Question 29.SE.2: Describe a way of preparing pentanoic acid from 1-bromobutan...
Describe a way of preparing pentanoic acid from 1-bromobutane.
Analyze We are asked to propose a synthesis of a carboxylic acid from a specific starting material and note that the target compound contains one more carbon atom than the reactant.
Plan There are two common methods of extending the number of carbon atoms in a chain: either a nucleophilic substitution by the cyanide ion or the addition of a Grignard reagent to a compound containing a C=O bond. Both could be extended to form a carboxylic acid. In the first case, hydrolysis of a nitrile under acid conditions will yield a carboxylic acid. In the second case, reaction of a Grignard reagent with carbon dioxide followed by dilute acid will furnish a carboxylic acid. We will choose the second route.
Solve Grignard reagents are very sensitive to moisture so are usually formed at the time they will be used in a “one pot” reaction. First, 1-bromobutane is treated with magnesium in diethyl ether solvent.
Next, carbon dioxide in the form of dry ice is added to the reaction vessel.
Finally, dilute acid is added to give the desired carboxylic acid.
