Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Amongst the trihalides of nitrogen which one is least basic? [1987 - 1 Mark] (a) \(\mathrm{NF}_{3}\) (b) \(\mathrm{NCl}_{3}\) (c) \(\mathrm{NBr}_{3}\) (d) \(\mathrm{NI}_{3}\)

Step-by-Step Solution

Answer

The least basic trihalide of nitrogen is \( \mathrm{NF}_{3} \).

Step 1: Understanding the Basicity Concept

Basicity refers to the ability of a compound to accept protons. For nitrogen trihalides, this characteristic is influenced by the stability of the lone pair on nitrogen and the electronegativity of the attached halogen atoms.

Step 2: Assessing Electronegativity

The electronegativity of the halogens affects the lone pair on nitrogen. Fluorine is the most electronegative, followed by chlorine, bromine, and iodine. Higher electronegativity stabilizes the lone pair, reducing basicity.

Step 3: Comparing the Electron Withdrawing Effect

In nitrogen trihalides, the halogen atoms withdraw electron density from nitrogen. As electronegativity increases (Fluorine > Chlorine > Bromine > Iodine), the electron withdrawing effect becomes stronger, further reducing basicity.

Step 4: Identifying the Least Basic Trihalide

Based on the electronegativity order, \(\mathrm{NF}_{3}\) has the most electronegative halogen (fluorine) which stabilizes the lone pair the best, making \(\mathrm{NF}_{3}\) the least basic among the nitrogen trihalides listed.

Key Concepts

Basicity

Basicity is a concept that describes how easily a compound can accept protons. In the context of nitrogen trihalides, basicity is specifically related to the presence of a lone pair of electrons on the nitrogen atom. This lone pair is key to attracting protons, similar to how a magnet attracts metal. But here's the twist: the basicity of nitrogen trihalides isn't just about what the nitrogen atom is willing to do.

Several factors affect basicity, especially the type of halogen bonded to nitrogen. In nitrogen trihalides, halogens like fluorine, chlorine, bromine, or iodine can influence the ability of nitrogen to accept a proton. The attachment of different halogens can stabilize or destabilize the lone pair, essentially making it more or less inclined to form a bond with a proton. This stabilizing action affects the overall basicity of the compound.

When comparing different nitrogen trihalides, the basicity decreases as the halogen’s electronegativity increases because the more stable the lone pair, the less likely it is to accept a proton. Thus, you could say the more electronegative the halogen, the less basic the compound becomes.

Electronegativity

Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond. It is crucial in understanding the behavior of nitrogen trihalides. The halogen atoms bonded to nitrogen have significant roles based on their electronegativity levels.

Fluorine, chlorine, bromine, and iodine are typical halogens seen in nitrogen trihalides. Amongst these, fluorine is the most electronegative, which means it holds onto electrons more tightly than the others. Here's why that matters:

• Fluorine's high electronegativity draws electron density towards itself.
• This action stabilizes the lone pair on nitrogen, making it less reactive.
• Consequently, nitrogen becomes less willing to accept additional protons.

Thus, the basicity of nitrogen trihalides decreases as we move from iodine to fluorine, reflecting the increased electronegativity and corresponding stabilization of nitrogen’s lone pair.

Electron Withdrawing Effect

The electron withdrawing effect refers to the ability of atoms or molecules to pull electron density away from a central atom. In nitrogen trihalides, this concept is profoundly applicable, as the halogen atoms bonded to nitrogen have varied electron withdrawing powers.

Halogens like fluorine, chlorine, bromine, and iodine display increasing levels of electron withdrawing effects as their electronegativity rises. This effect can have profound impacts:

• Electronegative halogens such as fluorine exhibit the strongest electron withdrawing effect.
• When the electron withdrawing effect is strong, as with fluorine, it stabilizes the lone pair on nitrogen.
• This stability reduces nitrogen's ability to act as a base and accept protons.

Thus, a high electron withdrawing effect generally equates to lower basicity in nitrogen trihalides, with \(\mathrm{NF}_{3}\) ending up on the low end of the basicity scale owing to fluorine's powerful electron withdrawing capabilities.