CHEM 2222 Chapter 13 Notes
CHEM 2222 Chapter 13 Notes Chem 212 - Organic Chemistry II
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Chem 212 - Organic Chemistry II
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This 48 page Class Notes was uploaded by annafen on Saturday May 28, 2016. The Class Notes belongs to Chem 212 - Organic Chemistry II at Vanderbilt University taught by Dr. Alissa Hare in Spring 2016. Since its upload, it has received 9 views. For similar materials see Organic Chemistry II in Chemistry at Vanderbilt University.
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Date Created: 05/28/16
13.1 Principles of Molecular Spectroscopy:Electromagnetic Radiation Electromagnetic Radiation • Electromagnetic radiation has properties of particles and waves. • Essential equations: 1 13.2 Principles of Molecular Spectroscopy Quantized Energy States • For electromagnetic radiation to be absorbed the energy must match the difference in energy levels • Not a continuum but • The molecule absorbs light when the photon has the energy that corresponds to the energy difference between two states 2 13.3 Introduction to H NMR Spectroscopy Effect of the Magnetic Field • Nuclei are positively charged and spin on an axis creating a small magnetic field • • 1H and 13C are the most common NMR nuclei in organic chemistry 1 13 • 99.9% of hydrogen is H but only 1.1% of carbon is C – Most is 12C (98.9%) 3 Effect of the Magnetic Field • The external magnetic field B cause the two nuclear spin states 0 to have different energies • The nuclear spin will either be aligned – lower energy – or opposed with the B – higher energy 0 • The difference in energy is small and dependent on B 0 4 The NMR Spectrometer • The magnetic flux B is meas0red in Tesla and NMRs have magnets as high as 20T • However the strength of most NMRs are reported in • ΔE is the energy required to flip one state to the other– this is called resonance • The ΔE atnomagnetci fl , β proton spin th e re is n o d iffe re n ce b e te(hig he r e ne α- andβ- states. be twe e n ∆E = h x 30 0 M Hz ∆E=hx500MHz ( y c n )E α proton spin (low e r e n e r 0T 7.05T 11.75T B o 5 Continuous Wave NMR • NMR used to be done with continuous wave NMR which would sweep the B 0from low to high and left to right with the EM radiation/radio frequency at a constant • As the magnet is changed, at some point the nuclei will absorb the rf frequency and the signal will appear • Most NMRs today have a constant B and vary the EM radiation, 0 6 Chemical Shift • Now with a constant B the0EM radiation is absorbed by different nuclei at different wavelengths – – • Additionally, a nuclei will resonate at different energies depending on its chemical and electronic environment • Chemical Shift is the field strength/radio frequency in ppm at which the nuclei is at resonance relative to the reference – 7 Chemical Shift • In a magnetic field, electrons • These electrons shield the nuclei from the external magnetic field causing them to resonate at a higher energy • Shielding is the influence of the groups around the nuclei 8 Chemical Shift • As a result, the position of the peak is dependent on the degree of shielding experienced by the nuclei • The chemical or position of the peak is reported as ppm on a δ scale 9 13.5 Effects of Molecular Structure on 1H Chemical Shifts Effect of Electronegativity • Protons in different parts of a molecule experience different degrees of shielding and therefore have different chemical shifts • Deshielding increases with the electronegative elements and is distance dependent 10 Effect of Electronegativity • The deshielding effect is cumulative 2 • Based on the generalization that sp carbons are more electronegative than sp 3 carbons leads to more deshielded allylic and benzylic hydrogens 11 Shielding of Protons • Hydrogen on alkenes or arenes are especially deshielded • The deshielding comes from anisotropy – chemical bonds have different properties along different axes • A local magnetic field can develop as a result • The main contributor to the deshielding of vinylic and aryl protons is the induced magnetic field associated with π electrons 12 Chemical Shift Tables 13 13.6 Interpreting H NMR Spectra 1 Structural Information from H NMR 1. The number of signals 2. The intensity of the signals as measured by the area under each peak, 3. The multiplicity, or splitting, of each signal, 14 1 Structural Information from H NMR • Chemically equivalent protons have the same chemical shift • The intensity of a peak is dependent on the number of equivalent hydrogens • The peak areas are compared and the integrated areas given • Integrated areas • 15 Chemically Nonequivalent Hydrogens • Nonequivalent hydrogens have different chemical environments • Their different chemical surroundings change the amount of deshielding that a given proton has • Nonequivalent protons have different chemical shifts and therefore you can tell how many different Hs you have by the number of signals 16 13.7 Spin–Spin Splitting and H NMR Spin-Spin Splitting • Protons on adjacent carbons will interact and • n+1 rule: 17 Spin-Spin Splitting • The hydrogen on the adjacent C can exist in one of two spin states • The spin either reinforces B or ohields the other nuclei • Half of the methyl protons feel are shielded by the single H and half are deshielded so a doublet is observed with signals of equal intensity • For the H seeing the 3 methyl Hs, the eight different combinations cause the signal to be split into a quartet with relative intensities 1:3:3:1. 18 Splitting • Multiplicity of the signal is the number of peaks due to splitting • Coupling constant: • Protons coupled to each other have the same coupling constantJ • 19 13.8 Splitting Patterns: The Ethyl Group Spin-Spin Splitting • In bromoethane there are two groups of equivalent protons BrCH C2 3 • The methyl protons are split by the 2 vicinal protons which can each have one of two spins and the signal will be split into • The three equivalent hydrogens split the signal into a quartet 20 13.9 Splitting Patterns: The Isopropyl Group The Isopropyl Group • The isopropyl group has 6 equivalent protons that are split into a doublet • The other signal is split into a septet 21 13.10 Splitting Patterns: Pairs of Doublets Distortion of Peaks: Pairs of Doublets • Splitting patterns don’t always have the ratio of intensities expected • The observed intensities depend on the relationship between the coupling constant J and the difference in chemical shift ∆ν between the two signals • As the ratio of ∆ν/J decreases the peaks get closer together and become distorted from the 1:1 intensities due to mixing of the magnetic fields 22 13.11 Complex Splitting Patterns Complex Splitting • Complex splitting may occur if the adjacent protons are not equivalent • The adjacent protons will split the signal independently causing a complex pattern to arise • For example • The second splitting from the cis proton is less and gives a doublet of each peak shown as a 1:1:1:1 split pattern 23 Complex Splitting 24 Complex Splitting 25 13.12 H NMR Spectra of Alcohols Spectra of Alcohols • The splitting of the hydroxyl proton is usually not observed • The chemical shift of the hydroxyl proton can vary between δ 0.5 and 5.0. • The signal is a 26 13.14 1C NMR Spectroscopy 13C NMR Spectroscopy • The 13C natural abundance is only 1.1% of the carbon atoms compared 1 to H which is 99.985% of the hydrogen atoms therefore the intensity of peaks is lower 13 • CNMR is much less sensitive • When you take many scans over time and 27 13 C NMR Spectroscopy • 13CNMR gives 13 • Integration is not used because the peak intensities in CNMR are a result of the surrounding spins (usually 1H) not the number of carbons 13 • CNMR 28 13.15 13C Chemical Shifts 13C Chemical Shifts 29 13.17 13C- H Coupling 13 13 13 1 C- C and C- H Coupling • Carbon peaks all appear as singlets even though it is reasonable to 13 13 1 13 expect C- C and H- C coupling. • 13C- C coupling isn’t observed because the natural abundance is too low 1 13 • H- C coupling is intentionally removed (broad band decoupling) in order 30 13.18 Using DEPT to Count Hydrogens Using DEPT to Count Hydrogens • Distortionless enhancement of polarization transfer (DEPT) is used to probe the number of protons attached to a carbon. • One 13C NMR spectrum is run as normal and then a second 13C spectrum run while simultaneously exciting the protons. (a) (b) (c) • Comparison of the two spectra allows the connectivity to be understood better 31 Using DEPT to Count Hydrogens 32 13.20 Introduction to Infrared Spectroscopy Infra Red Spectroscopy • IR is an important tool to identify • Structural units, including functional groups, vibrate in characteristic ways • IR causes the bonds to vibrate and is used qualitatively to determine with bonds are excited • The energy used is expressed as a frequency that resonates with the bond 33 Types of Vibrations • Two stretching vibrations • Four bending vibrations 34 Hooke’s Law 35 13.21 Infrared Spectra Aspects of an IR Spectrum • The IR spectrum is a plot showing the absorption of energy against the frequency of light 36 Regions of an IR Spectrum 37 13.22 Characteristic Absorption Frequencies Characteristic IR Frequencies 38 13.23 Ultraviolet-Visible Spectroscopy Ultraviolet-Visible Spectroscopy • UV-Vis depends on transitions between electronic energy leve– lsmainly conjugated π-systems • UV: 200-400 nm wavelength • Vis: 400-800 nm wavelength • λ max is the wavelength of maximum absorption 39 Energy of π-Orbitals and UV-Vis • Absorption of UV-vis radiation promotes an electron to a higher energy level (HOMO▯ LUMO) • Molecules with extended π-systems have λ maxin the visible range 40 Beer’s Law • Beer’s Law is used to determine the concentration of a molecule in solution • It is the linear relationship between absorbance and concentration 41 13.24 Mass Spectrometry Principles of Mass Spectrometry • Mass spectrometry leads to the mass • A molecule is hit by hi- ghnergy electron which transfers much of its energy to the molecule. • This energy-rich molecule ejects an electron forming a positively charged, odd-electron species called • Molecular ion passes between poles of a magnet and is deflected by the magnetic field • While travelling the unstable cation radical starts to decompose: 42 Schematic of a Mass Spectrometer 43 Principles of Mass Spectroscopy • Amount of deflection depends on mass-to-charge ratio • • Intensity of peak proportional to percentage of each ion of different mass in mixture • Parent Ion/Molecular ion M+ – • Base Peak 44 Fragmentation • The radical ion M is unstable and fragments into smaller groups • The fragmentation pattern of a molecule is characteristic • Fragmentation of propyl benzene is limited. The most abundant peak corresponds to the benzylic cation 45 Characteristics of Mass Spectra • Molecules containing chlorine and bromine spectra reflect the isotopic abundances of the halogen 35Cl : 37Cl = 100:32.7 and 79Br : 81Br = 100:97.5 • In chlorobenzene there is one peak atm/z 112 for CH 6 535Cl and one peak 37 at m/z 114 for CH 6 5 Cl • The ratio of the peak intensities must be 100:32.7 (about 3:1) 46 13.25 Molecular Formula as a Clue to Structure Molecular Weight and Structure • Odd molecular weight means that you have an odd number of nitrogens in your molecule • If you have even molecular weight, either have even number of nitrogens or none • • The exact masses of m/z are obtained from a high-resolution mass spectrometer and will give the molecular formula up to several significant figures after the decimal 47 Index of Hydrogen Deficiency • Alkanes have general formula C H n 2n+2 • Any time a ring or a double bond is present in an organic molecule, its molecular formula has two fewer hydrogen atoms than that of an alkane with the same number of carbons. • Index of hydrogen deficiency ½(C H n 2n + 2 C n )xwhere C H in yxur molecule • Oxygen atoms have no effect on the index of hydrogen deficiency • If a nitrogen is present, one hydrogen is taken away from the formula before calculation and halogens add a hydrogen before the calculation 48