A sample of (S)-CH3 CH(Cl) CH2 CH3 is dissolved in ethanol, and the resulting solution is optica

A sample of (S)-CH3 CH(Cl) CH2 CH3 is dissolved in ethanol,...

A sample of $(S)-\mathrm{CH}_{3} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{2} \mathrm{CH}_{3}$ is dissolved in ethanol, and the resulting solution is optically inactive. (a) Write the formula of the product. (b) By which mechanism does this nucleophilic substitution reaction occur?

Key Concepts

Nucleophilic Substitution Mechanisms

Nucleophilic substitution reactions can occur via two main pathways, SN1 and SN2. In an SN1 reaction, the rate?determining step is the formation of a carbocation intermediate, which is planar and can be attacked from either side, leading to racemization. In contrast, an SN2 reaction occurs in one concerted step with backside attack, resulting in inversion of configuration. The observation of an optically inactive product from an optically active starting material indicates that racemization has occurred, which is characteristic of the SN1 pathway.

Carbocation Intermediate and Racemization

When the leaving group departs in an SN1 reaction, a carbocation intermediate is formed. This intermediate is planar and symmetric, allowing the nucleophile to attack from either face with equal probability. This results in the formation of a racemic mixture, meaning the final product is optically inactive even if the starting material was optically active.

Solvent Effects Favoring SN1

The solvent can significantly influence the pathway of nucleophilic substitution reactions. Polar protic solvents, such as ethanol, stabilize charged species like carbocations and anions. This stabilization enhances the formation of a carbocation intermediate, thereby favoring the SN1 mechanism over the SN2 mechanism.

Product Structure in Nucleophilic Substitution

In a nucleophilic substitution reaction, the original leaving group (in this case, chloride) is replaced by the solvent acting as a nucleophile (ethanol). The final product is thus an ethoxy-substituted compound with the structure CH3CH(OEt)CH2CH3. This substitution, combined with the carbocation intermediate formation, results in a racemic mixture due to the equal probability of attack from both sides.

Transcript

00:01 So we're asked to write the formula of the products and determine how this reaction occurs.

00:06 So we've s2 chlorobutane.

00:09 This group is 1, the ethyls 2, the methyls 3, and group number 4 on the wedge.

00:15 So it should go in this direction, but because group number 4 is on the wedge, it goes in the opposite direction.

00:22 So it's s, and it's dissolved in ethanol.

00:26 So ch3, ch2oh, and this is known as a polar periodic solvent, and polar.

00:37 Puritic solvents are those that have hydrogen bonding.

00:41 As we can see, the oxygen is bound to the hydrogen.

00:46 We also have a secondary alkaliad.

00:53 So because of these two factors, we have the s -n -1 and e -1 mechanism, but the s -n -1 is the major product, and the e -1 is the minor.

01:03 So the first step is for the aquilide to leave.

01:07 Because remember, s -n -1 reactions occur in two steps.

01:15 So we have a secondary carbocation.

01:18 The next step is for, ethanol to attack the carbokadion, and it can attack from the back or from the front, leading to this.

01:54 So the oxygen has a positive charge because it donated two electrons that were negative, so it went from being neutral to positive.

02:04 So the next step is for the chlorine that was expelled in the first step to grab a hydrogen, and the electrons flow back to the oxygen, and this occurs in the same, the other configuration, too...

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