University:
Eastern Kentucky University
Course:
CHE 361 | Organic Chemistry I
Academic year:

2020
Views:

137

Pages:

1
Author:

Kolten Khan

Organic Chemistry 11: Spectroscop Infrared Spectroscopy Nuclear Magnetic Resonance Spectroscopy Measures absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding). Plotted as % transmittance vs. ! wavenumber ( l). Peaks to Know for MCAT: Bond Range (cm-1) Peak Type N-H 3300 Sharp O-H 3000 - 3300 Broad C O, C N 1900 – 2200 Medium C=O 1750 Sharp C=C 1600 – 1680 Weak IR Spectrum of Cyclohexanol NMR spectroscopy measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself. It is useful for determining the structure (connectivity) of a compound, including functional groups. Generally plotted as frequency vs. absorption energy. They are standardized by using chemical shift (d), measured in parts per million (ppm) of spectrophotometer frequency. TMS: NMR spectra are calibrated using tetramethylsilane (TMS), which has a chemical shift of 0 ppm Integration: Area under the curve. Proportional to the number of protons contained under the peak. Deshielding: Occurs when electron-withdrawing groups pull electron density away from the proton’s nucleus, allowing it to be more easily affected by the magnetic field. Deshielding moves a peak further downfield Downfield: LEFT. Deshielded by EWG or EN atom nearby. Upfield: RIGHT. More shielded, by EDG or less EN atom nearby. IR Spectrum of Benzaldehyde Spin-Spin When hydrogens are on adjacent atoms, they interfere with Coupling: each other’s magnetic environment, causing spin-spin coupling (splitting). A proton’s (or a group of protons’) peak is split into n+ 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest. Splitting patterns include doublets, triplets, and multiplets. 1H-NMR Shifts to Know for the MCAT IR Spectrum of Cyclohexylamine Mass Spectrometry Used to determine the molecular weight and aid in determining molecular structure. The charged molecule collides with an electron, resulting in the ejection of an electron from the molecule, making it a radical. Base Peak: Tallest peak (not always the intact molecule) Molecular Ion Peak: Peak that represents the molecule. Ultraviolet Spectroscopy UV spectroscopy is most useful for studying compounds containing double bonds and/or heteroatoms with lone pairs that create conjugated systems. Measures the absorption of UV light, which causes movement of electrons between molecular orbitals. UV spectra are generally plotted as percent transmittance or absorbance vs. Wavelength. M+1 Peak: Relative abundance of 13C. Found in relative abundance of 1.1%. So, if M+1 has an m/z value of 4.4, that means there are 4 carbons. 4.4/1.1 = 4. M+2 Peak: Relative abundance of either 81Br or 37Cl. Br has a 1:1 ratio relative to the M peak. Cl has a 3:1 ratio relative to the M peak. HOMO & LUMO To appear on a UV spectrum, a molecule must have a small enough energy difference between its HOMO and LUMO to permit an electron to move from one orbital to the other. The smaller the difference between HOMO and LUMO, the longer the wavelengths a molecule can absorb. Mass Spec of Bromoethane. M+ has similar intensity as M+2.