Video Transcript
The reaction of toluene with one equivalent of nitric acid can produce three different positional isomers, as shown. Which product is least likely to be produced from this reaction?
Let’s begin by examining the provided reaction. Toluene and nitric acid are the reactants. The reaction is carried out in the presence of concentrated sulfuric acid, which acts as a catalyst. Three possible products are shown, which are positional isomers. Our job is to determine which of these three products is least likely to form in the reaction.
The toluene molecule is classified as a monosubstituted arene. It consists of one substituent, which in the case of toluene is a methyl group. And this substituent is bonded directly to the aromatic ring. On the other hand, the products shown are disubstituted arenes. Each of these isomers contains two substituents bonded to the aromatic ring. By looking at these three positional isomers more closely, we notice that they differ only in the position of the NO2 or nitro group. A nitro group can become bonded to the aromatic ring of a toluene molecule during a nitro substitution or nitration reaction. During the reaction, one of the hydrogen atoms of the aromatic ring is replaced with a nitro group and a molecule of water is also produced.
However, since the focus of this problem is to determine which of the three products is least likely to form, let’s go ahead and remove water from the screen. When a monosubstituted arene, such as toluene, gains a second substituent during a chemical reaction, the identity of the original substituent, in this case the methyl group, determines the possible position of the new substituent. One carbon atom to the left or right of the original substituent are the ortho positions. Two carbon atoms to the left or right of the original substituent are the meta positions. And three carbon atoms from the original substituent is the para position. Chemists use these prefixes in the non-IUPAC names of only disubstituted arenes.
Let’s go ahead and write these common names for all three possible products. Product A is ortho-nitrotoluene, product B is meta-nitrotoluene, and product C is para-nitrotoluene. Some of the isomers shown are more likely to form during the reaction. The original substituent of the molecule before the reaction is a methyl group. An alkyl group like the methyl group is an ortho, para directing group. This means that during a reaction in which a second substituent becomes chemically bonded to an aromatic ring, the ortho and para products are more likely to form.
Since this question is asking us which product is least likely to form, we should select the meta product or meta-nitrotoluene as our answer. In conclusion, the product which is least likely to be produced from the nitration reaction shown is product B.
