Organic Chemistry Practice Problems and Problem Sets
EAS mechanism
Indicate the eletrophile formed by each set of reagents/conditions below.
Note that some textbooks don't draw EAS electrophiles with formal positive charges.
Some textbooks instead draw electrophiles with partial positive charges and a leaving group. It's just a convention, and doesn't affect the product of an EAS reaction. See the problem 595 for an explanation of the difference.
MendelSet practice problem # 596 submitted by Matt on July 9, 2011.
Phenol can be prepared from benzene and hydrogen peroxide in the presence of a really strong acid. Propose a mechanism for this reaction.
The electrophile we need for this EAS reaction is HO+. We can generate it by protonating one of the oxygens in peroxide, which causes it to act as a leaving group.
On the left side of this image I make a note about the structure of the electrophile. Some textbooks use electrophiles with formal positive charges, such as HO+. Other textbooks don't use formal positive charges, and instead use partial positive charges with a leaving group, such as HO-OH2+. Using HO+ as the electrophile is analogous to an SN1 reaction, while using HO-OH2+. is analogous to an SN2 reaction. When you draw mechanisms, you should use whichever convention your textbook uses. Both lead to the same product.
MendelSet practice problem # 595 submitted by Matt on July 9, 2011.
A chemist tried to prepare compound A from benzene via Friedel-Crafts alkylation and instead produced compound B.
Why did this happen? How could the chemist prepare compound A?
Friedel-Crafts alkylation is prone to carbocation rearrangement. In this case, alkylation produced a 1º carbocation which rearranged to a 3º carbocation, leading to compound B.
We can avoid this by instead doing Friedel-Crafts acylation. The intermediate in acylation is the acylium ion, which is stabilized by resonance and so won't rearrange.
But after the acylation reaction we have to get rid of the carbonyl (C=O) group, so we do a Wolff-Kishner reduction (N2H4/NaOH, heat).
MendelSet practice problem # 594 submitted by Matt on July 9, 2011.
a) Rationalize the relative stabilities of the cation species below.
b) Pyridine undergoes eletrophilic substitution at C-3. Let's compare the resonance forms of EAS carbocation intermediates to see why this is the case. Consider part a) in your explanation.
a) The species on the left are much more stable than the species on the right because they have complete octets.
Nitrogen is less electronegative than oxygen, so a positive nitrogen (with a full octect) is more stable than a positive oxygen; oxygen is electronegative and so doesn't "like" being positive (have low electron density).
The carbocation and the "nitrocation" are both very unstable because they only have 6 valence electrons. Because nitrogen is more electronegative than carbon, the nitrocation is even less stable than the carbocation, because its worse at handling a positive charge.
b) The cation intermediates that result from substitution at C-3 only contain carbocations, while the intermdiates from C-2 or C-4 substitution contain a "nitrocation," which is less stable than a regular carbocation, and so are not favored. So pyridine undergoes EAS reactions at C-3.
One more note: For substitution at C-2 and C-4, it's tempting to draw a resonance form where the positive charge is on the nitrogen, which is flanked by two double bonds. But this cyclic compound is too strained to exist, and so doesn't contribute to this analysis.
MendelSet practice problem # 593 submitted by Matt on July 9, 2011.
Naphthalene undergoes eletrophilic substitution at C-1.
Why is this the case, even though substitution at C-2 gives more resonance forms?
Substitution at C-1 and C-2 both lead to many resonance forms. But the resonance forms that arrise from substitution at C-2 are not aromatic, while the resonance forms from substitution at C-1 are aromatic (the benzene ring is maintained).
So the C-1 resonance forms are more stable than the C-2 resonance forms, so substitution occurs at C-1 instead of C-2.
MendelSet practice problem # 592 submitted by Matt on July 9, 2011.
Pyrrole undergoes eletrophilic aromatic substitution at C-2. Let's compare the resonance forms of EAS carbocation intermediates to see why this is the case. What do you think? Why C-2 and not C-3?
Electrophilic substitution at C-2 leads to a carbocation intermediate with three resonance forms, while substitution at C-3 leads to a carbocation intermediate with only two resonance forms.
The C-2 intermediate has more resonance forms than the C-3 intermediate, and so is more stable. Therefore, EAS occurs at C-2.
MendelSet practice problem # 591 submitted by Matt on July 9, 2011.
-NO2 is an EWG and a meta director. Let's draw an EAS reaction's cyclohexadienyl cation intermediates to demonstrate why this is true. I've started you off.
What's good about meta? What's bad about ortho/para?
Short answer: -NO2 is an meta director because the cyclohexadienyl intermediates that result from meta addtition are more stable than those that result from ortho or para addition.
Long answer: Addition at both the ortho or para positions lead to a cyclohexadienyl cation that contains two adjacent positive charges. This is very unstable, and so addition at the ortho or para positions is not favored.
The cyclohexadienyl cations that result from meta addition don't have this problem.
This will be the base for any substituent that has a positive charge (or partial positive charge). So -NO2, -C(O)R, -CF3, -NH3+, -CN, and -SO3R are all meta directors.
MendelSet practice problem # 590 submitted by Matt on July 9, 2011.
-OR is an EDG and an ortho-para director. Let's draw an EAS reaction's cyclohexadienyl cation intermediates to demonstrate why this is true. I've started you off.
What's good about ortho/para? What's bad about meta?
Short answer: -OR is an ortho/para director because the cyclohexadienyl intermediates that result from ortho and para addtitions are more stable than those that result from meta addition.
Long answer: The cyclohexadienyl intermediates from ortho and para addition include a resonance form where the oxygen is adjacent to the carbocation, while the cyclohexadienyl intermediates from a meta addition do not.
When an oxygen (or anything with a lone pair) is adjacent to a carbocation, it can share its lone pair and stabilize the positive charge. This is only possible when the electrophile adds to the ortho or para positions, so those positions are favored with -OR as a substituent. Hence, -OR is an ortho/para director.
Note that this logic holds for any substituent with a lone pair, so -OH, -OR, -NH2, -NR2, -F, -Cl, -Br, and -I are all ortho/para directors.
MendelSet practice problem # 589 submitted by Matt on July 9, 2011.