Chapter 13 Part 1 Spectroscopy IR UV and IHD 1 Introduction Spectroscopy is a highly valuable tool in organic chemistry to determine structure of compounds Absorption spectroscopy measures the amount of light absorbed by the sample as a function of the wavelength of light 2 Main Spectroscopic Methods Infrared IR Spectroscopy Measures bond vibration frequencies Determines functional groups Ultraviolet UV Spectroscopy Uses electronic transitions to determine bonding patterns Mass Spectrometry MS Fragments the molecule and measures mass Gives molecular weight of compound Provides information on functional groups 3 Main Spectroscopic Methods Nuclear Magnetic Resonance NMR Analyzes the environment of atoms in a molecule Useful clues to alkyl and other functional groups Can look at various elements i e H C P 4 Electromagnetic Radiation Electromagnetic radiation has properties of particles and waves Essential equations E hu h 6 63 x 10 34 J s c ul 5 Infrared Spectroscopy IR Section 13 20 Atoms and bonds vibrate like balls on springs 6 Infrared Spectroscopy IR Section 13 20 Important tool to identify functional groups IR photons cannot cause electronic transitions but will cause groups of atoms to vibrate with respect to bonds Structural units vibrate in characteristic ways This sensitivity to group vibrations is basis of IR 7 Infrared Spectroscopy IR Section 13 20 IR alone cannot determine structure Can usually identify functional groups The ABSENCE of a signal is definite proof the functional group is absent Comparison with known sample s IR spectrum confirms identity 8 Stretching Vibrations Molecules have different vibrations Stretching Bending 9 IR Spectrum of Methanol 10 Aspects of IR Spectrum Each vibration absorbs a particular frequency of IR radiation The IR spectrum is a plot showing the absorption of energy against the frequency 11 Regions of an IR Spectrum 12 IR Spectroscopy of Hydrocarbons Carbon Carbon bond stretching Occur at higher frequencies because bond is difficult to stretch C C 1200 cm 1 C C 1660 cm 1 C C 2200 cm 1 13 IR Spectroscopy of Hydrocarbons Carbon Carbon bond stretching Conjugation lowers the frequency isolated C C 1640 1680 cm 1 conjugated C C 1620 1640 cm 1 aromatic C C approx 1600 cm 1 14 IR Spectroscopy of Hydrocarbons Carbon Hydrogen bond stretching 15 IR Spectra of Hydrocarbons 16 IR Spectra of Hydrocarbons Alkene 17 IR Spectra of Hydrocarbons Alkyne 18 Characteristic IR Frequencies 19 Characteristic IR Frequencies 20 IR Spectroscopy O H and N H Stretching Both of these occur around 3300 cm 1 Alcohol O H is broad with rounded tip Secondary amine R2NH broad with one sharp spike Primary amine RNH2 broad with two sharp spikes No signal for tertiary amine R3N due to lack of hydrogen 21 IR Spectroscopy Ketones Aldehydes and Acids The C O bond of simple ketones aldehydes carboxylic acids absorb around 1710 cm 1 22 IR Spectroscopy Amides Carbonyl groups of amides absorbs about 1640 to 1680 cm 1 23 IR Spectroscopy Carbonyls above 1725 cm 1 Esters are usually around 1735 cm 1 Strained cyclic ketones absorb at higher frequency 24 IR Spectroscopy O H and N H Stretching 25 IR Spectroscopy O H and N H Stretching 26 IR Spectroscopy O and N Containing Functional Groups 27 IR Spectroscopy O and N Containing Functional Groups 28 IR Spectroscopy O and N Containing Functional Groups 29 IR Spectroscopy O and N Containing Functional Groups 30 IR Spectroscopy O and N Containing Functional Groups 31 IR Spectroscopy O and N Containing Functional Groups 32 IR Spectroscopy O and N Containing Functional Groups 33 Ultraviolet Visible UV Spectroscopy UV Vis depends on transitions between electronic energy levels mainly conjugated p systems UV 200 400 nm wavelength VIS 400 800 nm wavelength lmax is the wavelength of maximum absorption 34 Ultraviolet Visible UV Spectroscopy Absorption of UV Vis radiation promotes an electron to a higher energy level HOMO LUMO Energy levels for cis trans 1 3 cyclooctadiene 35 Ultraviolet Visible UV Spectroscopy Note increasing lmax with increasing conjugation Higher wavelength lower energy 36 Index of Hydrogen Deficiency Elements of Unsaturation 13 25 Unsaturation A Structural element that decreases the number of hydrogens in a molecule by two Double bonds and rings are elements of unsaturation Index of hydrogen deficiency for CnHx CnH2n 2 CnHx 37 Index of Hydrogen Deficiency Elements of Unsaturation Index of hydrogen deficiency for CnHx CnH2n 2 CnHx 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 38 Index of Hydrogen Deficiency Elements of Unsaturation To Calculate Find number of hydrogens if they were saturated subtract the actual number of hydrogens then divide by 2 This calculation cannot distinguish between unsaturations from multiple bonds and rings 39 Index of Hydrogen Deficiency Elements of Unsaturation Calculate the unsaturation for C5H8 First calculate the number of hydrogen atoms for a saturated compound with five carbons Now subtract the number of the actual number of hydrogen atoms in the formula and divide by 2 The compound has two unsaturations They can be two double bonds two rings or one double bond and one ring 40 Index of Hydrogen Deficiency Elements of Unsaturation Halogens replace hydrogen atoms in hydrocarbons so when calculating unsaturations count halides as hydrogen atoms Oxygen does not change the C H ratio so ignore oxygen in the formula Nitrogen is trivalent so it acts like half a carbon Add the number of nitrogen atoms when calculating unsaturations 41 Index of Hydrogen Deficiency Elements of Unsaturation Calculate the unsaturation for C4H7Br First calculate the number of hydrogen atoms for a saturated compound with five carbons Now subtract the number of the actual number of hydrogen atoms in the formula and divide by 2 Remember to count the halide as a hydrogen 42 Index of Hydrogen Deficiency Elements of Unsaturation Calculate the unsaturation for C6H7N 43