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1. Mass Spectrometry. 1. Mass Spectrometry (cont’d). C 6 x 12.0000 H 13 x 1.0078 F 1 x 18.9984 O 1 x 15.9949 120.0950. C 5 x 12.0000 H 12 x 1.0078 O 3 x 15.9949 120.0780. C 8 x 12.0000 H 8 x 1.0078 O 1 x 15.9949 120.0570.
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1. Mass Spectrometry (cont’d) C 6 x 12.0000 H 13 x 1.0078 F 1 x 18.9984 O 1 x 15.9949 120.0950 C 5 x 12.0000 H 12 x 1.0078 O 3 x 15.9949 120.0780 C 8 x 12.0000 H 8 x 1.0078 O 1 x 15.9949 120.0570 C 9 x 12.0000 H 12 x 1.0078 120.0940
1. Mass Spectrometry (cont’d) 2 88 = Molecular ion 73 = M+ -15 (loss of methyl) 70 = M+ -18 (loss of water) 45 = M+ -43 (loss of propyl) Loss of water indicates the compound is an alcohol. Alcohols also undergo α-cleavage. Alcohol that can lose methyl and propyl via α -cleavage:
2. Infrared Spectroscopy • a. Which signal would be more intense in an IR spectrum, a C-O stretch or a C-N stretch? Why? • Intensity of absorption is correlated to change in dipole moment with the vibration (stretch). The C-O bond is more polar and will thus experience a greater change in dipole moment during a stretching vibration, giving a more intense absorption. • b. Which would give a stronger C=C stretch in an IR spectrum, 1-butene or 2-butene? Why? • Intensity of absorption is correlated to change in dipole moment with the vibration (stretch). The 2-butene is symmetrical and will not experience a change in dipole moment during a stretching vibration. The 1-butene is not symmetrical about the C=C bond, and will this give a small change in dipole during stretching vibration and this the more intense absorption. • c. Which would have a higher frequency absorption, an N-H bond or a P-H bond? Why? • Absorption frequency is correlated to bond strength and atomic masses. Phosphorus is a larger atom than nitrogen, giving it a larger mass and weaker bonding via 3rd shell valence orbitals compared to nitrogen’s 2nd shell valence orbitals. Both of these factors cause the N-H bond to have a higher frequency absorption than the P-H bond.
2. Infrared Spectroscopy (continued) Cyclohexene - Presence of C=C at 1600 cm-1 - Absence of C=O around 1700 cm-1 - Absence of O-H around 3300 cm-1 Cyclohexenol - Presence of O-H at 3300 cm-1 - Absence of C=O around 1700 cm-1 - Absence of C=C around 1600 cm-1 Cyclohexanone - Presence of C=O at 1700 cm-1 - Absence of C=C around 1600 cm-1 - Absence of O-H around 3300 cm-1
3. Ultraviolet Spectroscopy (cont.) This would have the largest molar absorptivity (ε) and thus the most intense absorption
4. NMR Spectroscopy • If an average carbon requires irradiation at 75 MHz to resonate in a 7.046 T magnetic field, what frequency will be required to resonate an average carbon in a 14.092 T magnetic field? • ν = γB0 so doubling the magnetic field doubles the frequency needed to flip the same nucleus, 2π therefore 150 MHz is needed. • b. Use the following molecule to answer the following. Which carbon is furthest downfield? _____d_____ Which carbon is furthest upfield? _____a_____ Which carbon resonates at the highest frequency? _____d_____ Which carbon resonates at the highest chemical shift? _____d_____ Which carbon resonates at the lowest chemical shift? _____a_____ Which carbon is the most shielded? _____a_____ Which carbon is the most deshielded? _____d_____
