Cannizzaro reaction
The simplest organic compounds are called hydrocarbons which composes of two elements- carbon and hydrogen. Hydrocarbons can be of two types- aliphatic hydrocarbons and aromatic hydrocarbons. Depending upon the number of Carbon atoms chains, each type of hydrocarbon is further classified into three sub-types; alkanes, alkenes, and alkynes for aliphatic hydrocarbons and cycloalkanes, cycloalkenes, and cycloalkynes for aromatic hydrocarbons. However, when hydrogen atoms attaching to the carbon atom are replaced by functional groups, different classes of hydrocarbons are formed possessing varieties of physical and chemical properties. Alcohol or phenol is formed when a hydrogen atom in an aliphatic or aromatic hydrocarbon respectively is replaced by the hydroxyl (–OH) group. An aldehyde is a hydrocarbon in which a carbonyl group (>C=O) is bonded to carbon and hydrogen, while in ketones it is bonded to two carbon atoms. A carboxylic acid is a hydrocarbon with a carbonyl group, whose hydrogen atom is replaced by a -OH group.
Alcohols are reduced from aldehydes and ketones by catalytic hydrogenation. Catalytic hydrogenation is the addition of hydrogen to the carbonyl compounds in presence of catalysts. Finely divided metals like platinum, palladium, and nickel are usual catalysts. The formation of alcohols is by treating aldehydes and ketones with sodium borohydride or lithium aluminium hydride. Primary alcohols will yield from aldehydes while secondary alcohols will yield from ketones.
Aldehydes undergo oxidation to form carboxylic acids on treatment with oxidizing agents. Common oxidizing agents used are potassium permanganate, potassium dichromate, nitric acid, etc. Aldehydes can also oxidize with mild oxidizing agents, like Tollen’s reagent and Fehling’s reagent. Oxidation of an aldehyde in the presence of an alkali or base can occur due to the acidic nature of its alpha-hydrogen. The strong electron withdrawal effect of the carbonyl groups and resonance stabilization of the conjugate base makes the alpha-hydrogen atoms highly acidic. However, the aldehydes that possess no alpha-hydrogen atoms will undergo self- oxidation and reduction reaction upon heating with a concentrated alkali. During this redox reaction, a molecule of the aldehyde is reduced to alcohol while another aldehyde is oxidized to a carboxylic acid salt. Such a redox reaction in which a compound or intermediate oxidation state converts to different compounds, each of a higher and a lower oxidation state is called disproportionation.
The Cannizzaro Reaction, named after its discoverer, Stanislao Cannizzaro is an organic redox reaction involving an alkali-induced disproportionation of two molecules of non-elonizable aldehydes to form an alcohol and a carboxylic acid. The transformation was first accomplished by Cannizzaro in 1853 when a benzaldehyde reaction with potassium carbonate forms benzyl alcohol and potassium benzoate. Another example of a Cannizzaro reaction is the oxidation to benzoic acid and reduction to benzyl alcohol after two moles of aldehyde is treated with sodium hydroxide.
The mechanism behind the Reaction
The mechanism of a Cannizzaro reaction involves the following mechanisms:
- Firstly, a hydroxide ion that serves as a nucleophile, attacks the carbonyl group of the aldehyde leading to deprotonation to give a dianion. During this mechanism, the hydroxide attacks the electrophilic carbon atom of the carbonyl group. The nucleophile attacks from a direction perpendicular to the sp2 hybridized orbitals plane of the carbonyl group.
- The planar sp2 hybridized orbitals changes to a tetrahedral alkoxide intermediate, which is a dianion possessing sp3 hybridized orbitals. The resulting dianion then functions as a hydride reducing agent which releases a hydride anion that attacks another aldehyde molecule. Thereafter, the dianion converts to a carboxylate anion and the aldehyde becomes an alkoxide anion.
- Finally, the alkoxide anion picks a proton from water to form alcohol. The electrically neutral product of alcohol can be formed since alkoxide is more basic than water. However, carboxylate is less basic than water, for which it can only pick a proton upon acid workup. The carboxylate anion then only gets converted into the carboxylic acid. An acid workup is a method of protonation by adding an acid.
Under usual conditions formation of alcohol is not a perfect yield since involving aldehydes with alpha-hydrogen leads to deprotonation by the strong alkaline condition of the reaction, leading to enolates along with aldol reactions. Therefore, this ideal Cannizzaro reaction involving such two aldehydes has a low yield of alcohol and acid. To spike up the yield, crossed Cannizzaro reaction is conducted in which a sacrificial aldehyde, that is formaldehyde is used with another aldehyde. In this reaction, the formaldehyde works as a redundant which will completely oxidize to sodium formate, in the process while the other aldehyde completely reduces to alcohol. This phenomenon of spiking the product yield by using formaldehyde is due to this aldehyde do not possess an electron-donating group and an initial nucleophilic introduction of a hydroxide ion will immediately associate with formaldehyde, and easily oxidizing it to form formic acid. Simultaneously, the alcohol formed is benzyl alcohol.
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