Table of Contents
Introduction to Aldehydes and Ketones
Aldehydes and ketones are organic molecules that contain a carbon-oxygen double bond. The carbon atom in aldehydes and ketones is bonded to two hydrogen atoms and one oxygen atom. The carbon-oxygen double bond in aldehydes and ketones is referred to as a carbonyl group. Preparation of Aldehydes – From Alcohols and Hydrocarbons.
Aldehydes and ketones are structurally similar, but they have different chemical properties. Aldehydes are more reactive than ketones. Ketones are less reactive than aldehydes.
Aldehydes and ketones are used to make plastics, resins, and other chemicals. They are also used as flavoring agents and fragrance ingredients.
Aldehydes and ketones
Aldehydes and ketones are two types of organic compounds. Aldehydes are compounds that contain a carbon-oxygen double bond and a hydrogen atom attached to the carbon atom. Ketones are compounds that contain a carbon-oxygen double bond and two hydrogen atoms attached to the carbon atom. Preparation of Aldehydes – From Alcohols and Hydrocarbons.
Physical Properties and Characterization of Aldehydes
Aldehydes are organic compounds that have a carbonyl group (C=O) bonded to a hydrogen atom. They are colorless, flammable, and soluble in water. Aldehydes have a characteristic pungent odor and are used in the manufacture of perfume.
Aldehydes can be characterized by their physical properties, including their melting point, boiling point, and density. They can also be characterized by their chemical properties, including their reactivity and the types of bonds they form.
The General Method of Preparation of Aldehydes
The general method of preparation of aldehydes involves the oxidation of alcohols. The alcohol is oxidized to the corresponding aldehyde using a strong oxidizing agent such as chromic acid or potassium dichromate. The reaction is usually carried out in an aqueous acidic solution.
Functional Group Transformations
In organic chemistry, functional group transformations are the chemical changes that occur in a molecule’s functional group.
The most common type of functional group transformation is a substitution reaction, in which a functional group is replaced by another functional group. For example, in the reaction below, the hydroxyl group in ethanol is replaced by a chlorine atom.
This substitution reaction is catalyzed by the enzyme alcohol dehydrogenase.
Another common type of functional group transformation is an addition reaction, in which a functional group is added to a molecule. For example, in the reaction below, a molecule of hydrogen gas is added to an alkene to form a hydrocarbon.
This addition reaction is catalyzed by the enzyme hydroxyalkenyl dehydrogenase.
C-C Bond Cleavage
In the presence of a strong base, an sp3 carbon-carbon bond can be cleaved by the addition of a nucleophile. This process is known as beta-elimination.
The following steps describe the beta-elimination of propane:
1. A strong base (e.g. sodium hydride) is added to propane.
2. The strong base attacks the carbon-carbon bond, causing it to break.
3. The carbon-hydrogen bonds are also attacked by the strong base, causing them to break.
4. The products of the beta-elimination are propene and hydrogen gas.
Methods of Preparation of Aldehydes
The most common methods of preparing aldehydes are the oxidation of alcohols and the reduction of ketones.
1. The oxidation of alcohols: In this process, an alcohol is oxidized to an aldehyde by a strong oxidizing agent such as potassium dichromate (K2Cr2O7) or chromium trioxide (CrO3). The reaction is carried out in aqueous solution in the presence of a dilute acid such as sulfuric acid (H2SO4) or hydrochloric acid (HCl). The following equation illustrates the reaction:
2. The reduction of ketones: In this process, a ketone is reduced to an aldehyde by a reducing agent such as sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4). The reaction is carried out in an inert solvent such as diethyl ether (Et2O) or tetrahydrofuran (THF) in the presence of a base such as sodium hydroxide (NaOH) or lithium hydroxide (LiOH). The following equation illustrates the reaction:
Preparation of Aldehydes from Alcohols
The aldehyde can be prepared from the alcohol by two methods: the oxidation of the alcohol and the reduction of the carboxylic acid.
The oxidation of the alcohol can be accomplished by using an oxidizing agent such as chromic acid, permanganate ion, or ozone.
The reduction of the carboxylic acid can be accomplished by using a reducing agent such as sodium borohydride or lithium aluminum hydride.
By Collin’s Reagent
Collin’s reagent is a solution of copper sulfate pentahydrate and potassium iodide in water. It is used for the qualitative determination of starch.
The reagent is added to a sample of starch. If iodine is present, it will react with the starch to form a black-colored complex.
The presence of starch can be confirmed by adding a few drops of sodium hydroxide solution. If iodine is present, the solution will turn black.
PCC ( Pyridinium Chlorochromate)
PCC is a chemical compound that is used in various laboratory settings. It is a pyridinium salt that is often used as an oxidizing agent.
Dehydrogenation of Alcohols
The dehydrogenation of an alcohol is the removal of the hydrogen atom from the carbon atom that is bonded to the hydroxyl group.
The most common way to dehydrogenate an alcohol is to use a metal catalyst, such as palladium, to convert the alcohol into an alkene.
Preparation from Hydrocarbons
Preparation of benzene
Benzene is prepared commercially by the catalytic hydrogenation of toluene:
The toluene is converted to benzene and water.
Ozonolysis of Alkenes
The ozonolysis of an alkene is a reaction in which ozone is used to break the double bond of the alkene to form two new single bonds.
The ozonolysis of an alkene can be represented using the following equation:
O3 + C=C → O2 + CO2
In this equation, O3 represents ozone, C=C represents the double bond of the alkene, and O2 and CO2 represent the products of the reaction.
Hydration of Alkynes
To a suspension of the alkynes in anhydrous ether, a solution of lithium aluminum hydride is added. The hydride liberates hydrogen gas, which causes the alkynes to form the corresponding alkanes.
Preparation of Aromatic Aldehyde
The preparation of an aromatic aldehyde begins with the preparation of an aryl halide. Aryl halides are synthesized by the halogenation of an aromatic ring.
In the halogenation of an aromatic ring, an electrophile (such as a halogen) attacks an electron-rich aromatic ring, displacing a hydrogen atom. This process forms a new carbon-halogen bond and a new carbon-hydrogen bond.
The most common electrophile used in the halogenation of aromatic rings is chlorine. Bromine and iodine are also used, but to a lesser extent.
The preparation of an aryl halide begins with the preparation of an aromatic compound. The most common aromatic compound used in the preparation of aryl halides is benzene.
Benzene can be prepared by the benzene synthesis, the Friedel-Crafts alkylation, or the Friedel-Crafts acylation.
The benzene synthesis is the simplest method of preparing benzene. In the benzene synthesis, benzene is obtained by the reaction of acetylene with hydrogen gas in the presence of a nickel catalyst.
The Friedel-Crafts alkylation is the most common method of preparing alkylbenzenes. In the Friedel-Crafts alkylation, an alkyl halide is reacted with benzene in the presence of a Lewis acid catalyst.
Gattermann-koch Reaction
The Gattermann-Koch reaction is an organic reaction that involves the conversion of an aldehyde to an alkyl halide. The reaction is named for its two discoverers, Heinrich Gattermann and Friedrich Koch. The reaction is a two-step process in which an aldehyde is first converted to an imine, and then the imine is converted to the alkyl halide. The Gattermann-Koch reaction is a useful method for the synthesis of alkyl halides from aldehydes.
Side-Chain Halogenation
Side-chain halogenation is a type of organic reaction in which a halogen atom is added to the side chain of an organic molecule. The most common halogens used in this reaction are chlorine and bromine, but fluorine may also be used.
The mechanism for this reaction is typically a radical-mediated process in which a radical species is generated that attacks the side chain of the molecule. This radical then reacts with the halogen atom to form the halogenated product.
Side-chain halogenation is used to produce a variety of different products, including pharmaceuticals, agrochemicals, and surfactants.
