Flashcards
Chapter 16
Cards in this deck (14)
- 1Discovery of benzene-C:H ratio 1:1
-Molecular formula- C6H6
-Low C:H rations/conjugated/aromatic- has pleasant smell - 2Addition 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. - 3Why 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. - 4Resonance 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) - 5Resonance 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).
- 6Structure 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. - 7Failure 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.
- 8What 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. - 9Draw out MOs for Benzene
- 10First 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 - 11Intermediate MO of benzen-are degenerate (equal in energy) with 2 orbitals are each level
-π2 & π3 have one nodal plane (each) - 12All 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 - 13Describe the energy diagram of benzene- all bonding orbitals are filled "closed shell" -> extremely stable arrangement
-6 π electrons fill 3 bonding π orbitals - 14Draw MO of cyclobutadiene