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CHEM2213 Topic 8.docx

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Department
Chemistry
Course
Chemistry 1027A/B
Professor
Mark Workentin
Semester
Fall

Description
Topic 8 Structure Elucidation: IR, C-NMR, H-NMR 13 1 What Is Electromagnetic Radiation?  Electromagnetic Radiation: light and other forms of radiant energy  Wavelength (λ): distance between two consecutive identical points on a wave  Frequency (v): number of full cycles of wave that pass-1 point in a second  Hertz (Hz): unit in which wave frequency is reported, s  Wavelength and frequency are inversely proportional and we can calculate one from the other from the relationship o E = hv  High-energy radiation corresponds to short wavelengths and vice versa o Ultraviolet light (higher energy) has shorter wavelength o Infrared radiation (lower energy) has a longer wavelength What Is Molecular Spectroscopy?  Experimental process of measuring which frequencies of radiation are absorbed or emitted by a substance and correlating these patterns with details of molecular structure What Is Infrared Spectroscopy?  We use a portion of the electromagnetic spectrum called vibrational infrared region (extends from 4000 to 400 cm )1  Wavenumber: characteristic of electromagnetic radiation equal to the number of waves per cm o Higher wavenumber = higher energy of radiation  Baseline of an infrared spectrum is at the top of the chart  Peaks point downward in an infrared spectrum  100% transmittance of radiation through the sample = 0% absorption  For a molecule to absorb infrared radiation, bond undergoing vibration must be polar  Its vibration must cause a periodic change in the bond dipole o Greater polarity of the bond = more intense absorption  Simplest vibrational motions in molecules give rise to the absorption of infrared radiation are stretching and bending motions  Value of infrared spectra is to determine the presence or absence of particular functional groups  Fingerprint Region: portion of the vibrational infrared region that extends from 1000 to 400 cm -1  Characteristic IR absorptions of selected functional groups: Frequency Range (cm ) -1 Bond / Functional Group Intensity 3500 – 3200 O–H alcohol Strong & broad 3400 – 2400 O–H carboxylic acid Strong & broad 3500 – 3100 N–H amine Medium 3300 – 3270 C–H alkyne Medium 3100 – 3000 C–H alkene Medium 3000 – 2850 C–H alkane Medium to strong 2260 – 2100 C–C alkyne Weak 1800 – 1630 C=O carbonyl Strong 1680 – 1600 C–C alkene Weak 1250 – 1050 C–O ether Strong How Do We Interpret Infrared Spectra?  IR spectrum will reveal not only the functional groups that are present in a sample but also those that can be excluded from consideration  Because alkyl groups exist in most organic molecules, this peak will be observed in many IR spectra  Alcohols are easily recognized by characteristic O–H stretching absorption  Presence or absence of this O–H stretching can be used to distinguish between an ether and an alcohol  Primary amines have two peaks, one caused by symmetric stretching and other by asymmetric  Secondary amines give only one absorption and tertiary amines have no N–H are and therefore transparent  Aldehydes and ketones show characteristic strong infrared absorption associated with the stretching vibration of the C–O double bond  Carboxylic acids share that characteristic absorption with aldehydes and ketones  Index Of Hydrogen Deficiency: sum of number of rings and pi bonds in a molecule o (H referenceHmolecule/ 2 o Reference compound is the same number of C atoms with no rings or pi bonds  To determine the molecular formula of a reference compound containing elements besides C and H, write formula of reference hydrocarbon and add other elements contained in the unknown: o For each halogen atom, subtract one H o No correction is necessary for the addition of group 6 elements (O, S, Se) o For each atom of group 5 (N, P), add one H What Is Nuclear Magnetic Resonance?  Gives us information about the number and types of atoms in a molecule  An electron has a spin and a spinning charge creates an associated magnetic field  An atomic nucleus that has an odd mass / atomic number also has a spin and behaves as if it is a tiny bar magnet  Interactions between nuclear spins in an applied magnetic field are quantized and only two orientations are allowed  Resonance: absorption of electromagnetic radiation by a spinning nucleus and resulting “flip” of its spin for a lower energy state to a higher energy state  Resonance Signal: recording of nuclear m
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