Question

A sample of \((R)-\mathrm{CH}_{3} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) reacts with \(\mathrm{CH}_{3} \mathrm{S}^{-}\) in dimethyl sulfoxide, and the resulting solution is optically active. (a) Write the formula of the product. (b) By which mechanism does this nucleophilic substitution reaction occur?

Short answer

The resulting product of the reaction is \((R)-\mathrm{CH}_3 \mathrm{CH}(\mathrm{SCH}_3) \mathrm{CH}_2 \mathrm{CH}_3\) and the reaction mechanism is by SN2.

Step-by-step solution

Determine the product of the reaction

The given compound contains a chiral carbon atom attached to a chlorine atom. When the compound reacts with the nucleophile \(\mathrm{CH}_3 \mathrm{S}^-\), the latter will replace the chlorine atom in a nucleophilic substitution reaction. The resulting product will thus have the formula \((R)-\mathrm{CH}_3 \mathrm{CH}(\mathrm{SCH}_3) \mathrm{CH}_2 \mathrm{CH}_3\). This means the chlorine atom in the initial compound has been replaced by a \(\mathrm{SCH}_3\).

Identify the mechanism involved

In the context of nucleophilic substitution reactions, two main types of mechanisms can occur: SN1 or SN2. The SN1 mechanism is a two-step pathway that involves the formation of a carbocation intermediate, while the SN2 mechanism is a one-step pathway, where the nucleophile simultaneously attacks the substrate and expels the leaving group. Given that the reaction causes inversion of configuration at the chiral center, and the nucleophile is strong, this reaction likely follows an SN2 mechanism.

Provide the final answers

The product of the reaction is \((R)-\mathrm{CH}_3 \mathrm{CH}(\mathrm{SCH}_3) \mathrm{CH}_2 \mathrm{CH}_3\) and the mechanism of the reaction is SN2.

Key concepts

Chirality

Chirality is an essential concept in chemistry that refers to the geometric property where a molecule is not superimposable on its mirror image. Think of your hands; they are mirror images but cannot be perfectly aligned when overlapped. Similarly, chiral molecules have a carbon atom bonded to four different groups, creating two non-superimposable mirror images known as enantiomers. In the exercise, the compound is

• chiral due to its carbon bonded to different groups: a methyl group (\(\mathrm{CH}_3\)),
• a chlorine atom (\(\mathrm{Cl}\)),
• an ethyl group (\(\mathrm{CH}_2\mathrm{CH}_3\)), and
• a hydrogen atom (\(\mathrm{H}\)).

This particular arrangement is what gives the molecule its chirality. Understanding chirality is crucial because it affects how molecules interact with biological systems and other chemicals.

SN2 Mechanism

The SN2 mechanism, short for bimolecular nucleophilic substitution, is a one-step reaction process where a nucleophile attacks the substrate from the opposite side of the leaving group. It's like a dance where the nucleophile and leaving group switch places in a single, swift move. Here's how it works:

• The nucleophile (\(\mathrm{CH}_3\mathrm{S}^-\)) approaches the carbon atom bonded to chlorine.
• The approach happens from the opposite side of the chlorine since SN2 reactions require a 'backside attack.'
• As the nucleophile forms a bond with the carbon, the chlorine atom simultaneously leaves as a chloride ion (\(\mathrm{Cl}^-\)).

This path ensures the molecule undergoes inversion of configuration, meaning the spatial arrangement of the atoms around the carbon flips. The simple, concerted movement is why SN2 reactions occur with strong nucleophiles and usually in polar aprotic solvents, like dimethyl sulfoxide. Such conditions help avoid the formation of intermediates, making the reaction both efficient and predictable.

Optical Activity

Optical activity refers to a molecule's ability to rotate plane-polarized light. Chiral molecules, like those involved in nucleophilic substitution reactions, display optical activity because of their asymmetric structure. When a chiral compound is placed in a path of polarized light, the light's plane rotates either to the left (levorotatory) or right (dextrorotatory). In our scenario:

• The starting chiral molecule displays a certain level of optical activity.
• After the reaction, the new chiral center is formed in an inverted position, maintaining the optical activity but potentially altering its direction or magnitude.
• Unlike achiral molecules, these changes in optical properties provide vital clues about reaction pathways and products.

Being aware of optical activity is vital for chemists, particularly when determining the purity of chiral substances and understanding reaction mechanisms in organic synthesis.