Chemistry

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Uses of Aldehydes and Ketones

Formaldehyde

1. 40% aqueous solution of formaldehyde is called formalin. It is used for preserving biological specimens.
2. Formalin has hardening effct, hence it is used for tanning.
3. Formalin is used in the production of thermo setting plastic known as bakelite, which is obtained by heating phenol with formalin.

Acetaldehye

1. Acetaldehyde is used for silvering of mirrors
2. Paraldehyde is used in medicine as a hypnotic.
3. Acetaldehyde is used in the commercial preparation of number of organic compounds like acetic acid, ethyl acetate etc.,

Acetone

1. Acetone is used as a solvent, in the manufacture of smokeless gun powder (cordite)
2. It is used as a nail polish remover.
3. It is used in the preparation of sulphonal, a hypnotic.
4. It is used in the manufacture of thermosoftning plastic Perspex.

Benzaldehyde is Used

1. As a flavouring agent
2. In perfumes
3. In dye intermediates
4. As starting material for the synthesis of several other organic compounds like cinnamaldehyde, cinnamic acid, benzoyl chloride etc.

Aromatic Ketones

1. Acetophenone has been used in perfumery and as a hypnotic under the name hypnone.
2. Benzophenone is used in perfumery and in the preparation of benzhydrol eye drop.

Carboxylic Acids

Introduction

Carbon compounds containing a carboxyl function group, -COOH are called carboxylic acids. The Carboxyl group is the combination of carbonyl group and the hydroxyl group (-OH).

However, carboxyl group has its own characteristic reaction. Carboxylic acids may be aliphatic (R – COOH) or aromatic (Ar – COOH) depending on the alkyl or aryl group attached to carboxylic carbon. Some higher members of aliphatic carboxylic acids (C₁₂ to C₁₈) known as fatty acids occur in natural fats as esters of glycerol.

Aldehydes are currently used in the production of resins and plastics. The simplest ketone, propanone, is commonly called acetone. Acetone is a common organic solvent that was one used in most nail polish removers, but has largely been replaced by other solvents.

It is used in tanning, preserving, and embalming and as a germicide, fungicide, and insecticide for plants and vegetables, but its largest application is in the production of certain polymeric materials.

1. Ketone behaves as an excellent solvent for certain types of plastics and synthetic fibres.
2. Acetone act as a paint thinner and a nail paint remover.
3. It also is used for medicinal purposes such as chemical peeling procedure as well as acne treatments.

Example of Ketone

Ketones contain a carbonyl group (a carbon-oxygen double bond). The simplest ketone is acetone (R = R’ = methyl), with the formula CH₃C(O)CH₃. Many ketones are of great importance in biology and in industry. Examples include many sugars (ketoses), many steroids (e.g., testosterone), and the solvent acetone.

Example of Aldehyde

Aldehydes are given the same name but with the suffix – ic acid replaced by – aldehyde. Two examples are formaldehyde and benzaldehyde. As another example, the common name of CH₂ = CHCHO, for which the IUPAC name is 2-propenal, is acrolein, a name derived from that of acrylic acid, the parent carboxylic acid.

Generally, the common names of ketones consist of the names of the groups attached to the carbonyl group, followed by the word ketone. (Note the similarity to the naming of ethers). Another name for acetone, then, is dimethyl ketone. The ketone with four carbon atoms is ethyl methyl ketone.

Common Ketones are Acetone and Methyl Ethyl Ketone. They have different uses. Acetone is known as fingernail polish remover but is also commonly used as lacquer and varnish solvent.

Aldehydes are made by oxidising primary alcohols. The aldehyde produced can be oxidised further to a carboxylic acid by the acidified potassium dichromate (VI) solution used as the oxidising agent. In order to stop at the aldehyde, you have to prevent this from happening.

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Test for Aldehydes

(i) Tollens Reagent Test

Tollens reagent is an ammonical silver nitrate solution. When an aldehyde is warmed with Tollens reagent a bright silver mirror is produced due to the formation of silver metal. This reaction is also called silver mirror test for aldehydes.

(ii) Fehlings Solution Test

Fehlings solution is prepared by mixing equal volumes of Fehlings solution ‘A’ containing aqueous copper sulphate and Fehlings solution ‘B’ containing alkaline solution of sodium potassium tartarate (Rochelle salt)

When aldehyde is warmed with Fehlings solution deep blue colour solution is changed to red precipitate of cuprous oxide.

(iii) Benedict’s Solution Test:

Benedicts solution is a mixture of CuSO₄ + sodium citrate + NaOH.Cu²⁺ is reduced by aldehyde to give red
precipitate of cuprous oxide.

(iv) Schiff’ Reagent Test

Dilute solution of aldehydes when added to schiff’ reagent (Rosaniline hydrochloride dissolved in water and its red colour decolourised by passing SO₂) yields its red colour. This is known as Schiff’ test for aldehydes. Ketones do not give this test. Acetone however gives a positive test but slowly.

An aldehyde is similar to a ketone, except that instead of two side groups connected to the carbonyl carbon, they have at least one hydrogen (RCOH). The simplest aldehyde is formaldehyde (HCOH), as it has two hydrogens connected to the carbonyl group.

Tollen’s reagent is a classical organic laboratory technique to test for the presence of an aldehyde. The reagent consists of silver (I) ions dissolved in dilute ammonia. When the aldehyde is oxidized, the silver (I) ions are reduced to silver metal.

The Schiff test is a chemical test used to check for the presence of aldehydes in a given analyte. This is done by reacting the analyte with a small quantity of a Schiff reagent (which is the product formed in certain dye formulation reactions such as the reaction between sodium bisulfite and fuchsin).

Aldehyde, any of a class of organic compounds in which a carbon atom shares a double bond with an oxygen atom, a single bond with a hydrogen atom, and a single bond with another atom or group of atoms (designated R in general chemical formulas and structure diagrams).

Fehling’s solution can be used to distinguish aldehyde vs ketone functional groups. The compound to be tested is added to the Fehling’s solution and the mixture is heated. Aldehydes are oxidized, giving a positive result, but ketones do not react, unless they are α-hydroxy ketones.

Take the given organic compound in a clean test tube. Add 1ml of chromic acid reagent to the given organic compound. The appearance of a green or blue colour precipitate indicates the presence of aldehydes.

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Chemical Properties of Aldehydes and Ketones

A. Nucleophilic Addition Reactions

This reaction is the most common reactions of aldehydes and ketones. The carbonyl carbon carries a small degree of positive charge. Nucleophile such as CN^– can attack the carbonyl carbon and uses its lone pair to form a new carbon – nucleophile ‘σ’ bond, at the same time two electrons from the carbon – oxygen double bond move to the most electronegative oxygen atom. This results in the formation of an alkoxide ion. In this process, the hybridisation of carbon changes from sp² to sp³.

The tetrahedral intermediate can be protonated by water or an acid to form an alcohol.

In general, aldehydes are more reactive than ketones towards nucleophilic addition reactions due to +I and steric effect of alkyl groups.

Examples

1. Addition of HCN

Attack of CN^– on carbonyl carbon followed by protonation gives cyanohydrins.

The cyanohydrins can be converted into hydroxy acid by acid hydrolysis. Reduction of cyanohydrins gives hydroxy amines.

2. Addition of NaHSO₃

This reaction finds application in the separation and purification of carbonyl compound. The bisulphate addition compound is water soluble and the solution is treated with mineral acid to regenerate the carbonyl compounds.

3. Addition of Alcohol

When aldehydes / ketones is treated with 2 equivalents of an alcohol in the presence of an acid catalyst to form acetals.

Example

When acetaldehyde is treated with 2 equivalent of methanol in presence of HCl, 1, 1, – dimethoxy ethane is obtained.

Mechanism

4. Addition of Ammonia and its Derivatives

When the nucleophiles, such as ammonia and its derivative image 6 is treated with carbonyl compound, nuceophilic addition takes place, the carbonyl oxygen atom is protonated and then elimination takes place to form carbon – nitrogen double bond image 7

When G – alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂ etc…

(i) Reaction with Hydroxyl Amine

Aldehyde and ketones react with hydroxylamine to form oxime.

Example:

(ii) Reaction with Hydrazine

Aldehydes and ketones react with hydrazine to form hydrazone.

Example:

(iii) Reaction with Phenyl Hydrazine

Aldehydes and ketones react with phenyl hydrazine to form phenyl hydrazone.

Example:

5. Reaction with NH₃

(i) Aliphatic aldehydes (except formaldehyde) react with an ethereal solution of ammonia to form aldimines.

(ii) Formaldehyde reacts with ammonia to form hexa methylene tetramine, which is also known as Urotropine.

Structure

Uses

1. Urotropine is used as a medicine to treat urinary infection.

2. Nitration of Urotropine under controlled condition gives an explosive RDX (Research and development explosive). It is also called cyclonite or cyclotri methylene trinitramine.

3. Acetone reacts with ammonia to form diacetone amine.

4. Benzaldehyde form a complex condensation product with ammonia.

B. Oxidation of Aldehydes and Ketones

(a) Oxidation of Aldehydes

Aldehydes are easily oxidised to carboxylic acid containing the same number of carbon atom, as in parent aldehyde. The common oxidising agents are acidified K₂Cr₂O₇, acidic or alkaline KMnO₄ or chromic oxide.

Example

(b) Oxidation of Ketone

Ketones are not easily oxidised. Under drastic condition or with powerful oxidising agent like Con. HNO₃, H⁺/KMnO₄, H⁺/K₂Cr₂O₇, cleavage of carbon-carbon bond takes place to give a mixture of carboxylic acids having less number of carbon atom than the parent ketone.

The oxidation of unsymmetrical ketones is governed by Popoff ’s rule. It states that during the oxidation of an unsymmetrical ketone, a (C-CO) bond is cleaved in such a way that the keto group stays with the smaller alkyl group.

C. Reduction Reactions

(i) Reduction to Alcohols

We have already learnt that aldehydes and ketones can be easily reduced to primary and secondary alcohols respectively. The most commonly used reducing agents are Lithium Aluminium hydride (LiAlH₄), and Sodium borohydride (NaBH₄).

(a) Aldehyde are Reduced to Primary Alcohols

Example

(b) Ketone are Reduced to Secondary Alcohols.

Example

The above reactions can also be carried out with hydrogen in the presence of metal catalyst like Pt, Pd, or Ni. LiAlH₄ and NaBH₄ do not reduce isolated carbon – carbon double bonds and double bond of benzene rings. In case of α, β unsaturated aldehyde and ketones, LiAlH₄ reduces only C = O group leaving C = C bond as such.

(ii) Reduction to Hydrocarbon

The carbonyl group of aldehydes and ketones can be reduced to methylene group using suitable reducing agents to give hydrocarbons.

(a) Clemmensen Reduction

Aldehydes and Ketones when heated with zinc amalgam and concentrated hydrochloric acid gives hydrocarbons.

Example

(b) Wolf Kishner Reduction

Aldehydes and Ketones when heated with hydrazine (NH₂NH₂) and sodium ethoxide, hydrocarbons are formed Hydrazine acts as a reducing agent and sodium ethoxide as a catalyst.

Example

Aldehyde (or) ketones is first converted to its hydrazone which on heating with strong base gives hydrocarbons.

(iii) Reduction to Pinacols:

Ketones, on reduction with magnesium amalgam and water, are reduced to symmetrical diols known as pinacol.

D. Haloform Reaction

Acetaldehyde and methyl ketones, containing image 26 group, when treated with halogen and alkali give the corresponding haloform. This is known as Haloform reaction.

E. Reaction Involving Alkylgroup

(i) Aldol Condensation

The carbon attached to carbonyl carbon is called α – carbon and the hydrogen atom attached to α – carbon is called α – hydrogen.

In presence of dilute base NaOH, or KOH, two molecules of an aldehyde or ketone having α – hydrogen add together to give β – hydroxyl aldehyde (aldol) or β – hydroxyl ketone (ketol). The reaction is called aldol condensation reaction. The aldol or ketol readily loses water to give α, β – unsaturated compounds which are aldol condensation products.

(a) Acetaldehyde when warmed with dil NaOH gives β – hydroxyl butyraldehyde (acetaldol)

Mechanism

The mechanism of aldol condensation of acetaldehyde takes place in three steps.

Step 1:

The carbanion is formed as the α – hydrogen atom is removed as a proton by the base.

Step 2:

The carbanion attacks the carbonyl carbon of another unionized aldehyde to form an alkoxide ion.

Step 3:

The alkoxide ion formed is protonated by water to form aldol.

The aldol rapidly undergoes dehydration on heating with acid to form α – β unsaturated aldehyde.

(ii) Crossed Aldol Condensation

Aldol condensation can also take place between two different aldehydes or ketones or between one aldehyde and one ketone such an aldol condensation is called crossed or mixed aldol condensation. This reaction is not very useful as the product is usually a mixture of all possible condensation products and cannot be separated easily.

Example

F. Some Important Reactions of Benzaldehyde

(i) Claisen – Schmidt Condensation

Benzaldehye condenses with aliphatic aldehyde or methyl ketone in the presence of dil. alkali at room temperature to form unsaturated aldehyde or ketone. This type of reaction is called Claisen – Schmidt condensation.

Example

(ii) Cannizaro Reaction

In the presence of concentrated aqueous or alcoholic alkali, aldehydes which do not have α – hydrogen atom undergo self oxidation and reduction (disproportionation) to give a mixture of alcohol and a salt of carboxylic acid. This reaction is called Cannizaro reaction.

Benzaldehyde on treatment with concentrated NaOH (50%) gives benzyl alcohol and sodium benzoate.

This reaction is an example disproportionation reaction

Mechanism of Cannizaro Reaction

Cannizaro reaction involves three steps.

Step 1:

Attack of OH^– on the carbonyl carbon.

Step 2:

Hydride ion transfer

Step 3:

Acid – base reaction.

Cannizaro reaction is a characteristic of aldehyde having no α – hydrogen.

Crossed Cannizaro Reaction

When Cannizaro reaction takes place between two different aldehydes (neither containing an α hydrogen atom), the reaction is called as crossed cannizaro reaction.

In crossed cannizaro reaction more reactive aldehyde is oxidized and less reactive aldehyde is reduced.

3. Benzoin Condensation

The Benzoin condensation involves the treatment of an aromatic aldehyde with aqueous alcoholic KCN. The products are a hydroxy ketone.

Example

Benzaldehyde reacts with alcoholic KCN to form benzoin

4. Perkins’ Reaction

When an aromatic aldehyde is heated with an aliphatic acid anhydride in the presence of the sodium salt of the acid corresponding to the anhydride, condensation takes place and an α, β unsaturated acid is obtained. This reaction is known as Perkin’s reaction.

Example

5. Knoevenagal Reaction

Benzaldehyde condenses with malonic acid in presence of pyridine forming cinnamic acid, Pyridine act as the basic catalyst.

6. Reaction with Amine

Aromatic aldehydes react with primary amines (aliphatic or aromatic) in the presence of an acid to form schiff’s base.

Example

7. Condensation with Tertiary Aromatic Amines

Benzaldehyde condenses with tertiary aromatic amines like N, N – dimethyl aniline in the presence of strong acids to form triphenyl methane dye.

Example

8. Electrophilic Substitution Reactions of Benzaldehyde

Electrophilic Substitution Reaction of Acetophenone

Acetophenone reacts with Nitrating mixture to form m – nitroacetophenone.

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Physical Properties of Aldehydes and Ketones

1. Physical State:

Formaldehyde is a gas at room temperature and acetaldehyde is a volatile liquid. All other aldehydes and ketones upto to C₁₁ are colourless liquids while the higher ones are solids.

2. Boiling Points

Aldehydes and ketones have relatively high boiling point as compared to hydrocarbons and ethers of comparable molecular mass. It is due to the weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.

These dipole-dipole interactions are weaker than intermolecular H-bonding. The boiling points of aldehydes and ketones are much lower than those of corresponding alcohols and carboxylic acids which possess inter molecular hydrogen bonding.

3. Solubility

Lower members of aldehydes and ketones like formaldehyde, acetaldehyde and acetone are miscible with water in all proportions because they form hydrogen bond with water. Solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl chain.

4. Dipolemoment

The carbonyl group of aldehydes and ketones contains a double bond between carbon and oxygen. Oxygen is more electronegative than carbon and it attracts the shared pair of electron which makes the carbonyl group as polar and hence aldehydes and ketones have high dipole moments.

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.

Physical Properties of Ketones

Ketones are highly reactive, although less so than aldehydes, to which they are closely related. Much of their chemical activity results from the nature of the carbonyl group. Ketones readily undergo a wide variety of chemical reactions.

Properties of Aldehydes

The reactivity of these compounds arises largely through two features of their structures:

The polarity of the carbonyl group and the acidity of any α-hydrogens that are present. Aldehydes are polar molecules, and many reagents seek atoms with a deficiency of electrons.

Both aldehydes and ketones contain a carbonyl group. That means that their reactions are very similar in this respect. An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidise.

Carboxylic acids have high boiling points compared to other substances of comparable molar mass. Boiling points increase with molar mass. Carboxylic acids having one to four carbon atoms are completely miscible with water. Solubility decreases with molar mass.

Aldehydes and ketones have a much higher boiling point than the alkanes. As the molecules get larger, the difference between an aldehyde/ketone and its corresponding alkane gets smaller. The reason for this is that the non-polar region of the carbon chain is getting larger as the polar region (C=O) is staying the same.

Examples of Aldehydes

Aldehydes are given the same name but with the suffix – ic acid replaced by – aldehyde. Two examples are formaldehyde and benzaldehyde. As another example, the common name of CH₂=CHCHO, for which the IUPAC name is 2-propenal, is acrolein, a name derived from that of acrylic acid, the parent carboxylic acid.

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General Methods of Preparation of Aldehydes and Ketones

A. Preparation of Aldehydes and Ketones

1. Oxidation and Catalytic Dehydrogenation of Alcohols

We have already learnt that the oxidation of primary alcohol gives aldehydes and secondary alcohol gives a ketone. Oxidising agents such as acidified Na₂Cr₂O₇, KMnO₄, PCC are used for oxidation. Oxidation using PCC yield aldehydes. Other oxidising agents further oxidise the aldhydes / ketones in to carboxylic acids.

When vapours of alcohols are passed over heavy metal catalyst such as Cu, Ag, alcohols give aldehydes and ketons.

Catalytic Dehydrogenation of Alcohols

2. Ozonolysis of Alkenes

We have already learnt in XIth standard that the reductive ozonolysis of alkenes gives aldehydes and ketones. Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc and water gives aldehydes and ketones. Zinc dust removes H₂O₂ formed, which otherwise can oxidise aldehydes/ketones.

Terminal olefies give formaldehyde as one of the product.

3. Hydration of Alkynes

We have already learnt in XI standard that the hydration of alkynes in presence of 40% dilute sulphuric acid and 1% HgSO₄ to give the corresponding aldehydes/ketones.

(a) Hydration of acetylene yields acetaldehyde

(b) Hydration of alkynes, other than acetylene gives ketones

4. From Calcium Salts of Carboxylic Acids

Aldehydes and ketones may be prepared by the dry distillation of calcium salts of carboxylic acids.

(a) Aldehydes are obtained when the mixture of calcium salt of carboxylic acid and calcium formate is subjected to dry distillation.

(b) Symmetrical ketones can be obtained by dry distillation of the calcium salt of carboxylic acid (except formic acid)

B. Preparation of Aldehydes

1. Rosenmund Reduction

(a) Aldehydes can be prepared by the hydrogenation of acid chloride, in the presence of palladium supported by barium sulphate. This reaction is called Rosenmund reduction.

2. Stephen’s Reaction

When alkylcyanides are reduced using SnCl₂/HCl, imines are formed, which on hydrolysis gives corresponding aldehyde.

3. Selective Reduction of Cyanides

Diisobutyl aluminium hydride (DIBAL -H) selectively reduces the alkyl cyanides to form imines which on hydrolysis gives aldehydes.

(c) Preparation of Benzaldehyde

1. Side chain oxidation of toluene and its derivatives by strong oxidising agents such as KMnO₄ gives benzoic acid. When chromylchloride is used as an oxidising agent, toluene gives benzaldehyde. This reaction is called Etard reaction. Acetic anhydride and CrO₃ can also be used for this reaction.

Oxidation of toluene by chromic oxide gives benzylidine diacetate which on hydrolysis gives benzaldehyde.

2. Gattermann – Koch Reaction

This reaction is a variant of Friedel – Crafts acylation reaction. In this method, reaction of carbon monoxide and HCl generate an intermediate which reacts like formyl chloride.

3. Manufacture of Benzaldehyde from Toluene

Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde.

This is the commercial method for the manufacture of benzaldehye.

(d) Preparation of Ketones

1. Ketones can be prepared by the action of acid chloride with dialkyl cadmium.

2. Preparation of Phenyl Ketones

Friedel – Craft Acylation

It is the best method for preparing alkyl aryl ketones or diaryl ketones. This reaction succeeds only with benzene and activated benzene derivatives.