CHEM 286 Theory Exam Covered Topics: SPECTROSCOPY, Labeling NMR spectrum, TECHNIQUES, and THEORETICAL YIELD CALCULATIONS

CHEM 286 Theory Exam Covered Topics:

SPECTROSCOPY
Labeling IR spectrum:
 Draw cmpd structure on spectrum, ID all fxnal groups present, assign important bands in spectrum (str +
bends)
 If several bands exist for same fxnal group, use bracket to specifically cover correct set of signals
 MEMORIZE
Vibration type Functional Group Wavelength Characteristic
OH stretch Alcohol/Phenol 3200-3600 Broad/strong
Acid (-COOH centered 2500-3200 Broad/strong
~3000)
NH stretch 3200-3500 Broad/strong
CH stretch Sp2 3000-3100 Medium
Sp3 2800-3000 Strong
Aldehyde 2700-2850 Medium
C triple bond C Alkyne 2100-2260 Weak
C triple bond N Cyano 2110-2260 Medium
C=O All 1660-1750 Very strong
C=C Alkene/aromatic 1500-1650 Weak to strong
Aromatic 1400-1515 Variable
CH bend CH2/CH3 1350-1475 Medium/multiple band
OH bend Phenols 1315-1390 Medium-strong
Acids 870-950 Broad/strong
NH bend Amines/amides (weaker and 1500-1650 Broad/strong
sharper than OH)
660-910 Broad/medium
C-O stretch “Conjugated” (double-single 1160-1310 Medium-strong
partial bond character
Alcohol/ether 1000-1200 Medium
C-N stretch 1020-1340 Weak-medium
CH oop Polarized 950-1150 Strong
Bends Aromatic 675-900 Strong
 STRONG ACTIVATORS: groups w/ accessible lone pair e- (N, O)
 WEAK ACTIVATORS: alkyl groups (inductive only), alkene/other aromatics (minor resonance)
 STRONG DEACTIVATORS: groups w/ (+) charge, NO2, CN
 MODERATE DEACTIVATORS: groups w/ carbonyl functionality
 One oxygen = aldehyde/ketone
 Aldehyde – look for signal in NMR (9-10ppm, SHARP, singlet/doublet –depends on neighbors)
 Ketone – wide, broad signal
 STRONGER SIGNALS = determined by POLARITY; H-BONDING WIDENS SIGNAL
 Amides (NH, one loop) shifted closer to R than amines (NH2, 2 loops) on IR
 If OH clearly separated = alcohol, if closer = phenol
 CHECKLIST EXAMPLE
 4000-1300: look at CH stretch bands around 3000
 Are any/all to R of 3000? = Alkyl groups
 Are any/all to L of 3000? = C double bond C/aromatic grp in molecule
 Look for carbonyl in 1760-1690. If there is…
 Is OH also present? = Carboxylic acid grp
 C-O? = Ester
 Aldehydic C-H? = Aldehyde
 N-H = Amide
 None of the above = Ketone
 Look for broad OH band in 3500-3200. If there is, = alcohol/phenol
 Other structural features to check for:
 C-O stretches? = Ether (or ester if carbonyl present too)
 C double bond C stretch? = Alkene
 C triple bond C band? = Alkyne
 If absence of major fxnal grps in 4000-1300, other than CH stretches, cmpd’s probably strict
hydrocarbon

Labeling NMR spectrum:
 Draw cmpd structure on spectrum, assign letter to each H/group of equivalent H w/i molecule
 Make sure to include:
1. Proton ID (based on structure – can’t do until structure is found, draw if not given)
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, 2. # of H’s (based on integral)
 Divide all by lowest integration value, round to nearest 0.5 then multiply all to get whole #’s
(match to chemical formula)
 3 H integral = 3 substituents
 4H integral = 2 substituents
 5 H integral = 1 substituent
3. Splitting pattern (singlet, doublet, triplet, quartet, quintet, sextet, multiplet…etc.)
 2 doublets = para
 N+1 (= *splitting pattern*, solve for n) rule for finding neighboring hydrogens
 Hydrogen Deficiency Index (HDI):
 # of H = [2 x (# of C)] + 2
 Each multiple bond or ring reduces H count by 2 (“degree of unsaturation”) – can be calc. from
molecular formula using:
 (2C + 2 + N – H – X) / 2
 C = carbons, N = nitrogens, H = hydrogens, X = halogens
 If HDI=4, consider benzene ring
 7-8 ppm = Aromatic (i.e. benzene), 2.0-2.5=proton next to carbonyl
 De-shielding (EWG) closer to 8ppm (maybe above) than 7ppm  higher in aromatic region (De-shielded = L)
 Shielded (EDG) might shift belo2w 7ppm (Shielded = R)
 Hydrogen bonding hydrogens don’t couple (OH, NH, FH)

TECHNIQUES
 Reflux
 Standard Reflux
 The reflux temp is near bp of solvent
 Place reagents in round bottom flask large enough to hold both reagents + enough
solvent to dissolve them w/o being much more than ½ full
 Choose solvent that…
 Dissolves reactants @ boiling temp
 Doesn’t react w/ reagents
 Boils @ temp high enough to cause rapid rxn rate
 Dissolve reactants in solvent and add stirbar
 Place condenser upright on flask, connect to water faucet and run water through (in
bottom, out top)
 Put suitable heat source under flask. Reflux time starts when solution boils and solvent
in flask starts condensing + falling back into flask
 When rxn time’s up, turn off heat, let setup cool, dismantle, collect + purify product
 Dry Reflux
 Fill drying tube with drying agent – CaCl2, surrounded by loose cotton on either side
 Have round bottom flask flame dried, then quickly place drying tube back onto flask (air
moisture then trapped by drying agent)
 Allow it to cool down, assemble apparatus as quickly as possible to avoid moisture condensing
back
 Quickly add reagents/solvents to reaction flask and assemble system
 Carry out reaction like standard reflux
 Extraction (same operation as washing  EXTRACT good material from impure matrix, WASH impurities from
good material)
 Solutions must be insoluble in each other (to form distinct layers when mixed)
 Something like 10% NaOH means it’s 90% water, therefore density will be very close to water’s
 Examples:
 Strong organic acid
 Extract into sat’d sodium bicarbonate – foaming + fizzing b/c WB turns SA into salt that
dissolves into water bicarbonate solution – invert funnel, point stem up and toward
back of hood and open stopcock to vent funnel
 To recover acid, add conc’d HCl until solution is acidic on litmus
 To wash out SA, throw bicarbonate away
 Weakly acidic organic acid
 Extract into 10% NaOH water sol’n – SB needed to rip H off WA + turn them into salts
that go into NaOH water layer
 To recover acid, add conc’d HCl until acidic on litmus
 To wash out WA, throw NaOH water sol’n away
 Organic base
 Extract w/ 10% HCl water sol’n – SA turns base into salt that goes into water layer
 To recover base, add ammonium OH to water soln until basic on litmus
 To wash out organic base, wash as previously noted and throw out soln
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