Haloform Reaction

Topics: Chlorine, Hydrogen, Acid Pages: 2 (379 words) Published: November 13, 2008
Discussion

The reaction through which 4-mrthaoxyacetophenone is converted to 4-methoxybenzoic acid is a haloform reaction. This reaction involves two distinct stages that occur in the same environment. The first stage occurs through the based-catalyzed halogenation of the ketone that forms a trihalo ketone. First, the bleach is transformed into chlorine and hydroxide ions through the following reaction:

Then, the hydroxyl is able to attack the rather acidic (pKa ~ 20) α-hydrogen of the ketone. This acidity is due to the stability of the resonance structures of the corresponding enolate ion that forms upon dissociation. When the hydroxide is able to subtract a hydrogen atom, a carbanion is formed. This carbanion is a very strong base and is able to attack the chlorine thus taking one of the chlorine atoms. The introduction of one chlorine atom catalyzes the attack of another hydroxide molecule on the remaining α-hydrogens of the ketone. The subsequent attacks occur faster because with every introduction of a chlorine atom, the remaining hydrogens are more acidic because of the inductive effect of the halogens. The second part of the reaction occurs through a nucleophilic acyl substitution. Once the trihalo ketone is formed, the hydroxide ions attack the carbonyl carbon and the carbon-carbon bond between the carbonyl carbon and the halomethyl carbon is cleaved. The trihalomethyl group acts as a good leaving group where the three electronegative chlorine atoms help to disperse the negative charge efficiently. The end reaction is the formation of chloroform and the salt of the 4-methoxybenzoic acid through a simple acid-base reaction.

Throughout the reaction, it was important to test for the presence of NaOCl. Its presence in excess is vital to the ability of the reaction to take place as it is the source of the two needed reagents, namely OH- and Cl2. To test for its presence, starch iodide paper is inserted in the reaction flask and if sodium...
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