Question

Which of these bonds to carbon would you expect to be relatively reactive: \(\mathrm{C}-\mathrm{H}, \mathrm{C}-\mathrm{C}, \mathrm{C}-\mathrm{I}, \mathrm{C}=\mathrm{O}, \mathrm{C}-\mathrm{Li} ?\) Explain.

Short answer

The \(\mathrm{C}-\mathrm{Li}\) bond is the most reactive due to its high polarity and weak nature.

Step-by-step solution

Understand Bond Reactivity

To determine which bond is relatively reactive, consider factors such as bond strength, bond length, and the electronegativity of the atoms involved. Weaker bonds are generally more reactive.

Analyze the \(\mathrm{C}-\mathrm{H}\) Bond

The \(\mathrm{C}-\mathrm{H}\) bond is quite strong and less polar because of the similar electronegativities of carbon and hydrogen. This makes it relatively less reactive.

Analyze the \(\mathrm{C}-\mathrm{C}\) Bond

The \(\mathrm{C}-\mathrm{C}\) bond is also strong due to the similarity in electronegativity of carbon atoms. It is generally not very reactive.

Analyze the \(\mathrm{C}-\mathrm{I}\) Bond

The \(\mathrm{C}-\mathrm{I}\) bond is weaker and longer because iodine is large and less electronegative. This bond is more reactive than \(\mathrm{C}-\mathrm{H}\) and \(\mathrm{C}-\mathrm{C}\) bonds.

Analyze the \(\mathrm{C}=\mathrm{O}\) Bond

The double bond \(\mathrm{C}=\mathrm{O}\) is very strong and holds significant polarity, which makes it less reactive compared to others but reactive under specific conditions like nucleophilic attack.

Analyze the \(\mathrm{C}-\mathrm{Li}\) Bond

The \(\mathrm{C}-\mathrm{Li}\) bond is highly reactive because lithium is very electropositive, making the bond highly polarized and easy to break.

Conclusion

Among the bonds given, the \(\mathrm{C}-\mathrm{Li}\) bond is expected to be the most reactive due to the high polarity and weak nature of the bond.

Key concepts

bond strength

Bond strength refers to how strongly the atoms in a bond are held together. This depends on the types of atoms involved and the number of bonds between them.
Single bonds tend to be weaker, while double and triple bonds are stronger.
For instance, in the given exercise, the C-H and C-C bonds are strong because both pairs of atoms are similar in size and electronegativity.

On the other hand, the C-I bond is weaker because iodine is much larger than carbon. Similarly, the C-Li bond is weak because lithium is electropositive, making it easier to break the bond.
Understanding bond strength allows us to predict reactivity. Weaker bonds react more easily.

electronegativity

Electronegativity describes an atom's ability to attract electrons towards itself.
In organic chemistry, it helps us understand how shared electrons are distributed in covalent bonds.
Atoms with similar electronegativities form non-polar bonds, while those with different electronegativities form polar bonds.
The C-H bond is an example of a non-polar bond because carbon and hydrogen have similar electronegativities.

In contrast, the C-Li bond is highly polar. This is because lithium is less electronegative compared to carbon, causing an uneven electron distribution.
The higher the difference in electronegativity, the more reactive the bond. Hence, the highly polarized C-Li bond is also highly reactive.

bond length

Bond length is the average distance between the nuclei of two bonded atoms.
It generally increases with the size of the atoms involved and decreases with multiple bonds (double and triple bonds are shorter).

A longer bond is weaker, making it more reactive.
For example, the C-I bond is longer and weaker because iodine is significantly larger than carbon.
On the other hand, the C-O double bond is shorter and stronger because of the double-bond nature and smaller size of oxygen.
Therefore, bond length directly affects the reactivity of a molecule.

polarity

Polarity in a bond depends on the difference in electronegativity between the atoms making up the bond.
In polar bonds, electrons are more attracted to one atom than the other.
For instance, the C-Li bond is highly polarized because of the significant electronegativity difference between carbon and lithium.
This bond is very reactive as the electron distribution is uneven.

In contrast, the C-H bond is non-polar due to the similar electronegativities of carbon and hydrogen.
Therefore, it's less reactive.
Understanding bond polarity helps us predict and explain the reactivity of different bonds in organic molecules.