Potassium tert-butoxide, K^+ OCMe C3, is used as a base in E2 reactions. (a) How does it compare

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Potassium tert-butoxide, K^+ OCMe C3, is used as a base in...

Potassium tert-butoxide, $\mathrm{K}^{+} \overline{\mathrm{OCMe}} \mathrm{C}_3$, is used as a base in E2 reactions. (a) How does it compare in effectiveness with ethylamine, $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2$ ? (b) Compare its effectiveness in the solvents tert-butyl alcohol and dimethylsulfoxide (DMSO). (c) Give the major alkene product when it reacts with $\left(\mathrm{CH}_3\right)_2 \mathrm{CClCH}_2 \mathrm{CH}_3$. (a) $\mathrm{K}^{+} \mathrm{OCMe}_3$ is more effective because it is more basic. Its larger size also precludes $\mathrm{S}_{\mathrm{N}} 2$ reactions. (b) Its reactivity is greater in aprotic DMSO because its basic anion is not solvated. $\mathrm{Me}_3 \mathrm{COH}$ reduces the effectiveness of $\mathrm{Me}_3 \mathrm{CO}^{-}$by $\mathrm{H}$-bonding. (c) $\mathrm{Me}_3 \mathrm{CO}^{-}$is a bulky base and gives the anti-Saytzeff (Hofmann) product $\mathrm{CH}_2=\mathrm{C}\left(\mathrm{CH}_3\right) \mathrm{CH}_2 \mathrm{CH}_3$.

Potassium tert-butoxide, $\mathrm{K}^{+} \overline{\mathrm{OCMe}} \mathrm{C}_3$, is used as a base in E2 reactions. (a) How does it compare in effectiveness with ethylamine, $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2$ ? (b) Compare its effectiveness in the solvents tert-butyl alcohol and dimethylsulfoxide (DMSO). (c) Give the major alkene product when it reacts with $\left(\mathrm{CH}_3\right)_2 \mathrm{CClCH}_2 \mathrm{CH}_3$.
(a) $\mathrm{K}^{+} \mathrm{OCMe}_3$ is more effective because it is more basic. Its larger size also precludes $\mathrm{S}_{\mathrm{N}} 2$ reactions.
(b) Its reactivity is greater in aprotic DMSO because its basic anion is not solvated. $\mathrm{Me}_3 \mathrm{COH}$ reduces the effectiveness of $\mathrm{Me}_3 \mathrm{CO}^{-}$by $\mathrm{H}$-bonding.
(c) $\mathrm{Me}_3 \mathrm{CO}^{-}$is a bulky base and gives the anti-Saytzeff (Hofmann) product $\mathrm{CH}_2=\mathrm{C}\left(\mathrm{CH}_3\right) \mathrm{CH}_2 \mathrm{CH}_3$.

Key Concepts

E2 Reaction Mechanism

An E2 reaction is a single-step, concerted elimination process where a base abstracts a proton as the leaving group departs simultaneously, forming a double bond. This mechanism requires a proper geometric arrangement, usually an anti-periplanar configuration, to facilitate the removal of the proton and the leaving group in a synchronized manner.

Basicity Versus Steric Hindrance

The strength of a base in driving an E2 reaction is not solely dependent on its ability to abstract a proton, but also on its steric profile. Bulky bases, due to their large substituents, are more hindered and less effective as nucleophiles, which makes them favor elimination reactions over substitution. This steric hindrance influences the selectivity and outcome of the reaction.

Solvent Effects on Reaction Efficiency

The reactivity of a base in an E2 reaction can be significantly influenced by the solvent. Aprotic solvents, which do not strongly solvate anions through hydrogen bonding, allow the base to remain more reactive and thus more effective in proton abstraction. In contrast, protic solvents can hydrogen-bond with the base, reducing its availability and reactivity in elimination reactions.

Formation of Anti-Saytzeff (Hofmann) Products

When bulky bases are used in elimination reactions, they tend to favor less substituted alkenes, leading to the anti-Saytzeff product. This outcome arises because steric hindrance makes it more difficult for the base to access the more crowded, more substituted proton, resulting in the formation of the less substituted, typically less stable, alkene.

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