Question

Consider the given reaction, $BX_3 + NH_3 \rightarrow BX_3 \cdot NH_3$ + Heat of adduct ($\Delta H$). The numerical value of $\Delta H$ is found to be maximum for

• A. $BF_3$
• B. $BCl_3$
• C. $BBr_3$
• D. $BI_3$

Answer 1

Answer: (D)

$BI_3$

Answer 2

The reaction is a Lewis acid-base reaction between $BX_3$ (Lewis acid) and $NH_3$ (Lewis base) to form an adduct $BX_3 \cdot NH_3$:

$BX_3 + NH_3 \rightarrow BX_3 \cdot NH_3$

The heat of adduct formation ($\Delta H$) is a measure of the strength of the Lewis acid-base interaction. Since the formation of a bond releases energy, this reaction is exothermic, and $\Delta H$ is negative. A stronger interaction leads to a more stable adduct and a larger amount of heat released, meaning a more negative $\Delta H$. The numerical value of $\Delta H$ refers to its magnitude, $|\Delta H|$. We are looking for the $BX_3$ that gives the maximum numerical value of $\Delta H$, which corresponds to the strongest Lewis acid-base interaction and thus the strongest Lewis acid among the $BX_3$ series, as the Lewis base ($NH_3$) is constant.

The Lewis acidity of $BX_3$ compounds is influenced by the electronegativity of the halogen (X) and the extent of backbonding from the halogen to boron.

1. Electronegativity: Electronegativity decreases down the group (F > Cl > Br > I). A more electronegative halogen withdraws electron density from boron through sigma bonds, increasing the positive charge on boron and enhancing its Lewis acidity. Based on this effect alone, the order of Lewis acidity would be $BF_3 > BCl_3 > BBr_3 > BI_3$.

2. Backbonding (pπ-pπ interaction): The halogens have lone pairs in their p orbitals which can overlap with the vacant 2p orbital of boron. This backbonding donates electron density from the halogen to boron, reducing the electron deficiency on boron and decreasing its Lewis acidity. The effectiveness of backbonding depends on the size and energy match of the orbitals involved. The overlap between B(2p) and X(np) decreases as the size of the halogen increases (n increases from 2 for F to 5 for I). Thus, backbonding is strongest in $BF_3$ (2p-2p overlap) and weakest in $BI_3$ (2p-5p overlap).

The observed trend in Lewis acidity of $BX_3$ is determined by the dominant effect. Experimental evidence shows that the Lewis acidity increases in the order $BF_3 < BCl_3 < BBr_3 < BI_3$. This indicates that the effect of backbonding is more significant than the inductive effect of electronegativity. The strong backbonding in $BF_3$ makes it a relatively weak Lewis acid compared to the others. As backbonding decreases from $BF_3$ to $BI_3$, the Lewis acidity increases. $BI_3$ has the weakest backbonding and is therefore the strongest Lewis acid among the tetrahalides.

A stronger Lewis acid forms a stronger coordinate covalent bond with the same Lewis base ($NH_3$), leading to a more stable adduct and a larger release of heat. Thus, the magnitude of the heat of adduct formation ($|\Delta H|$) is maximum for the strongest Lewis acid.

Since $BI_3$ is the strongest Lewis acid among $BF_3, BCl_3, BBr_3,$ and $BI_3$, the adduct formation with $NH_3$ will be strongest for $BI_3$. This means the reaction $BI_3 + NH_3 \rightarrow BI_3 \cdot NH_3$ will release the most heat, resulting in the maximum numerical value of $\Delta H$.

The order of increasing Lewis acidity is $BF_3 < BCl_3 < BBr_3 < BI_3$. The order of increasing magnitude of heat of adduct formation ($|\Delta H|$) is $BF_3 < BCl_3 < BBr_3 < BI_3$.

Therefore, the numerical value of $\Delta H$ is found to be maximum for $BI_3$.