Part IV — Addition Reactions

Five chapters. The first major variation on the themes of Part III:

Alkenes: Structure, Stability, and Electrophilic Addition — The $\pi$ bond is a new kind of nucleophile.
Addition Reactions of Alkenes — HX, hydration, halogenation, hydroboration, oxidation. All electrophilic, all predictable.
Alkynes — Triple bonds are two $\pi$ bonds stacked. The chemistry is alkene chemistry, squared.
Radical Reactions — A different mechanism for a different selectivity — and a different set of industrial applications.
Conjugated Systems, Diels-Alder Reactions, and Orbital Symmetry — Where organic chemistry touches quantum mechanics. The Diels-Alder is the closest thing in chemistry to a magic trick.

The move from substitution to addition

In Part III, the alkyl halide was the electrophile and a nucleophile attacked a carbon that already had all four bonds filled. The attack pushed the leaving group off.

In Part IV, the alkene is the nucleophile. The $\pi$ bond — an electron pair in a molecular orbital spread out above and below the plane of the double bond — reaches up and attacks an electrophile. No leaving group leaves; instead, the $\pi$ bond opens, and two new $\sigma$ bonds form across the old double bond.

This is worth pausing on. It is the same chemistry Part III built — nucleophile attacks electrophile — but with the roles assigned differently. The alkene looks electron-poor from the outside (it is a hydrocarbon) but is electron-rich above and below the C=C plane, where its $\pi$ electrons live. Anything that wants electrons (an acid, a halogen, a borane) will find them there.

Once you see the electronic structure, the products fall out. This is how all of organic chemistry works, and Part IV is where the insight stops being abstract.

Markovnikov, anti-Markovnikov, and why the rules exist

Markovnikov's rule ("H goes to the carbon with more H's; X goes to the other carbon") is the first selectivity rule most students meet, and it is usually taught as a memorizable observation. It is not. Markovnikov selectivity is a consequence of carbocation stability — the more stable carbocation forms preferentially in the first step, and the regiochemistry of the product follows.

Once you know this, "anti-Markovnikov" additions — hydroboration and peroxide-catalyzed HBr — are not exceptions to be memorized separately. They are what you predict from a different first step: hydroboration has a four-center transition state where steric bulk forces the boron to the less-substituted carbon, and radical HBr addition has a bromine radical adding first, which picks the more-substituted carbon because the more-stable radical forms.

Every regiochemical outcome in Part IV is derivable. Learn the mechanism; the selectivity falls out.

The Diels-Alder and orbital symmetry

Chapter 19 is the one place in a standard undergraduate course where quantum mechanics shows up directly. The Diels-Alder reaction — a diene plus a dienophile making a cyclohexene in one step — works only because the $\pi$ molecular orbitals of the two partners have the right symmetry to overlap simultaneously at both ends. Get the symmetry wrong (a $[2+2]$ cycloaddition instead of a $[4+2]$) and the reaction does not work thermally; it requires UV light and an excited-state electron configuration.

Woodward and Hoffmann won the Nobel Prize for realizing this. It is extraordinary. A small handful of rules about orbital phases, derived from quantum mechanics, dictates which thermal and photochemical cycloadditions happen and which do not. We introduce them briefly in Chapter 19 and return to them in depth in Chapter 39.

What you can do at the end of Part IV

Predict the product of any electrophilic addition to an alkene or alkyne, including regiochemistry and stereochemistry, from the mechanism alone.
Distinguish electrophilic, nucleophilic, and radical additions by their characteristic mechanisms and selectivities.
Predict which of two competing cycloadditions will happen thermally vs. photochemically, using the $[4+2]$/$[2+2]$ distinction from orbital symmetry.
Add these reactions to the growing Synthesis Toolkit you started in Chapter 14.

How Part IV connects to the rest of the book

Part V (aromatic chemistry) builds on the electrophilic-addition mechanism of Chapter 15 and modifies it: aromatic rings are too stable to complete a full addition, so they undergo substitution instead. Same first step, different second.
Part VI (carbonyl chemistry) will remind you that a C=O is a $\pi$ bond too — but now the carbon is $\delta^+$ and the oxygen is $\delta^-$, so nucleophiles attack the carbon directly without needing an initial electrophilic step.
Chapter 36 (oxidation and reduction) revisits the hydroboration and osmium-tetroxide oxidations of Chapter 16 in a more unified framework.
Chapter 39 (pericyclic reactions) gives you the full treatment of Diels-Alder and its cousins — sigmatropic rearrangements, cycloadditions, electrocyclizations — under the Woodward-Hoffmann rules.

Part IV is where the book becomes genuinely predictive. If you understand Chapter 15, you can look at any new alkene chemistry in Chapter 16 and know what is going to happen before you read the mechanism. That is what we are training.