CHEM 2211 1nd Edition Lecture 12 Outline of Last Lecture I Chapter 4 a Optically active b Cyclic and meso compounds c Naming with more than one asymmetric center Outline of Current Lecture I Chapter 15 Current Lecture NMR spectroscopy Helps to identify the carbon hydrogen framework of an organic compound We will be looking at how NMR works and two forms of it H NMR for hydrogens and C NMR for Carbons The number of signals in an H NMR The number of signals acquired in an H NMR are based on the environment around the Hydrogens Protons in the same environment are called chemically equivalent protons o These protons will only give one signal no matter how many Hydrogens there are Each set of chemically equivalent protons will produce a signal These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Let s look at some examples CH3CH2CH2Br The above compound will have 3 signals because there are 3 different chemically equivalent proton groups We can label them using A B C etc CH3CH2CH2Br A b c A is CH3 All 3 H are the same environment B is CH2 C is CH2 These two CH2 groups do not have the same environment because the first one is between a CH3 and a CH2 and the second one is between a CH2 and a Br Those are not the same environment so even though they are both are SP2 carbons with secondary Hydrogens they produce two different signals for their hydrogens Another example to show how environment matter A B A CH3CHCH3 Br This atom is symmetrical both CH3 atoms have the same environment they are next to CHBrCH3 In this atom we would only see 2 signals because the two CH3 atoms would produce one combined signal Chemical shifts Is how far the signal is from the reference signal The farther away the signal the more deshielded the hydrogens Shielding The greater the electron density of the environment in which the proton is located the more the proton is shielded So on H NMR spectrum the high the frequency the more deshielded the proton Scale of shielded protons Most shielded o CH3 o R CH2 R o C CH3 o O CH3 o C CH2 o Halogen C H o Benzene H Least shielded Another way to look at it is in this table
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