14278662
Description
Flashcards by Umaimah Kazi, updated more than 1 year ago
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Created by Umaimah Kazi
almost 6 years ago
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| Question | Answer |
| Discovery of benzene | -C:H ratio 1:1 -Molecular formula- C6H6 -Low C:H rations/conjugated/aromatic- has pleasant smell |
| Addition of bromine to benzene: What catalyst is needed to add benzene? Substitution or addition reaction? | When bromine adds to benzene, a catalyst such as FeBr3 is needed. The reaction that occurs is the substitution of a hydrogen by a bromine. |
| Why does benzene undergo substitution with Br2 while cyclohexene undergoes an addition reaction? | -Addition of Br2 to benzene is unfavorable. -It would result in a non-aromatic product. -Addition of Br2 to the double bond is not observed. |
| Resonance energy of Benzene | -Benzene does not have the predicted heat of hydrogenation of –359 kJ/mol. -The observed heat of hydrogenation is –208 kJ/mol, a difference of 151 kJ. -This difference between the predicted and the observed value is called the resonance energy. (molar heats of hydrogenation) |
| Resonance structures of benzene | -Benzene is actually a resonance hybrid of the two Kekulé structures. -The C—C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths (bond order 1.5). |
| Structure of Benzene | -Each sp2 carbon has an unhybridized p orbital perpendicular to the ring that overlaps around the ring. -The six pi electrons are delocalized over the six carbons. |
| Failure of Resonance Picture | -All cyclic conjugated hydrocarbons were proposed to be aromatic. -However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. -Cyclooctatetraene adds Br2 readily to the double bonds. |
| What are the MO rules for benzene? | 1. Six overlapping p orbitals must form six molecular orbitals (MOs). 2. Three will be bonding, three antibonding. 3. Lowest-energy MO will have all bonding interactions, no nodes. 4. As energy of MO increases, the number of nodes increases. |
| Draw out MOs for Benzene | |
| First MO of benzene | - is entirely bonding, no nodes -has very low energy because it has 6 bonding interactions and the electrons are delocalized over all 6 carbon atoms |
| Intermediate MO of benzen | -are degenerate (equal in energy) with 2 orbitals are each level -π2 & π3 have one nodal plane (each) |
| All antibonding MO of benzene | -The all antibonding π6* has 3 nodal planes -Each pair of adjacent P orbital is out of the phase in a destructive manner |
| Describe the energy diagram of benzene | - all bonding orbitals are filled "closed shell" -> extremely stable arrangement -6 π electrons fill 3 bonding π orbitals |
| Draw MO of cyclobutadiene |
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