Aromaticity

(Speaking to my 15 year old brother)

Benzene is the easiest compound to use when trying to explain aromaticity. Benzene is a six carbon ring with alternating double bonds (3 π bonds).

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 This is a picture of Benzene. Each point of the ring represents a carbon atom. The three extra lines you see represent double bonds, also called π bonds. A chemical bond is simply and attraction between two atoms. A double bond simply means there are two bonds between the atoms and the electrons are shared.

Aromaticity describes the special stability that Benzene has because of the structure. Since Benzene is considered Aromatic is follows Huckel’s rule. This criteria is as follows:

The molecule must cyclic which means there is a ring of atoms. It is easy to tell that the atoms are placed in a ring by drawing a circle around the molecule.

The molecule must be planar which means that all atoms in the molecule must lie in the same plane.

3.      The molecule has to be completely conjugated which means there has to a p orbital on very atom. A p orbital explains the wave-like behavior of the electrons. Below is a picture of the Benzene molecule drawn with a p orbital on each atom.

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 And finally, the molecule must contain 4n+2 π electrons. This confirms that the molecule is stable because all the bonding orbitals are filled with paired electrons. You can count the number of π electrons a molecule has and set this equation equal to that number. If n is equal to 0 or any other positive number the rule has been met.  A π electron resides in a π bond. 3 π bonds = 6 π electrons because there are 2 π electrons in each double bond. Here is an example of how to confirm this rule applies using Benzene.

4n + 2 = 6

4n = 4

n = 1

Since n is a positive integer, the rule is confirmed. 

Sample Question Not Seen On Test

I anticipated a problem on determining the number or protons giving rise to an NMR signal. The example listed in chapter 14 on page 505 is a good example of this type of problem. The problem and solution are listed below.

Question:
A compound of molecular formula C9H10O2 gives the following integrated 1H NMR spectrum. How many protons give rise to each signal?

(The spectrum is shown in the book on page 505 at the bottom of the page.)

Answer:
Step 1: Determine the number of integration units per proton by dividing the total number of integration units by the total number of protons.

• Total number of integration units: 54+23+33=110 units
• Total number of protons = 10
• Divide: 110 units/10 protons = 11 units per proton

Step 2: Determine the number of protons giving rise to each signal.

• To determine the number of H atoms giving rise to each signal, divide each integration value by the answer of Step 1 and round to the nearest whole number. 

                      Solution: Signal A - 54/11 = 4.9 = 5 H
                                    Signal B - 23/11 = 2.1 = 2 H
                                    Signal C - 33/11 = 3H

The area under an NMR signal is proportional to the number or absorbing protons. The height of each step is proportional to the area under the peak, which is in turn proportional to the number of absorbing protons. Knowing the molecular formula of a compound and integration values from its 1H NMR spectrum gives the actual number of protons responsible for a particular signal.