The nitrosyl cation is shown below. Also shown are several proposed resonance arrows, only one of which is correct.
Draw the resonance forms that would follow from each set of arrows, and include formal charges. Which one is the correct resonance form? Explain your reasoning.
When evaluating possible Lewis structures the most important rule to follow is the octet rule: every atom must be surrounded by 8 and only 8 electrons (atoms in period 3 or below on the periodic table have d orbitals and so can contain more than 8 electrons, but that exception does not apply here).
Do any of the resonance forms drawn have atoms that break the octet rule? Yes- they all do! Except for resonance arrows in D. So D is the correct resonance form.
MendelSet practice problem # 755 submitted by Matt on July 27, 2011.
Draw in the arrows to show the electron flow and resonance forms in the nucleophilic aromatic substitution reaction below.
Note: Depending on the textbook, nucleophilic aromatic substitution is referred to as NAS, SNAr, or addition-elimination.
SNAr is sort of like SN2, except the leaving group doesn't leave right away; a tetrahedral intermediate is formed first.
The trick with SNAr (or NAS, addition-elimination, etc.) is to draw resonance forms that stabilizes the negative charge that forms. That's why EWG increase the rate of SNAr (they stabilize negative charges).
MendelSet practice problem # 611 submitted by Matt on July 10, 2011.
Draw a mechanism for the nucleophilic aromatic substitution (SNAr) reaction below. Show all resonance forms of the intermediate.
For SNAr to work you need to have electron withdrawing groups (EWG) either ortho or para to the leaving group.
This molecule has two leaving groups (chlorines), but only one chlorine has EWG ortho/para to it. So that's the carbon where the nucleophile (NH3) will attack.
Why do EWG need to be ortho/para? Because that's the only way for the negative charge to be stabilized by resonance. Try it- if you have the NH3 attack the other carbon with a chlorine, you will not be able to draw a resonance form that places the negative charge on one of the EWG.
MendelSet practice problem # 612 submitted by Matt on July 10, 2011.
Let's go through a benzyne reaction (also called elimination-addition).
In the reaction below, the strong base (NaNH2) will form a benzyne intermediate, which when forms either ortho nitroaniline or meta nitroaniline.
Used curved arrows to show the formation of each intermediate and the final products.
First the -NH2 acts as a base to eliminate a leaving group and form Benzyne. Then the -NH2 acts as a nucleophile and attacks the benzyne, and the "leaving group" is the triple bond.
MendelSet practice problem # 615 submitted by Matt on July 10, 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.
-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.
First, let's form a Grignard reagent. Then, let's elminate to form benzyne.
Halogens get more reactive towards Grignard formation as we go down the periodic table, so Mg0 will form a Grignard with Br before F.
Grignards behave like negatively charged carbon atoms- very unstable. So even though fluorine is a poor leaving group, the negative carbon will cause it to eliminate to form benzyne.
MendelSet practice problem # 616 submitted by Matt on July 10, 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.