About this Video

• Video Title: Quick revision - Structure and bonding in benzene
• Channel: MaChemGuy
• Speakers: MaChemGuy (assumed based on channel name)
• Duration: 00:04:16

Introduction

This video provides a quick revision of the structure and bonding in benzene, comparing Kekulé's 1865 model with the updated delocalized model. The video uses experimental evidence to demonstrate why the delocalized model is a more accurate representation of benzene's structure and bonding.

Key Takeaways

• Kekulé's model: Proposed a hexagonal ring with alternating carbon-carbon double and single bonds. This model predicted benzene would undergo addition reactions and have alternating bond lengths. The enthalpy change of hydrogenation was predicted to be three times more exothermic than cyclohexene.
• Experimental evidence contradicts Kekulé's model: Benzene undergoes substitution reactions, not addition reactions; requires a catalyst to react with halogens; and exhibits uniform carbon-carbon bond lengths. The enthalpy change of hydrogenation is less exothermic than predicted by Kekulé's model.
• Delocalized model: Benzene's six carbon atoms form three sigma bonds each. The remaining p-orbital electrons overlap to form a delocalized pi electron cloud above and below the ring. This delocalized pi electron system explains the observed properties of benzene, including its stability, uniform bond lengths, and substitution reactions.

Here are 10 questions based on the provided video transcript about benzene's structure and bonding:

1. What was Kekulé's proposed structure for benzene, and what type of reactions did it predict?
2. What experimental observations contradicted Kekulé's model of benzene?
3. What is the key difference in reactivity between Kekulé's predicted benzene and the actual behavior of benzene?
4. How do the carbon-carbon bond lengths in benzene differ from those predicted by Kekulé's model?
5. What is the enthalpy change of hydrogenation for cyclohexene, and how does this compare to the expected value for Kekulé's benzene structure?
6. Describe the formation of sigma bonds in the delocalized model of benzene.
7. Explain the role of p-orbitals in forming the delocalized pi electron cloud in benzene.
8. What is meant by "delocalized electrons" in the context of benzene's structure?
9. How does the delocalized pi electron cloud in benzene explain its stability compared to a molecule with localized double bonds?
10. Why does benzene require a catalyst to react with halogens, unlike the prediction from Kekulé's model?

Let's match your points with direct answers from the transcript. Note that some of your points are summaries or inferences, making direct, verbatim quotes difficult. I'll provide the closest related information from the transcript instead.

1. alternating localised pi bonds . cycloex-1-3-5 triene: The transcript refers to Kekulé's 1865 proposal as having "a hexagonal ring with alternating carbon-carbon double and carbon-carbon single bonds." The comparison to cyclohexa-1,3,5-triene is an accurate analogy but not explicitly stated.

2. all bond lengths of benzene are the same, benzene didnt decolorise bromine, benzene has a less exothermic enthalpy of hydrogeneation than expected.: The transcript states: "X-ray diffraction shows that all of the carbon-carbon bond lengths in the ring are the same length...and the enthalpy change of hydrogenation was found to be less exothermic than predicted from cyclohexene." The lack of decolorization with bromine is implied by the statement that benzene "undergoes substitution reactions and not addition reactions," indicating the pi electrons are not readily available for addition.

3. doesnt do electrophillic substitution without a catalyst: The transcript says: "...it was found that it needed a catalyst...when it reacts with halogens it needs a halogen carrier catalyst."

4. all bonds lengths are the same: See answer 2.

5. enthalpy change of hydrogenation of cyclohexene times 3 is more exo that enthalpy change of hydrgoenation of benzene: The transcript states that for Kekulé's structure, "we'd expect minus 360 kilojoules per mole," three times the value for cyclohexene (-120 kJ/mol). It then notes that the actual enthalpy change for benzene was "only minus 208," significantly less exothermic.

6. head to head overlap of orbintals: This isn't directly mentioned in the transcript. The description focuses on the sideways overlap of p-orbitals.

7. they do sideways overlap to form a delocalised electron pi bonding system above and below the plane: The transcript describes: "the P orbitals overlap...that forms PI bonds...the pair of electrons is now shared between two P orbitals...and that continues around the ring...and so we get this doughnut shaped orbital which is the PI electron cloud."

8. not local on the carbons: The transcript explains: "the electrons are no longer shared between two atoms and so they are delocalized electrons."

9. less reactive as it is less electron dense, and cannot polarise: The transcript explains benzene's reaction with halogens: "that's because the electron density in the ring is unable to polarize the halogen molecules." This lack of polarization explains the need for a catalyst.

10. idk: This requires a new question based on the transcript.