Question

As we saw in Chapter 6, carbon-carbon double bonds are attacked by electrophiles but not by nucleophiles. An exception to this generalization is the reactivity of \(\alpha, \beta\)-unsaturated aldehydes and ketones toward nucleophiles. Even though an isolated carbon-carbon double bond does not react with \(2^{\circ}\) amines such as dimethylamine, 3 -buten-2-one reacts readily by regioselective addition.

Short answer

Question: Analyze the reaction between 3-buten-2-one and dimethylamine, and discuss the regioselective addition involved in this reaction. Answer: The reaction between 3-buten-2-one, an α,β-unsaturated ketone, and dimethylamine, a secondary amine, involves nucleophilic addition of nitrogen in dimethylamine to the electrophilic β-carbon of 3-buten-2-one. This regioselective addition occurs due to the higher partial positive charge on the β-carbon, which makes it more electrophilic and attractive to nucleophiles like the nitrogen in dimethylamine. The final product is \(\displaystyle \mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} (\mathrm{CH}_{ 3})_{ 2}\mathrm{-CH}_{ 2}\mathrm{-CH} (\mathrm{CH}_{ 3})\mathrm{-C}(\mathrm{OH}).\)

Step-by-step solution

Identifying the relevant molecules and functional groups

First, we need to identify the reactants and their functional groups. In this case, we have 3-buten-2-one, which is an α,β-unsaturated ketone, and dimethylamine, which is a secondary amine. 3-buten-2-one: \(\displaystyle \mathrm{CH}_{3}\mathrm{CH}=\mathrm{CH} - \mathrm{C}=\mathrm{O}\) Dimethylamine: \(\displaystyle \mathrm{H}_{3}\mathrm{C}-\mathrm{N}(\mathrm{CH}_{3})_{2}\)

Understanding the reaction mechanism

Next, we need to understand the reaction mechanism between the given reactants. In this case, we know that α,β-unsaturated ketones can react with secondary amines like dimethylamine. The nucleophilic nitrogen in dimethylamine will attack the electrophilic carbon in the ketone double bond. In this reaction, the regioselectivity can be explained by the preference of the nucleophile to attack the β-carbon over the α-carbon. This is because the β-carbon has a higher partial positive charge due to the presence of the adjacent carbonyl group.

Writing the reaction and observing regioselectivity

Now, we can write the reaction between 3-buten-2-one and dimethylamine, showing the steps that lead to the product. The reaction proceeds as follows: 1. Nucleophilic addition of the nitrogen in dimethylamine to the β-carbon of the 3-buten-2-one. \(\displaystyle \mathrm{CH}_{ 3}\mathrm{CH} =\mathrm{CH}\mathrm{-C}=\mathrm{O}+\mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} ( \mathrm{CH}_{ 3})_{ 2}\rightarrow\mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} ( \mathrm{CH}_{ 3})_{ 2}\mathrm{-CH}_{ 2}\mathrm{-CH} ( \mathrm{CH}_{ 3})\mathrm{-C} (\mathrm{O}^{-}).\) 2. Proton transfer from the amine to the negatively charged oxygen atom, resulting in the final product. \(\displaystyle \mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} ( \mathrm{CH}_{ 3})_{ 2}\mathrm{-CH}_{ 2}\mathrm{-CH} (\mathrm{CH}_{ 3})\mathrm{-C} (\mathrm{(O}^{-}) )\rightarrow\mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} (\mathrm{CH}_{ 3})_{ 2}\mathrm{-CH}_{ 2}\mathrm{-CH} (\mathrm{CH}_{ 3})\mathrm{-C}(\mathrm{OH}).\) Our final product is: \(\displaystyle \mathrm{H}_{ 3}\mathrm{C}\mathrm{-N} (\mathrm{CH}_{ 3})_{ 2}\mathrm{-CH}_{ 2}\mathrm{-CH} (\mathrm{CH}_{ 3})\mathrm{-C}(\mathrm{OH}).\) This reaction demonstrates regioselectivity because the nucleophile attacks the β-carbon rather than the α-carbon within the 3-buten-2-one molecule. The selectivity of this reaction is due to the partial positive charge on the β-carbon, which makes it more electrophilic and attractive to nucleophiles like the nitrogen in dimethylamine.