Substitution Reaction Examples in Organic Chemistry

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  • 0:04 Substitution Reactions
  • 0:54 Nucleophilic…
  • 2:28 Sodium Nitrate &…
  • 4:00 Electrophilic…
  • 5:25 Benzocaine &…
  • 5:55 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will learn about substitution reactions. We'll explore the difference between electrophilic substitution reactions and nucleophilic substitution reactions, and we'll look at examples of both types.

Substitution Reactions in Organic Chemistry

Have you ever wondered why a hot dog can cause cancer? Or how some topical anesthetics like benzocaine, commonly used for toothaches, can reduce pain? Both of these compounds can be formed through substitution reactions.

A substitution reaction is a reaction between molecules where an atom or a group of atoms replaces a current atom in the original molecule. For example, a hydrogen atom might get kicked off so that a different atom can be put on. There are two types of substitution reactions: nucleophilic and electrophilic. These two reactions differ in the type of atom that is attaching to the original molecule. In nucleophilic reactions, the new atom is electron-rich, while in electrophilic reactions, the new atom is electron-deficient.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions can occur with any carbon chain that has a good leaving group. Often this leaving group is a halide, usually bromine or chlorine, but oxygen groups, typically in the form of water, can be the leaving group as well. The less basic a leaving group is, the more its to leave increases. In other words, when it has a lower pKa, it is a better leaving group.

A nucleophile is similar to a base in that they both have a lone pair, or a pie bond. They differ only in which atom they attack. Nucleophiles attack the atom that is deficient in electrons, usually the carbon atom, while bases attack protons. A stronger base will be a strong nucleophile.

So remember the leaving group needs to be a poor base and have a lower pKa, while the nucleophile needs to be a stronger base.

Figure 1: The general reaction for a nucleophilic substitution
General nucleophilic substitution

In this equation, 'Nu' stands for the nucleophile, 'R' represents a chain of carbons, and 'X' refers to the leaving group. The nucleophile attacks the carbon, which can then kick off the leaving group. The arrows show the flow of electrons. The electrons from the electron-rich nucleophile attach to the carbon. Then the electrons from the C-X bond leave with the leaving group.

If the leaving group is a particularly good leaving group, then it can leave on its own before the nucleophile attacks. Then even a weak nucleophile can attack, since the carbon will have a full positive charge and be highly deficient in electrons.

Sodium Nitrite & Nucleophilic Substitution

Sodium nitrite is used as a preservative in many foods, such as hot dogs, smoked fish, and Spam. Without the use of sodium nitrites, these products would easily form a toxin from Clostridium botulinum, which is a lethal food poisoning. Even when it is caught quickly and the anti-toxin is administered, victims cannot ever completely recover from the effects, which can be anything from difficulty breathing to paralysis.

Due to the danger associated with Clostridium botulinum, food governing agencies have continued to allow sodium nitrite to be used. After all, without the use of sodium nitrite these products would be impossible to safely make. Yet sodium nitrite can also be dangerous when consumed. It can form a cancer-causing substance called nitrosamines.

These nitrosamines are dangerous because of how they're formed through a nucleophilic substitution reaction. Nitrosamines can form a structure where an R-group is bonded to a nitrogen that is triple bonded to another nitrogen. This nitrogen is a very good leaving group and can leave the R-group without any attack from a nucleophile. This leaves the R-group with a positive charge. Now, remember, any nucleophile, even a weak nucleophile, can attack that R-group. Often, this R-group gets attacks with biological compounds that can act as a nucleophile, such as DNA. This disrupts the structure of DNA, which can lead to the formation of cancer cells.

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