Lecture 5 - CH 310 N T Th 2-3:30 LECTURE 5 Textbook...

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Unformatted text preview: CH 310 N T Th 2-3:30 LECTURE 5 Textbook Assignment: Chapter 13 finish Homework (for credit): POW 2 postedToday's Topics: Equivalent protons; splitting and integration Notice & Announcements: Conflict Exam List was emailed Kyle Williams Mon: 3-4 WEL 5.330 Cubicle "B" Cubicle "B" Cubicle "A" Lisa Alexander Mon: 10-11 Dan Propheter Tues: 8-9 Matt Sanderson Tues: 12-1 Organic Lecture Series spins = -1/2 E0 = E E B0 Applied spins = +1/2 If a sample is irradiated with electromagnetic radiation of energy E0, exactly equal to E, that energy can be absorbed by some of the nuclei in the spin = +state. The result is "spin flip"nuclei that absorb energy end up in the higher energy spin = state. Where E0 = E; spin "flip" occurs and the nuclei are in "resonance" 2 Organic Lecture Series NMR Spectrum 1H-NMR spectrum of methyl acetate O H3C O CH3 O H3C O CH3 Downfield: the shift of an NMR signal to the left on the chart paper Upfield: the shift of an NMR signal to the right on the chart paper 3 1 Organic Lecture Series Equivalent Hydrogens Equivalent hydrogens: have the same chemical environment a molecule with 1 set of equivalent hydrogens gives 1 NMR signal O CH 3 CCH 3 Propanone (Acetone) H3 C ClCH 2 CH2 Cl H3 C 1,2-Dichloroethane Cyclopentane C C CH3 CH3 2,3-Dimethyl2-butene 4 Organic Lecture Series Equivalent Hydrogens a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set Cl CH3 CHCl 1,1-D ich loroeth ane (2 signals ) O Cyclop entan on e (2 s ign als) Cl C C H H (Z)-1-Ch loroprop ene (3 signals) Cyclohexen e (3 signals) CH3 5 Organic Lecture Series Signal Areas Relative areas of signals are proportional to the number of H giving rise to each signal Modern NMR spectrometers electronically integrate and record the relative area of each signal 6 2 Organic Lecture Series Chemical Typ e of Hydrogen Shifts (CH3 ) 4 Si 1H-NMR RCH3 RCH2 R R3 CH R2 C=CRCHR2 RC CH ArCH3 ArCH2 R ROH RCH2 OH RCH2 OR R2 NH O RCCH3 O RCCH2 R Ch emical Shift () 0 (by definition) 0.8-1.0 1.2-1.4 1.4-1.7 1.6-2.6 2.0-3.0 2.2-2.5 2.3-2.8 0.5-6.0 3.4-4.0 3.3-4.0 0.5-5.0 2.1-2.3 2.2-2.6 Typ e of Hyd rogen O RCOCH3 O RCOCH2 R RCH2 I RCH2 Br RCH2 Cl RCH2 F ArOH R2 C=CH2 R2 C=CHR ArH O RCH O RCOH Ch emical Shift () 3.7-3.9 4.1-4.7 3.1-3.3 3.4-3.6 3.6-3.8 4.4-4.5 4.5-4.7 4.6-5.0 5.0-5.7 6.5-8.5 9.5-10.1 10-13 7 Organic Lecture Series Chemical Shift - 1H-NMR 8 Organic Lecture Series Chemical Shift Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds Electron eg- Chemical Electronegativity CH -X ativity of X Shift () 3 CH3 F CH3 OH CH3 Cl CH3 Br CH3 I (CH3 ) 4 C (CH3 ) 4 Si 4.0 3.5 3.1 2.8 2.5 2.1 1.8 4.26 3.47 3.05 2.68 2.16 0.86 0.00 9 3 Organic Lecture Series Chemical Shift The higher s character, the greater the deshielding Type of Hydrogen (R = alkyl) RCH3 , R2 CH2 , R3 CH R2 C=C(R)CHR2 RC CH R2 C=CHR, R2 C=CH2 RCHO N ame of Hydrogen Alk yl Allylic Acetylen ic Vin ylic Ald ehydic Chemical Sh ift () 0.8 - 1.7 1.6 - 2.6 2.0 - 3.0 4.6 - 5.7 9.5-10.1 10 Organic Lecture Series Chemical Shift Diamagnetic effects of pi bonds a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a smaller value a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield (to the left) to a larger value Type of H RCH3 RC CH R2 C=CH2 Chemical N ame Shift () Alk yl 0.8- 1.0 Acetylenic 2.0 - 3.0 Vin ylic 4.6 - 5.7 11 Organic Lecture Series Chemical Shift magnetic induction in the pi bonds of a carboncarbon triple bond (Fig 13.9) Triple bond will shield this proton 2.0-3.0 12 4 Organic Lecture Series Chemical Shift magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.10) 4.6-5.7 13 Organic Lecture Series Chemical Shift magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11) 6.5-8.5 14 Organic Lecture Series Signal Splitting & the (n + 1) Rule Peak: the units into which an NMR signal is split; doublet, triplet, quartet, etc. Signal splitting: splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens (n + 1) rule: if a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks 15 5 Signal Splitting (n + 1) Organic Lecture Series 1H-NMR spectrum of 1,1-dichloroethane For thes e hyd rogens, n = 1; their signal is s plit into (1 + 1) = 2 p eaks; a d ou blet CH3 -CH-Cl Cl For this h yd rogen, n = 3; its s ignal is split into (3 + 1) = 4 p eaks; a q uartet 16 Signal Splitting (n + 1) Organic Lecture Series Problem: predict the number of 1H-NMR signals Problem O and the splitting pattern of each O (a) CH3 CCH2 CH 3 NMR a H3 C O c C H2 CH3 b O H 3C (b) CH3 CH2 CCH2 CH3 NMR a a CH3 C H2 C H2 b O b O (c) CH3 CCH( CH3 ) 2 NMR a H3C b CH CH3 c CH3 c 17 Organic Lecture Series O a singlet n = 0 H3C CH3 C H2 c triplet n=2 a b quartet n=3 c b 18 6 a triplet n=2 O H3C C H2 C H2 CH3 Organic Lecture Series b quartet n=3 b a 19 Organic Lecture Series a singlet n=0 H3 C O b septet n=6 CH3 CH CH3 a c doublet n=1 c b 20 7 ...
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This note was uploaded on 12/08/2008 for the course BIO 365R taught by Professor Draper during the Fall '08 term at University of Texas.

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