Video Transcript
The tosyl group is used extensively in organic synthesis, and it is the anionic derivative of the tosylic acid shown. Which two reagents would react when heated to form tosylic acid and water? (A) Benzene and sulfuric acid, (B) toluene and hydrogen sulfide, (C) benzene and sulfur dioxide, (D) toluene and sulfuric acid, (E) nitrobenzene and sulfuric acid.
The chemical structure shown is tosylic acid. To solve this problem, we need to determine the identity of two reagents, which we will call X and Y here, that will react when heated to form tosylic acid and water. Tosylic acid is what we call a multisubstituted arene. It contains two substituents directly bonded to an aromatic ring. The two substituents of the tosylic acid molecule are a methyl or CH3 group and a sulfo or SO3H group.
Let’s clear some space to discuss an important reaction involving the sulfo group. When benzene and concentrated sulfuric acid react under reflux, one of the hydrogen atoms of the benzene ring is replaced with a sulfo group, forming benzenesulfonic acid. The other product of the reaction is water. The reaction is known as a sulfo substitution reaction or sulfonation. Carrying a reaction out under reflux involves heating the chemical reaction at a constant temperature for a period of time. The vapors produced are continually cooled using a condenser so that the liquid returns to the reaction flask. Using reflux prevents the escape of flammable vapors and allows the control of concentration and temperature during the reaction.
By looking carefully at the structure of tosylic acid, we can recognize the structure of the aromatic compound toluene. If toluene and concentrated sulfuric acid were heated, a sulfo substitution reaction would take place and one hydrogen atom of the aromatic ring would be replaced with a sulfo group. So the products of the reaction are tosylic acid and water. Remember that an alkyl group, such as a methyl group, is an ortho, para directing group. So the position of the sulfo or SO3H group is most likely to be found in the ortho or para position. In tosylic acid, the sulfo group is located in the para position. In fact, another name for tosylic acid is para-toluenesulfonic acid.
In conclusion, we have determined that toluene and sulfuric acid are the two reagents that would react when heated to form tosylic acid and water. Therefore, the correct answer is answer choice (D).
