Chemistry 30A

Topics: Stereochemistry, Conformational isomerism, Valence bond theory Pages: 9 (1307 words) Published: April 29, 2014
Chem 30A Ch.1-4: Condensed Chang’s Class Notes
Chapter 1
Valence electrons
octet rule
exceptions
P and S can have 10 or 12 valence e-
B and Al (Group 3) can have 6 valence e-
electronegativity- how much an atom wants e-
< 0.5 = nonpolar, covalent (i.e. H2)
0.5 - 1.9 = polar covalent
direction of polarity (using arrow and delta symbol)
overall dipole
> 1.9 = ionic
formation of ions
fish-hook arrow
Lewis structure
formal charge
Functional Groups
Molecular Shape
VSEPR (Valence Shell e- Pair Repulsion)
bonds contain e- (negative)
these repel one another
sp3
i.e. CH4
4 regions of e-
tetrahedral
109.5°
25% s-character, 75% p-character
sp2
i.e. CH2O
3 regions of e-
trigonal planar
120°
33% s-character, 66% p-character
sp
i.e. CO2
2 regions of e-
linear
180°
50% s-character, 50% p-character
Orbitals
Einstein: photoelectric effect → photons
de Broglie → wave-particle duality
Schrodinger → wave functions 𝞿→ orbitals
Atomic orbitals
Simple Molecular Orbital Theory
# MO’s = #AO’s
MO’s arranged by relative E
Filling follows (1: Aufbau, 2: Pauli)
Bond Order
(e- in bonding orbital - e- in antibonding) / 2
Valence Bond Theory
places e- pairs between atoms
problems
incorrect bond geometry
energy
C-H 101 kcal/mol
2s + 1s H
2px + 1s H
2py + 1s H
2pz + 1s H
Hybridization
look over diagrams in notes/book
Combined MO + VB Theory
MO = bonding/antibonding
VB = hybridization
look over diagrams in notes/book
ON MIDTERM: “Draw full combined valence bond/molecular orbital bonding picture “ in NOTES
try these:
CH3--C ≡ N
CO2
Resonance
arrow movement
formal charge
brackets
double-headed arrow
hybridization
greater contributing structure?
filled valences
more covalent bonds
minimize charge separation
charges on atoms of appropriate EN
NOTE: Hybrid looks like major contributor
ON MIDTERM
TRY THESE:
CH3N3
O3
CH3CO2
Chapter 2
Saturated alkanes: maximum of C-H bonds
Representations
ball + stick
line angle
General formula
straight chained/branched → CnH2n+2
Constitutional Isomers
same molecular formula, different connectivity
isomers have different physical properties
Nomenclature
IUPAC
common name
Roots (meth, eth, prop, etc)
Naming Rules
KNOW THESE (look in notes for picture)
1-methylethyl (isopropyl)
2-methylpropyl (isobutyl)
1-methylpropyl (sec-butyl)
1,1-dimethylethyl (tert-butyl)
2,2-dimethylpropyl (neo-pentyl)
Classification of Alkanes
1°, 2°, 3°, 4°
Cycloalkanes
CnH2n
Cyclopropane- frozen in eclipsed conformation, ring strain
Cyclobutane- puckered, ring strain
cyclopentane- envelope, price 1 eclipsing interaction
cyclohexane- “strain free”, chair conformation
Drawing organic molecules HANDOUT ON CCLE
other conformations
boat → flagpole
axial, equatorial, chair flip
Gibbs Free Energy
G= -RTlnKeq (R=8.31)
Keq = product /reactant
express as %
i.e if Keq = 18.9, then product = 18.9, reactant = 1
so total = 19.9, then 18.9 / 19.9 = 95% equatorial, and 5% axial Table 2.4 → given on exam
REMEMBER: G axial → equatorial
RULE: axial = negative (-), equatorial = positive (+)
look in notes for examples
Bicycloalkanes
CnH2n-2
format of naming
i.e. bicyclo[4.3.0]nonane (look in notes for picture)
ON MIDTERM
Look in notes for TRY THESE
know how to draw Bicycloalkanes
NO CHAIR FLIP, ONLY ONE
difference between drawing CIS and TRANS (look in notes)

steroids?
Conformations of Alkanes/Cycloalkanes
strain = measure of stored energy due to structural distortion Torsional
eclipsing interactions
donation of e density in σ*
e delocalization → stability
→ resonance?
Angle
where bond angle is compressed/expanded compared to optimal values i.e. cyclobutane
angle is 90°
but optimal value for sp3 is 109.5°
Steric
where nonbonded atoms come into close contact
steric effect
steric hindrance
steric interaction ions
Newman projection
relationship between staggered and eclipsed
conformers (conformational isomers)...
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