[Chemistry Class Notes] on Decarboxylation Reaction Pdf for Exam

A Decarboxylation Reaction is a chemical reaction in which a carboxyl group is removed and carbon dioxide is released ([CO_{2}]). Decarboxylation is a process in which carboxylic acids remove a carbon atom from a carbon chain. Carboxylation is the split chemical step in photosynthesis, where [CO_{2}] is added to the substance. It is a completely reversible reaction. Decarboxylases, on the other hand, are enzymes that catalyze decarboxylation.

One of the oldest known organic transformations is decarboxylation. It is one of the processes that is thought to occur in conjunction with destructive distillation and pyrolysis (thermal decomposition of substances). Through the intermediacy of metal carboxylate complexes, metal salts, particularly copper compounds, enhance the process. Aryl carboxylates can be decarboxylated to yield the equivalent aryl anion, which can then undergo cross-coupling processes.

The term decarboxylation means the removal of a carboxyl group (-COOH) from any reactant molecule. The chemical reaction where a carboxyl group (-COOH) gets eliminated and carbon dioxide ([CO_{2}]) is released at the product end is called Decarboxylation. The liberation of [CO_{2}] makes the reaction almost irreversible in many cases. However, the reverse process i.e. Carboxylation is the addition of [CO_{2}]. Carboxylation accounts for the very first step of Photosynthesis after the intake of [CO_{2}]. Carboxylation results in the formation of Carboxylic acid. Most Decarboxylation Reactions involve carboxylic acids, where a carbon atom is broken off from the carbon chain. This carbon atom is released in the form of [CO_{2}].

Decarboxylation – Carboxylic Acid

Carboxylic acids are the organic decarboxylation acids written as RCOOH, where R stands for an alkyl group or Hydrogen. The decarboxylation of a carboxylic acid is one of the oldest reactions discovered in organic chemistry. The reaction process involves the removal of the -COOH group or a carboxylate salt of the given acid. The reaction gives the product RH along with [CO_{2}]. 

 

[RCO_{2}H  rightarrow  RH  +  CO_{2}]

 

Decarboxylation Reactions are observed with a slightly categorized form in many compounds. The following are some of them.

• Krapcho Decarboxylation: This reaction involves activated esters with an electron-withdrawing group and halide anions. The ester is later replaced by a proton or an electrophile.

• Hunsdiecker Reaction: It is a reaction where silver salts of carboxylic acid undergo decarboxylation to give an organic halide byproduct. It is also called the Halogenation reaction for the addition of halogen.

 

Decarboxylation Reaction Mechanism

• The decarboxylation mechanism replaces the carboxyl group in a carboxylic acid with hydrogen. The reaction is facilitated by a group of enzymes called decarboxylases or carboxy-lyases. 

• The regent that helps with the reaction is Soda-lime. It is a mixture of caustic soda and quick lime.

• The mechanism of the reaction takes place in three steps. The first one begins with the removal of [H^{+}] ion from the carboxylic acid and the addition of Na. Na salt of carboxylic acid and water molecule was released in this step.

• In the second step, a negative charge from Oxygen is moved in between the carbon-oxygen bond. So, the bond between the alkyl group(-R) and carbon breaks. [CO_{2}] is liberated in this step.

• The third step takes place with help of the [H_{2}O] generated in the first step. It breaks to give a proton ion that combines with an alkyl group to give an alkane.

 

Decarboxylation Enzyme

The enzymes that help in the process of Decarboxylation of several organic acids are called decarboxylases or carboxy-lyases. They function in both adding and removing a carboxyl group from the organic compound. They are usually named according to the substrate they catalyze. Some examples of these enzymes are Ornithine decarboxylase, RuBisCO, Pyruvate decarboxylase, Histidine decarboxylase, etc.  

 

Decarboxylation of Amino Acids

Decarboxylation of amino acid results in the formation of an Amine by the removal of the carboxyl group (-COOH) from the amino acid. Due to the removal of the organic acid group, the byproduct moves up the pH scale for being of alkaline nature. The decarboxylase enzymes facilitate the decarboxylation mechanism i.e. removal of acidic groups. Deaminases, on the other hand, remove the amino groups to give out chemicals acidic in nature. 

 

Decarboxylation Tests

• These are the biochemical tests involving the production of enzyme decarboxylase. So, they are also known as decarboxylase tests. 

• The test is used to differentiate various members of Enterobacteriaceae that produce decarboxylase, from other gram-negative bacteria.

• Organisms that can metabolize amino acid by decarboxylation are identified by the formation of decarboxylase enzymes namely, arginine decarboxylase, ornithine decarboxylase, and lysine decarboxylase. 

• The members are then further differentiated based on their abilities to produce these enzymes. 

• The basal medium used in the test is Moeller’s formula. Meat peptones and beef extract present in the medium provide nitrogenous nutrition for bacterial growth.

• Cresol red and Bromocresol purple are the pH indicators in the media.

• Substrates like arginine, ornithine, and lysine are added to the medium to detect decarboxylation.