In the reaction of 2-phenylacetic acid with ethanol in the presence of sulfuric
ID: 608066 • Letter: I
Question
In the reaction of 2-phenylacetic acid with ethanol in the presence of sulfuric acid catalyst, ethyl-2-phenyl acetate is formed. In this experiment (a) which group in the acyl compound is being substituted and (b) what is the substituting group? a. (a) the carbonyl group of 2-phenylacetic acid (b) the hydroxyl group of ethanol b. (a) the hydroxyl group of ethanol (b) the carbonyl group of 2-phenylacetic acid c. (a) the hydroxyl group of 2-phenylacetic acid (b) the oxygen of ethanol d. (a) the hydroxyl group of ethanol (b) the oxygen of the hydroxyl group of 2-phenylacetic acid I understand the hydroxyl (OH) group of the 2-phenylacetic acid is being replaced with the ethyl oxide since the single-bonded oxygen in the ethyl-2-phenyl acetate is the same oxygen that was part of the ethanol, but the way it's worded, that's not an option! Given the choices, I want to choose (c), but the substituting group is more than just the oxygen. Can someone help explain what I'm missing and/or over-thinking? :-/Explanation / Answer
Step 1 In the first step, the ethanoic acid takes a proton (a hydrogen ion) from the concentrated sulphuric acid. The proton becomes attached to one of the lone pairs on the oxygen which is double-bonded to the carbon. The transfer of the proton to the oxygen gives it a positive charge, but it is actually misleading to draw the structure in this way (although nearly everybody does!). The positive charge is delocalised over the whole of the right-hand end of the ion, with a fair amount of positiveness on the carbon atom. In other words, you can think of an electron pair shifting to give this structure: You could also imagine another electron pair shift producing a third structure: So which of these is the correct structure of the ion formed? None of them! The truth lies somewhere in between all of them. One way of writing the delocalised structure of the ion is like this: The double headed arrows are telling you that each of the individual structures makes a contribution to the real structure of the ion. They don't mean that the bonds are flipping back and forth between one structure and another. The various structures are known as resonance structures or canonical forms. There will be some degree of positive charge on both of the oxygen atoms, and also on the carbon atom. Each of the bonds between the carbon and the two oxygens will be the same - somewhere between a single bond and a double bond. For the purposes of the rest of this discussion, we are going to use the structure where the positive charge is on the carbon atom. Step 2 The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen of the ethanol molecule.