Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is strongest nucleophile? (a) \(\mathrm{Br}\) (b) : \(\mathrm{OH}^{-}\) (c) : \(\overline{\mathrm{CN}}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \overline{\mathrm{O}}:\)

Step-by-Step Solution

Answer

:\(\overline{\mathrm{CN}}\) is the strongest nucleophile.

Step 1: Define Nucleophile

A nucleophile is a species that donates an electron pair to an electrophile to form a chemical bond. Nucleophiles are typically negatively charged ions or molecules with a lone pair of electrons that seek out positively charged centers.

Step 2: Consider Charge

Nucleophilic strength is influenced by charge; generally, the more negatively charged the nucleophile, the stronger it is. Among the given options, both :\(\mathrm{OH}^{-}\) and :\(\overline{\mathrm{CN}}\) carry a negative charge, making them potentially strong nucleophiles.

Step 3: Analyze Electronegativity

A less electronegative atom is more likely to donate its electrons, thus being a stronger nucleophile. Among the elements being considered, carbon (in :\(\overline{\mathrm{CN}}\)) is less electronegative compared to oxygen (in :\(\mathrm{OH}^{-}\)). This contributes to the greater nucleophilicity of :\(\overline{\mathrm{CN}}\).

Step 4: Consider Resonance

: \(\overline{\mathrm{CN}}\) is also less stabilized by resonance compared to a lone pair on an oxygen, which contributes to its higher reactivity as a nucleophile. This lack of resonance stabilization makes :\(\overline{\mathrm{CN}}\) a stronger nucleophile in reactive situations.

Step 5: Compare Solvent Effects

In protic solvents, :\(\overline{\mathrm{CN}}\) will remain a strong nucleophile as it is less impacted by hydrogen bonding compared to :\(\mathrm{OH}^{-}\) due to the less polar nature of the carbon-nitrogen bond.

Step 6: Conclusion

Based on the combined aspects of charge, electronegativity, resonance, and solvent effects, :\(\overline{\mathrm{CN}}\) is the strongest nucleophile among the given options.

Key Concepts

Nucleophile

A nucleophile is an important player in chemical reactions. It is a species, usually an ion or a molecule, that gives away an electron pair to attach to an electrophile, forming a new bond. Nucleophiles are electron-rich, and they seek out electron-deficient areas, which are usually positively charged or partially positive. In simpler terms, think of a nucleophile as someone with extra cash seeking to invest in a new opportunity.

• They typically carry a negative charge or have lone pairs of electrons ready to be shared.
• They are like the 'donors' in the reaction process.
• The stronger a nucleophile is, the more eager and effective it is in forming bonds.

To understand their strength and reactivity, we source their power from their charge, structure, and ability to stabilize after reaction. Nucleophilicity directly affects how molecules interact in our reactions, impacting the rate and outcome of chemical processes.

Electronegativity

Electronegativity is a measure of how much an atom wants electrons. Think of it as the tug-of-war strength between atoms for electrons. When discussing nucleophiles, a key point is how electronegativity impacts their readiness to share their electron pair.

• Less electronegative atoms are better nucleophiles because they hold onto their electrons more loosely, making them ready to donate.
• For instance, carbon is less electronegative than oxygen, which is why carbon-based nucleophiles, like \(\overline{\mathrm{CN}}\), can be stronger than oxygen-based ones.
• More electronegative atoms will hold onto their electrons more tightly, reducing their nucleophilicity.

So, in a list of potential nucleophiles, pay attention to the atoms involved. The less electronegative the atom is, the more likely it will be to act quickly and efficiently as a nucleophile.

Resonance

Resonance involves the delocalization of electrons across a molecule. This means that the electrons are not just fixed to a particular atom but can move across different atoms, resulting in resonance stabilization. In the case of nucleophiles, when electrons are delocalized, it can impact their reactivity.

• Nucleophiles that have their lone pairs involved in resonance are often less reactive because their electrons are spread out and stabilized.
• For example, with \(\overline{\mathrm{CN}}\), the lone pair on the carbon is not involved in resonance, unlike some other structures. This means those electrons are more available for reaction.
• Compounds that lack resonance stabilization, such as \(\overline{\mathrm{CN}}\), tend to be more "desperate" to donate their electrons, increasing their nucleophilicity.

When assessing nucleophiles, check if resonance plays a role in the molecule's stability or reactivity. Lesser resonance often equates to greater reactivity in terms of donating electrons.

Solvent Effects

The effect of the surrounding solvent can drastically alter the strength of a nucleophile. Solvents can be divided into protic and aprotic categories. Understanding their interaction with nucleophiles helps predict which nucleophiles will be stronger in a given environment.

• **Protic solvents** can form hydrogen bonds, often hindering nucleophile strength by "trapping" them in a network of interactions.
• This is why \(\overline{\mathrm{CN}}\) remains strong, as it is less solvated compared to others like \(\mathrm{OH}^{-}\).
• **Aprotic solvents**, on the other hand, do not engage in hydrogen bonding and thus do not inhibit nucleophile strength.

When deciding on nucleophile strength, it's essential to understand the solvent environment. For instance, in polar aprotic solvents, many nucleophiles will exhibit their full potential, whereas, in polar protic solvents, their strength may be concealed by solvent interactions.