Problem 109 - Chemistry The Central Science

Question

Ammonia reacts with boron trifluoride to form a stable compound, as we saw in Section 8.7 . (a) Draw the Lewis structure of the ammonia-boron trifluoride reaction product. (b) The B-N bond is obviously more polar than the \(\mathrm{C}-\mathrm{C}\) bond. Draw the charge distribution you expect on the \(\mathrm{B}-\mathrm{N}\) bond within the molecule (using the delta plus and delta minus symbols mentioned in Section 8.4\()\) . ( ) Boron trichloride also reacts with ammonia in a similar way to the trifluoride. Predict whether the \(B-N\) bond in the trichloride reaction product would be more or less polar than the \(B-N\) bond in the trifluoride product, and justify your reasoning.

Step-by-Step Solution

Answer

The Lewis structure for the ammonia-boron trifluoride reaction product, \(\mathrm{NH_3BF_3}\), is as follows: ``` F F H | | | B - N - B - N | | | F F H ``` The charge distribution on the B-N bond indicates that it is polar, with nitrogen having a delta minus (\(δ^-\)) and boron having a delta plus (\(δ^+\)) charge. Comparing the polarity of the B-N bond in the reaction products of boron trifluoride and boron trichloride with ammonia, the B-N bond in the trifluoride product, \(\mathrm{NH_3BF_3}\), is more polar than the B-N bond in the trichloride product, \(\mathrm{NH_3BCl_3}\), due to the higher electronegativity of fluorine.

Step 1: Draw the Lewis structure of the ammonia-boron trifluoride reaction product

To draw the Lewis structure, we need to know the chemical formula of the product. Ammonia (\(\mathrm{NH_3}\)) has a lone electron pair, and boron trifluoride (\(\mathrm{BF_3}\)) has an empty valence orbital. Thus, the lone pair from the nitrogen atom in ammonia can form a dative bond with the boron atom in boron trifluoride. This results in the formation of the compound ammonium boron trifuoride (\(\mathrm{NH_3BF_3}\)). The Lewis structure for \(\mathrm{NH_3BF_3}\) can be drawn as follows: ``` F F H | | | B - N - B - N | | | F F H ```

Step 2: Determine the charge distribution on the B-N bond

To determine the charge distribution on the B-N bond, we need to consider the differences in electronegativity between boron and nitrogen. Nitrogen is more electronegative than boron, so the electron cloud in the B-N bond will be distributed more towards the nitrogen atom, making the nitrogen partially negatively charged and the boron partially positively charged. Thus, the B-N bond will be polar, using the delta plus (\(δ^+\)) and delta minus (\(δ^-\)) symbols: ``` F F δ+ | | | B - N - B - N - δ- | | | F F δ+ ```

Step 3: Compare the polarity of the B-N bond in the reaction products between boron trifluoride and boron trichloride

First, let's predict the reaction product of ammonia and boron trichloride. Similar to the reaction with boron trifluoride, ammonia will form a dative bond with boron trichloride, resulting in the formation of ammonium boron trichloride (\(\mathrm{NH_3BCl_3}\)). Now, we need to compare the polarity of the B-N bond in the two reaction products, \(\mathrm{NH_3BF_3}\) and \(\mathrm{NH_3BCl_3}\). The difference in electronegativities between boron and nitrogen is the same in both compounds. However, the electronegativity of fluorine is higher than that of chlorine. As a result, fluorine atoms will draw electron density away from the boron in \(\mathrm{NH_3BF_3}\). This will lead to a larger partial positive charge on the boron atom in \(\mathrm{NH_3BF_3}\) compared to \(\mathrm{NH_3BCl_3}\), which in turn makes the B-N bond in \(\mathrm{NH_3BF_3}\) more polar than the B-N bond in \(\mathrm{NH_3BCl_3}\). In conclusion, the B-N bond in the trifluoride product is more polar than the B-N bond in the trichloride product.

Key Concepts

Ammonia-Boron Trifluoride Reaction

Understanding the ammonia-boron trifluoride reaction is fundamental when studying chemical reactions involving coordination compounds. In this specific reaction, ammonia ( NH_3) acts as a Lewis base due to its lone pair of electrons, which can be donated to another molecule. Conversely, boron trifluoride ( BF_3) serves as a Lewis acid because it has an empty orbital that can accept a pair of electrons.

When NH_3 comes into contact with BF_3, the lone pair on the nitrogen atom is donated to the empty p-orbital of the boron atom in BF_3, forming a dative or coordinate bond. This process results in the formation of a stable complex called ammonium boron trifluoride (NH_3BF_3). The Lewis structure of this compound is distinct as it includes a bond that arises from the sharing of two electrons from the same atom, which is a key feature of a coordinate bond.

The interesting aspect of this reaction is how the properties of the resulting compound differ from its reactants. Not only does the reaction showcase an important chemical interaction, but it also illustrates the concept of electron sharing and the formation of new, stable compounds through Lewis acid-base reactions.

Polarity of Chemical Bonds

The concept of polarity in chemical bonds is crucial for understanding molecular interactions. A polar bond is the result of unequal sharing of electrons between two atoms with different electronegativities.

In the context of the ammonia-boron trifluoride reaction product, the B-N bond is an excellent example to discuss. Nitrogen, which has a higher electronegativity compared to boron, will attract the shared electrons in the bond more strongly, creating a dipole moment. This dipole is represented with the symbols δ+ (delta plus) for the less electronegative atom and δ- (delta minus) for the more electronegative atom.

The polarity of a bond affects many physical properties of a compound, such as solubility, boiling point, and reactivity. For students to fully grasp the intricacies of polarity, it is helpful to visualize the charge distribution within the molecule, something that Lewis structures can aid in understanding. Furthermore, recognizing the polar nature of a bond can help predict how a molecule will interact with other substances or how it might behave in different environments.

Electronegativity Differences

Electronegativity refers to the ability of an atom to attract shared electrons in a chemical bond. The greater the difference in electronegativity between two bonded atoms, the more polar the bond becomes. It's a fundamental principle that helps explain a wide range of chemical phenomena, including bond polarity and molecular reactivity.

For instance, in the compounds NH_3BF_3 and NH_3BCl_3, the key difference lies in the identity of the halogen attached to boron. Fluorine has a higher electronegative value than chlorine. This means that in the boron trifluoride complex, the fluorine atoms will create more polarization through their increased electron-withdrawing power. This causes the B-N bond to become more polar in NH_3BF_3 than NH_3BCl_3. Students using this concept can predict and rationalize the polarity direction in bonds and understand the changes in chemical behavior that occur when different atoms are introduced into a molecule.