Which base should be used for alkylation next to a ketone? NaNH2, NH3

Which base should be used for an aldol condensation NaOH, H2O

Question

Which base should be used for alkylation next to a ketone? NaNH2, NH3

Which base should be used for an aldol condensation NaOH, H2O

BlackTom AI · Accepted Answer

The question presents two separate base-choice scenarios in a single item. We will examine each dropdown option in turn and explain why the chosen base is appropriate or inappropriate for the stated transformation.

First dropdown (which base should be used for alkylation next to a ketone?):
- Option 1: NaNH2, NH3. This combination is a strong non-nucleophilic base system (sodium amide in liquid ammonia) commonly used to generate enolates from ketones under very basic conditions. Once the ketone is deprotonated at the alpha position to form the enolate, it can undergo alkylation with an alkyl halide or related electrophile. The ammonia solvent stabilizes the base and the enolate can be subsequently alkylated, making this a standard method for alpha-alkylation of ketones.
- Option 2: NaOH, H2O. Sodium hydroxide in water is a much weaker and protic base system compared to NaNH2 in NH3. While NaOH can promote enolate formation only in the presence of very activated substrates or under forcing conditions, it is not typically used to generate the strongly reactive enolate required for efficient alkylation of a ketone in a controlled manner. In short, this base system is not the preferred choice for direct alkylation at the alpha position of a ketone.

Second dropdown (which base should be used for an aldol condensation?):
- Option 1: NaNH2, NH3. Again, sodium amide in ammonia is an extremely strong base, but aldol condensations are efficiently promoted under basic yet aqueous, milder conditions that generate enolates and allow subsequent aldol addition. Using NaNH2 in NH3 would be far too harsh and moisture-sensitive for a typical aldol reaction; it could lead to side reactions or decomposition rather than a clean aldol condensation.
- Option 2: NaOH, H2O. This is the conventional base/solvent combination for aldol condensations. Hydroxide in water deprotonates the alpha position to form the enolate (or carbanion character) under aqueous basic conditions, enabling nucleophilic addition to another carbonyl compound to yield the aldol product after workup. This setup is standard in many aldol reaction protocols because it provides the right balance of base strength and reaction medium compatibility.

In summary, for alkylation next to a ketone, the NaNH2 in NH3 choice (Option 1 of the first dropdown) is the appropriate strong base/enolate-forming system, whereas NaOH in H2O (Option 2 of the first dropdown) is not ideal for this transformation. For aldol condensation, the NaOH in H2O option (Option 2 of the second dropdown) is the standard base/solvent system, while NaNH2 in NH3 (Option 1 of the second dropdown) is not suitable due to excessive basicity and incompatibility with aqueous aldol chemistry.

CHE 008B A01 SS1 2025 Lecture 9 - Reactions of Enolate Ions

Which base should be used for alkylation next to a ketone? NaNH2, NH3 Which base should be used for an aldol condensation NaOH, H2O

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