This article is on the Aldehydes and Ketones Class 12 Notes of Chemistry. The notes on Aldehydes and Ketones of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the Aldehydes and Ketones chapter of Class 12 for the students so that they can give a quick glance of the chapter.
The chapter has been divided into two articles. This article (Part 1) is on Aldehydes and Ketones. The second article (Part 2) is on Carboxylic acids.
ALDEHYDES AND KETONES PART 1
Carbonyl compounds have a general formula CnH2nO and contain a >C=O group which is present in aldehydes H-C=O as well as in R2C=O ketones. They are constituents of fabrics, plastics and drugs. These are also used as reagents and solvents.
NOMENCLATURE OF ALDEHYDES AND KETONES
There are two systems of nomenclature of aldehydes and ketones.
1. Common Names
Common names of aldehydes are derived from common names of carboxylic acids just by replacing the ending -‘ic’ of a carboxylic acid with ‘aldehyde’.
2. IUPAC Names
The word ‘al’ and ‘one’ replaces the ending ‘e’ of corresponding alkanes while naming the open chain aliphatic aldehydes and ketones, respectively. In aldehydes, the longest carbon chain is numbered starts from the carbon of the aldehyde group whereas numbering starts from the end nearer to the carbonyl group in ketones.
Structure of Carbonyl Compounds
The carbonyl group with the C atom is sp2 hybridised. One of the sp2 hybrid orbitals of one carbon atom overlaps with one of the sp2 hybrid orbitals of oxygen atom forming C–O σ-bond. The remaining two sp2 hybrid orbitals of C atom overlap with either sp3 orbital of C-atoms (as in the case of ketone) or one with an sp3 orbital of carbon and other with s orbital of hydrogen (as in the case of aldehyde) thus forming 2 more σ-bonds. Each of two sp3 hybrid orbitals of oxygen atom contains a lone pair of electrons. The unhybrid orbitals of the C atom form a π- bond with the oxygen atom by sideways overlapping. The structure can be represented as:
Thus C = O group contains one σ-bond and one π-bond as
The C-O double bond is shorter, stronger and polarized. The double bond of the carbonyl group has a large dipole moment because of the high electronegativity of oxygen than carbon. The diﬀerence between C=O and C=C is because of O-atom in the carbonyl group is more electronegative than carbon as a result polarity is developed as – >C+= O–.
Thus, the double bond of the carbonyl group has a large dipole moment. This polarity conforms that there is a nucleophilic addition reaction in the carbonyl compound. On the other hand, in alkene (C = C) there is an electrophilic addition reaction.
METHOD OF PREPARATION OF ALDEHYDES AND KETONES
1. Oxidation of Alcohols
1° Alcohol on oxidation using PCC gives an aldehyde. 2° alcohol on oxidation by Na2Cr2O7 gives a ketone.
RCH2OH – [O] —-> R-CH=O
2. By Dehydrogenation of Alcohols
Dehydrogenation means removal of hydrogen and the reagent used is heated copper.
1° alcohol (RCH2OH) — Cu/300°C —> Aldehyde (R-CH=O)
2° alcohol (R2CHOH) — Cu/300°C —> Ketone (R2C=O)
3° alcohol (R3COH) — Cu/300°C —> Alkene
3. From Hydrocarbons
(i) Hydration of Alkynes
It is the addition of water in the presence of a heavy metal ion. Acetylene on hydration gives aldehyde while any higher alkyne gives a ketone.
For example, in the case shown below, by varying the Alkyl (–R) group, the product also varies accordingly.
In the above reaction, the carbonyl group will be formed on that carbon of the alkyne which is easy to attack by the nucleophile (water in this case). Thus, a less crowded carbon will favour the formation of a carbonyl group whereas a more crowded carbon will not favour it. Therefore, in case (iii), the carbonyl group is formed on that carbon which is easy to attack.
(ii) Ozonolysis of Alkenes
Ozonolysis is used to get carbonyl compounds from alkenes. The alkenes undergo ozonolysis followed by the reaction of it with zinc (Zn) dust and water. The product obtained is either an aldehyde or a ketone depending on the substituent and also on the substitution pattern of it.
For many years, ozonolysis was used as a method for determining the structures of unknown molecules. By “stitching” the fragments together and analysing them, it is then possible to deduce what the original structure was.
Methods Used for the Preparation of Aldehydes Only
1. From Acyl Chloride (Rosenmund’s Reaction)
Here, acyl chlorides are hydrogenated over palladium on barium sulphate catalyst.
2. From Nitriles and Esters (Stephen’s Reduction)
3. By the oxidation of Methylbenzene
(a) Use of Chromyl Chloride (CrO2Cl2)
This reaction is also known as the Etard Reaction.
(b) Use of Chromic Oxide (CrO3)
(c) By the side chain Chlorination followed by Hydrolysis
(d) By Gattermann-Koch Reaction
Methods used for the Preparation of Ketones
1. From Acyl Chlorides (By the Action of suitable dialkyl cadmium on acyl chloride)
This reaction is known as the Friedel-Craft acylation reaction.
3. From Nitriles
PHYSICAL PROPERTIES OF CARBONYL COMPOUNDS
(a) Physical state
Methanal is a gas at room temperature. Ethanal is a volatile liquid, with a boiling point of 294 K.
Other aldehydes and ketones containing up to 11 carbon atoms are colourless liquids while higher members are solids.
Lower aldehydes have unpleasant odours but with the increase in molecular size aldehydes and ketones generally, have a pleasant smell. Many naturally occurring aldehyde and ketones are used in blending perfumes and ﬂavourings agents.
Aldehydes and ketones up to 4 C-atoms are miscible with water due to the presence of hydrogen bonding between the polar carbonyl group and H2O molecules.
The solubility decreases with the increase in the size of the alkyl group.
(d) Boiling points:
The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses because weak intermolecular association arises in aldehydes and ketones due to dipole-dipole interaction.
Among the carbonyl compounds, ketones have a slightly higher boiling point than the isomeric aldehydes due to the presence of two electron releasing groups around the carbonyl carbon, which makes them more polar.
The density of aldehydes and ketones is less than that of water
CHEMICAL REACTIONS OF CARBONYL COMPOUNDS
Aldehydes and ketones are highly reactive compounds. Since both these possess the same functional group i.e., a polarized carbonyl group, they undergo a number of common reactions.
Nucleophilic Addition Reactions
Both alkanes, aldehydes and ketones are unsaturated compounds. However, unlike alkanes which show electrophilic addition reactions, aldehydes and ketones undergo nucleophilic addition reactions. In short, it is the addition of a nucleophile and a proton across the (C = O) double bond.
1. Reaction with Alcohols
Aldehydes and ketones react with alcohol in the presence of dry HCl gas to yield an unstable intermediate, known as hemiacetal and hemiketal respectively, which further react with another molecule of alcohol to give acetal (in case of aldehyde) and ketal (in case of ketone). Acetal is formed to protect aldehyde (as a functional group) and ketal to protect ketone for a long time. On treating with ethylene glycol we get cyclic acetal or ketal (1, 3-dioxolane). Acetal formation is found to be more favourable than ketal formation if both the carbonyl groups are present within the molecule.
2. Addition of HCN
It Is a Base Catalyzed Addition. Addition of HCN over aldehyde and ketones gives cyanohydrin and cyanohydrin on acid hydrolysis gives α-hydroxy acid.
3. Addition of Sodium Bisulphite (NaHSO3)
The addition of sodium bisulphite to aldehydes and ketones results in the formation of a salt. The salt on acidification gives us an addition product.
Nucleophiles such as ammonia and its derivatives add to the carbonyl group of aldehydes and ketones to form derivatives that are essential for the characterization and identification of aldehydes and ketones, the product contains a carbon-nitrogen double bond resulting from the elimination of a molecule of water from the initial addition products.
1. Reaction with ammonia derivatives (H2N–Z)
This reaction is a nucleophilic addition followed by water elimination. This reaction is reversible and is catalyzed by acid which will generate a nucleophilic centre for weak base ammonia derivatives.
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidized to carboxylic acids containing the same number of carbon atoms. Because aldehydes contain H-atom attached to the carbonyl group, which can be converted into –OH group without involving cleavage of any other bond. So these also oxidized by weak oxidizing agents like Tollen’s reagent, Fehling’s solution and Benedict’s reagent.
(a) Tollen’s reagent: It is ammoniacal silver nitrate solution, prepared by the addition of ammonium hydroxide to the AgNO3 solution. During the reaction, first Ag2O is formed which is dissolved in ammonium hydroxide to give Tollen’s reagent.
RCHO + [2Ag(NH3)2] + 3OH– → RCOO– + 2Ag + 2H2O +4NH3
(b) Fehling’s solution: It is an alkaline solution of cupric ion complexed with sodium potassium tartarate. Two solutions are kept by naming Fehling solution A (Aqueous CuSO4 solution) and Fehling solution B (Alkaline solution of sodium potassium tartarate). When these two solutions are mixed we get a deep blue coloured solution.
RCHO + 2Cu2+ + 3OH– → RCOO– + Cu2O + 2H2O
(c) Benedict’s Test
Benedict’s solution is an alkaline solution of Cu2+ ions complexed with citrate ions (whereas Fehling’s solution is an alkaline solution of Cu2+ ions complexed with tartarate ions). It reacts in the same way as Fehling’s solution.
Reduction of Carbonyl Compounds
(a) Reduction to alcohols
Aldehydes and ketones are reduced to 10 and 20 alcohols, respectively. The reduction is carried out either catalytically (H2 + Ni/Pt/Pd) or chemically (LiAlH4 or NaBH4).
(b) Reduction to hydrocarbons
It involves reduction of the carbonyl group of aldehydes or ketones to methylene (CH2) group to form a hydrocarbon. It is carried out in the presence of Zn amalgam and conc. HCl.
Wolf Kishner Reduction
It involves reduction of the carbonyl group of aldehydes or ketones to methylene (CH2) group to form a hydrocarbon. It is carried out with hydrazine followed by heating with NaOH or KOH in a high boiling solvent such as ethylene glycol.
Reaction due to α- Hydrogen
It is given by aldehydes and ketones that have at least one α-hydrogen atom in presence of dilute basic media to obtain α, β-unsaturated aldehyde/ketone via the formation of β-hydroxy aldehyde/ketone.
Cross Aldol Condensation
On using two type of carbonyl compounds both having α-hydrogen atoms in an aldol condensation, a mixture of four condensed products is obtained. It is called cross aldol condensation. Ketones can also be used as one component in the cross aldol. Cross aldol condensation reactions are of great synthetic use if one of the carbonyl compounds do not possess α-H atoms.
Carbonyl compounds without α-hydrogen atoms undergo disproportionation or redox reactions in presence of a strong basic media.
USES OF ALDEHYDES AND KETONES
In the Chemical industry, aldehydes and ketones are used as solvents and reagents for the synthesis of products. To preserve biological specimens formalin (40% solution of formaldehyde) solution is used and also to prepare Bakelite, urea-formaldehyde glues and other polymeric products. Acetaldehyde is used in the making of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs. Some are also known for their odours and flavours like butyraldehyde, vanillin, acetophenone, camphor, etc.
This article has tried to highlight all the important parts of aldehydes and ketones in the form of short notes for class 12 students in order to understand the basic concepts of the chapter. The notes aldehydes and ketones have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.
Check Part 2 of the Chapter here: CARBOXYLIC ACIDS