Question

The major product obtained on treatment of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{F}) \mathrm{CH}_{3}\) with \(\mathrm{CH}_{3} \mathrm{O}^{-} / \mathrm{CH}_{3} \mathrm{OH}\) is: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{OCH}_{3}\right) \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OCH}_{3}\)

Short answer

The major product is (b) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCH}_{3}\).

Step-by-step solution

Identify the Reaction Type

The reaction involves an alkyl halide \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{F}) \mathrm{CH}_{3}\) treated with a methoxide ion \(\mathrm{CH}_{3} \mathrm{O}^{-}\) in methanol. This setup is typical for nucleophilic substitution or elimination reactions.

Determine the Major Reaction Pathway

Considering the structure of the substrate and the conditions, elimination reactions are favored over substitution due to the poor leaving group ability of fluoride ions. Therefore, the major reaction is likely an elimination reaction leading to the formation of an alkene.

Apply Saytzeff's Rule for Elimination

According to Saytzeff's rule, the formation of the more substituted alkene is favored in elimination reactions. The substrate \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{F}) \mathrm{CH}_{3}\) can form either of two alkenes: \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCH}_{3}\) (but-2-ene) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CH}_{2}\) (but-1-ene). Saytzeff's rule predicts the formation of the more substituted \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHCH}_{3}\) as the major product.

Verify Possible Products

Review the possible answer choices: (a) is a substitution product, (b) represents the more substituted alkene formed by elimination, (c) represents the less substituted alkene from elimination, and (d) is another substitution product. Based on Saytzeff's rule, option (b) is the major product.

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction mechanism in organic chemistry. In these reactions, a nucleophile, which is an electron-rich species, replaces a leaving group in a molecule. The leaving group is typically an atom or group that can easily depart with an electron pair.

For example, in the presence of a strong nucleophile like the methoxide ion (\( \mathrm{CH}_{3} \mathrm{O}^{-} \)), one might expect a nucleophilic substitution reaction. The methoxide ion would attack the electrophilic carbon that is bonded to the leaving group, in this case, fluoride.

However, fluoride is a poor leaving group due to its high electronegativity and strong bond with carbon, making nucleophilic substitution less likely in this specific reaction. This is why, instead of substitution, elimination becomes the favored pathway.

Overall, nucleophilic substitutions are key for various synthetic applications, creating a wide range of functional groups and providing diverse synthetic strategies. Understanding the factors affecting these reactions, such as the strength of nucleophiles and the ability of leaving groups, is crucial for mastering organic chemistry mechanisms.

Saytzeff's Rule

Saytzeff's rule, also known as Zaitsev's rule, is a critical concept in elimination reactions. It predicts the formation of the more substituted alkene as the major product. This rule states that during elimination, the hydrogen is typically removed from the adjacent carbon with the fewest hydrogens. As a result, a more stable, highly substituted alkene is formed.

In the reaction of \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{F}) \mathrm{CH}_{3}\) with methoxide ions, we apply Saytzeff's rule to predict the outcome. Two possible alkenes could form in this elimination reaction: but-2-ene (\(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\)) and but-1-ene(\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\)).

Saytzeff's rule favors the production of the more substituted but-2-ene.
This is because but-2-ene has a higher degree of alkyl substitution, which confers greater stability due to hyperconjugation and the overall distribution of electron density. Understanding and applying Saytzeff's rule helps predict the favored direction of many elimination reactions, guiding chemists in synthetic strategy.

Reaction Mechanism in Organic Chemistry

Understanding the various reaction mechanisms in organic chemistry is crucial for predicting and rationalizing the outcome of chemical transformations. A reaction mechanism describes the step-by-step process by which reactants are converted to products. It details the bonds that are broken and formed, the intermediates that appear, and the order in which these events occur.

For the reaction involving \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{F}) \mathrm{CH}_{3}\), we consider the competition between substitution and elimination mechanisms. While substitution involves a direct exchange of components, elimination typically involves the formation of double bonds by the removal of atoms or groups.

In this case, an elimination mechanism is favored, primarily due to the poor leaving group nature of fluoride. The reaction mechanism involves the strong base,\(\mathrm{CH}_{3} \mathrm{O}^{-}\), abstracting a proton from a neighboring carbon atom. This process generates a π-bond and leads to the formation of an alkene.

Meticulously studying reaction mechanisms allows chemists to design reactions with desired outcomes and troubleshoot unexpected reactionresults, making it a fundamental skill in organic synthesis.