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
Rate the relative stabilities of all the carbocations you know
What is a bonding molecular orbital?
side-to-side overlap of in-phase p-orbitals
What is the anti-bonding molecular orbital?
side-to-side interaction between out-of-phase p orbitals
In what two situations does resonance occur?
1) A structure with three or more contiguous atoms that each have an unhybridized p-orbital (sp or sp2)
2) A structure with two adjacent atoms that each have an unhybridized p-orbital next to an atom with non-bonding electrons (like a lone pair)
What are the steps to drawing resonance structures?
What is donation of electrons by resonance?
If a substituent has a lone pair on the atom directly attached to a benzene ring, then the lone pair can be delocalized into the ring. Substituents such as NH2, OH, OR, and Cl donate electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring is more electronegative than a hydrogen.
What is withdrawing electrons by resonance?
If a substituent is attached to a benzene ring by an atom that is doubly or triply bonded to a more electronegative atom, then the electrons of the ring can be delocalized onto the substituent. Substituents such as C=O, C=-N, SO3H, and NO2 withdraw electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring has a full or partial positive charge and is, therefore, more electronegative than a H.
Does a methoxy group attached to a benzene ring donate electrons by resonance or withdraw electrons inductively? How does it effect the pKa of a methoxy-substituted phenol?
Both. There’s a lone pair on the atom attached to the ring so it can donate electrons by resonance. Because oxygen is more electronegative than hydrogen, the methoxy group withdraws electrons inductively. The fact that methoxy-substituted phenol is a weaker acid than phenol indicates that the substituent’s resonance electron donation into the ring is more significant than its inductive electron withdrawal from the ring.
Does a Cl group attached to a benzene ring donate electrons by resonance or withdraw electrons inductively? How does it effect the pKa of a Cl-substituted phenol?
Cl has a lone pair that can donate electrons by resonance and, because it is more electronegative than H, Cl withdraws electrons inductively. The fact that chloro-substituted phenol is a stronger acid than phenol indicates that Cl’s inductive electron withdrawal from the ring is more significant than its resonance electron donation into the ring.
How does an electron donating substituent affect the COOH, an OH, or an NH3+ group attached to a benzene ring?
It decreases the acidity of the group
How does an electron withdrawing substituent affect the acidity of a COOH, an OH, or an NH3+ group attached to a benzene ring?
It increases the acidity of the group
Describe the mechanism for the reaction of an isolated diene with excess HBr.
When a diene with conjugated double bonds, such as 1,3-butadiene, reacts with a limited amount of electrophilic reagent so that addition can occur at only one of the double bonds, how many products are formed and what are they?
2 products, a 1,2 addition and 1,4 addition
1,2 addition = direct addition
1,4 addition = conjugate addition
Describe the mechanism of reaction of a conjugated diene with HBr.
What is the mechanism for the diels-alder reaction?
Rank the following dienes from most to least reactive in a Diels-Alder reaction.
Describe the stereochemistry of the Diels-Alder reaction.
How do you do a retrosynthetic analysis of a Diels-Alder reaction?
What are the two criteria for aromaticity?
Describe the reaction mechanism of an electrophilic aromatic substitution reaction.
