Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds is resistant to nucleophilic attack by hydroxyl ions? (a) acetonitrile (b) diethyl ether (c) acetamide (d) methyl acetate

Step-by-Step Solution

Answer

Acetonitrile is resistant to nucleophilic attack by hydroxyl ions.

Step 1: Understanding the Problem

The problem asks us to identify which compound among the given choices is resistant to nucleophilic attack by hydroxyl ions (OH-). A nucleophilic attack involves a nucleophile donating a pair of electrons to an electrophile.

Step 2: Analyzing the Reactivity of Each Compound

Consider each option: (a) Acetonitrile (CH3CN) has a triple bond between carbon and nitrogen, providing stability and resistance to nucleophilic attack due to its linear geometry and lack of suitable electrophilic center. (b) Diethyl ether (C2H5OC2H5) has an oxygen atom that creates a polar environment, though the ether linkage itself is relatively stable against nucleophiles. (c) Acetamide (CH3CONH2) has an amide linkage, which is typically resistant to nucleophilic attack due to resonance stabilization of the carbonyl group. (d) Methyl acetate (CH3COOCH3) is an ester that can be subjected to nucleophilic attack at the carbonyl carbon, especially by strong bases like hydroxyl ions.

Step 3: Identifying the Most Resistant Compound

Evaluate the likelihood of a nucleophilic attack on each compound. Acetonitrile has no significant electrophilic centers to invite attack due to its triple bond and minimal charge separation. Amides are also highly resistant, but lesser compared to nitriles, which are notably inert towards nucleophilic attack because of their electronic structure.

Step 4: Conclusion

Among the given compounds, acetonitrile, with its strong triple bond and lack of a reactive electrophilic site, is the most resistant to nucleophilic attack by hydroxyl ions.

Key Concepts

Electrophilic Centers

In organic chemistry, understanding electrophilic centers is essential to grasp reactions involving nucleophiles. Electrophilic centers are atoms within a molecule that offer a region of low electron density. This makes them attractive targets for nucleophiles, which are rich in electrons. An electrophile is typically characterized by a partial positive charge or a high affinity for electrons. Some common electrophilic centers in molecules include:

• The carbon atom in carbonyl groups (e.g., in aldehydes, ketones, carboxylic acids, and esters).
• The carbon atom next to a leaving group (such as in alkyl halides).

In the exercise, analyzing which compound is resistant to nucleophilic attack involves examining these electrophilic centers. Acetonitrile, for instance, lacks such suitable electrophilic sites, due to its sp-hybridized carbon in the triple bond with nitrogen. This structure provides neither a positive region nor an accessible electron-deficient site, making it less reactive to nucleophiles like hydroxyl ions.

Reactivity of Compounds

The reactivity of compounds depends significantly on their structure and the presence of certain functional groups. Different functional groups can influence how a molecule interacts with a nucleophile or an electrophile. Reactivity is generally understood in terms of how easily a compound undergoes a chemical reaction with a nucleophile. For example:

• Esters, like methyl acetate, have a carbonyl carbon that is reactive toward nucleophiles.
• Amides, like acetamide, are typically less reactive due to resonance stabilization of the carbonyl group.
• Nitriles, such as acetonitrile, exhibit minimal reactivity because of the full delocalization of electrons across the triple bond.

In our problem, acetonitrile's minimal reactivity is due to its triple bond's strength and the linear geometry of its structure. This setup imparts a high degree of stability, making nucleophilic attacks unlikely, especially by weak bases like hydroxyl ions.

Nucleophiles

Nucleophiles are chemical species that possess a pair of electrons which can be donated to form a bond with electrophiles. This characteristic is crucial in various chemical reactions, particularly nucleophilic substitution and addition reactions. Nucleophiles can be either anions, like hydroxyl ions (OH-), or neutral molecules that have lone electron pairs, such as water (H2O). They are represented by a region of high electron density:

• Examples of effective nucleophiles include halide ions (Cl⁻, Br⁻), cyanide ion (CN⁻), and strong bases like hydroxide ion (OH⁻).
• Nucleophiles often target positive or partially positive carbon atoms in molecules, seeking to donate electrons.

In the exercise, identifying a compound resistant to nucleophilic attack by hydroxyl ions involves analyzing which compound lacks accessible electrophilic centers. Since acetonitrile's structure offers no such attractive centers for hydroxyl ions, it remains mostly unaffected by nucleophilic attack. This demonstrates how a nucleophile's ability to initiate a reaction heavily depends on the presence of suitable sites within a compound.