a few basic concepts
• molecules are made of atoms
• atom = nucleus + electrons
• valence shell: outermost shell of electrons (e )
• valence e : e in valence shell
• octet rule: tendency to gain to lose e to form
-
-
-
-
ions with 8 e- in valence shell
• electronegativity value: tendency of an atom
to attract e- to itself
• Lewis structure
• bond: attraction between atoms
ionic bonds
covalent bonds
from last lecture
• valence shell
• valence electron
• octet rule: Na , F , O (2s 2p )
• electronegativity value Li, Na, K(metal): 0.98-0.7
+
-
2-
2
6
H: 2.2
C: 2.5
N: 3.0
O: 3.5
F: 4.0
Cl: 3.2
Br: 3.0
ionic bond
• electrostatic attractions between ions;
• same in all directions
• broken when compound dissolved in water
Examples:
NaCl or (NaCl)n: Na+ClCuSO4: Cu2+(SO4)2MgBr2: Mg2+Br-2
covalent bond
• shared electron pair
• electron lone pair (LP)
• Lewis structure
Lego approach:
H:1, C:4, O:2, N:3, B:3/4
F/Cl/Br/I(halide):1
Nbond = [(8xNnon-H+2xNH)-NVe+charge]/2
Practice: H2O, CO2, CH4O, BF3, BF4-, CH4, O3,
HOCl, NO3-, CO, HN3
• Formal= Charge
group number - 2xN
LP
- Nbond
sum of the formal charges of all atoms = charge of the molecule/ion
• Resonance structures
less charged atoms
negative charge on the most electronegative atom
positive charge on the least electronegative atom
Example: NCO-
• Polar covalent bond (electronegativity)
• Molecular geometry
Molecular Geometry
Central atom
VSEPR (valence shell electron pair repulsion) theory
NLP + Nbonding atoms
2: linear (CO2)
3: trigonal planar (H2CO) or bent (SO2)
4: tetrahedron (CH4)
or trigonal pyramidal (NH3)
or bent (H2O)
NLP + Nbonding atoms
2 (sp):
3 (sp2):
shape: linear (CO2)
shape: trigonal planar (H2CO)
1 lp shape: bent (SO2)
4 (sp3):
shape: tetrahedron (CH4)
1 lp shape: trigonal pyramidal (NH3)
2 lp shape: bent (H2O)
Atomic Orbitals (AOs)
Electrons are part matter and part wave
Atomic Orbital describes the wave-like behavior of
electrons: 1s, 2s, 2p, 3s, 3p ...
represent the probability of finding the electron in
any specific region around the nucleus
Molecular Orbitals (MOs)
Molecular Orbital: describes the wave-like behavior of
an electron in a molecule
represents probability of finding an electron in any
specific region
2H
H2
MOs of CH4 (hybridization of AOs)
AOs of carbon
AO hybridization determines molecular shape
isomers: different compounds that have the
same molecular formula
constitutional isomers: isomers that differ in
the connectivity of their atoms
nomenclature (no more than 10C)
classification of carbons:
primary, secondary, tertiary, quaternary
Lewis base: compounds with the ability to
donate electron pairs (F-, H2O)
Brønsted base: molecule that donates an
electron pair to H+
Nucleophile: a Lewis base that donates an
electron pair to an atom other than H+
Brønsted acid: proton or a molecule that
can donate an proton (HCl, ethanol, acetic
acid)
Electrophile: a species (molecule or ion) that
receives an electron pair
pKa values
pKa = -log(Ka),
Ka: equilibrium constant
logKa = -(ΔGo)/(2.3RT)
Ka
H A
Ka
H + + A-
pKa
C H
10 -55
55
C
very unstable anion
N H
10 -35
35
N
unstable anion
O H
10 -15.7
15.7
F H
10 -3
3
F
Cl
10 6
-6
Cl
H
H
O
stable
Can you explain the trend with electronegativity values?
How about pKa values of HBr and HI?
Arrows represent flow of electrons in rx
Rules to draw polar organic mechanisms:
1. attack with electrons (start arrows
from electrons, arrows indicate movement
of electrons);
2. electrons attack empty (or partially
empty) orbitals;
3. bonds break towards more
electronegative atom (electron attacks less
electronegative atom in most cases).
• Formal Charge
= group number - 2xNLP - Nbond
• Resonance structures
octet rule
less charged atoms
negative charge on the most electronegative atom
positive charge on the least electronegative atom
• Molecular geometry
curved arrow notation
A
B
A
+
B
A
B
A
+
B
A
B
A
+
B
A
+
B
A
B
A
+
B
A
B
A
+
B
A
B
how to draw product: analyze one arrow at a time
A
B
C
how to draw arrows: identify bonds to break and
bonds to form before drawing arrows
1. Draw the best Lewis structure for each species shown below.
Lone pairs and formal charges much be included. (10 pts)
2. Provide the shape of the following species and hybridization
of the central atom (10 pts).
3. Provide all constitutional isomers for alkane C6H14 using
bond-line formulas (10 pts)
4. Provide the IUPAC names of the following structures and
circle all tertiary carbons (9 pts).
5. Draw the structure of the following molecules and circle all
quaternary carbons (6 pts).
6. Draw the energy diagram as the dihedral angle (θ) of the
indicated carbon-carbon bond of X varies from 0o (as shown) to
360o. Draw appropriate Newman projections for conformations
whose dihedral angles are 60o, 120o, 180o, 240o, 300o, and 360o
(18 pts).
7. Draw the best resonance structure of the following species.
Indicate the movement of electrons between resonance
structures using the curved arrow notation. (12 pts)
8. Compare the pKa values of each of the following series of
acids (10 pts).
9. Provide the products of each of the following reactions based
on the curved arrow notations (9 pts).
10. Draw the missing arrows based on the starting materials and
products of each reaction (6 pts).