes alcohols?
Answer:
Alcohol with its —OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. Alcohol with its —OH group attached to two other carbon atoms will form a ketone.
Neeraj Anand, Param Anand
Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations.
In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS".
He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.
Below is the CBSE Class 12 Syllabus along with the marking scheme and time duration of the Chemistry exam.
S.No
Title
No. of Periods
Marks
1
Solutions
10
7
2
Electrochemistry
12
9
3
Chemical Kinetics
10
7
4
d -and f -Block Elements
12
7
5
Coordination Compounds
12
7
6
Haloalkanes and Haloarenes
10
6
7
Alcohols, Phenols and Ethers
10
6
8
Aldehydes, Ketones and Carboxylic Acids
10
8
9
Amines
10
6
10
Biomolecules
12
7
Total
70
CBSE Class 12 Chemistry Practical Syllabus along with Marking Scheme
The following is a breakdown of the marks for practical, project work, class records, and viva. The total number of marks for all parts is 15. The marks for both terms are provided in the table below.
Evaluation Scheme for Examination
Marks
Volumetric Analysis
08
Salt Analysis
08
Content-Based Experiment
06
Project Work and Viva
04
Class record and Viva
04
Total
30
CBSE Class 12 Chemistry Syllabus (Chapter-wise)
Unit -1: Solutions
Raoult's law.
Colligative properties - relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure, determination of molecular masses using colligative properties, abnormal molecular mass.
Solutions, Types of solutions, expression of concentration of solutions of solids in liquids, solubility of gases in liquids, solid solutions.
Van't Hoff factor.
Unit -2: Electrochemistry
Redox reactions, EMF of a cell, standard electrode potential
Nernst equation and its application to chemical cells
Relation between Gibbs energy change and EMF of a cell
Kohlrausch's Law
Electrolysis and law of electrolysis (elementary idea)
Dry cell-electrolytic cells and Galvanic cells
Conductance in electrolytic solutions, specific and molar conductivity, variations of conductivity with concentration.
Lead accumulator
Fuel cells
Unit -3: Chemical Kinetics
Rate of a reaction (Average and instantaneous)
Rate law and specific rate constant
Integrated rate equations and half-life (only for zerfirst-order order reactions)
Concept of collision theory (elementary idea, no mathematical treatment)
Factors affecting rate of reaction: concentration, temperature, catalyst;
Order and molecularity of a reaction
Activation energy
Arrhenius equation
Unit -4: d and f Block Elements
Lanthanoids- Electronic configuration, oxidation states, chemical reactivity and lanthanoid contraction and its consequences.
Actinoids- Electronic configuration, oxidation states and comparison with lanthanoids.
General introduction, electronic configuration, occurrence and characteristics of transition metals, general trends in properties of the first-row transition metals – metallic character, ionization enthalpy, oxidation states, ionic radii, color, catalytic property, magnetic properties, interstitial compounds, alloy formation, preparation and properties of K2Cr2O7 and KMnO4.
Unit -5: Coordination Compounds
Coordination compounds - Introduction, ligands, coordination number, color, magnetic properties and shapes
The importance of coordination compounds (in qualitative analysis, extraction of metals and biological system).
IUPAC nomenclature of mononuclear coordination compounds.
Bonding
Werner's theory, VBT, and CFT; structure and stereoisomerism
Unit -6: Haloalkanes and Haloarenes
Haloarenes: Nature of C–X bond, substitution reactions (Directive influence of halogen in monosubstituted compounds only). Uses and environmental effects of - dichloromethane, trichloro methane, tetrachloromethane, iodoform, freons, DDT.
Haloalkanes: Nomenclature, nature of C–X bond, physical and chemical properties, optical rotation mechanism of substitution reactions.
Unit -7: Alcohols, Phenols and Ethers
Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophilic substitution reactions, uses of phenols.
Ethers: Nomenclature, methods of preparation, physical and chemical properties, uses.
Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only), identification of primary, secondary and tertiary alcohols, mechanism of dehydration, and uses with special reference to methanol and ethanol.
Unit -8: Aldehydes, Ketones and Carboxylic Acids
Carboxylic Acids: Nomenclature, acidic nature, methods of preparation, physical and chemical properties; uses.
Aldehydes and Ketones: Nomenclature, nature of carbonyl group, methods of preparation, physical and chemical properties, mechanism of nucleophilic addition, the reactivity of alpha hydrogen in aldehydes, uses.
Unit -9: Amines
Diazonium salts: Preparation, chemical reactions and importance in synthetic organic chemistry.
Amines: Nomenclature, classification, structure, methods of preparation, physical and chemical properties, uses, and identification of primary, secondary and tertiary amines.
Unit -10: Biomolecules
Proteins -Elementary idea of - amino acids, peptide bond, polypeptides, proteins, structure of proteins - primary, secondary, tertiary structure and quaternary structures (qualitative idea only), denaturation of proteins; enzymes. Hormones - Elementary idea excluding structure.
Vitamins - Classification and functions.
Carbohydrates - Classification (aldoses and ketoses), monosaccharides (glucose and fructose), D-L configuration oligosaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen); Importance of carbohydrates.
Nucleic Acids: DNA and RNA.
The syllabus is divided into three parts: Part A, Part B, and Part C. Part A consist of Basic Concepts of Chemistry, which covers topics such as atomic structure, chemical bonding, states of matter, and thermochemistry. Part B consists of Topics in Physical Chemistry, which includes topics such as chemical kinetics, equilibrium, and electrochemistry. Part C consists of Topics in Organic Chemistry, which covers topics such as alkanes, alkenes, alkynes, and aromatic compounds.
Basic Concepts of Chemistry:
Atomic structure: This section covers the fundamental concepts of atomic structure, including the electronic configuration of atoms, the Bohr model of the atom, and the wave nature of matter.
Chemical bonding: This section covers the different types of chemical bonds, including ionic, covalent, and metallic bonds, as well as the concept of hybridization.
States of the matter: This section covers the three states of matter - solid, liquid, and gas - and the factors that influence their properties.
Thermochemistry: This section covers the principles of thermochemistry, including the laws of thermodynamics and the concept of enthalpy.
Chapters in Physical Chemistry:
Chemical kinetics: This section covers the study of the rate of chemical reactions and the factors that influence it, including the concentration of reactants, temperature, and the presence of catalysts.
Equilibrium: This section covers the principles of chemical equilibrium, including the concept of Le Chatelier's principle and the equilibrium constant.
Electrochemistry: This section covers the principles of electrochemistry, including the concept of half-cell reactions, galvanic cells, and electrolysis.
Chapters in Organic Chemistry:
Alkanes: This section covers the properties and reactions of alkanes, including their structure, isomerism, and combustion.
Alkenes: This section covers the properties and reactions of alkenes, including their structure, isomerism, and addition reactions.
Alkynes: This section covers the properties and reactions of alkynes, including their structure, isomerism, and addition reactions.
Aromatic compounds: This section covers the properties and reactions of aromatic compounds, including their structure, isomerism, and electrophilic substitution reactions.
In addition to the topics covered in the syllabus, the CBSE Class 12 Chemistry exam also tests students on their analytical and problem-solving skills, as well as their ability to apply the concepts learned in the classroom to real-world situations.
Students can also check out the Tips for the Class 12 Chemistry Exam. They can easily access the Class 12 study material in one place by visiting the CBSE Class 12 page at ANAND CLASSES (A School Of Competitions). Moreover, to get interactive lessons and study videos, download the ANAND CLASSES (A School Of Competitions) App.
Frequently Asked Questions on CBSE Class 12 Chemistry Syllabus
Q1
How many chapters are there in the CBSE Class 12 Chemistry as per the syllabus?
There are 10 chapters in the CBSE Class 12 Chemistry as per Syllabus. Students can learn all these chapters efficiently using the study materials provided at ANAND CLASSES (A School Of Competitions).
Q2
What is the marking scheme for CBSE Class 12 Chemistry practical exam according to the syllabus?
The marking scheme for CBSE Class 12 Chemistry practical exam, according to the syllabus, is 8 marks for volumetric analysis, 8 marks for salt analysis, 6 marks for the content-based experiment, 4 marks for the project and viva and 4 marks for class record and viva.
Q3
Which is the scoring chapter in Chemistry as per CBSE Class 12 syllabus?
The chapter Electrochemistry in Chemistry is the scoring chapter as per CBSE Class 12 syllabus.
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