CANNIZZARO REACTION MECHANISM

The Cannizzaro reaction is a disproportionation reaction that produces a primary alcohol as well as a carboxylic acid by combining two molecules of an aldehyde with a hydroxide base. This is demonstrated by the reaction of benzaldehyde with benzyl alcohol to produce benzoic acid. 

And disproportion is a redox process in which a chemical in the middle of its oxidation cycle splits into two different molecules, one high and the other low in oxidation Number.

Vanillin, benzaldehyde, syringaldehyde, and formaldehyde are examples of Cannizzaro reactions that lack active hydrogen. They are oxidized intramolecularly and intermolecularly with a strong base (NaOH) to produce a carboxylic acid as well as alcohol molecule.

CANNIZARO REACTION MECHANISM: 

The Cannizzaro reaction process explains how two aldehyde molecules are converted into one alcohol and one carboxylic acid molecule. Benzyl alcohol & potassium benzoate were discovered in 1853 by scientist Stanislao Cannizzaro from benzaldehyde.

 In the leaving group, where another aldehyde is attacked, an aldehyde is replaced by a nucleophile acyl. A tetrahedral intermediate is the result of a hydroxide attack on a carbonyl. This tetrahedral intermediate collapses & reforms the carbonyl, causing a hydride to attack another colony.

A proton from an acid and an alcoholic ion is now switched. If a base with a high concentration is provided, the aldehyde produces an anion with a charge of two. A hydride ion composed of carboxylate & alcohol is transferred to a second aldehyde molecule. The solvent also provides a proton to the alcohol ion for the reaction.

STEPS IN THE CANNIZZARO REACTION MECHANISM:

Step 1: A nucleophile, such as hydroxide, is used to attack the carbonyl group in the aldehyde in concern, resulting in a disproportionate reaction as well as a two-negative-charged anion.

Step 2: The intermediate product now can function as a hydride reduction. Because of its unstable nature, the intermediate produces a hydride anion. Another aldehyde molecule is attacked by this anion hydride. The aldehyde becomes an anion for carboxylate, and the doubly charged anion has become an anion for alcohol.

Step 3: Water gives a proton to the alcohol anion in this third stage, resulting in the final alcoholic product. So, because alcohol is basic instead of water, the reaction may occur. When acid is used, the carboxylate ion produces the carboxylic acid’s final product (the acid workup is needed because carboxylate is less basic than water & therefore cannot obtain a proton from water).

In general, the response follows third-order kinetics. It is second in aldehyde and first in basic:

k[RCHO]2[OH] = temporary rate

For very high foundation, a second way (k’) is required, which is 2nd order in base:

k[RCHO] = Temporary rate

k'[OH]-2 + 2[RCHO]

In the k’ trajectory, a reaction involving a doubly charged anion (RCHO22) and aldehyde is involved. The fact that no deuterium is bonded to the -carbon in the recovered alcohol is when reaction is carried out only when D2O occurs indicates direct transmission of hydrogen ions.

CROSS CANNIZZARO REACTION:

It’s not surprising that the Crossed Cannizzaro reaction produces only half of the alcohol and carboxylic acid required under ideal conditions. As a result, the Cannizzaro crossover reaction is more extensively used. 

Formaldehyde reduces sodium oxidation by mixing a sacrificial aldehyde with a more valuable molecule. The required alcohol is obtained by reducing other aldehyde compounds. When two distinct aldehydes may be entirely converted into the desired product, the output of the beneficial chemical is increased. 

Finally, the Cannizzaro reaction can be used to disproportionate a non-enolizable aldehyde. To increase the yield of the precious product, the Cross Cannizzaro reaction is used. Example:

PhCHO +HCHO      PhC(OH)+ HCOONa

SCOPE: 

Due to extremely alkaline reactions, aldehydes with alpha hydrogen atom(s) have been decongested, resulting in enolates and possible aldol reactions. Under ideal conditions, the procedure yields just 50% alcohol and carboxylic acid (it takes two aldehydes to produce one acid and one alcohol). 

The cross Cannizzaro reaction, which mixes sacrifice aldehyde with a much more desirable molecule, can be performed more frequently to avoid low yields.

Formaldehyde is the reducer in this version, which is oxidized to formate sodium whereas alcohol is converted to another aldehyde molecule. In this situation, instead of losing half of a single reactant for each of two products, one aldehyde can be entirely converted to its corresponding product. As a result, while the atomic economics is still low, the output of the precious chemical is high.

USES AND APPLICATIONS:

• Polyols are manufactured in the industry using a mixture of crossed Cannizzaro reaction & aldol condensation. Polyols are extremely valuable and have a wide range of industrial applications.
• Neopentyl glycol is used in polyesters to make resins for use in planes or on board, varnish coatings, synthetic lubricants, and plasticizers. The formula of neopentyl makes it resistant to light, heat, and hydrolysis.
• Pentaerythritol is a hot commodity in the varnish business as a raw material. Pentaerythritol esters are used as oil additives, plastifying agents, and emulsifiers in products with higher fatty acids.
• Trimethylolpropane is used as a glycerin substitute in a variety of applications, including the manufacture of alkaline resins, polyesters, and polyurethanes.

CONCLUSION:

Polyols are made from formaldehyde and other aldehydes using a combination of aldol condensation and the crossed-Cannizaro reaction. The synthesis of Pentaerythritol from acetaldehyde is a common use of the process. Polyols are indeed very useful and have various applications in business