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Nucleophilic Substitution: SN1 & SN2
Substitution reactions swap one group on a carbon for another. You will learn what makes a good nucleophile and leaving group, then how to tell SN1 from SN2 from the substrate, solvent, and stereochemistry.
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What this chapter covers
- Nucleophiles, Electrophiles & Leaving Groups. Every substitution question starts by naming three players: who attacks, who is attacked, and who leaves.
- The SN2 Mechanism. SN2 is a single, crowded step, so sterics and stereochemistry decide almost every exam question about it.
- The SN1 Mechanism & Choosing a Pathway. SN1 goes through a carbocation. Once you can judge that carbocation, the rest of the chapter follows.
Key points to know
- Nucleophile = electron-pair donor; electrophile = electron-poor carbon
- Good leaving group = weak base = conjugate base of a strong acid
- SN2: one step, backside attack, rate = k[R–X][Nu⁻]
- Reactivity: methyl > 1° > 2° ≫ 3° (tertiary never does SN2)
- SN1: two steps, carbocation intermediate, rate = k[R–X]
- Favored by tertiary substrates, polar protic solvents, good leaving groups
Try a practice question
Which of these alkyl halides has the best leaving group?
- mol:CF{fluoromethane}
- mol:CCl{chloromethane}
- mol:CBr{bromomethane}
- mol:CI{iodomethane}
Show the answer
D. Iodide is the conjugate base of HI, the strongest of these acids, so it is the weakest base and best leaving group. Fluoride is the worst of the halides.
Common questions
What does this chapter cover?
Nucleophilic Substitution: SN1 & SN2 covers Nucleophiles, Electrophiles & Leaving Groups, The SN2 Mechanism, The SN1 Mechanism & Choosing a Pathway.
What should I know first?
Nucleophile = electron-pair donor; electrophile = electron-poor carbon Good leaving group = weak base = conjugate base of a strong acid SN2: one step, backside attack, rate = k[R–X][Nu⁻]
17 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.