OChemNMR

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victimsofadown
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186239
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OChemNMR
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2012-11-30 11:53:56
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OChemNMR
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OChemNMR
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  1. What are the characteristics of 1H NMR spectrum, and what they represent (brief)?
    • Number of signals: number of electrons with different electronic environments
    • Chemical shift: based on electronic environment of protons
    • Integration: number of protons giving rise to the signal
    • multiplicity: Number of neighboring protons
  2. What are the rules regarding number of signals in an NMR
    • Number of signals indicates the number of protons with a different electronic environment
    • If protons are homotopic or enantiotopic they are equivalent (a single signal) - can be interchanged by rotation or reflection
    • The 3 protons of a CH3 group are always equivalent
    • The 2 protons of a CH2 group are equivalent if the compound has no chirality centers
    • The 2 protons of a CH2 group are not equivalent if the compound has chirality centers
  3. Values to memorize regarding induction and other chemical shifts.
    • Methyl (CH3): ~.9ppm
    • Methylene (CH2): ~1.2ppm
    • Methine (CH): ~1.7
    • Higher electronegativity = greater shift
    • Beta protons affected 1/5 as much as alpha protons
    • Oxygen of alcohol or ether (ROH, ROR): +2.5
    • Oxygen of ester (R(C=O)OR): +3
    • Carbonyl group (C=O): +1
    • Aromatic methyl (arene-CH3): ~2.5
    • Alkyl halide (R2-XH): ~2-4
    • Aryl (arene-H): ~6.5-8
    • Aledhyde (R(C=O)H): ~10
    • Carboxylic acid (RCOOH): ~12
  4. What is the process for determining integration?
    • Measure the step curve, or the area given
    • Divide all areas by the smallest, this gives the relative number
    • Match the relative number to the given molecular formula by multiplying all by same integer.
  5. Describe the methods behind coupling
    • Singlet, doublet, triplet - based on nonequivalent neighboring protons
    • Splitting is most common 2-3 sigma bonds apart, nothing highter than 3 sigma bonds apart will split
    • Intensity of the peaks follows the pyramid
  6. Coupling constant and familiar patterns
    • J value: the distance between peaks of a splitting pattern.  Will be equal between protons that split eachother.
    • ethyl group pattern: triplet integration 3 upfield from quartet integration 2
    • isopropyl group pattern: doublet integration 6 upfield from septet integration 1
    • tert-buytl group pattern: singlet with integration 9
  7. Complex splitting
    • When a proton has two different kinds of nonequivalent neighboring protons
    • Example - if Hb is split by 3 Ha and 2 Hc...
    • if Jab >> Jbc the signal will appear as a quartet of triplets
    • If Jbc >> Jab the signal will appear as a triplet of quartets
  8. How to analyze and interpet NMR spectra
    • inspect the molecular formula - calculate the degrees unsaturation.  4+ = aromatic ring
    • consider the number of signals and their integration (clues about symmetry)
    • analyze each signal (chemical shift, integration, multiplicity) and then draw fragments consistent with each signal. 
    • assemble the fragments into a molecular structure
  9. All 13C NMR data
    • Requires more sample
    • Only number of signals and chemical shift is considered
    • 220 - 0 ppm
    • Each signal is C w/ unique electronic environment
    • 0-50: R2-C-R2
    • 50-100: C=C, C-N, C-O
    • 100-150: C=C, aromatic-R
    • 150+: C=O

    • DEPT-90: only CH signals
    • DEPT-135: CH3 and CH give + signals, CH2 give - signals

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