Part III — Substitution and Elimination: The First Mechanisms

This is where the real chemistry starts. Five chapters:

Nucleophilic Substitution ($S_{N}2$) — One step, backside attack, inversion.
Nucleophilic Substitution ($S_{N}1$) — Two steps, carbocation intermediate, racemization.
Elimination Reactions ($E2$ and $E1$) — The alkene-forming sisters of $S_{N}2$ and $S_{N}1$.
Predicting Substitution vs. Elimination: The Decision Framework — Given a substrate, nucleophile/base, and solvent, which pathway wins?
Synthesis Workshop 1 — Your first designed multi-step synthesis, using only the tools of Part III.

Why these four reactions first

Substitution and elimination are where most organic courses hit their first wall. They are also the gateway to the rest of the subject. Every reaction you will meet in later parts — addition, aromatic substitution, carbonyl chemistry — is a variation on the same fundamental moves:

An electron source (a nucleophile, a base, a pi bond, a lone pair) attacks an electron sink (an electrophile, an acid, an electrophilic carbon).
Bonds form and bonds break. Electrons flow from source to sink. The geometry of the attack determines the product.

$S_{N}2$, $S_{N}1$, $E2$, and $E1$ are the four reactions where these moves are easiest to see and easiest to isolate from everything else. Once you have them, the rest is largely pattern recognition.

The decision framework (Chapter 13) is the most important chapter in Part III

Most students think of $S_{N}2$, $S_{N}1$, $E2$, and $E1$ as four separate reactions to memorize. That is backward. They are four outcomes of a single question:

When a nucleophile/base meets an alkyl halide (or similar substrate), which pathway dominates?

The answer depends on five factors, all introduced in Part III:

The substrate — primary, secondary, or tertiary? Allylic or benzylic?
The nucleophile/base — strong or weak? Bulky or small? Charged or neutral?
The solvent — polar protic or polar aprotic?
The temperature — low favors substitution, high favors elimination.
The leaving group — good or bad? (This connects straight back to Chapter 3's $pK_a$ framework.)

Chapter 13 gives you a single flowchart for this decision. You will use it in every part of the book after this one — whenever you face a molecule that could substitute or eliminate, you will work through this flowchart in your head. By the time you reach the synthesis workshops, it will be automatic.

Anchor example in Part III

The $S_{N}2$/$S_{N}1$/$E2$/$E1$ decision framework is this book's second anchor example, built explicitly across these five chapters. It will return in almost every chapter from here on.

When you see an alkyl halide in Chapter 20, you will still be asking: is this $S_{N}2$, $S_{N}1$, $E2$, or $E1$? When you see an amine reacting with an alkyl halide in Chapter 30, you will be asking the same question. When you see a Grignard reagent displacing a leaving group in Chapter 37, you will be recognizing $S_{N}2$ geometry with a new nucleophile.

What you can do at the end of Part III

Draw every arrow of every one of the four mechanisms — $S_{N}2$, $S_{N}1$, $E2$, $E1$ — without looking them up. The arrows should feel mandatory, not memorized. Given the substrate and nucleophile, the arrows should be forced by what electrons want to do.
Predict the stereochemistry of the product. Inversion, retention, racemization, elimination geometry — each follows from the mechanism's geometry.
Predict which pathway will dominate for a given combination of substrate, nucleophile/base, and solvent.
Design a two- or three-step synthesis that uses substitution or elimination to install a functional group, and justify each step mechanistically.

What Part III does not yet give you

Addition reactions — the move from alkyl halide to alkene back to alkyl halide (or alcohol, etc.) requires Part IV.
Carbonyl chemistry — everything involving $C=O$ is Part VI.
Aromatic substitution — $S_{N}2$ and $E2$ do not work on benzene; you need a different framework, which Part V builds.

How to read Part III

Draw every mechanism. Not "read about" — draw. The arrows must be in your hand, not on the page. A good practice is to close the book after each worked example and redraw the mechanism from memory. If you cannot, re-read and try again.

The exercises in these chapters are not optional. The skill of mechanism-drawing is the central skill of organic chemistry, and Part III is where you build it.