Organic Chemistry 2 Flashcards
infrared spectroscopy (IR)
bonds with dipole exposed to infrared radiation (used to create an oscillating electric field) → causes polar bonds to oscillate at a specific vibrational frequency → when IR radiation matches the frequency of vibration of a particular bond, that bond is in resonance and it will absorb some of the IR energy → absorbance picked up by detector vibrational frequency determined by: strength of bond, molecular weight of bonded atoms bond ~ mass on a spring *** a bond with no dipole will not be detected by IR
C = O IR absorbance
1700 cm- sharp, deep
O-H IR absorbance
3300 cm- broad, separate from CH
C-H, saturated alkane IR absorbance
2800 cm- sharp, deep
O-H, carboxylic acid IR absorbance
3000 cm- broad, overlaps CH
N-H, amine IR absorbance
3300 cm- broad, shallow
N-H, amide IR absorbance
3300 cm- broad, deep
C≡N, nitrile IR absorbance
2250 cm- sharp, deep
ultraviolet spectroscopy (UV)
molecule exposed to UV radiation → e- within that molecule will absorb that energy and excite to the next highest energy level → absorbance is recorded on a UV spectrum shows energy difference in J between two adjacent molecular orbitals 1) molecules containing only single bonds show low or no UV absorbance 2) double and triple bonds absorb UV strongly (triple > double) 3) conjugated systems absorb even more than isolated triple and double bonds 4) graph of absorbance vs wavelength (nm) 5) the greater the degree of conjugation → the farther to the right the species will absorb
mass spectrometry
molecules of sample are bombarded with e- → both break apart into smaller pieces randomly and ionize → fragments with different masses and charges → fragments accelerated through a flight tube, only particles with certain mass/charge ratio (m/z) will follow the exact curve and reach the detector → strength of magnetic field is varied → changes the curvature height of each peak gives relative abundance of that fragment parent peak = original molecule minus one e- (molecular ion peak) base peak = most common and stable fragment → 100 % relative abundance
nuclear magnetic resonance spectroscopy (NMR)
atom must have either an odd atomic number or an odd mass number to register on an NMR ~ MRI all nuclei with odd atomic or mass number have nuclear spin → causes changes in electric field → magnetic field external magnetic field applied → nuclei will align their own magnetic fields with the direction of the external field → exposed to photons, some nuclei absorb this photon energy and flip their orientation so they are aligned agains the external field → absorbance picked up by NMR different nuclei require different frequency of photon to cause this flip in orientation → difference caused by degree to which each nucleus is shielded by neighboring hydrogens
H-NMR
spin-spin splitting: n = # of subpeaks - 1 n = number of non-equivalent hydrogen neighbors area under peak: relative representation of number of hydrogens represented by that peak absorbance range: 0 - 12 ppm 12 ppm = downfield = deshielded 0 ppm = upfield = shielded
C13-NMR
absorbance range: 0 - 220 ppm 220 ppm = downfield = deshielded 0 ppm = upfield = shielded peaks: represent carbon functional groups ~ IR *** no spin-spin splitting *** area under curve not representative of relative number of carbons
C13-NMR absorbances
C-C → 0 - 50 C-O → 50 - 100 C=C → 100 - 150 C=O → 150 - 200
improving separation
repetition fractional extraction: extracting with 5 mL ten different times will produce a better separation than extracting with 50 mL once addition of an acid to protonate the product / addition of a base to deprotonate the product → depends on what molecule and what you want the result to be
