Video Transcript
Picric acid, also known as
2,4,6-trinitrophenol, is a benzene derivative containing several nitro groups. Given the chemical groups it
contains, what primary use might picric acid have? (A) Fuel for cars, (B) fertilizers,
(C) explosives, (D) food preservation, (E) paints.
Picric acid is called a benzene
derivative because it contains functional groups chemically bonded to the benzene
ring. In fact, in this molecule, there
are a total of four functional groups bonded to the benzene ring. For this reason, this molecule is
also classified as a multisubstituted arene.
Let’s identify the four functional
groups present in picric acid. First, there is one hydroxy or OH
group, and there are three nitro or NO2 groups. The IUPAC name of picric acid is
2,4,6-trinitrophenol. The numbers two, four, and six
refer to the carbon atom positions of the nitro groups on the benzene ring. Notice that the hydroxy group is
given priority and is on carbon number one. The trinitro- portion of the name
refers to the presence of three nitro groups. Phenol represents the remaining
portion of the molecule, which we can recognize as being composed of one hydroxy
group bonded to a benzene ring.
Nitro groups can be added to phenol
using a nitration reaction, which is a type of substitution reaction. During the nitration reaction,
phenol reacts with nitric acid in the presence of sulfuric acid. If enough nitric acid is present,
three hydrogen atoms of the aromatic ring in phenol will be replaced with nitro
groups. Here we show the nitro groups with
formal charges.
The nitro groups exhibit
resonance. Here is an equivalent resonant
structure. We can also represent the nitro
group using a resonance hybrid. However, for the purpose of this
video, let’s just represent them using one of the resonant structures. Because picric acid contains
several nitro groups, it is often called a polynitrated compound. Another very similar polynitrated
compound that we’re probably familiar with is TNT, or 2,4,6-trinitrotoluene. We know TNT is an explosive. When TNT is detonated, an explosion
occurs and a very large amount of energy is released. But why is this?
Chemical bonds in the TNT molecule
are somewhat strained and weak. However, the bonds in the molecules
that form when TNT detonates — carbon dioxide, water, and nitrogen gas — are quite
strong, especially the triple bond in the nitrogen gas molecules. Because significantly less energy
is needed to break the weak bonds in TNT than is released when the products form, a
very large amount of energy is released to the surroundings. We’ve used a very basic unbalanced
chemical equation to represent the decomposition of TNT, which is quite a bit more
complicated than what we’ve discussed in this video.
Now, since picric acid contains
three nitro groups, like TNT, it’s highly likely that picric acid is also an
explosive. It turns out that picric acid is an
even stronger explosive than TNT. It explodes more easily and is
therefore quite dangerous unless stored properly. In conclusion, the primary use of
picric acid is in explosives. So the correct answer is answer
choice (C).
