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What is Physical Organic Chemistry?

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Sometimes scientists use what they know about physical chemistry to study how organic molecules behave. Such scientists often belong to a scientific discipline called physical organic chemistry. In this lesson, discover and learn more about this emerging field.

What is Physical Organic Chemistry?

Let's say you are working in a laboratory performing a chemical reaction using an organic molecule and oxygen. You realize that when both chemicals react together you get carbon dioxide, water, and heat. If there is an insufficient supply of oxygen then the products formed will include carbon monoxide. Why does the reaction proceed in this manner if the levels of oxygen change? How does the structure of the organic molecule influence chemical reactivity and reaction rate?

Chemical Equation for an Organic Molecule That Reacts With Oxygen
carbon

When a scientist is interested in studying why organic molecules behave the way they do, as in chemical reactions, they are called physical organic chemists. Physical organic chemistry is the application of physical chemistry techniques to the study of organic chemical reactions. let's dissect this defintion:

  • A physical chemist looks at the physical aspects that control how an atom or molecule behaves. They often use physics to study the behavior of chemicals.
  • organic molecules will always contain a carbon atom bonded to a hydrogen atom in its structure. For example, the compound butanol (CH3CH2CH2CH2OH) is an organic compound as it contains carbon-hydrogen bonded atoms.

This tells you that a physical organic chemist may use math and physics to quantify the rate at which a chemical reaction occurs; this is referred to as kinetics. They may study how electrons are arranged in an atom or molecule or model the way organic molecules behave as they are converted from reactants to products. Sometimes they evaluate the thermodynamics of a reaction, looking at the influence of chemical stability on the formation of products.

Example of How Chemical Kinetics Affects Molecules
collision

Physical organic chemists may also investigate what happens at the atomic level to cause an organic molecule to react violently during a reaction. Lastly, they may focus on how atoms rearrange themselves during the reaction.

A Late Bloomer: Discovery of This Field

It was during the early twentieth century that the world witnessed the birth of this new field. A scientist by the name of Louis Hammett wrote and published several papers highlighting the relationship between the structure and reaction of organic molecules. He eventually pioneered the development of a new branch of chemistry: physical organic chemistry.

Scientists were busy discovering organic molecules and explaining the mechanisms used to synthesize or create them. Hammett, on the other hand, was looking far beyond an organic chemistry mechanism. In fact, he designed a mathematical equation that proved you could quantify chemical reactivity. Beyond the transfer of electrons and arrangement of atoms, his initial discovery encouraged scientists to consider many different aspects of a chemical reaction such as:

  • how many products are made at the end of a chemical reaction
  • how much energy is released or absorbed from molecules during a reaction
  • how fast does a reaction occur

Why Is This Field Important to Science?

Within the scientific community, the significance of this branch of chemistry is often questioned. We already have a physical chemistry field that uses physics to explain chemical behavior. Meanwhile, organic chemists are on the hunt to modify or update mechanisms as new discoveries are made.

As uniquely different as both organic and physical chemistry are, physical organic chemistry bridges a connection between both fields. That is, scientists such as Hammett recognized the need to take physical chemistry techniques (such as thermodynamics or kinetics) and apply that to organic molecule behavior in chemical reactions.

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