Question

Which of the following alkyl halide can form ylide with \(\mathrm{PPh}_{3}\) ? (A) Clc1ccccc1 (B) Clc1ccccc1 (C) CC(C)Cl (D) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{F}\)

Short answer

The correct answer is (C). Only isopropyl chloride meets the criteria to form a ylide with \(\mathrm{PPh}_{3}\), as it has a chlorine atom as a suitable leaving group and a stable carbocation when the chlorine leaves.

Step-by-step solution

Understand the concept of ylides

Ylides are neutral molecules with positive and negative charges on adjacent atoms, frequently involving phosphorus or sulfur atoms. In this case, we are specifically considering the reaction between an alkyl halide and triphenylphosphine (\(\mathrm{PPh}_{3}\)) to form a phosphonium ylide. This reaction can be represented as: $$\mathrm{RX + PPh}_{3} \rightarrow \mathrm{R^{+}PPh_{3}X^{-}}$$ Where: - R is the alkyl group - X is the halogen in the alkyl halide

Determine the necessary conditions for ylide formation

For successful ylide formation, an alkyl halide must meet the following conditions: 1. The halogen (X) must be a good leaving group. Common halogens that fulfill this requirement include chlorine, bromine, and iodine. Fluorine is typically considered a poor leaving group. 2. The alkyl part (R) should be stable when forming a cation or capable of stabilizing the positive charge developed on the phosphorus atom after the halogen leaves.

Apply the conditions to the given options

Let's evaluate each option based on the conditions described in Step 2. (A) Clc1ccccc1 This compound is chlorobenzene, which has a chlorine atom as the halogen – a suitable leaving group. However, the benzene ring cannot form a stable cation as the positive charge developed due to the delocalization of benzene π electrons does not allow it to stabilize the positive charge on the phosphorus atom. Thus, it cannot form a ylide with \(\mathrm{PPh}_{3}\). (B) Clc1ccccc1 This compound is identical to option (A) and has the same issues. It cannot form a ylide with \(\mathrm{PPh}_{3}\). (C) CC(C)Cl This compound is isopropyl chloride, which has a chlorine atom as the halogen – a suitable leaving group – and a stable carbocation when the chlorine leaves. The 2° carbocation (isopropyl cation) can stabilize the positive charge on the phosphorus atom. Therefore, this compound can form a ylide with \(\mathrm{PPh}_{3}\). (D) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{F}\) This compound is vinyl fluoride, which has a fluorine atom as the halogen – a poor leaving group. The vinyl group cannot form a stable cation, and the fluorine is a poor leaving group, so it cannot form a ylide with \(\mathrm{PPh}_{3}\).

Conclusion

Among the given options, only option (C), isopropyl chloride, meets the criteria to form a ylide with \(\mathrm{PPh}_{3}\). Therefore, the correct answer is (C).

Key concepts

Alkyl Halide

Alkyl halides are organic compounds containing a halogen atom (like chlorine, bromine, iodine, or fluorine) attached to an alkyl group. The structure of these compounds is expressed as \[\text{R - X}\]where R is the alkyl group and X is a halogen. These halides are notable in reactions because halogens are highly electronegative, making the carbon-halogen bond polarized. This polarization renders the carbon atom susceptible to nucleophilic attacks, a key feature in various substitution and elimination reactions.Alkyl halides are essential precursors in many chemical reactions, particularly for formations like ylide creation. In ylide formations, the halogen acts as the leaving group, facilitating the reaction with nucleophiles like triphenylphosphine (\(\mathrm{PPh}_{3}\)). Hence, the presence of a good leaving group in the alkyl halide is crucial to ensure successful reactionation.

Triphenylphosphine

Triphenylphosphine is a widely used reagent in organic chemistry, symbolized as \(\mathrm{PPh}_{3}\).It is a phosphorus-centered molecule bonded to three phenyl groups. Thanks to the lone pair of electrons on the phosphorus, triphenylphosphine acts as a potent nucleophile, meaning it can donate electrons to other atoms or molecules efficiently. In the context of ylide formation, triphenylphosphine interacts with alkyl halides to produce a phosphonium ylide. This process involves triphenylphosphine displacing the halide ion from the alkyl halide. The resulting ylide is characterized by the presence of both a positive and a negative charge localized on adjacent atoms, with the phosphorus atom carrying a positive charge and an adjacent carbon atom typically supporting the negative charge.Ylides formed through this reaction are not only academically interesting but also indispensable in synthetic chemistry, particularly in the Wittig reaction, which is extensively employed to form carbon-carbon double bonds.

Leaving Group

The concept of a leaving group is integral in substitution and elimination reactions involving alkyl halides. A leaving group is an atom or a group of atoms that can be displaced from the parent molecule, forming a stable structure upon leaving. For a successful ylide formation, the leaving group must be excellent, meaning it should be able to leave with ease while stabilizing itself as an ion or a neutral molecule. Halogen atoms such as chlorine, bromine, and iodine are often considered good leaving groups due to their ability to stabilize the negative charge after leaving the carbon-based molecule. Conversely, fluorine is a poor leaving group due to its strong bond with carbon and its relatively high charge density, which makes it less inclined to leave. The choice of leaving group impacts the efficiency of the reaction, and in cases like ylide formation, it is crucial to choose an alkyl halide with a suitable leaving group to facilitate the creation of the desired phosphorus-based ylide.

Carbocation Stability

Carbocations are positively charged carbon molecules that form as intermediates during various organic reactions. The stability of a carbocation is critical as it determines the feasibility and direction of the reaction pathway. In the case of ylide formation, the ability of the alkyl group to accommodate and stabilize a positive charge is essential. Different factors influence carbocation stability, including:

• Hyperconjugation: The presence of adjacent carbon-hydrogen bonds that can donate electron density helps stabilize the positive charge.
• Resonance: Delocalization of the positive charge across multiple atoms provides stability through resonance structures.
• Inductive Effects: Electronegative atoms or groups can delocalize the charge through sigma bonds, offering additional stabilization.

Secondary and tertiary carbocations are more stable than primary carbocations due to these stabilizing factors. For instance, in the case of isopropyl chloride, the secondary carbocation that forms upon halide departure is relatively stable, facilitating the reaction with triphenylphosphine to form an ylide. Understanding these stability factors is crucial for predicting and explaining outcomes in chemical syntheses involving carbocations.