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| of bromination of an alkene and catalyzed bromination of an arene, which of the two is faster and why? |
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bromination of an alkene is faster, because the beginning state is less stable (more energy), and therefore closer to the high-energy carbocation intermediate.
benzene is so stable and low in energy that its bromination takes a lot longer because the difference between benzene and the carbocation intermediate is so much greater - even though the intermediate is more stable than that of the alkene carbocation, the benzene is MUCH more stable. |
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| why doesn't an addition product form when you brominate benzene? |
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| because it is MUCH less stable than the substitution product, which is still aromatic (the addition product with a double bromine is not.) |
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| what halogen can't be added via substitution to a benzene? |
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| what must be present in order to iodonate a benzene? |
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| what are the four limits of FC alkylation? |
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- ONLY alkyl halides work (no double or triple bonds allowed next to the halide)
- EWG and amino groups PREVENT THE REACTION
- and it's hard to stop after one sub (polyalkylation)
- skeletal rearrangement (you get mixed products) |
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| why do actors activate, and deactivators deactivate? |
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| anything that makes the intermediate ring carbocation more stable ACTIVATES by donating electrons. Anything that makes it less stable and takes away electrons DEACTIVATES. |
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| why does making the benzene ring electron poor deactivate it? |
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| because the carbocation intermediate is destabilized, raising the activation energy |
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| why are the meta and para positions more stable for substitution to a phenol? |
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| because in those positions the O can donate electrons to the ring with resonance. In the meta position, it can't. |
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| why do deactivators direct to the meta position? |
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| because the positive charge on the ring then can't be directly on where they are attached - so they can't draw the electrons out and destabilize as much |
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| how do we know that nucleophilic aromatic substitution doesn't take place by SN1 or SN2? |
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| can't take place by SN2, because the carbocation intermediate (after the halogen leaves spontaneously) would be too unstable and so it never happens. Can't take place by SN1 because the nucleophile can't come in through the back of the molecule because it's blocked by the ring structure. |
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| nucleophilic aromatic substitution can happen to a benzene that... |
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| has electron withdrawing substituents |
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| resonance-stabled, negatively charged (carbanion) intermediate after the -OH- attacks in nucleophilic aromatic substitution |
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| what are the three things that make a compound more acidic? |
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1. if the conjugate base (anion) is smaller - this means it's easier to solvate and therefore more stable
2. if it has larger atoms in it - a sulfur containing compound will be more acidic than an oxygen-containing one, for example. This is because the negative charge of the conjugate base will be dispersed over a larger volume.
3. If it has electron withdrawing groups, these will also stabilize the anion. |
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| why are phenols so much more acidic than alcohols? |
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| because their conjugate ion is resonance stabilized - the negative charge is delocalized. |
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| what sort of substituent group would make a substituted phenol more acidic? |
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| to make it MORE acidic you would stabilize the anion by adding an electron-withdrawing group |
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| how do you turn the name of an alcohol into the name of its anion? |
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| add -ate to the end of the whole name |
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| what happens when you attempt to react a gringnard reagent with X-[c]-F where F = -OH, NH, SH, or CO2H? |
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| the grignard reagent is protonated by the functional groups |
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| what reagent would you use to convert a TERTIARY alcohol to an alkyl halide? What about PRIMARY/SECONDARY? |
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HCl or HBr (SN1 mechanism) for tertiary;
SOCl2 or PBr3 (SN2 mechanism) for primary/secondary |
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| to perform an alkene dehydration with H3O+/THF, what is the problem with doing it on a primary alcohol and how do we get around it? |
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| there is a carbocation intermediate that is too unstable on a primary carbon, so instead we use POCl3/pyridine |
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| what kind of alcohol refuses to be oxidized? |
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| what is the difference between oxidizing primary, secondary, and tertiary alcohols? |
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primary: PCC will oxidize to an aldehyde, but CrO3 will oxidize all the way to a carboxylic acid
secondary: both PCC and CrO3 will reduce to a ketone
tertiary: will not react with either |
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| how would you protect a reactive functional group (such as an alcohol) in preparation for reacting a molecule with a gringnard reagent? |
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| use chlorotrimethyl silane to protect it |
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| what is the potential problem with using chlorotrimethyl silane to protect a functional group, and how is it averted? |
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| Si is tertiary - we might think this would prevent the SN2 reaction from happening, but since Si's bonds to the methyl groups are so much longer than C's would be, the reaction can happen anyway (not enough hindrance to stop it) |
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| how do you oxidize a phenol? |
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| by using Fremy's salt (potassium disulfonate) (KSO3)2NO, which gives you a =O arranged opposite each other on the ring. it's reversed with SnCl2 to give a double phenol (hydroquinone) |
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| in NMR, why do we never see splitting for an alcohol hydrogen? |
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| because it exchanges rapidly with the H in the solvent |
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| pyridinium chloro chromate |
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