Using bond energies from Table 8.4, estimate the barrier to...

Using bond energies from Table $8.4$, estimate the barrier to rotation about a carbon-carbon double bond. To do this, consider what must happen to go from
in terms of making and breaking chemical bonds; that is, what must happen in terms of the $\pi$ bond?

Key Concepts

Barrier to Rotation

The barrier to rotation refers to the energy required to rotate around a bond, particularly a double bond. For carbon-carbon double bonds, the rotation is hindered by the need to break the pi bond during the process. Estimating this barrier involves calculating the energy cost associated with breaking the pi bond, as this disruption must occur before the bond can re-form, resulting in a high energy barrier that maintains the double bond’s rigidity.

Pi Bond

The pi bond is an essential component of a double bond that arises from the lateral overlap of p-orbitals. It is generally weaker than the sigma bond and is more readily disrupted. When considering the rotation about a carbon-carbon double bond, breaking or disrupting the pi bond is the primary energy-demanding step, which accounts for the high barrier to rotation in such bonds.

Carbon-Carbon Double Bond

A carbon-carbon double bond consists of one sigma (?) bond and one pi (?) bond. The sigma bond is formed by the head-on overlap of orbitals, creating a strong bond, while the pi bond results from the sideways overlap of p-orbitals. The presence of the pi bond restricts rotation around the double bond, contributing significantly to the molecule’s rigidity.

Bond Energy

Bond energy is the amount of energy required to break a chemical bond between two atoms. It is a quantitative measure of bond strength and is typically expressed in kilojoules per mole. In the context of this problem, bond energies are used to estimate the energy cost associated with breaking specific bonds during chemical reactions or conformational changes.

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