Lecture 12 Saturday February 7 2015 10 08 AM less shielded higher frequency more shielded lower frequency Acidic protons can in NMR show up at various chemical shifts and NMR is not particularly useful for figuring those out Signals are often broad Shielding ability of a group depends on its electronegativity i e its ability to withdraw electrons away from the nucleus The large deshielding effect of alkenes and arenes on the protons that are attached to sp2 hybridized carbons is attributed to the induced magnetic field that results from the current pi electrons create when placed in the external magnetic field The direction of this induced magnetic field at the space occupied by protons is in the same direction as the external field Pi electrons of alkynes also create the induced magnetic field but the protons feel it as shielding it has opposite direction from the external field Most importantly all the shielding and dishielding effects are additive For any proton what it feels is a sum of the external magnetic field and all the shielding and dishielding effects of electrons that are around For export Page 1 Lecture 12 Saturday February 7 2015 10 22 AM Combination where all 3 red protons are parallel is more deshielding than the other combinations That s why this combination gives a signal with highest chemical shift Each of the 3 combinations creates the same overall field felt by the blue proton and all 3 give 1 signal For export Page 2 Reflects the probability of the particular state Lecture 12 Saturday February 7 2015 10 31 AM Protons with a same chemical shift or chemically equivalent protons do not split each other ethane For export Page 3 Lecture 12 Saturday February 7 2015 10 40 AM Doublet of doublets When there are multiple types of protons around a proton of interest they all independantly split that proton In this case Ha will be split by Hb into a doublet Then each peak of the boublet is further split into a doublet The end result is that there will be 4 lines but all of the same area All peaks are equally likely This type of splitting pattern is called doublet of doublets In this case Ha and Hb are diastereotopic and split each other However their chemical shifts are very similar As a result the situation becomes more complicated and instead of getting two doublets we get something that looks like a quartet This splitting pattern is called AM doublet If chemical shifts are the same we get a singlet Remember protons with the same chemical shift do not split each other In this case the question is what would we see by NMR One way so think about this is to think about how long it takes NMR to see a proton relative to the time that is needed for the chair flip the equilibrium above It turns out that chair flip is much faster than the NMR and chair flip happens many times while NMR is registering protons As a result NMR sees the average state of protons over time Ha and Hb have the same average chemical shifts there is no coupling and we get only one peak in proton NMR of cyclohexane Overall when time scale of an equilibrium is shorter than the time scale of the NMR experiment we see an average of the possible states If the scale of the change is much longer than the time scale of the NMR experiment we register each state individually And if time scales are close we can get broad signals For export Page 4 Lecture 12 Saturday February 7 2015 10 56 AM In carbon NMR we observe 13C isotope which in nature occurs with abundance of 1 1 The rest is 12C isotope which does not produce a signal in NMR In carbon NMR we care about the number of peaks and about their chemical shifts Number of peaks tells us how many chemically non equivalent carbons are there Their chemical shifts tell us about the environment in which those carbons find themselves In 13C NMR there is no splitting and no integrals Integrals can be obtained but it is technically a hard thing to do For export Page 5 Lecture 12 Saturday February 7 2015 11 03 AM Chemical shifts work in the same way as in proton NMR Electron withdrawing groups close to the carbon deshield it by withdrawing elecrtons away from the nucleus Groups with Pi electrons create induced magnetic field that has a similar effect as in proton NMR and explains why sp2 hybridized carbons have very high chemical shifts 110 145 For export Page 6 Lecture 12 Saturday February 7 2015 10 35 AM This part I did not mention in the lecture but I will at the beginning of the next one Area under the signal Corresponds to the number of protons that can absorb at that frequency Integral gives us the relative ratio of the number of protons that contribute to each signal Notice that isopropyl group will always give a doublet for methyl groups and septet for the proton in the middle with integration of 6 to 1 Ethyl group will give a triplet for the methyl group and a quartet for the two protons Relative integration will be 3 to 2 For export Page 7 Lecture 12 Saturday February 7 2015 10 55 AM try to assign all the peaks which peak belongs to which proton For export Page 8 Lecture 12 Saturday February 7 2015 11 06 AM Based on the given molecular formula C3H8O2 and proton and carbon NMRs propose the structure of the compound For export Page 9