Ochem final

jopokohu's version from 2015-12-10 06:29


Question Answer
shielding and deshielding effectsdetermine position of proton signal; shielding shifts absorption upfield
deshieldingdownfield (left), decreased electron density
shieldingupfield (right), increased electron density
effects of electronegative atoms on nmr shiftwithdrawl of e- density, deshield a nucleus
protons on benzene rings/double bondsdeshielded by pi e-
protons on triple bondsshielded by pi e-
a set of n nonequivalent protons on the same carbon or adjacent carbonssplits an nmr signal into n + 1 peaks
nonequivalent protons come frompi bonds
splitting is not caused byOH, NH protons
# peaks for proton with two sets of nearby nonequivalent (to each other) protons(n+1)(m+1)
# peaks for protons on flexible alkyl chainspeak overlap often occurs, resulting in n + m + 1

1HNMR Characteristic shifts

Question Answer
alkane-H0.9-1.7, increasing R:H groups
C, O, or N=C-C-H1.5-2.5
-alkyne H~2.5
N, O, or X-C(sp3)-H2.5-4
alkene H4.5-6
aldehide H9-10
carboxylic acid H10-12
RO-H or R-N-H1-5


Question Answer
heteroatomshave lone pairs and create electron-deficient sites on C
pi bonds make a moleculea base and a nucleophile
C-Z sigma bondselectronegative heteroatom (Z) creates polar bond, making carbon electron-deficient/partial positive charge
Carbonyl (C=O) functional groupspolar C=O bond makes carbon electron-deficient/partial positive, while lone pairs on O allow it to react as a nucleophile and a base
determining hybrid orbitalscount # groups (atoms and nonbonded e-) around atom
2 groups2 sp orbitals
3 groups3 sp2 orbitals
4 groups4 sp3 orbitals
increased % s-characterincreased bond strength, decreased length


Question Answer
acidity trends on ptleft/down
electronegativity trends on ptleft/up
bonds between atoms w/ different electronegativitiesare polar
dipoleseparation of charge in polar bonds
polar moleculeseither 1 polar bond or 2+ polar bonds that reinforce each other, eg H2O
nonpolar moleculeseither no polar bonds, or two or more bond dipoles that cancel (eg CO2- double bonds cause linear molecule with opposing dipoles)
inductive effects on acidityelectron-withdrawing substituents can increase acidity of a nearby atom, which increases w/ strength of electronegativity and decreases w/ distance
%s character and acidity%s-character of a carbon increases acidity of protons on that carbon
pka and aciditypka is inversely related to acidity

Correlated factors

Question Answer
substitution on carbocationcarbocation stability
steric hindrance of alkyl halideratio of e2 to sn2
electron density of aromaticrate of electrophilic aromatic substitution
pka of acid strength of conjugate base and stability of conjugate base
electron density of carbonyl carbonunreactivity with nucleophiles
difference in bonded atoms electronegativitydipole movement
polarityboiling point
electron density on nucleophilenucleophilicity
acidity%s-character, electronegativity of substituents
chiralityplane of symmetry, super-imposable mirror image
number (n) of stereogenic centersno more than 2^n stereoisomers
%s-characterwavenumber, bond strength
alkyl substitution (lowered by resonance structures)chemical shift


Question Answer
base peaktallest peak in mass spectrum
methane m/z17
hexane m+1 and base peak (C4H9+)87; 57
nitrogen rulean odd m+1 generally indicates that a compound contains nitrogen
A compound that contains an even number of N atoms gives aneven molecular ion.
alkyl chloride molecular ion3:1 ratio of M to M+2
Br atom molecular ion1:1 ratio of M to M+2


Question Answer
constitutional isomers have differentchemical and physical properties
chiral moleculescannot be rotated/flipped to be superimposed on it's mirror image
stereogenic centerstetrahedral carbons with 3 different groups (only one may be H)
meso compoundsachiral compounds with two identical halves
same molecular formulaisomers
same cis/trans or r/sstereoisomers
mirror imagesenantiomers
enantiomeric excess% of one enantiomer - % of other enantiomer OR (a mixture/a pure enantiomer) x 100%

Characteristic IR spectra

Question Answer
bonds to hydrogen4000-2500
triple bonds2500-2000
double bonds2000-1500
single bonds1500-400
O-H3600-3200, strong, broad
C(sp3)-H3000-2850, strong
C-C or C-N triple bond2250
benzene ring1600, 1500

Identifying unknown compounds with IR and MS

Question Answer
step 1divide molecular ion m/z by 12 to get maximum amount of C(remainder is H). Add 1+ O(s) and remove C's and H's to keep weight the same. Do this to find all possible formulas
if molecular ion/weight is odda nitrogen is present
step 2determine degrees of unsaturation (fully saturated = 2 + 2n where n = # C)
degrees of unsaturation1 degree/ -2 H; indicate presence of rings or pi bonds
step 3determine functional groups from ir spectrum, try to find functional groups that explain unsaturated groups