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 is shorter. (b) C-O is shorter. (c) C-F is shorter. (d) C≡N is shorter.

Step 1: Comparing Bond Lengths

To determine which bond is shorter, consider the following factors: bond order (the number of bonds between two atoms), the size of the atoms involved, and the electronegativity difference between the atoms in the bond. Bonds with higher bond orders are shorter due to stronger attractions between the atoms.

Step 1: Compare Si-N and Si-O Bond Lengths

Both the \( ext{Si}- ext{N}\) and \( ext{Si}- ext{O}\) bonds are single bonds. Oxygen is more electronegative than nitrogen and has a smaller atomic radius. This suggests that \( ext{Si}- ext{O}\) will be shorter due to the stronger attractive forces.

Step 2: Compare Si-O and C-O Bond Lengths

Oxygen is bonded to silicon and carbon, forming \( ext{Si}- ext{O}\) and \( ext{C}- ext{O}\) bonds, respectively. Carbon has a smaller atomic radius than silicon, which makes the \( ext{C}- ext{O}\) bond shorter due to the closer proximity of the atoms.

Step 3: Compare C-F and C-Br Bond Lengths

The \( ext{C}- ext{F}\) bond involves carbon and a small fluorine atom, while \( ext{C}- ext{Br}\) involves bromine, which is larger than fluorine. Thus, \( ext{C}- ext{F}\) is shorter due to the smaller size of fluorine.

Step 4: Compare C-N and C≡N Bond Lengths

Between \( ext{C}- ext{N}\) and \( ext{C} \equiv \text{N}\) in \( ext{H}_{2} ext{NCH}_{2} ext{C} \equiv \text{N}\), the triple bond \( ext{C} \equiv \text{N}\) has a bond order of three, making it significantly shorter than the single bond \( ext{C}- ext{N}\) due to stronger attractive forces.

Key Concepts

Bond Order

Bond order is a fundamental concept in chemistry that relates to the number of bonds between two atoms in a molecule. In simpler terms, it's like counting the number of ropes connecting two climbers. The more ropes – or bonds – the stronger and closer the climbers are.

• A single bond has a bond order of one.
• A double bond means a bond order of two.
• A triple bond is the strongest, with a bond order of three.

Higher bond order usually means a shorter bond length. This is because more electrons are shared between the atoms, increasing the attractive force, which pulls the atoms closer together. For example, in a comparison between a single C-N bond and a triple C≡N bond, the latter has a higher bond order and is thus shorter.

Atomic Size

Atomic size, or atomic radius, refers to the size of an atom. Atoms, like people, come in different sizes, and this can impact how close they can get to each other, a bit like two people standing in a small elevator versus a spacious room. Smaller atoms can bond more closely to other atoms, leading to shorter bond lengths.

• For instance, fluorine is smaller than bromine, making the C-F bond shorter than the C-Br bond, simply because fluorine can snuggle up closer to carbon than bromine can.
• In another example, compare carbon and silicon. Carbon has a smaller atomic radius than silicon, which results in a shorter C-O bond when compared to an Si-O bond.

Electronegativity

Electronegativity describes how strongly an atom attracts bonding electrons towards itself. Imagine a tug-of-war where the stronger team pulls the rope closer to their side. Atoms with higher electronegativity values can pull the shared electron cloud towards themselves more effectively.

• Oxygen, for example, is more electronegative than nitrogen.
• This greater pull contributes to a shorter and stronger bond because the electron cloud effectively holds the bonded atoms more tightly together.

In the context of Si-N versus Si-O, oxygen being more electronegative means the Si-O bond is shorter.

Chemical Bonding

Chemical bonding is like the glue that holds atoms together in molecules. It's a fundamental aspect of chemistry that describes how atoms interact to form compounds. There are different types of chemical bonds, such as:

• Covalent bonds: sharing of electron pairs between atoms.
• Ionic bonds: formed when electrons are transferred from one atom to another.
• Metallic bonds: electrons flow freely between a lattice of metal atoms.

The type of bond and the interactions between atoms determine the length and strength of the bond. In most of the examples given, we are dealing with covalent bonds, where atoms share electrons to establish a bond.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. Picture a molecule as a tiny building where the atoms are the bricks and the bonds are like how the bricks are connected and arranged. Understanding molecular structure helps in predicting and explaining the properties of a substance, such as reactivity, polarity, phase, color, magnetism, and biological activity.

• A triple bond, for instance, not only shortens the bond length but also affects the shape and angles in a molecule.
• The spatial arrangement of atoms can also influence how a molecule interacts with others, which is vital in fields like drug design.

The arrangement of these bonds and atoms impacts the molecular size and shape, directly influencing properties such as boiling and melting points.