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Biology Question for 6R - asking for help please

Discussion in 'MCAT: Medical College Admissions Test' started by qinnu, Apr 7, 2004.

  1. qinnu

    qinnu Member
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    Question 207

    It was asking about which of graph is the NMR spectra for
    CH3OC(CH3)3.

    The key is A(the one 3 is down field). I chose B(the one 9 is down field). Oxygen is electron donating. The 9 hydrogen on
    -C(CH3) is further from the oxygen atom. Hence have less electron on them, and then down field. I just don't get it how could the key be A. Could someone tell me what I am missing here.

    Question 190
    It was asking which change would not interfere with the repeated transmission of an impuse.

    The key is C. I chose A(add cholinesterase blocker). I don't understand how could it be A.

    Thanks a lot

    Qinnu
     
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  3. jhk43

    jhk43 Senior Member
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    Question 207

    It was asking about which of graph is the NMR spectra for
    CH3OC(CH3)3.

    The key is A(the one 3 is down field). I chose B(the one 9 is down field). Oxygen is electron donating. The 9 hydrogen on
    -C(CH3) is further from the oxygen atom. Hence have less electron on them, and then down field. I just don't get it how could the key be A. Could someone tell me what I am missing here.


    in this case of shielding and deshielding electrons, oxygen is electronegative or electron withdrawing. I guess the electrons in ether Oxygen are localized (as opposed to an ester with delocalized e's resonanced to the carbonyl pi orbitals). can someone confirm this?


    Question 190
    It was asking which change would not interfere with the repeated transmission of an impuse.

    The key is C. I chose A(add cholinesterase blocker). I don't understand how could it be c..


    The keyword is "REPEATED TRANSMITTION", ie 1,2,3,4...cholinisterase blocker would be a tetanus or one long ass transmittion.

    More receptors -->increase in pulsed transmission (not deconstructivly interfere with the constant pulse)
     
  4. rugbyboy

    rugbyboy Member
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    You are in luck I just took 6R, correction, got raped by 6R. I got them both right.

    1) 207 was simple enough, I suggest you review NMR theory. I remembered that ch3 always hovers around 0 unless it is bonded to an ewg.

    2) 190 This one was tricky. I am assuming you chose A and are wondering why C is correct. It said " repeated transmission of an impulse at the neuro muscular junction". Everything else would interfere with this process, but if you increase the number of receptors, the junction would become a little sensitive, but it won't interfere with "repeated transmission". If you add cholinesterase blocker you are interfering with "repeated transmission" the acytocholine once discharged would remain there, and a second transmission cannot occur.
     
  5. rugbyboy

    rugbyboy Member
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    I think I just added a redundant post :-( oh well!.
     
  6. qinnu

    qinnu Member
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    If you guys are using EK review book, in Chapter 2 Benzene section, they listed -O as electron donating. The audiomosis CD, Jordan and John mentioned that when you do NMR analysis, the same electron donating/withdrawing rules apply there. I understand Oxygen is very electronegative. However I feel we also have to consider the fact it has lone electron pair. So, I am still not quite convinced about the key.

    qinnu
     
  7. CanIMakeIt

    CanIMakeIt Fellow
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    I think you are getting confused by "downfield" and "upfield" with shielding of electrons ..... As rugbyboy said.... review the NMR basics and you should be golden ..... the key is correct and it makes sense if you get the language right ..... it should not take more than few minutes of reading. ..... by ur explanation, it seems you understand what is going on, just read up on downfield and upfield in terms of NMR scale

    All the best

    Tick tock tick tock
     

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