Organic Chemistry H-Nuclear Magnetic Resonance Spectroscopy NMR 8 Min NMR machine Royal RSC http://www.rsc.org/learn-chemistry/resource/res00001041/ spectroscopy-videos#!cmpid=CMP00001774 Or 8 Min NMR machine https://www.youtube.com/watch?v=YDgjbyGm6zM
Chem Study https://www.youtube.com/watch?v=mNaipM3WEO0 2 HYDROGEN-NMR (OR H-NMR) The molecular formula and functional groups are determined using the mass spectrum and the IR spectrum. The 1H NMR spectrum is used to put the entire structure together. It can tell us:
1) HOW MANY - Hydrogen's (Protons) we have 2) WHERE - Hydrogen's (Protons) are located SO we can: tell a molecules structure (C3H6O) Is it Methoxyethane (aka, ethylmethylether) or 1-propanol or 2-propanol We will study 2 nuclei: 1H and 13C 3 Nuclear Spin A nucleus with an odd atomic number has a nuclear spin. The spinning
charged nucleus generates a magnetic field. 1H and 13C have odd numbers ODD, so POSSESS SPIN 1 H 1 EVEN, so DON T POSSESS SPIN 12 C 6
13 C 6 Chapter 13 => 4 Hans Christian Orsteds Discovery A moving charge generates a magnetic field
When placed in an external field, spinning protons act like bar magnets. 5 Magnetic Shielding e - create their own magnetic field e- surround protons (H+) and shield them from the external field. Polarity also affects this shielding, when it strips protective hydrogens away Polar Less electrons
6 The NMR Spectrometer 7 => The NMR Graph More energy needed Less energy needed
8 => Nuclear Shielding and 1 H Chemical Shifts What do we mean by "shielding?" What do we mean by "chemical shift?"
Proton NMR Chemical Shift Values TMS More Energy Polar De shielded Electron Poor for H+ LESS Energy NON-Polar Shielded Electron RICH for H+
10 Tetramethylsilane Downfield from what????? Silicon is less electronegative than carbon, TMS protons are highly shielded. Organic protons absorb downfield (to the left) of the TMS signal. All the protons are chemically equivalent - PRODUCES A SINGLE PEAK
Non-toxic liquid - SAFE TO USE Inert - DOESNT REACT WITH COMPOUND BEING ANALYSED THE BEST SHEILDING ANYWHERE. WHY? Carbon actually is MORE electronegative than Silicon, thus all the electrons are pulled into the protons (H+) and it is well shielded from external magnetic fields. The molecule contains four methyl groups attached to a silicon atom in a tetrahedral arrangement. All the hydrogen atoms are chemically equivalent. THUS only 1 signal is given off.
CH3 H3C Si CH3 CH3 11 Chemical Shift
Measured in parts per million (ppm) Ratio of shift downfield from TMS (Hz) Called the delta scale ( ppm). ppm). Based on polarity, the more polar atoms strip away the 1H shielding e-, leaving them naked to the external magnetic fields effects. Unprotected the H+ are held well, thus we need more RF energy to make them resonate. The more polar the more downfield shifted from TMS a molecule is (R-COOH) ppm). 10-12 ppm, while CH3 is 0.5 12 Signals
What is the patttern? More electronegative atoms deshield more and give larger shift values. Additional electronegative atoms cause increase in chemical shift. 13 Electronegative substituents Deshield Methyl groups DO YOU SEE A PATTERN HERE?
CH3CH3 2 CH3N(CH3)2 1 0 CH3Si(CH3)3 Does the Chemical Shift Make Sense?
Thus R-X hydrogens = 3 to 4 ppm , R-CH3 is 0.9. What shifted it? The highly electronegative halogen desheilds it. Now we need more energy from RF to resonate it 15 Aldehyde Proton, 9-10 In C=O the electronegative oxygen pulls shielding Electrons away from protons (H+) thus more energy has to be put into these protons to make them resonate. You will find C=O (carbonyls) shifted FAR downfield from TMS Please locate on your chart the chemical shift
Electronegative oxygen atom => 16 Simple Aldehyde Chemical Shift Propanal Do these values make sense for the chemical shift? Why is the methylene (CH2) and shifted up field, by a factor of 1ppm? Notice the CH3 group is not affected because it is too far away from the electronegative carbonyl. Why are there 3 peaks?
17 Aromatic Protons Shift from TMS, 7-8 Benzenes electrons are pulled inwards towards the center of the ring and thus away from any protons (H+) Please locate on your chart the chemical shift Chapter 13 18 Ethylbenzene
Equivalent Hs on the aromatic CH3 CH2, shifted up from 1.2 To 2.5 19 Vinyl Protons, 5-6
C=C deshield those groups attached to it, they pull the electrons away. Please locate on your chart the chemical shift 20 => Identifying the O-H or N-H Peak Singlet Arises because the H on the OH, rapidly exchanges with protons on other molecules (such as water or acids) and is not attached to any particular oxygen long enough to register a super strong as expected signal.
21 O-H and N-H Signals These both are a very special case and quite unexpected in their inability to shift downfield away from TMS. Oxygen (and N) are very electronegative and should deshield greatly. The problem is they hydrogen bond with the water and anything else around them so they actually leave the molecule entirely. These proton exchanges weaken the expected effect, so O-H hard to predict..broad range 1 to5.5 ppm. 22
Carboxylic Acid Proton, 10+ TMS 23 => Number of Signals Chemical Shifts.
Why is the aCH3 shifted to almost 4 ppm while cCH3 is only 2.5? Answer: Polarity of 2 oxygens (similar to Carboxylic Acid) for a protons greater than c 24 1,1,2-Tribromoethane Examples of protons on adjacent carbons. 2 Brs more electronegative Than 1 Br
Chapter 13 25 => Spin-Spin Splitting or (Multiplicity) or Coupling What do we mean by Spin-Spin Splitting?
Spin-Spin Splitting Protons (H+) on adjacent carbons have magnetic fields that may align with other adjacent protons and this increases their signal. Hydrogen's in similar environments will have the same magnetic spin This magnetic coupling causes the proton peaks to split into doublets, triplets, quintet. (n+1) EXAMPLES of Proton Coupling or peak splitting Singlet has 0 Proton neighbors or n=0 but n+1 = 1 peak Doublet has 1 Proton neighbor or n=1 but n+1 = 2 peaks Triplet has 2 Proton neighbors or n=2 but n+1 = 3 peaks Quartet has 3 Proton neighbors or n=3 but n+1 = 4 peaks Quintet (Pentet) has 4 Proton neighbors or n=4 but n+1 = 5 peaks
27 1,1,2-Tribromoethane LETS LOOK AT THIS AGAIN Examples of protons on adjacent carbons. 2 Brs more electronegative Than 1 Br Chapter 13
28 => n+1 rule Hydrogen's in similar environments will have the same magnetic spin. Ex CH3 here: CH3CH2CH3. The CH3 will send out the SAME signals they are equivalent. SO, we say we have only 2 environments for hydrogen Different environments in neighboring hydrogen's often do not have the assume signal, so they, being different have slightly different signals. Thus different neighbors interfere slightly with each others signal (called coupling) and cause them to be split (called splitting, note splitting = coupling). They are split in a specific pattern called: (splits) = n+1, where n=protons
2 Hs TMS Signal for Hs on carbon 3 is shifted furthest downfield from TMS due to proximity of the electronegative halogen 5 4
3 2 1 0 Answer the following: 1)Is there any symmetry? 2)How many proton environments?
3)How much spin spin coupling 4)Where are the chemical shifts? 3-methyl-2-butanone 34 3-methyl-2- butanone 2 . 1 4 p p m Chemical Shift s in g le t H 3C
Chemical Shift1 O CH C C .1 1 p p m d o u b le t
3 (Symmetry) C H 3 H 2 .5 8 p p m s e p te t
Chemical Shift 3 environments, so 3 peaks on NEXT page graph 35 Why so tiny ? Chapter 13 Only 1 hydrogens 36
Determine for 4-methlyphenol 1)# of environments for hydrogen 2)The chemical shifts 3)Spin-Spin Coupling 4)# of hydrogen environments Answer ON NEXT SLIDE 37 5 .2 0 p p m s in g le t
4-methlyphenol 4 environments O H H H H H C H
6 .7 2 p p m d o u b le t 7 .0 0 p p m d o u b le t 3 2 .2 5 p p m sin g le t 38 (4-methlyphenol)
CH3 2 doublets , very close Not a quartet OH 39 2 Environments 40 Splitting Examples
A=1 peak (no neighbor) B=2 peaks(1 neighbor) C=7 peaks (6 neighbors) AND SO 3 Environments => 41 Hydroxyl Proton Non -splitting Ultrapure samples of ethanol show splitting and be downfield. Nice, just what we expect. BUT ultrapure. Ethanol with a small amount of
acidic or basic impurities will not show splitting. We will get proton exchanges and 1 signal back towards TMS 42 => Identifying the O-H or N-H Peak Singlet Arises because the H on the OH, rapidly exchanges with protons on other molecules (such as water or acids) and is not attached to any particular oxygen
long enough to register a splitting signal. The signal due to the hydroxyl (OH) hydrogen is a singlet, there is no splitting Hs on OH groups do not couple with adjacent hydrogen atoms 43 O-H bonds and the D2O shake As has been pointed out, the signal due to the hydroxyl (OH) hydrogen is a singlet. A small amount of deuterium oxide D2O, a form of water, is added to the sample and the spectrum is re-run. Any signal due to O-H proton disappears. The H in the O-H bond changes places with a deuterium atom, 2H or D, from D2O Deuterium doesnt exhibit nuclear magnetic resonance under the conditions used for
proton nmr so the signal is removed to another part of the spectrum. . O-H bonds and splitting patterns OH and NH Bonds are Singlet's and do NOT spilt OH hydrogens are always seen as a singlet ... there is no splitting This is a quartet despite the fact that there are 4 Hs on adjacent atoms the H on the OH doesnt couple
MRI USES Magnetic resonance imaging, noninvasive Nuclear is omitted because of publics fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Tumors readily detected. 46 Analyze the peaks and chemical shifts here please C2H5Br
WHAT IS THE FINAL STRUCTURE? C2H4O2 WHAT IS IT! C2H4O2 WHAT IS IT, NOW? C4H8O2
WHAT IS IT! C4H8O2 WHAT IS IT! C3H6O WHAT IS IT! C3H6O WHAT IS IT!
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