Review 10 Slides e2a 2013 - Chemistry E-2a Review Lecture Week 10 December 9 2013 & HowdoesNMRwork?( smallmagnets)(NMR)th

Review 10 Slides e2a 2013 - Chemistry E-2a Review Lecture...

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Chemistry E-2a Review Lecture, Week 10 December 9 , 201 3
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Infrared   Spectroscopy   &   Nuclear   Magnetic   Resonance   Spectroscopy   How   does   NMR   work?   Certain   atomic   nuclei   possess   a   nuclear   spin   (think   of   the   nuclei   behaving   like   small   magnets)   and   when   applied   to   an   immense   magnetic   field   (NMR)   there   are   2   possible   energy   states   (see   page   9,   this   is   a   great   illustration).    The   larger   the   applied   field,   the   larger   the   energy   difference.    Thus,   if   we   apply   radiation   of   the   correct   frequency   we   can   promote   nuclei   in   the   lower   spin   state   to   the   higher     energy   will   be   absorbed,   so   we   can   measure   the   NUCLEAR   MAGNETIC   RESONANCE   SPECTRUM!    NMR   is   truly   remarkable   because   it   allows   us   to   see   protons   in   organic   molecules.    With   respect   to   the   final   exam   your   primary   focus   should   be   how   to   solve   problems   based   on   IR   Data   and   the   NMR   spectrum.     Guidelines   for   Solving   Spec   Problems   for   the   Final   Exam:   The   general   idea   is   you   need   to   predict   the   structure   of   a   molecule   based   on   three   pieces   of   information/data.    This   will   always   be   provided   on   the   exam;   (1)   the   molecular   formula,   (2)   the   IR   Data,   and   (3)   the   proton   NMR   spectrum.    The   Molecular   formula   allows   you   to   calculate   the   index   of   unsaturation.    The   “U”   value   is   the   number   of   PI   bonds   and/or   rings.    The   IR   data   will   allow   you   to   determine   the   functional   groups   that   are   present   (it   is   also   wise   to   make   note   of   the   functional   groups   that   are   absent   as   well).    The   NMR   data   will   reveal   chemical   shifts,   integrals,   and   spin spin   coupling.    Recall   from   lecture   that   chemical   shifts   are   due   to   electron   shielding.    Shielded   protons   will   show   up   further   upfield   on   the   NMR   spectrum   and   de shielded   protons   will   show   up   downfield   ( this   “de shielding”   is   often   due   to   electronegative   groups ).    In   addition   protons   attached   directly   to   sp2   carbons   absorb   at   a   lower   field   (down field).    Protons   attached   directly   to   sp3   carbons   will   have   shifts   further   up field.    Thus   signals   on   the   left   side   of   the   spectrum   are   produced   at   lower   field   strength   (downfield     requires   a   weaker   magnetic   field   to   induce   resonance)   whereas  
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  • Winter '14
  • Molecular formula, Harshad number, chemical shifts

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