Chapter 9: Addition Reactions
Transforming Multiple Bonds
Introduction
Alkenes and alkynes are more reactive than alkanes because their π bonds are relatively easy to break.
Addition reactions convert multiple bonds into new single bonds while introducing additional atoms into molecules.
These reactions form an important bridge between Organic Chemistry I and Organic Chemistry II.
The Big Idea
Substitution replaces groups. Elimination forms multiple bonds. Addition reactions consume multiple bonds and create new functional groups.
Why Addition Reactions Matter
Addition reactions allow chemists to transform relatively simple molecules into more complex ones.
They make possible the synthesis of:
- alcohols,
- halides,
- carbonyl compounds,
- and many other functional groups.
Common Addition Reactions
Hydrogenation
Addition of hydrogen.
Hydrohalogenation
Addition of HX.
Hydration
Addition of water.
Halogenation
Addition of halogens.
Carbocations and Stability
Many addition reactions involve carbocation intermediates.
Consequently, ideas from earlier chapters remain important:
- resonance,
- stability,
- acids and bases,
- electron flow.
Organic chemistry repeatedly revisits the same foundational principles.
Regiochemistry
Not all additions occur in the same way.
When an electrophile adds to an unsymmetrical alkene, the two carbons of the double bond are not equivalent, and addition can proceed through two different carbocation intermediates. The pathway that forms the more stable intermediate is strongly preferred, because a more stable intermediate corresponds to a lower-energy transition state and a faster reaction.
Carbocation stability increases with the number of attached alkyl groups, so the more-substituted carbon is better able to stabilize a positive charge. As a result, hydrogen tends to add to the less-substituted carbon, leaving the more-substituted carbon to accept the other group. This outcome is known as Markovnikov’s rule — but it is not an independent rule to memorize. It is a direct consequence of reasoning through intermediate stability.
Important questions include:
- Which carbon receives hydrogen?
- Which carbon receives another group?
- Which pathway leads to the most stable intermediate?
Answering the third question answers the first two.
Thinking About Addition
Helpful questions include:
- Where are the π electrons?
- Which atom is electron-poor?
- Which intermediate is most stable?
Gentle Exercises
Identify: π bonds, electrophiles, nucleophiles.
Predict: where additions are likely to occur.
Predict: for an unsymmetrical alkene, which carbon ends up with the new group, by comparing the stability of the two possible carbocation intermediates.
Common Mistakes
Applying Addition Reactions to Aromatic Rings
Better approach:
Aromatic compounds strongly resist addition because addition would destroy aromatic stabilization. When a double bond is part of a benzene ring, substitution — not addition — is the expected pathway.
Predicting Regiochemistry Without Reasoning Through Intermediate Stability
Better approach:
When an electrophile adds to an unsymmetrical alkene, the intermediate carbocation forms preferentially at the more substituted carbon — the more stable one. Identifying the more stable intermediate predicts the major product. This principle underlies Markovnikov’s rule.
Self-Assessment
I can:
☐ Recognize addition reactions.
☐ Understand why π bonds are reactive.
☐ Appreciate carbocation stability.
☐ Predict regiochemistry by comparing carbocation intermediates (Markovnikov’s rule).
☐ Recognize common addition patterns.
Further Study
Reading
LibreTexts Organic Chemistry — Ch. 7, Alkenes: Structure and Reactivity — Electrophilic addition; Markovnikov’s rule; carbocation stability.
Videos
Khan Academy — Alkenes and Alkynes — Addition reactions.
Looking Ahead
Although individual reactions are useful, experienced chemists rarely memorize isolated mechanisms.
Instead, they compare patterns and develop intuition.
The next chapter explores the relationships among substitution, elimination, and addition reactions.