Video Transcript
Benzene has unusual chemical properties that can be rationalized if we consider how carbon atoms are bonded to adjoining hydrogen and carbon atoms. How many 𝜎 bonds are there in benzene?
Benzene is a cyclic hydrocarbon that contains six carbon atoms and six hydrogen atoms. Because benzene forms a six-membered cyclic carbon ring, we must first put all of our carbons in a hexagonal shape. Putting our carbons in a hexagon allows us to show two things: first that all of our carbons are equally spaced apart and second that all the bond angles are the same. In this molecule, each carbon atom is bonded to two other carbon atoms and one single hydrogen atom. And so we can place one hydrogen atom next to each of the carbon atoms, equally spaced out.
Next, we need to draw all of the single bonds that bind these 12 atoms together. But we’re not done yet. In order to finish drawing this molecule, we have to look at the total number of valence electrons. And we know that each carbon atom has four valence electrons and each hydrogen atom has one valence electron. And since we know that there are six of each type of atom, we can calculate that there should be a total number of 30 valence electrons, meaning that there are 30 total electrons that should either be in bonds or in a lone pair. Every bond and every lone pair that we draw in a molecule represents a pair of two electrons. And so far, in our benzene molecule, we only drawn 12 bonds representing 24 total electrons. That means that we still have six electrons that we have to include to complete our benzene molecule.
In order to use the remaining six electrons, we have to remember that each carbon has to have or share a total of eight electrons. This is also known as the octet rule. And we will do so by adding a second bond to every other pair of carbon atoms in this ring. And at this point, we’ve drawn a structure that has a six-carbon ring, has 30 valence electrons, where each carbon has its octet filled.
Now, the question is asking how many 𝜎 bonds are there in benzene. It’s important to know that all of the bonds that were drawn in pink or the initial bonds that bonded all of the atoms together are 𝜎 bonds and the double bonds that have been drawn in yellow are 𝜋 bonds. And with this information, we can answer the question. Benzene has unusual chemical properties that can be rationalized if we consider how carbon atoms are bonded to adjoining hydrogen and carbon atoms. How many 𝜎 bonds are there in benzene? And if we count all of the 𝜎 or pink-colored bonds, we find that the correct answer is 12 𝜎 bonds.
