Alkoxides & tert-Butoxide Reagents

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Alkoxides & tert-Butoxide

The masters of substitution ($S_N2$) and elimination ($E2$).

By chemca Team • Updated Oct 2026

Alkoxides ($RO^-$) are the conjugate bases of alcohols. They are strong bases and strong nucleophiles. The outcome of their reaction with alkyl halides—whether they perform Substitution ($S_N2$) or Elimination ($E2$)—depends entirely on the steric bulk of the alkoxide and the degree of substitution of the alkyl halide.

1. Small Alkoxides: Williamson Ether Synthesis

Nucleophilic Substitution ($S_N2$)

Reagents: Sodium methoxide ($CH_3O^-Na^+$) or Sodium ethoxide ($CH_3CH_2O^-Na^+$).

Action: Small, unhindered alkoxides act as excellent nucleophiles. When reacted with primary ($1^\circ$) or methyl halides, they undergo $S_N2$ substitution to form ethers.

$$ \underset{\text{Sodium ethoxide}}{CH_3CH_2O^-Na^+} + \underset{\text{Methyl bromide}}{CH_3-Br} \xrightarrow{S_N2} \underset{\text{Ethyl methyl ether}}{CH_3CH_2-O-CH_3} + NaBr $$

The $3^\circ$ Halide Trap: If a small alkoxide is reacted with a tertiary ($3^\circ$) or secondary ($2^\circ$) alkyl halide, the steric hindrance of the halide blocks the $S_N2$ attack. The alkoxide is forced to act as a base, plucking a proton and causing $E2$ elimination (Zaitsev product major).

2. Bulky Alkoxides: Hofmann Elimination

Potassium tert-Butoxide ($t-BuOK$)

Context: $t-BuOK$ (or $KOC(CH_3)_3$) is an extremely sterically hindered strong base. Because it is so bulky, it is a terrible nucleophile. It will almost always act as a base, favoring $E2$ elimination even with primary halides.

Action (Hofmann's Rule): When performing an $E2$ elimination on an unsymmetrical alkyl halide, standard small bases (like $EtO^-$) remove an internal proton to give the more substituted, stable alkene (Zaitsev's Rule). However, bulky bases like $t-BuOK$ cannot easily reach internal protons due to steric clash. They abstract the most accessible, terminal proton instead!

$$ \underset{\text{2-Bromo-2-methylbutane}}{CH_3-C(Br)(CH_3)-CH_2-CH_3} + \underset{\text{Bulky Base}}{t\text{-}BuO^-K^+} \xrightarrow{E2} \underset{\text{2-Methyl-1-butene (Hofmann Product, MAJOR)}}{CH_2=C(CH_3)-CH_2-CH_3} $$

Golden Rule for Alkyl Halides:
Small Base + Alkyl Halide $\rightarrow$ Zaitsev Alkene (More substituted).
Bulky Base ($t-BuOK$) + Alkyl Halide $\rightarrow$ Hofmann Alkene (Less substituted).

3. Preparation of Alkoxides

Acid-Base Reaction with Metals

Alcohols are very weak acids (weaker than water, except for methanol). They do not react with standard bases like $NaOH$ to form alkoxides. They must be reacted with highly active metals (like $Na$ or $K$) or metal hydrides (like $NaH$).

With Active Metals ($Na$ or $K$):

$$ 2 \ CH_3CH_2OH + 2 \ Na \longrightarrow \underset{\text{Sodium ethoxide}}{2 \ CH_3CH_2O^-Na^+} + H_2 \uparrow $$

With Sodium Hydride ($NaH$): (Preferred as $H_2$ gas bubbles out, driving the reaction to completion).

$$ (CH_3)_3C-OH + NaH \longrightarrow \underset{\text{Sodium tert-butoxide}}{(CH_3)_3C-O^-Na^+} + H_2 \uparrow $$

4. Reaction Matrix: Alkoxide vs Alkyl Halide

Alkyl Halide Type	Small Alkoxide (e.g., $CH_3O^-$)	Bulky Alkoxide (e.g., $t-BuO^-$)
Methyl ($CH_3-X$)	$S_N2$ (Ether)	$S_N2$ (Ether) (No $\beta$-hydrogens for E2)
Primary ($1^\circ$) ($RCH_2-X$)	$S_N2$ (Ether)	$E2$ (Alkene) (Base is too bulky for $S_N2$)
Secondary ($2^\circ$) ($R_2CH-X$)	$E2$ (Zaitsev Alkene) major, some $S_N2$	$E2$ (Hofmann Alkene) exclusive
Tertiary ($3^\circ$) ($R_3C-X$)	$E2$ (Zaitsev Alkene) exclusive	$E2$ (Hofmann Alkene) exclusive

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