Substitution Reactions — Revision Notes

⚡ 30-Second Revision

S$_N$1 — 2 steps, unimolecular, Rate = $k$[RX], carbocation intermediate, racemization, favors $3^circ > 2^circ$, weak Nu, polar protic solvent.
S$_N$2 — 1 step, bimolecular, Rate = $k$[RX][Nu], transition state, Walden inversion, favors Methyl > $1^circ > 2^circ$, strong Nu, polar aprotic solvent.
Leaving Group — I$^{-}$ > Br$^{-}$ > Cl$^{-}$ > F$^{-}$ (weak bases are good LGs).
Haloarenes — Unreactive to S$_N$1/S$_N$2. Undergo S$_N$Ar (activated by ortho/para EWGs) or Benzyne mechanism.

2-Minute Revision

Substitution reactions involve replacing one group with another. The two main types are S$_N$1 and S$_N$2. S$_N$1 is a two-step process where the leaving group departs first to form a carbocation intermediate, followed by nucleophilic attack.

Its rate depends only on the substrate concentration, and it leads to racemization if the carbon is chiral. S$_N$1 is favored by tertiary alkyl halides (due to stable carbocations), weak nucleophiles, and polar protic solvents.

S$_N$2 is a one-step, concerted process where the nucleophile attacks from the backside as the leaving group departs, forming a single transition state. Its rate depends on both substrate and nucleophile concentrations, and it results in Walden inversion of configuration.

S$_N$2 is favored by methyl and primary alkyl halides (due to less steric hindrance), strong nucleophiles, and polar aprotic solvents. Good leaving groups are weak bases (e.g., I$^{-}$ > Br$^{-}$ > Cl$^{-}$).

Haloarenes are generally unreactive to these mechanisms but can undergo S$_N$Ar if activated by electron-withdrawing groups at ortho/para positions.

5-Minute Revision

Substitution reactions are core to organic chemistry, primarily nucleophilic substitution (S$_N$). We differentiate between S$_N$1 and S$_N$2 based on their mechanisms, kinetics, and stereochemistry.

S$_N$1 (Substitution Nucleophilic Unimolecular):

Mechanism — Two steps. Step 1 (slow, rate-determining) is the ionization of the alkyl halide to form a planar carbocation intermediate. Step 2 (fast) is the attack of the nucleophile on the carbocation.
Kinetics — Rate = $k$[Alkyl Halide]. Unimolecular in the rate-determining step.
Substrate Reactivity — Favors stable carbocations: $3^circ > 2^circ > 1^circ > \text{Methyl}$. Allylic and benzylic halides are also highly reactive.
Nucleophile — Strength is not critical; weak nucleophiles (often the solvent, e.g., H$_2$O, ROH) are sufficient.
Solvent — Polar protic solvents (e.g., H$_2$O, alcohols) stabilize the carbocation intermediate and the leaving group, accelerating the reaction.
Stereochemistry — If the $\alpha$-carbon is chiral, racemization occurs as the nucleophile can attack the planar carbocation from either face.

S$_N$2 (Substitution Nucleophilic Bimolecular):

Mechanism — One step, concerted. The nucleophile attacks the $\alpha$-carbon from the backside simultaneously as the leaving group departs, forming a single transition state.
Kinetics — Rate = $k$[Alkyl Halide][Nucleophile]. Bimolecular.
Substrate Reactivity — Favors less sterically hindered substrates: Methyl > $1^circ > 2^circ > 3^circ$. Tertiary halides are essentially unreactive.
Nucleophile — Requires strong nucleophiles (e.g., OH$^{-}$, CN$^{-}$, I$^{-}$) for efficient reaction.
Solvent — Polar aprotic solvents (e.g., DMSO, acetone, DMF) enhance nucleophile reactivity by not solvating them strongly.
Stereochemistry — If the $\alpha$-carbon is chiral, Walden inversion of configuration occurs due to backside attack.

Common Factors:

Leaving Group Ability — For both S$_N$1 and S$_N$2, good leaving groups are crucial. Weak bases are good leaving groups. Order for halides: I$^{-}$ > Br$^{-}$ > Cl$^{-}$ > F$^{-}$.

Haloarenes:

Generally unreactive towards S$_N$1 and S$_N$2 due to partial double bond character of C-X bond and instability of aryl carbocations/steric hindrance.
Can undergo **Nucleophilic Aromatic Substitution (S$_N$Ar)** via a Meisenheimer complex if activated by strong electron-withdrawing groups (like -NO$_2$) at ortho and para positions.
Can also react via the Benzyne mechanism under very harsh basic conditions (e.g., NaNH$_2$ in liquid NH$_3$).

Example: Consider 2-bromopropane reacting with NaOH. If in acetone (polar aprotic), it's S$_N$2. If in water (polar protic), it's S$_N$1. The product will be propan-2-ol in both cases, but stereochemistry would differ if 2-bromopropane was chiral.

Prelims Revision Notes

Nucleophilic Substitution Reactions (S$_N$1 & S$_N$2)

1. S$_N$1 Reaction (Substitution Nucleophilic Unimolecular)

Mechanism — Two steps.

1. Slow (RDS): Leaving group (LG) departs, forming a planar carbocation ($R^+$). 2. Fast: Nucleophile ($Nu^-$) attacks the carbocation from either side.

Kinetics — Rate = $k$[Alkyl Halide]. Unimolecular.
Substrate Reactivity — $3^circ > 2^circ > 1^circ > \text{Methyl}$. Favors stable carbocations. (Allylic and Benzylic are highly reactive due to resonance stabilization).
Nucleophile — Weak nucleophiles are sufficient (e.g., H$_2$O, ROH, CH$_3$COOH).
Solvent — Polar protic solvents (e.g., H$_2$O, alcohols, acetic acid) stabilize carbocation and LG via solvation.
Stereochemistry — Racemization (formation of a racemic mixture) if the $\alpha$-carbon is chiral.
Energy Profile — Two transition states, one carbocation intermediate.

2. S$_N$2 Reaction (Substitution Nucleophilic Bimolecular)

Mechanism — One step, concerted. Backside attack of nucleophile and simultaneous departure of LG. Forms a single transition state.
Kinetics — Rate = $k$[Alkyl Halide][Nucleophile]. Bimolecular.
Substrate Reactivity — Methyl > $1^circ > 2^circ > 3^circ$. Favors less sterically hindered substrates.
Nucleophile — Strong nucleophiles required (e.g., OH$^{-}$, CN$^{-}$, I$^{-}$, CH$_3$O$^{-}$).
Solvent — Polar aprotic solvents (e.g., DMSO, acetone, DMF, acetonitrile) enhance nucleophile reactivity by not solvating them.
Stereochemistry — Walden inversion (inversion of configuration) if the $\alpha$-carbon is chiral.
Energy Profile — One transition state, no intermediates.

3. Common Factors for S$_N$1 & S$_N$2

Leaving Group Ability — Good leaving groups are weak bases. Order: I$^{-}$ > Br$^{-}$ > Cl$^{-}$ > F$^{-}$. (Tosylates, Mesylates are also excellent LGs).

4. Haloarenes

Unreactive to S$_N$1/S$_N$2 — Due to partial double bond character of C-X bond (resonance) and instability of aryl carbocations ($sp^2$ carbon).
Nucleophilic Aromatic Substitution (S$_N$Ar) — Occurs via addition-elimination mechanism (Meisenheimer complex intermediate). Activated by strong electron-withdrawing groups (EWGs) at ortho and para positions (e.g., -NO$_2$, -CN).
Benzyne Mechanism — Occurs under very harsh basic conditions (e.g., NaNH$_2$ in liquid NH$_3$), forming a benzyne intermediate.

5. Competition with Elimination (E1/E2)

Strong bases/nucleophiles and higher temperatures often favor elimination over substitution. (Important for $2^circ$ and $3^circ$ substrates).

Key to Success: Practice identifying the mechanism based on the substrate, nucleophile, and solvent, then predict the product and its stereochemistry.

Vyyuha Quick Recall

S$_N$1 vs. S$_N$2 Checklist (SN-CHECK)

Substrate: 1 for $3^circ$, 2 for $1^circ$/Methyl Nucleophile: 1 for Weak, 2 for Strong Carbocation: 1 has it, 2 doesn't Hindrance: 1 tolerates, 2 hates Energy: 1 has 2 humps, 2 has 1 hump Configuration: 1 Racemizes, 2 Inverts Kinetics: 1 Unimolecular, 2 Bimolecular