Problem 46 - Chemistry and Chemical Reactivity

Question

In each pair of bonds, predict which is shorter. (a) \(\mathrm{Si}-\mathrm{N}\) or \(\mathrm{Si}-\mathrm{O}\) (b) \(\mathrm{Si}-\mathrm{O}\) or \(\mathrm{C}-\mathrm{O}\) (c) \(\mathrm{C}-\mathrm{F}\) or \(\mathrm{C}-\mathrm{Br}\) (d) The \(\mathrm{C}-\mathrm{N}\) bond or the \(\mathrm{C} \equiv \mathrm{N}\) bond in \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{C} \equiv \mathrm{N}\)

Step-by-Step Solution

Answer

(a) Si-O, (b) C-O, (c) C-F, (d) C≡N

Step 1: Understanding Bond Length

Bond length is the distance between the nuclei of two bonded atoms. Generally, bond length decreases with increasing bond order and decreases as the atoms involved become smaller or more electronegative.

Step 2: Compare Si-N and Si-O (a)

The bond length is shorter when the atom is smaller or more electronegative. Oxygen is smaller and more electronegative than nitrogen, so the Si-O bond is shorter than the Si-N bond.

Step 3: Compare Si-O and C-O (b)

Carbon and silicon are in the same group, with silicon being larger. The C-O bond is shorter than the Si-O bond because carbon is smaller and forms a stronger bond with oxygen.

Step 4: Compare C-F and C-Br (c)

Fluorine is smaller and more electronegative than bromine. Therefore, the C-F bond is shorter than the C-Br bond.

Step 5: Compare C-N and C≡N in H₂NCH₂C≡N (d)

Triple bonds are generally shorter than single bonds. In the molecule H₂NCH₂C≡N, the C≡N triple bond is shorter than the C-N single bond.

Key Concepts

Bond Order

Bond order is a key concept that helps us to understand the length of bonds between atoms. It refers to the number of chemical bonds between a pair of atoms. For example, a single bond has a bond order of one, a double bond has a bond order of two, and a triple bond has a bond order of three.
The higher the bond order, the stronger the attraction between atoms, which typically results in a shorter bond length. This is because more electrons are shared between the two atoms, pulling them closer together.

• Single bonds are generally the longest because there is less electron sharing.
• Double bonds are shorter than single bonds but longer than triple bonds due to increased electron sharing.
• Triple bonds are the shortest because there is the most electron sharing.

Understanding bond order is crucial for predicting bond lengths, as shorter bond lengths often indicate stronger bonds.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. The more electronegative an atom is, the stronger its pull on the electron cloud shared with another atom. This can significantly influence bond length.

• Highly electronegative atoms will pull shared electrons closer to themselves.
• This "electron cloud draw" makes the bond shorter because the nuclei of the two atoms are pulled closer together.

In the context of bond length, two elements with differing electronegativities can result in a shorter bond compared to those with similar electronegativities. For example, in part of the original problem, oxygen's higher electronegativity compared to nitrogen results in a shorter \(\text{Si-O}\) bond compared to \(\text{Si-N}\). Understanding the concept of electronegativity is key for accurately predicting and analyzing bond characteristics.

Bond Strength

Bond strength refers to the energy required to break a bond between two atoms. It is closely related to bond order and electronegativity, and it plays a vital role in determining bond length. A stronger bond will typically be shorter because stronger bonds result from greater electron sharing between the atoms, which pulls them closer together.
Some factors affecting bond strength include:

• Bond Order: Higher bond orders mean stronger bonds due to increased electron sharing.
• Electronegativity: More electronegative atoms form stronger bonds as they pull bonding electrons closer, creating a stronger attraction.
• Atomic Size: Smaller atoms can form stronger bonds as their nuclei can get closer together, allowing their electron clouds to overlap more extensively.

By understanding bond strength, one can make better predictions about bond lengths and stability.

Atomic Size

Atomic size is a fundamental property that influences bond length directly. It refers to the distance from the nucleus of an atom to the outer boundary of the surrounding electron cloud. When considering bond length, the size of the atoms involved is crucial:

• Smaller atoms can form shorter bonds because their electron clouds can overlap more efficiently with those of other atoms.
• Larger atoms have more distant nuclei, resulting in longer bonds due to less effective overlap of electron clouds.

In the examples given in the original exercise, fluorine forms a shorter bond with carbon compared to bromine because fluorine is smaller. Similarly, oxygen forms a shorter bond with silicon than nitrogen because oxygen is smaller. Understanding atomic size helps clarify the relationship between atomic dimensions and bond measurements, allowing more precise predictions of bond lengths.