Ch. 8: Delocalized Electrons, Aromaticity and Electronic Effects Flashcards
What are localized electrons?
electrons that are restricted to a particular region, belong to either a single atom or are shared by two atoms
What are delocalized electrons? And how do they come to be?
Electrons shared by three or more atoms. They result from a p orbital overlapping the p orbitals of two adjacent atoms.
How many degrees of unsaturation does benzene have?
C6H6, so 4 degrees of unsaturation so the total number of rings and pi bonds in benzene is 4
How many products does a benzene form when you replace one H with a different atom?
one monosubstituted product
How many products does a benzene form when you replace two Hs with two different atoms?
three disubstituted compounds
1,3
1,4
1,2 (enantiomers)
Is benzene planar? Why or why not?
Each of benzene’s carbons is sp2 hydridized. An sp2 carbon forms a 120 bond angle which is the same as a planar hexagon. Benzene is planar.
What is a resonance contributor/resonance structure/contributing resonance structure?
the approximate structure with localized electrons
What is a resonance hybrid?
The actual structure of a molecule with delocalized electrons
What are the 3 rules for drawing resonance contributors?
1) only electrons move. atoms never move.
2) only pi electrons (electrons in pi bonds) and long-pair electrons can move. (never move sigma electrons)
3) the total number of electrons in the molecule does not change. therefore, each of the resonance contributors should have the same net charge.
What hybridizations can be involved in resonance contributing?
ONLY sp and sp2
What are the features that decrease the predicted stability of resonance contributors?
1) an atom with an incomplete octet
2) a negative charge that is not on the most electronegative atom
3) a positive charge that is on an electronegative atom
4) charge separation
Which carbocation is more stable and why?
What’s more stable a tertiary or secondary carbocation?
tertiary
What does delocalization/resonance energy depend on?
It depends on the number (of relatively stable ones) and the predicted stability of the resonance contributors.
The greater the number of relatively stable resonance contributors, the greater the delocalization energy.
The more nearly equivalent the structures of the resonance contributors, the greater the delocalization energy.
Name two things about resonance contributors?
1) the greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid
2) the greater the number of relatively stable resonance contributors and the more nearly equivalent their structures, the greater the delocalization energy
What are dienes?
hydrocarbons with two double bonds
What is an isolated diene?
they have isolated double bonds (separated by more than one single bond)
What is a conjugated diene?
have conjugated double bonds (separated by one single bond)
What’s more stable: an isolated or conjugated diene? Why?
A conjugated diene.
1) Electron delocalization: The pi electrons in each of the double bonds of an isolated diene are localized between two carbons. In a conjugated diene, the pi electrons are delocalized, and electron delocalization stabilizes a compound.
2) Hybridization of the orbitals that form the C–C single bonds: Because a 2s electron is closer to the nucleus, on average, than a 2p electron, a bond formed by sp2-sp2 overlap is shorter and stronger than one formed by sp3-sp2. (sp2 has more s character). Thus, one of the single bonds in a conjugated diene is a stronger single bond than those in an isolated diene, and stronger bonds cause a compound to be stable.
What is an allene?
A compound with cumulated double bonds. These are double bonds that are adjacent to one another.
What is an allylic cation?
It’s a carbocation with a positive charge on an allylic carbon; an allylic carbon is a carbon adjacent to an sp2 carbon of an alkene.
What is a benzylic cation?
A carbocation with a positive charge on a benzylic carbon; a benzylic carbon is a carbon adjacent to an sp2 carbon of a benzene ring.
How many resonance contributors does an allylic cation have?
two
How many resonance contributors does a benzylic cation have?
five