Explain how the conjugate base of thiamine might react with pyruvic acid to produce an intermediate which could easily decarboxylate. Complete the reaction sequence leading to the formation of acetoin.

Short Answer

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The conjugate base of thiamine, formed by losing a proton from the sulfur atom, acts as a nucleophile and reacts with pyruvic acid to create a tetrahedral intermediate. This intermediate decarboxylates, losing a CO2 molecule, and forms an enol species. The enol species then attacks another pyruvic acid molecule, resulting in the formation of acetoin (CH3CHOHCOCH3).

Step by step solution

01

Understand the concept of a conjugate base

A conjugate base is formed when an acid loses a proton (H+ ion). In this case, we are considering the conjugate base of thiamine, which means that the thiamine molecule has lost a proton and has become negatively charged.
02

Identify the structure of thiamine and pyruvic acid

Thiamine, also known as vitamin B1, has a complex structure containing a positively charged nitrogen (N) atom and an acidic sulfur (S) atom. Its structure can be simplified as R-S-CH2-CH2-NH2 where R is a complex substituent. Pyruvic acid is an organic acid with the structure CH3COCOOH. For the reaction, we need the conjugate base of thiamine, which is formed by losing a proton from the sulfur atom: R-S-CH2-CH2-NH2 ⇌ R-S^(-)+H^+
03

Reaction of conjugate base of thiamine with pyruvic acid

The conjugate base of thiamine, having a negative charge on the sulfur atom, will act as a nucleophile and attack the carbonyl carbon of pyruvic acid. This creates a tetrahedral intermediate, which will eventually lose a hydroxide ion (OH-) to form a new carbonyl group. The resulting intermediate contains a carbanion, which is adjacent to the carbonyl group.
04

Decarboxylation of the intermediate

Decarboxylation is the process of losing a carboxyl group (-COOH) from a molecule, forming carbon dioxide (CO2) as a byproduct. Due to the presence of the carbanion mentioned in step 3, the intermediate is now more prone to decarboxylation. When decarboxylation occurs, the intermediate loses a CO2 molecule, and the negative charge on the carbanion is pushed back to the carbonyl carbon while hydrogen from the adjacent carbon is abstracted to form a new molecule.
05

Formation of acetoin

After the decarboxylation process, we have an enol species, which is a nucleophile and can attack another molecule of pyruvic acid, forming a new intermediate that loses a hydroxide ion (OH-) to form a new carbonyl group. Lastly, a proton (H+) is transferred within the molecule to generate acetoin, which has the structure CH3CHOHCOCH3. In summary, the conjugate base of thiamine reacts with pyruvic acid, forming an intermediate that undergoes decarboxylation, eventually leading to the formation of acetoin.

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