Chapter 16 Flashcards
(36 cards)
Michael Faraday _______ C6H6
isolated
Eilhard Mitscherlich ________ benzene (mw=78)
synthesized
Fredrich Kekule _______ benzene
found structure (dream about snake eating tail)
Benzene is actually a _______ hybrid between the two Kekule structures
resonance
The C-C bond lengths in benzene are _______ than typical single-bond lengths, yet _______ than typical double-bond lengths
shorter
longer
Each sp2 hydridized C in the ring contains an unhybridized _____ orbital perpendicular to the ring that overlaps around the ring. The _______ pi electrons are delocalized over the 6 carbons
p
6
Rules for creating MO diagrams for cyclic, conjugated molecules:
1) # of orbitals = # of _______ in ring
2) Place _______ orbital at lowest energy, then place orbitals in _______ (degenerate orbitals) at the same energy, until they run out. If there is a leftover unpaired orbital, it is placed alone at highest energy
3) make sure the orbitals are _______ distributed. Orbitals will only be found at energy = 0 if the molecule is perfectly symmetrical (eg 4,8,12)
1) atoms
2) one ; pairs
3) symmetrically
The Polygon Rule: the energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the _______
bottom
General rule for MO conditions (Huckel’s Rule):
1) aromatic compounds have _______ electrons and the orbitals are _______
2) antiaromatic compounds have _______ electrons and have _______ electrons in two degenerate orbitals
1) 4N+2 ; filled
2) 4N ; unpaired (diradical)
What are the 3 characteristics for all molecules classified as aromatic and antiaromatic?
1) cyclic
2) fully conjugated
3) planar
Fully conjugated rings between ___ and ___ atoms can be assumed to be planar
3 and 7
If the ring size is bigger or equal to _____, more information is needed to describe if planar
- depends on info and whether 4N or 4N+2
8
A 4N annulene that has ring size bigger or equal to 8 are _______
non-aromatic
- flexible and nonplanar
A 4N+2 annulene that has ring size bigger or equal to 8 are _______
Need more info
- usually about bonding of Br2
- or reaction with KMnO4
Huckels rule also applies to system bearing a _______ or _______ charge
positive
negative
In a cyclopentadienyl ion the cation has an empty p orbital and 4 pi electrons, so it is _______
antiaromatic
In a cyclopentadienyl ion the anion has a BN pair of electrons in a p orbital for a total of 6 pi electrons, so it is _______
aromatic
Deprotonation turns a cyclopentadiene from nonaromatic to _______
anion aromatic
Huckel’s rule is a better predictor of _______ for conjugated cyclic systems than resonance
stability
_______ compounds: rings containing sp2-hybridized atoms other than C
Heterocyclic
- Most common are N, O, S
Electrons in a conjugated cyclic structure are either part of the _______ cycle or they are _______
p-orbital
stuck
(If the molecule is aromatic to begin with)
If the lone pair is on a nitrogen involved in a pi bond then it is _______ and _______
basic and stuck
(If the molecule is aromatic to begin with)
If the lone pair is one away from an atom involved in a pi bond, then it is part of the conjugated system = in a ____- orbital
p
Only if aromatic @ beginning, if the lone pair is _____ a nitrogen involved in a pi bond then it is basic
on
- of AA or NA then all N are basic
- if N is not involved in pi bond then its electron is needed in the p orbital, and it is Nonbasic
