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Electron DONATING and WITHDRAWING

PiBond

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    Electron donating groups generally have a lone pair on the atom directly bonded to the aromatic ring. Examples include: OH, NR2, OR, NHCOR (amides), OCOR (esters), and alkyl groups.

    Electron withdrawing groups make it more difficult to introduce new groups onto the ring. Examples include: COR, NO2, CN, CONH2, and NH3.

    Electron withdrawing groups are meta-directors, electron donating are ortho-para directors.

    How can you differentiate between electron donating and withdrawing groups? How do you tell the difference? What are some examples of each group? How can you tell which group does what? THanks
     

    TurkSurg

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      Those are some good examples listed by PiBond. Be careful with halides. EDG are ortho/para directors. EWG are meta directors. There's one exception: Halides are electron withdrawing BUT they are ortho/para directors.

      How can you differentiate between electron donating and withdrawing groups? How do you tell the difference? What are some examples of each group? How can you tell which group does what? THanks
       
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      PiBond

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        Strongest activators: NR2 and OH
        Next/Moderate activators: OR, amides, esters, alkyl groups
        -remember that the atom with the lone pair in the above functional groups is directly attached to the aromatic ring

        Strongest deactivators: NR3, NO2, CN
        Next/Moderate deactivators: SO3H, COOH, COOR, CONH2 (some are the same as above but the functional groups are attached by the carbonyl carbon where there is no lone pair--thus it is the carbonyl group that is withdrawing the electron density)
        Weak Deactivators: halides like Cl, Br, I
         

        BennieBlanco

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          I would also like to know the RANK of the different groups.
          Id like to know which groups donate more than others..... and which groups withdraw more than others... rank-wise
          THanks

          You should aim to understand why one is stronger than another.

          Then you don't need to memorize lists, your "list" is understanding why it is EDG or EWG and then why functional groups are donating or withdrawing more or less.
           

          BennieBlanco

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            thanks, so why are some groups stronger than others? could you please elaborate? how can i find out?

            Short answer:

            Electrons (lone pairs) and resonance structures. Draw each group out, you will notice that all the donating group have an electron lone pair on the atom connected to the ring (or whatever). The withdrawing groups will typically have a positive charge on the atom connected to the ring or structure.

            Methyl groups are slight activators because the hydrogen atoms have low electronegativity, therefore carbons will have a slight density of electrons.

            Think electron density... which of these groups do you feel would have a lot of electrons on the connecting atom and which will have low electron density.

            That is really all it is.

            If you're not understanding some of this then Ochem becomes difficult, throughout your 2 semesters you constantly are building on fundamental ideas. This application is where the learning occurs and you understand the nature of Ochem. If you try to memorize lists then your brain will explode.
             

            PiBond

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              This is beyond the scope of the MCAT...

              The ring activating/deactivating effects of substituents boils down to resonance and, to some degree, the inductive effect.

              We already said that ring activators have an unshared pair of electrons on the atom bonded directly to the aromatic ring. The reason they are called activators is because they stabilize the positive charge of the cation intermediate when the ring acts as a nucleophile when it introduces new substituents onto the ring. Stabilizing the intermediate is a reoccurring theme in organic chemistry that should appear in your prep book.

              Thus, NR2, OH, OR, NHR, NH2 are strongly activating because they have lone pairs that can directly stabilize the intermediate. I would bet that NR2 is slightly more activating than NH2 due to the inductive effect of the alkyl groups attached--at least that's what occurs in basicity trends.

              Amides, Esters, and other substituents that have carbonyl groups are slightly less activating because of the carbonyl group which withdraws electron density. Yet, because of the available lone pairs these groups are still considered activating.

              Strong deactivators are so because of their electronegativity and lack of a lone pair to donate--thus they take away electron density. The strongest deactivators occur when highly electronegative groups are attached to carbon (such as Fluorine and chlorine). Also, the strongest deactivators result when nitrogen has a positive charge on it (ie NH3+) or when a resonance structure containing nitrogen (ie NO2) contains a contributor where nitrogen has a positive charge. Because nitrogen is very electronegative, the positive charge will cause it to "suck" more electron density away from the aromatic ring, thus further deactivating it.

              sorry, i still dont understand
              could you please elaborate
              thanks
              whats the reason behind the ranking?
               

              PiBond

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                i am very confused
                you say that activating groups have lones pairs... but COOH has lone pairs but it is a deactivator?
                how does that make sense?
                why are methyl groups deactivators?

                Carboxylic acids are deactivators because the atom directly bonded to the aromatic ring is the carbonyl carbon...there's no lone pairs on the carbon. Thus, the carbonyl group is electron withdrawing.

                Methyl group are activators, not deactivators, due to the inductive effect. The donate a small amount of electron density to the aromatic ring and thus make it easier to introduce new substituents.
                 

                BennieBlanco

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                  i am very confused
                  you say that activating groups have lones pairs... but COOH has lone pairs but it is a deactivator?
                  how does that make sense?
                  why are methyl groups activators?

                  I am not trying to be mean, but did you take 1st and 2nd semester Ochem?

                  How well did you study if you did.

                  The explanations you are being given are pretty clear and may require a bit of thinking but I think you may have too many holes in your Ochem game for the MCAT.

                  Guy buy Berkley Review and take your time through the MCAT section.
                   

                  JohnnyBravo

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                    How can you differentiate between electron donating and withdrawing groups? How do you tell the difference? What are some examples of each group? How can you tell which group does what? THanks

                    From the sounds of it, either you never took O-chem, or you memorized your way to decent grades with no comprehension. Unfortunately, there are kids at my school who get A's via the latter format by finding old tests.

                    You will have to get TBR Books. Also, get TPR BS Book. Read through that first and do the within chapter exercises. Then go through TBR O-chem and you'll be good. Definitely don't take any test before March. TAke 2 months and make o-chem solid. Based on your questions, I surmise right now you would miss 90% of the o-chem questions which limits you to a 9 or 10 at best.
                     

                    PiBond

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                      Which one is more electron donating: R group or Hydrogen? Why?

                      Thus, which would be a stronger withdrawer: COOR or COOH?

                      R group is more electron-donating...hydrogen has no electron density to give.

                      The MCAT wouldn't ask you to differentiate between these two because they have the same effect on the ring and are extremely close in structure. But I'm pretty sure COOH would be more withdrawing because it lacks the R group
                       

                      PiBond

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                        What do you mean stability of the intermediate
                        could you please explain
                        thanks

                        The double bonds acts as a nucleophile and attacks an electrophile. A carbocation intermediate results--this is electron deficient. Carbocations are stabilized by electron-donating substituents (the R groups in your example help stabilize the carbocation). Thus their stability is ranked 3 > 2 > 1

                        When CH2=CH2 acts as a nucleophile a primary carbocation intermediate results. This is highly unstable and highly reactive because it is not substituted.
                         
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