Question

Renzene exhibits resonance. The carbon-carbon bonds of benzene are: A. shorter and stronger than the double bond of an alkene. B. longer and weaker than the double bond of an alkene. C. longer and stronger than the carbon-carbon bond of an alkane. D. longer and weaker than the carbon-carbon bond of an alkane.

Short answer

B. longer and weaker than the double bond of an alkene.

Step-by-step solution

Understand Resonance in Benzene

Resonance in benzene means that the electrons in the double bonds are delocalized over all six carbon atoms in the ring. This creates a situation where all the carbon-carbon bonds are equivalent and are not true double bonds or single bonds.

Compare Bond Lengths

Double bonds are generally shorter than single bonds. In benzene, due to resonance, the carbon-carbon bonds are intermediate in length between single and double bonds. Therefore, they are longer than a typical double bond found in alkenes.

Compare Bond Strengths

The delocalization in benzene also makes the carbon-carbon bonds stronger than typical single bonds (seen in alkanes), but weaker than a standalone double bond in alkenes.

Determine the Correct Answer

Based on the previous steps, benzene's carbon-carbon bonds are longer and weaker than the double bonds in alkenes, but stronger than the single bonds in alkanes. Thus, the correct answer is: B. longer and weaker than the double bond of an alkene.

Key concepts

carbon-carbon bond lengths

In a molecule, bond lengths are the distances between the nuclei of two bonded atoms. These distances vary depending on the type of bond. There are mainly three types of carbon-carbon bonds: single bonds, double bonds, and triple bonds.

A single bond (like in alkanes) has the longest bond length. Double bonds (like in alkenes) are shorter because more electrons are shared between the two atoms, pulling them closer together. Triple bonds are even shorter for the same reason.

In benzene, a special situation occurs. Benzene shows resonance, meaning the electrons are delocalized across all six carbon atoms. This delocalization makes each bond in benzene neither a true single nor double bond but something in between. As a result, the carbon-carbon bond lengths in benzene are intermediate between the longer single bonds and shorter double bonds.

bond strength comparison

Bond strength refers to the amount of energy required to break a bond. Like bond lengths, bond strengths also vary depending on the type of bond.

• Single bonds (like in alkanes) are the weakest and require less energy to break.
• Double bonds (like in alkenes) are stronger than single bonds and need more energy to break.
• Triple bonds are the strongest among the three.

Benzene's unique structure due to resonance also affects its bond strength. The delocalization of electrons gives benzene's carbon-carbon bonds a strength that is intermediate. They are stronger than the single bonds but not as strong as the double bonds. Thus, benzene's bonds are strong but still easier to break than a regular double bond found in alkenes.

alkanes vs. alkenes

Let's understand the differences between alkanes and alkenes, focusing on their bond types and stability.

• Alkanes: These are saturated hydrocarbons with single bonds between carbon atoms. They have the formula CnH2n+2. Their single bonds make them relatively less reactive and more stable under normal conditions.


• Alkenes: These are unsaturated hydrocarbons with at least one double bond between carbon atoms. They have the formula CnH2n. The presence of double bonds means more electrons are shared, leading to shorter and stronger bonds. However, these double bonds make alkenes more reactive than alkanes.

In comparison, benzene holds a special place as it exhibits characteristics of both alkanes and alkenes due to resonance. Its carbon-carbon bonds are shorter and stronger than those in alkanes but longer and weaker than those in standard alkenes.