CHEM 286 Theory Exam Covered Topics: SPECTROSCOPY, Labeling NMR spectrum, TECHNIQUES, and THEORETICAL YIELD CALCULATIONS
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CHEM 286 Theory Covered Topics
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CHEM 286 Theory Covered Topics
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 s everal bands exist for same fxnal group, use bracket to specifically cover correct set of signals • M...
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)
This study source was downloaded by 100000861003072 from CourseHero.com on 03-29-2023 23:06:20 GMT -05:00
, 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
This study source was downloaded by 100000861003072 from CourseHero.com on 03-29-2023 23:06:20 GMT -05:00
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