EM 232 rganic hemistry I at hicago Lecture 13 rganic hemistry 1 Professor Duncan Wardrop February 23, 2010 1
EM 232 rganic hemistry I at hicago Spectroscopy & Spectrometry hapter 13 2
EM 232 rganic hemistry I at hicago Introduction to Analytical Methods Sections: 13.1-13.2 3
Spectroscopy vs. Spectrometry Spectroscopy study of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation Spectrometry evaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation at hicago EM 232, Spring 2010 Slide 4 4
Spectroscopic Methods Method Infrared Spectroscopy Ultraviolet-Visible (UV-vis) Spectroscopy Mass Spectrometry Nuclear Magnetic Resonance Spectroscopy Measurement/Application vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment functional group determination electronic states: energy associated with promotion of an electron in a ground state to an exited state chromophore determination molecular weight: of parent molecule and fragments produced by bombardment with free electrons fragment and isotope determination nuclear spin states: energy associated with spin states of nuclei in the presence of a magnetic field determine structural groups and connectivity at hicago EM 232, Spring 2010 Slide 5 5
Absorption/Transmission Spectroscopy: Simplified Principles sample absorbs different frequencies of light corresponding to molecular vibrations (IR) or electronic transitions (UV-vis) detector determines what frequencies of light passed through (transmittance) and what frequencies of light were absorbed (absorbance) at hicago EM 232, Spring 2010 Slide 6 6
Electromagnetic Spectrum shorter wavelength (λ) higher frequency (ν) higher energy (E) longer wavelength (λ) lower frequency (ν) lower energy (E) Electromagnetic Radiation propagated at the speed of light (3 x10 8 m/s) has properties of particles and waves energy is directly proportional to frequency energy is indirectly proportional to wavelength E = hν c = νλ at hicago EM 232, Spring 2010 Slide 7 7
Quantized Energy States Types of States Energy Range (λ) Spectroscopic Method Increasing Energy nuclear spin rotational vibrational radiofrequency 1-10 m microwave 10-100 cm infrared 0.78-1000 μm NMR Microwave IR electronic ultraviolet 800-200 nm UV-vis at hicago EM 232, Spring 2010 Slide 8 8
EM 232 rganic hemistry I at hicago Infrared Spectroscopy Sections: 13.20-13.22 9
Principles of Infrared Spectroscopy IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment (µ). Stretching δ + δ + δ + δ δ δ δ Bending δ δ δ + δ + δ + at hicago EM 232, Spring 2010 Slide 10 10
Stretching & Bending Vibrations at hicago EM 232, Spring 2010 Slide 11 11
Dipole Moment more electronegative atom covalent 2 electron bond dipole arrow less electronegative atom δ (partially negatively charged) δ (partially positively charged) In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ). at hicago EM 232, Spring 2010 Slide 12 12
ooke s Law: Bonds are Like Springs Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached ~ ν = k f * (m1 + m2) (m1 * m2) ~ ν = vibrational frequency in wavenumbers (cm -1 ) k = constant (1/2πc) f = force constant; strength of bond (spring) Trends: bond strength = frequency mass = frequency m 1, m 2 = masses (not molecular weights) of attached atoms at hicago EM 232, Spring 2010 Slide 13 13
Spring Analogy smaller mass = higher frequency = higher energy stronger spring (bond) = higher frequency = higher energy at hicago EM 232, Spring 2010 Slide 14 14
Wavenumber (ῡ) and Infrared Scale ῡ (cm -1 ) = 1 λ (cm) higher wavenumber (ῡ) = higher frequency (υ) = lower wavelength (λ) = higher energy (E) lower wavenumber (ῡ) = lower frequency (υ) = longer wavelength (λ) = lower energy (E) N- - (sp)- (sp2)- (sp3)- 2 (2380) N N fingerprint region 3400 3000 2600 2200 1800 1400 1000 wavenumber (cm -1 ) wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency at hicago EM 232, Spring 2010 Slide 15 15
Infrared Spectrum 100 80 % Transmission 60 40 20 Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations Frequency: typically measured in wavenumbers; higher wavenumber = higher frequency = higher energy vibration Bands: frequency of vibration absorbed by molecules; can be broad or narrow; number of bands does not equal number of bonds 4000 3500 3000 2500 2000 Wavenumbers at hicago EM 232, Spring 2010 1500 1000 500 Slide 16 16
haracteristic Stretches - Alkanes 100 80 2 3 1 1 2 3 hexane 3 2 60 2 = sp 3 - bond stretching motion; general absorb around 2850-2950 cm -1 40 1 = - rocking motion when atom is part of a methyl group (- 3 ); 1370-1350 cm -1 20 4000 3500 3000 2500 2000 Wavenumbers 1500 1000 500 3 = scissor motion of - 3 hydrogen atoms; 1470-1450 cm -1 1300-900 cm -1 = fingerprint region for organic molecules; typically complex and unhelpful at hicago EM 232, Spring 2010 Slide 17 17
haracteristic Stretches - Alkenes 100 80 60 40 20 5 4 3 1-hexene 5 5: notice sp 2 - (~3100 cm -1 ) at higher frequency than sp3 - (~2950 cm -1 ) more s-character = stronger bond = higher frequency 4: also, = bond at higher frequency than - bond; ~1600 cm -1 4 4000 3500 3000 2500 2000 1500 1000 Wavenumbers at hicago EM 232, Spring 2010 500 Slide 18 18
haracteristic Stretches - Alkynes 6 6 3 7 7 1-hexyne 7: notice sp - (~3300 cm -1 ) at higher frequency than sp 2 - (~3100 cm -1 ), which was higher than sp 3 - (~2950 cm -1 ) 6: stretch is very weak because carbons have almost identical electronegativities = small dipole moment at hicago EM 232, Spring 2010 Slide 19 19
haracteristic Stretches - Alcohols 100 80 60 5 4 9 4 prop-2-en-1-ol (allyl alcohol) 5 9: hydroxyl groups (-) exhibit strong broad bands; ~3300 cm -1 40 20 9 broad peak is a result of hydrogen bonding; width depends on solution concentration lower concentration = less hydrogen bonding = more narrow - band 4000 3500 3000 2500 2000 1500 1000 Wavenumbers at hicago EM 232, Spring 2010 500 Slide 20 20
haracteristic Stretches - NItriles 100 80 8 9 N 8 3-hydroxy-propionitrile 60 40 20 9 8: nitriles ~2200 cm -1 nitriles ( N) absorb a greater magnitude of energy than alkynes ( ) because they have a larger dipole moment larger dipole moment = more intense peak 4000 3500 3000 2500 2000 Wavenumbers 1500 1000 500 size of the dipole does NT affect frequency of vibration at hicago EM 232, Spring 2010 Slide 21 21
Example: Ester, Amine, Benzene 100 80 4 N 10 11 2-amino-benzoic acid butyl ester 60 10: strong carbonyl (=) band ~1700 cm -1 40 20 11 10 4 11: amines; secondary amines (-N) give one band; primary amines (- N2) gives two bands 4: several alkene bands ~1600 cm -1 for benzene ring = double bonds 4000 3500 3000 2500 2000 1500 1000 Wavenumbers at hicago EM 232, Spring 2010 500 Slide 22 22
haracteristic Stretches - arboxylic Acids 100 80 9 4 5 4 10 9 cyclohex-2-enecarboxylic acid 60 10: strong carbonyl (=) band ~1700 cm -1 40 9: hydroxyl band (-) can be less intense and sharper in carboxylic acids 20 10 4: weak alkene band (=) since small dipole moment 4000 3500 3000 2500 2000 1500 1000 Wavenumbers at hicago EM 232, Spring 2010 500 Slide 23 23
haracteristic Stretches - Aldehydes 100 10 80 4 4 hept-2-enal 12 60 40 12 12: usually two bands for - of aldehydes; may overlap with sp 3 - bands 20 10 4000 3500 3000 2500 2000 1500 1000 Wavenumbers at hicago EM 232, Spring 2010 500 Slide 24 24
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 25 25
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 26 26
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 27 27
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 28 28
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 29 29
Self Test Questions Which molecule is represented by the IR below? 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. A. a B. b.c D.d at hicago EM 232, Spring 2010 Slide 30 30
Example 3 3 2 3 2 3 cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal a. b. c. d. e. Greater s character = stronger, shorter bonds = higher frequency at hicago EM 232, Spring 2010 A. a B. b.c D.d Slide 31 31
EM 232 rganic hemistry I at hicago Ultraviolet-Visible Spectroscopy Section: 13.23 This topic will be covered in hapter 10. 32
EM 232 rganic hemistry I at hicago Next Lecture... hapter 13: Sections 13.23,13.24, 13.25 Problem Set 1 has been posted Quiz This Week... hapter 5 & 6 33