A few organic reaction mechanisms that chemistry majors must understand
If an alcoholic solution of sodium alcohol is used, carboxylic acid esters are obtained:
This method can be used to synthesize tensile four-membered rings.
Reaction Mechanisms+
Fries rearrangement
Phenolic esters can undergo acyl rearrangement by heating in the presence of Lewis acid to give a mixture of o-hydroxy and p-hydroxy aryl ketones. The rearrangement can be carried out in solvents such as nitrobenzene and nitromethane, or by direct heating without solvent.
The ratio of the o- and para-products depends on the structure of the phenolic ester, the reaction conditions and the catalyst. For example, catalysis with polyphosphoric acid produces mainly para rearrangement products, while catalysis with titanium tetrachloride produces mainly o-ratio rearrangement products. The effect of reaction temperature on the ratio of neighboring and para products is relatively large. Generally speaking, the rearrangement at lower temperatures (e.g. room temperature) favors the formation of para isomerization products (kinetic control), and the rearrangement at higher temperatures favors the formation of neighboring isomerization products (thermodynamic control).
Reaction Mechanisms
Hofmann rearrangement (degradation)
Amides treated with bromine (or chlorine) under alkaline conditions are transformed into primary amines with one less carbon atom: the reaction is controlled by the Hofmann reaction mechanism.
Reaction mechanism
Reaction Example
Knoevenagel reaction
Compounds containing active methylene are condensed with aldehydes or ketones in the presence of a weakly basic catalyst (ammonia, primary amines, secondary amines, pyridine, and other organic bases) to give a,b-unsaturated compounds.
Reaction Mechanism
Gabriel Synthesis
Phthalimide is converted to phthalimide salt by the action of potassium hydroxide solution, and this salt reacts with haloalkanes to form N-alkyl phthalimide, and then hydrolyzed under acidic or alkaline conditions to obtain primary amine and phthalic acid, which is a method to prepare pure primary amine.
In some cases, hydrolysis is difficult and can be replaced by hydrazinolysis:
Reaction mechanism
The reaction between phthalimide salts and haloalkanes is a nucleophilic substitution reaction, and the hydrolysis of the substitution reaction products is similar to the hydrolysis of amides.
Mannich reaction
Aldehydes and ketones containing an a-active hydrogen react with formaldehyde and an amine (primary amine, secondary amine, or ammonia), resulting in the substitution of an a-active hydrogen by an amine methyl group; this reaction is also known as an amine methylation reaction, and the resulting product is known as a Mannich base.
Reaction Mechanism
Oppenauer oxidation
Secondary alcohols are oxidized to the corresponding ketone and reduced to isopropanol in the presence of aluminum tert-butoxide or aluminum isopropoxide. This reaction corresponds to the reverse of the Meerwein-Ponndorf reaction.
A special intramolecular rearrangement reaction occurs when katana alcohols are heated and dehydrated in the presence of dilute H2SO4 to produce katana ketones. The same result can be obtained by using dehydrating-translocating agents such as, oxalic acid, I2/CH3COOH, CH3COOH, etc. instead of H2SO4:
Reaction Mechanism
The key to the reaction is the generation of the carbon positive ion: the
Aldehydes or ketones react with a-halogenates and zinc in an inert solvent and undergo hydrolysis to give b-hydroxy acid esters.
Reaction mechanism
First, the a-halogenate and zinc react to form the intermediate organozinc reagent, which then undergoes addition to the carbonyl group of the aldehyde or ketone, followed by hydrolysis:
Reaction examples
Reimer-Tiemann reaction
Phenol and chloroform are heated in alkaline solution to form o- and para-hydroxybenzoic acids. Heterocyclic compounds such as quinoline, pyrrole, and indene, which contain hydroxyl groups, can also undergo this reaction.
The commonly used alkaline solutions are aqueous solutions of sodium hydroxide, potassium carbonate, and sodium carbonate, and the products are generally dominated by the neighboring site, with a small amount of the para product. If both neighboring sites are occupied then the para site is entered. Compounds that cannot react in water can be carried out in pyridine, when only the neighboring products are obtained.
Reaction mechanism
First chloroform in alkali solution to form dichlorocarbin, which is an electron-deficient electrophilic reagent, and the negative ion of phenol (Ⅱ) electrophilic substitution occurs to form an intermediate (Ⅲ), (Ⅲ) from the solvent or the reaction system to obtain a proton, and at the same time, the α-hydrogen of the carbonyl group to leave the formation of (Ⅳ) or (Ⅴ), (Ⅴ) hydrolyzed to obtain aldehydes.
The diazonium salt is treated with cuprous chloride or cuprous bromide to give chlorinated or brominated aromatics:
This reaction can also be realized with freshly prepared copper powder and HCl or HBr (Gattermann reaction).
Reaction mechanism
Wittig reaction
The Wittig reagent reacts with the carbonyl groups of aldehydes and ketones in a nucleophilic addition reaction to form olefins:
Reaction Mechanism
When the b-carbon atom of the hydroxyl group of an alcohol is a secondary (secondary carbon atom) or tertiary (tertiary carbon atom), a rearrangement reaction often occurs in acid-catalyzed dehydration reactions to give a rearrangement product:
