Question

Which of the following is the correct order of ease of displacement of groups in substitution reactions? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}\) (b) \(\mathrm{CH}_{3} \mathrm{COO}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3}^{-}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}^{-}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{SO}_{3}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}\)

Short answer

Question: Determine the correct order of ease of displacement of groups in substitution reactions among the following options: (a) \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}>\mathrm{CH}_{3}\mathrm{COO}^{-}>\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}<\mathrm{CH}_{3}\mathrm{COO}^{-}<\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}>\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}>\mathrm{CH}_{3}\mathrm{COO}^{-}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}>\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}>\mathrm{CH}_{3}\mathrm{COO}^{-}\) Answer: (a)

Step-by-step solution

Leaving Group Stability

We will first assess the stability of each leaving group and rank them. The stability of the leaving groups is as follows: aromatic groups > carboxylate ion (\(\mathrm{COO}^{-}\)) > phenoxide ion (\(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\)). In this case, - \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}\) is an aromatic sulfonate group, which is considered very stable. - \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is a carboxylate ion, which is considered moderately stable. - \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\) is a phenoxide ion, which is considered less stable compared to the other two.

Rank the Leaving Groups

Now let's rank these leaving groups based on their stability: 1. \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}\) (most stable) 2. \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) 3. \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\) (least stable)

Compare the Options

Now let's compare the options given in the exercise with our ranking. Option (a) has the order: \(\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{SO}_{3}^{-}>\mathrm{CH}_{3}\mathrm{COO}^{-}>\mathrm{C}_{6} \mathrm{H}_{5}\mathrm{O}^{-}\) which matches our ranking. The other options (b), (c), and (d) do not match our ranking as they have different orders for the leaving groups. The correct order of ease of displacement of groups in substitution reactions is given by option (a).

Key concepts

Leaving Group Stability

Understanding the stability of leaving groups is essential in organic chemistry, especially when analyzing substitution reactions. A 'leaving group' is an atom or group of atoms that is displaced as a stable species in a substitution reaction. The stability of these groups can greatly influence how easily they are displaced.

For instance, sulfate groups like \(\mathrm{C}_6\mathrm{H}_5\mathrm{SO}_3^{-}\) are known for their exceptional stability due to resonance and inductive effects, which help to delocalize the negative charge over a larger area, making them less reactive and excellent leaving groups. Carboxylate ions, denoted as \(\mathrm{CH}_3\mathrm{COO}^{-}\), while less stable than sulfonate groups, are still quite stable due to resonance stabilization where the negative charge is shared between two oxygen atoms. Lastly, phenoxide ions (\(\mathrm{C}_6\mathrm{H}_5\mathrm{O}^{-}\)) have less resonance stabilization compared to the sulfonate and carboxylate groups, as the negative charge is not as effectively delocalized, making them the least stable of the three.

Overall, the ease of displacement for leaving groups can be correlated with their stability. More stable leaving groups are displaced more readily in nucleophilic substitution reactions. Thus, when ranking leaving groups, it is the stability which we observe to predict the reactivity of different compounds in these reactions.

Nucleophilic Substitution

Nucleophilic substitution is a cornerstone mechanism in organic chemistry where a nucleophile, a species with a lone pair of electrons, replaces a leaving group in a molecule. The process can be either bimolecular (SN2) or unimolecular (SN1), depending on various factors such as the structure of the substrate, the nature of the leaving group, and the solvent.

In a bimolecular reaction (SN2), the nucleophile attacks the substrate and the leaving group departs simultaneously. SN2 reactions are stereospecific and often lead to an inversion of stereochemistry. Factors that favor SN2 reactions include strong nucleophiles, primary substrates, and polar aprotic solvents.

Unimolecular reactions (SN1), on the other hand, involve a two-step mechanism where the leaving group detaches first, forming a carbocation intermediate, followed by the nucleophile attaching to this positively charged intermediate. SN1 reactions are characterized by their reliance on tertiary or secondary substrates, weak nucleophiles, and protic solvents.

Understanding both mechanisms and the factors that influence them is crucial for predicting the outcomes of nucleophilic substitution reactions and is indispensable in the synthesis of various organic compounds.

Organic Chemistry Education

Organic chemistry education is pivotal in shaping a strong foundation for students aspiring to work in fields like pharmaceuticals, forensics, and biochemistry. Central to mastering this discipline is grasping the multitude of reactions, mechanisms, and principles that dictate organic processes.

Effective education in organic chemistry involves not just memorization, but also understanding and application of concepts. One way to enhance comprehension is through problem-solving exercises, which reinforce the theoretical knowledge by applying it to practical scenarios. Engaging with exercises similar to the one discussed here, where students discern the correct order of leaving group displacement, develops their analytical skills.

Additionally, using visual tools such as reaction diagrams and molecular models can help students better visualize molecular structures and the dynamic nature of chemical reactions. The integration of digital platforms and interactive simulations can further enrich students' learning experiences, making difficult concepts more accessible and easy to understand.

Lastly, continual practice, engagement with the scientific community, and real-life application of organic chemistry principles are essential components for students to not only learn but also retain and utilize the knowledge effectively in their future careers.