How can you tell when a reaction goes through the meisenheimer complex or benzyne? (The Meisenheimer complex is formed only in the presence of a resonance acceptor in an ortho- or para-position to the leaving group. Otherwise the reaction requires a strong base, harsher conditions, and proceeds through benzyne - GS).

What should we know regarding fluorescence? I recall discussing the topic in the past but am unable to find my notes regarding. (When a molecule absorbs the energy of light, it can cause a chemical reaction (photochemistry), or re-emit back as a light of different energy (fluorescence), or “wiggle out” by molecular vibrations to heat. Rigidity of a molecule reduces “wiggling out” the energy, which increases the efficiency of fluorescence. Therefore, we can control fluorescence by affecting the molecule by pH, presence of metals, DNA, etc. That is how many chemical sensors work. - GS).

Just to be sure, can we go over why cumulated is more stable than isolated? (Cumulated dienes are less stable than isolated, because they have fewer sigma-pi conjugations. - GS).

I’m still a little confused on why benzene will not react with bromine without a catalyst but cyclohexene will react readily without one? (This is an excellent demonstration of lower activity of aromatic compounds due to their stability. - GS).

Is the a difference in the mechanism of bromination and nitration? (Yes, only on the step of the electrophile preparation, which also leads to a different by-product. - GS).

What is an Allylic Carbon/Carbocation? (It is a carbon bonded to a C=C bond and everything (positive charge, atoms) attached to that carbon. If that C=C bond is a part of an aromatic ring, the term switches to “benzylic”. - GS).

Substitution is more favorable right because it restores aromaticity? (Yes, for the second step of aromatic electrophilic substitution. - GS).

Is there a way to predict the major product of an electrophilic substitution reaction for aromatic compounds without doing all of the steps in the reaction mechanism? (Yes. Determine if the existing substituent is ortho-para- or meta-director. - GS).

What is n in 4n+2? I understand that it is an integer but what number does it represent? Principal Quantum number? (No, it comes from the mathematical definition of an odd number of orbitals (2n+1). - GS). It is still not clear. (Each orbitals holds 2 electron, so the number of electrons = 2X(2n+1) - 4n+2. - GS).

Can a substituent be an electron donor and acceptor? (Yes. - GS). If it can, does the electron donor effects always override the acceptor effects? (No. Resonance effects override the inductive effects for activators or deactivators, except for F with its huge inductive effect. - GS).

How do you determine the order of reactivity when comparing cyclodienes, cis-dienes, and trans-dienes? (By their ability to produce the reactive (in the Diels-Alder reaction) s-cis conformation. - GS).

Can you please review how to determine if a compound is aromatic or not depending on the number of conjugated electrons?  I understand how to count the number of conjugated electrons but am unsure of how that relates to aromaticity. Thanks (An aromatic system must contain 4n+2 electrons (n=0,1,2,....) in a cyclic conjugation (uninterrupted by sp3-hybridized atoms. - GS).

Why is having a donor in the Meta position so unfavorable? (Since the sigma-complex has the formal positive charge distributed between ortho- and para-positions, the groups at those positions have the strongest effect on the reaction, so the donor speeds up ortho- and para-substitution much more than meta-substitution. - GS).

How do you identify a “pure acceptor?” How are these different than regular acceptors? (They have no electron-donating effects. - GS).

What does resonance have to do with the weak influence of the donor in the meta position? (Since the sigma-complex has the formal positive charge distributed between ortho- and para-positions, the groups at those positions have the strongest effect on the reaction. - GS).

To clarify- if you have an incoming substituents in the metha or ortho position, there is a strong influence and if you have an incoming substituent in the ortho position there is little influence? (No. Since the sigma-complex has the formal positive charge distributed between ortho- and para-positions, the groups at those positions have the strongest effect on the reaction.The incoming substituent has almost no influence, because it attaches to an sp3-carbon with no formal charge - GS).

Just to clarify something with more substituents is more stable than something with less? (That is true for C=C bonds and carbocations. There are other examples. - GS).

So far we learned about ortho and para positions but do compounds ever go to the meta position and if so when does that happen? (Electrophiles always enter the meta-position,, but preferably - when the existing group has no donating effects. - GS). Also can you go over why meta isn’t as favorable as ortho or para please, I’m still confused why it isn’t? (The sigma-complex has the positive charge distributed over the ortho- and para-positions, so it is stabilized better by ortho- and para- electron-donating substituents, which makes the ortho- and para-orientation preferred. Ortho- and para-substituents with no electron-donating effects are are having trouble withdrawing electrons from the positively-charged ortho- and para-positions, which makes ortho- and para-orientation unfavorable, so the meta-substitution is preferred. - GS).  

How do you tell if something has a resonance electron withdrawing effect? (There is a X=O bond or empty d-orbital (like in Cl, Br, and I) in conjugation with the benzene ring - GS).

Can you go over another example of the pi-complex and sigma-complex mechanisms? I don’t really know the distinction between the two quite yet. Thanks!! (They are steps of the same mechanism of aromatic electrophilic substitution: http://ochem.orgfree.com/SEAr.html  - GS).