Question

Reaction of ( \(S\) )-3-methylpentan-2-one with methylmagnesium bromide followed by acidification yields 2,3-dimethylpentan-2-ol. What is the stereochemistry of the product? Is the product optically active?

Short answer

The product is a racemic mixture and is not optically active.

Step-by-step solution

Recognizing the Reaction Type

The reaction involves the addition of a Grignard reagent, methylmagnesium bromide, to a carbonyl group in ( \(S\) )-3-methylpentan-2-one, followed by acidification. This is a nucleophilic addition creating a tertiary alcohol.

Analyzing the Chiral Center Formation

The addition occurs at the carbonyl carbon, which is initially not a chiral center. Methylmagnesium bromide adds a methyl group, converting it into a chiral center because of the creation of a new stereogenic carbon atom.

Applying Stereochemical Considerations

When the methyl group from methylmagnesium bromide adds to the carbonyl, it could attack from either the top face or the bottom face of the carbonyl group. This results in the formation of two diastereomers if the original compound has another chiral center, like in the case here, resulting in a racemic mixture if no specific stereochemical control is applied.

Assessing Optical Activity

A racemic mixture, which contains equal amounts of both enantiomers, does not rotate plane polarized light because the optical activities of the individual enantiomers cancel each other out.

Identifying the Final Product

The final product is 2,3-dimethylpentan-2-ol. Due to the creation of a new chirality center and lack of stereochemical direction, both configurations (\(R\) and \(S\)) are formed, making the product a racemic mixture.

Key concepts

Nucleophilic Addition

In organic chemistry, nucleophilic addition is a fundamental reaction mechanism. It occurs when a nucleophile, an electron-rich species, adds to a positively charged or electron-deficient part of a molecule. In the context of Grignard reactions, the nucleophile is often a Grignard reagent like methylmagnesium bromide. The carbon atom in this reagent possesses a partial negative charge, making it a strong nucleophile. When it encounters a carbonyl group, such as the one found in (S)-3-methylpentan-2-one, the nucleophile targets the electron-deficient carbon of the carbonyl, leading to the formation of a new bond. This addition effectively transforms the carbonyl into an alcohol. - **Key Points:** - Nucleophilic addition is essential in creating new carbon-carbon bonds. - Grignard reagents are widely used nucleophiles in these reactions. - The result of nucleophilic addition is typically the conversion of a carbonyl to an alcohol.

Chiral Center

A chiral center is an atom in a molecule that has four different groups attached to it, allowing it to exist in two non-superimposable mirror-image forms, known as enantiomers. In our reaction, the carbonyl carbon in (S)-3-methylpentan-2-one is transformed into a chiral center after the nucleophilic addition of the Grignard reagent. Before the reaction, this carbon is planar and lacks chirality. However, once the methylmagnesium bromide adds its methyl group, the central carbon now has four different groups. - **Why Chiral Centers Matter:** - Chiral centers are critical in determining the 3D shape and optical properties of molecules. - The specific configuration at the chiral center affects how a compound interacts with biological systems.

Racemic Mixture

A racemic mixture is a 50:50 mixture of two enantiomers of a chiral molecule. In the reaction we are examining, after the nucleophilic addition, there is no preferential direction for the nucleophile to attack. As a result, the product formed—2,3-dimethylpentan-2-ol—exists in both possible enantiomeric forms (R and S), forming a racemic mixture. This outcome is typical in reactions where no specific stereochemical controls are applied. - **Implications of a Racemic Mixture:** - They do not exhibit optical activity because the effects of the two enantiomers cancel each other out. - In pharmaceuticals, a racemic mixture may have different biological properties compared to its enantiopure counterparts.

Optical Activity

Optical activity is a property of chiral compounds that allows them to rotate plane-polarized light. Compounds that show optical activity are called optically active. In the case of our reaction, the resultant racemic mixture of 2,3-dimethylpentan-2-ol contains equal amounts of both enantiomers. Each enantiomer would normally rotate plane-polarized light to an equal but opposite degree. However, in a racemic mixture, this results in zero net rotation because the rotations cancel each other out. - **Key Takeaways:** - Optical activity helps in determining the purity of a chiral compound. - In a racemic mixture, despite having chiral molecules, the mixture is optically inactive.