write the chemical equation for the bromination of trans-cinnamic acid using pyridinium tribromide and glacial acetic acid
The Correct Answer and Explanation is :
The bromination of trans-cinnamic acid using pyridinium tribromide (C(_5)H(_5)NHBr(_3)) in glacial acetic acid involves the addition of bromine (Br(_2)) across the double bond of trans-cinnamic acid. The reaction follows an anti-addition mechanism, leading to the formation of erythro-2,3-dibromo-3-phenylpropanoic acid as the major product.
Chemical Equation:
[
\text{C}_6\text{H}_5\text{CH=CHCOOH} + \text{Br}_2 \xrightarrow{\text{Glacial Acetic Acid}} \text{C}_6\text{H}_5\text{CHBrCHBrCOOH}
]
Expanded with pyridinium tribromide as the bromine source:
[
\text{C}_6\text{H}_5\text{CH=CHCOOH} + \text{C}_5\text{H}_5\text{NHBr}_3 \xrightarrow{\text{Glacial Acetic Acid}} \text{C}_6\text{H}_5\text{CHBrCHBrCOOH} + \text{C}_5\text{H}_5\text{NHBr}
]
Explanation:
- Reaction Mechanism:
- Pyridinium tribromide (C(_5)H(_5)NHBr(_3)) is a convenient and mild source of molecular bromine (Br(_2)). In the presence of glacial acetic acid, it releases Br(_2), which acts as an electrophile.
- The bromine molecule reacts with the electron-rich double bond of trans-cinnamic acid (C(_6)H(_5)CH=CHCOOH), forming a cyclic bromonium ion intermediate.
- Due to anti-addition, the nucleophilic attack by Br(^-) occurs from the opposite side, leading to the formation of erythro-2,3-dibromo-3-phenylpropanoic acid.
- Stereochemistry:
- The reaction follows an anti-addition mechanism, meaning the two bromine atoms attach to opposite sides of the double bond, leading to a stereospecific erythro-product.
- Since trans-cinnamic acid starts with a trans-configuration, anti-addition ensures that the major product is erythro-2,3-dibromo-3-phenylpropanoic acid.
- Solvent Role:
- Glacial acetic acid acts as a solvent, helping dissolve pyridinium tribromide and stabilizing the bromonium ion intermediate.
This reaction is commonly used in organic synthesis to study regio- and stereoselectivity in halogen addition reactions.
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