Chemical Intuition

While organic chemistry is arguably one of the toughest courses, it is also one of the coolest and most interesting subjects you will ever take. In this course, you will learn how to manipulate atoms and molecules on the quantum scale. You will learn to make molecules (pharmaceuticals, pesticides, dyes, etc.) and materials (plastics and other polymers). No other course, except maybe some art classes, will teach you how to create things in such a detailed and precise way.

I hope you will also learn to see these molecules and materials differently as you develop your chemical intuition. When an organic chemist looks at a molecule, they see it differently than other scientists—and even some chemists—would. As you begin to explore the fascinating world of organic chemistry, always keep in mind that molecular structure controls the physical and chemical properties of a given molecule. When you look at a molecule, ask yourself these questions, and in time, you will develop your chemical intuition:

• What structural features are present, and how do they relate to the physical properties (i.e., boiling point, melting point, solubility, and chromatography)?

• What are the important intermolecular (secondary) forces resulting from these structural features?

• What structural features are present, and how do they relate to the chemical reactivity?

• Where are the electrons located? Are they in lone pairs, π bonds, or in the highest-energy occupied orbitals (where electrons are most loosely held)?

• Where are electrons absent? Are there empty orbitals or lowest-energy unoccupied orbitals (where electrons can be accepted)?

• Are there acidic protons or basic lone-pair electrons?

Let’s briefly look at one of the first structures we will explore: ethene, a member of the alkene family. The four structures/models below show the different levels of detail for ethene. On the left is the simplest approximation. Notice how a π bond is no different from a σ bond in the simple line-bond structure. You should remember from general chemistry that π bonds are different from σ bonds, and this structure doesn’t provide that detail. The two structures on the right are the quantum mechanically (QM) derived electron density maps and electrostatic potential maps. When an organic chemist sees the structure on the left, they envision the two structures on the right. From these structures, we can easily understand why alkenes behave like nucleophiles in addition reactions. A nucleophile likes to react with something positive.

Line Bond representation	Line bond (showing HOMO)	QM derived HOMO	QM derived electrostatic potential