Nucleophilic Substitution and Elimination Reactions: Quick Revision

A brief note on Nucleophilic Substitution and Elimination Reactions. Read it before a test. Try each question at the end before you open its answer.

Substitution

• Nucleophile: gives a pair of electrons. Leaving group: takes the old bond pair with it. Leaving group order: I, Br, Cl, F.
• Products: aqueous KOH gives $\text{R}\text{-}\text{OH}$. $\text{R}'\text{ONa}$ gives ethers. $\text{KCN}$ gives $\text{R}\text{-}\text{CN}$ and $\text{AgCN}$ gives $\text{R}\text{-}\text{NC}$. $\text{KNO}_{2}$ gives $\text{R}\text{-}\text{ONO}$ and $\text{AgNO}_{2}$ gives $\text{R}\text{-}\text{NO}_{2}$. Excess $\text{NH}_{3}$ gives amines.

$S_{\text{N}}2$ and $S_{\text{N}}1$

• $S_{\text{N}}2$: one step, rate $= k[\text{RX}][\text{Nu}]$, inversion. Speed: methyl, then primary, then secondary, then tertiary. Polar aprotic solvents help.
• $S_{\text{N}}1$: two steps through a carbocation, rate $= k[\text{RX}]$, racemisation (often a small excess of inversion). Speed: tertiary, then secondary, then primary. Allylic and benzylic halides react easily. Polar protic solvents help. Rearrangement is possible.
• Optical activity: enantiomers turn polarised light in opposite directions by equal amounts. A racemic mixture does not turn it. $[\alpha] = \dfrac{\alpha}{l \times c}$. Enantiomeric excess is the difference between the two percentages.

Elimination

• E2: one step, rate $= k[\text{RX}][\text{base}]$. E1: two steps, with $S_{\text{N}}1$.
• Saytzeff: the more substituted alkene is the main product. A bulky base gives more of the less substituted one.
• Favoured by: alcoholic KOH, a strong or bulky base, a tertiary halide and a high temperature.
• A vicinal or gem-dihalide with alcoholic KOH or sodamide gives an alkyne.

A worked example

An optically pure chiral halide gives a product with $80\ \%$ inversion and $20\ \%$ retention. The pure product has $[\alpha] = +30.0$ degrees (units degree mL g$^{-1}$ dm$^{-1}$). What rotation does a solution of $0.100\ \text{g/mL}$ in a $1.00\ \text{dm}$ tube show?

1. Excess. $80 - 20 = 60\ \%$.
2. Pure sample. $30.0 \times 1.00 \times 0.100 = 3.00$ degrees.
3. Mixture. $3.00 \times 0.60 = 1.80$ degrees.

Practice with new numbers

A program to try

The program gives the main pathway for each class of halide with each type of reagent.

Mistakes to avoid

• $S_{\text{N}}2$ for a tertiary halide. It does not happen.
• Putting the nucleophile in the $S_{\text{N}}1$ rate law. It is not there.
• Mixing up aqueous and alcoholic KOH.
• Forgetting cis and trans when counting alkenes.

Check yourself

1. How does the rate of an $S_{\text{N}}2$ reaction change when only the nucleophile is doubled?

2. Which carbocation is the most stable: primary, secondary or tertiary?

3. Which KOH solution favours elimination, aqueous or alcoholic?

Sources

• NCERT, Chemistry Part II for Class XII, the chapter Haloalkanes and Haloarenes.
• OpenStax, Organic Chemistry, Chapter 11 (Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations).