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20 Oct 97 Bonding and structure (2) 1
Chemical Bonding and
Molecular Structure (Chapter 9)
• Ionic vs. covalent bonding• Molecular orbitals and the covalent bond (Ch. 10)• Valence electron Lewis dot structures
octet vs. non-octetresonance structuresformal charges
• VSEPR - predicting shapes of molecules• Bond properties
electronegativitypolarity, bond order, bond strength
20 Oct 97 Bonding and structure (2) 2
•OCTET RULE: #Bond Pairs + #Lone Pairs = 4 (except for H and atoms of 3rd and higher periods)
Rules for making Lewis dot structures
2. Place a bond pair (BP) between connected atoms3. Complete octets by using rest of e- as lone pairs (LP)4. For atoms with <8 e-, make multiple bonds to complete octets5. Assign formal charges : fc = Z - (#BP/2) - (#LP) Indicate equivalent (RESONANCE) structures6. Structures with smaller formal charges are preferred - consider non-octet alternatives (esp. for 3rd, 4th row)
— 2 for # of PAIRS# of PAIRS
1. Count no. of valence electrons (- don’t forget to include the charge on molecular ions!)
#lone pairs at central atom in AXn = {(#e-) - 8*n}/2
20 Oct 97 Bonding and structure (2) 3
Sulfur Dioxide, SO2
••O OS
••
••
••
••••••
bring inleft pair
OR bring inright pair
These equivalent structuresare called:
RESONANCE STRUCTURES. The proper Lewis structure
is a HYBRID of the two.
Each atom has OCTET . . . . . BUT there is a +1 and -1 formal charge
••O OS
••
••
••
••••
••O OS••
••
••
••
••
+— —+
Rules 1-3 O—S —O
20 Oct 97 Bonding and structure (2) 4
SO2 (2)
Alternate Lewis structure for SO2 uses 2 double bonds
NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S+-O- and O=S=O structures
O = S = O Sulfur does not obey OCTET ruleBUT the formal charge = 0
This is better structure than O=S+-O- since it reduces formal charge (rule 6). 3rd row S atom can have 5 or 6 electron pairs
20 Oct 97 Bonding and structure (2) 5
A. S=C=N
Thiocyanate ion, (SCN)-
Which of three possible resonance structuresis most important?
-0.52 -0.32-0.16
Calculated partial charges
ANSWER:
C > A > B
C. S-C N
B. S=C - N
20 Oct 97 Bonding and structure (2) 6
VSEPR • Valence Shell Electron Pair
Repulsion theory.
• Most important factor in determining geometry is relative repulsion between electron pairs.
MOLECULAR GEOMETRY
Molecule adopts the shape that minimizes the electron pair repulsions.
6_VSEPR.mov
20 Oct 97 Bonding and structure (2) 7
H
HH
H
tetrahedral
109o
C4
F
120o
planar trigonalFF
B3
GeometryExample
No. of e- PairsAround CentralAtom
180o
linear2 F—Be—F
CACheimage
20 Oct 97 Bonding and structure (2) 8
Structure Determination by VSEPR
There are 4 electron pairs at the corners of a tetrahedron.
lone pair of electronsin tetrahedral position
H
H HNH
••
H
H
N
The ELECTRON PAIR GEOMETRY is tetrahedral.
Ammonia, NH3
20 Oct 97 Bonding and structure (2) 9
Although the electron pair geometry is tetrahedral . . .
VSEPR - ammonia
Ammonia, NH3
. . . the MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL.
lone pair of electronsin tetrahedral position
H
H HN
20 Oct 97 Bonding and structure (2) 10
AXnEm notation
• a good way to distinguish between electron pair and molecular geometries is the AXnEm notationwhere: A - atom whose local geometry is of interest
(typically the CENTRAL ATOM) Xn - n atoms bonded to A Em - m lone pair electrons at A
NH3 is AX3E system pyramidal
(NB this notation not used by Kotz)
20 Oct 97 Bonding and structure (2) 11
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the corners of a tetrahedron.
H
HO
The electron pair geometry is TETRAHEDRAL.
H - O - H••
••
VSEPR - water
Water, H2O
1. Draw electron dot structure
20 Oct 97 Bonding and structure (2) 12
H
HO
. . . the molecular . . . the molecular geometry is bent.geometry is bent.
VSEPR - water (2)
Although the electron pair geometry is TETRAHEDRAL . . .
H - O - H••
••
H2O - AX2E2 system - angular geometry
20 Oct 97 Bonding and structure (2) 13
2. Count BP’s and LP’s: At Carbon there are 4 BP but . . .
3. These are distributed in ONLY 3 regions. Double bond electron pairs are in same region.There are 3 regions of electron densityElectron repulsion places them at the corners of a planar triangle.
Both the electron pair geometry and the molecular geometry are PLANAR TRIGONAL 120o bond angles.
••
C HH
••OVSEPR - formaldehyde Formaldehyde, CH2O
1. Draw electron dot structure
• ••
CHH
•O
H2CO at the C atom is an AXAX33 species species
20 Oct 97 Bonding and structure (2) 14
AXnEm designation ?
at C
at O
Define bond angles 1 and 2
Angle 1 = H-C-H = ?
Angle 2 = H-O-C = ?
Answer:
VSEPR - Bond Angles
H
HAngle 2
••H—C—O—H
Angle 1
6_CH3OH.mov
Methanol, CH3OH
109o because both the C and O atoms are surrounded by 4 electron pairs.
••
AX4 = tetrahedral
AX2E2 = bent
20 Oct 97 Bonding and structure (2) 15
AXnEm designation ?
at CH3 carbon
at CN carbon
Define bond angles 1 and 2
VSEPR - bond angles (2) Acetonitrile, CH3CN
Angle 1 = ?
H 1
H—C—C
2
H
••N
Why ? : The CH3 carbon is surrounded by 4 bond charges
The CN carbon is surrounded by 2 bond charges
Angle 2 = ?
AX4 = tetrahedral
AX2 = linear
109o
180o
20 Oct 97 Bonding and structure (2) 16
What about:STRUCTURES WITH CENTRAL ATOMS
THAT DO NOT OBEY THE OCTET RULE ?
BF3
SF4
PF5
20 Oct 97 Bonding and structure (2) 17
Geometry for non-octet species also obey VSEPR rules
The B atom is surrounded by only 3 electron pairs.
Bond angles are 120o F••
••
••
F
F
B••
••
••
••
••
••
Molecular Geometry is planar trigonalBF3 is an AX3 species
Consider boron trifluoride, BF3
20 Oct 97 Bonding and structure (2) 18
FF
F
FF
Trigonal bipyramid
120
90
P 5 electron pairs
FF F
Octahedron90 F
FF90S6 electron pairs
Compounds with 5 or 6 Pairs Around the Central Atom 6_VSEPR.mov
AX5 system
AX6 system
20 Oct 97 Bonding and structure (2) 19
There are 5 (BP + LP)e- pairs around the STHEREFORE:electron pair geometry ?
F
F
F
F
••• •••
••••••
•••
••
••••
••
••••••
••
S
Sulfur Tetrafluoride, SF4
F
FFF•• S
Number of valence e- = 34No. of S lone pairs =
{17 - 4 b.p. - 3x4 l.p.(F)}= 1 lone pair on S
= trigonal bipyramid
AX4E system. Molecular geometry ?
F
FFF
••
SOR
20 Oct 97 Bonding and structure (2) 20
120
90
F
F
F
F•• S
Sulfur Tetrafluoride, SF4 (2)
axial
equatorial
Molecular geometry of SF4 is “see-saw”
Q: What is molecular geometry of SO2 ?
Lone pair is in the equatorial position because it requires more room than a bond pair.
20 Oct 97 Bonding and structure (2) 21
Bonding with Hybrid Atomic Orbitals
4 C atom orbitals hybridize to form four4 C atom orbitals hybridize to form four
equivalent spequivalent sp33 hybrid atomic orbitals. hybrid atomic orbitals.
6_CH4.mov
But atomic carbon has an s2p2 configurationWhy can it make more than 2 bonds ?
- Carbon prefers to make 4 bonds as in CH4
20 Oct 97 Bonding and structure (2) 22
Orbital Hybridization
BONDS SHAPE HYBRID REMAIN e.g.
2 linear {2 x sp &2 p’s} C2H2
3 trigonal {3 x sp2 & 1 p} C2H4
planar
4 tetrahedral {4 xsp3 } CH4
s2p2
20 Oct 97 Bonding and structure (2) 23
Multiple Bonds and Bonding in C2H4
• The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the bond.
p orbital
3 sp2
hybrid orbitals
2p2s
C atom orbitals are COMBINED (= re-hybridized) to form orbitalsbetter suited for BONDING
• The 3 sp2 hybrid orbitals are used to make the C-C and two C-H bonds
6_C2H4-sg.mov
6_C2H4.mov
H HC
H Hsp2120 C6_C2H4-pi.mov
20 Oct 97 Bonding and structure (2) 24
Consequences of Multiple Consequences of Multiple BondingBonding
Restricted rotation around C=C bond in1-butene = CH2=CH-CH2-CH3.
27
233
E (
kJ/m
ol)
-180 0 180C-C=C angle (o)
P. 475 - Photo-rotation about double bonds lets us see !!
See Butene.Map in ENER_MAP in CAChe models.
20 Oct 97 Bonding and structure (2) 25
Bond Properties• What is the effect of bonding and structure on
molecular properties ?
Buckyball in HIV-protease, see page 107
- bond order - bond length - bond strength - bond polarity - MOLECULAR polarity
20 Oct 97 Bonding and structure (2) 26
H
H
H
C C NC
Bond Order
• the number of bonds between a pair of atoms.
singleBO = 11
triple, BO = 31 and 2
double, BO = 21 and 1
CH2CHCNAcrylonitrile
20 Oct 97 Bonding and structure (2) 27
Bond order = Total # of e - pairs used for a type of bond
Total # of bonds of that type
Bond Order (2)
Fractional bond orders occur in molecules with resonance structures.
Consider NO2-
Bond order in NO2- = 3 (e - pairs in N-O bonds)
2 (N - O bonds)N-O bond order in NO2
- = 1.5
O O O O
N••
••••
••••
••••••••••
••••
••N
20 Oct 97 Bonding and structure (2) 28
Bond Order and Bond Length
Bond order is related to two important bond properties:
(a) bond strength
as given by DE
(b) Bond length - the distance between the nuclei of two bonded atoms.
745 kJ745 kJ
414 kJ414 kJ 123 pm123 pm
110 pm110 pm
Formaldehye
20 Oct 97 Bonding and structure (2) 29
Bond Length
- depends on size of bonded atoms:
Molecule R(H-X)H- F 104 pmH- Cl 131 pmH- I 165 pm
- depends on bond order.
Molecule R(C-O)CH3C- OH 141 pmO=C=O 132 pm
C O 119 pm
20 Oct 97 Bonding and structure (2) 30
Bond Strength• Bond Dissociation energy (DE) - energy required to
break a bond in gas phase. • See Table 9.5
BOND STRENGTH (kJ/mol) LENGTH (pm)H—H 436 74
C—C 347 154C=C 611 134CC 837 121
NN 946 110
The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.
20 Oct 97 Bonding and structure (2) 31
Bond Strength (2)
Bond Order Length Strength
HO—OH 1 149 pm 210 kJ/mol
O=O 2 121 498 kJ/mol
O O•••••••
••••
••O 1.5 128 ?
HOW TO CALCULATE ?
Hrxn = {3xHf(O) - Hf(O3)} = {3x249.2 - 142.7} = 605 kJ/mol 2 O-O bonds in O3 DE (O3) = 605/2 = 302.5 kJ/mol
O3 (g) 3 O(g)
303 kJ/mol
20 Oct 97 Bonding and structure (2) 32
Bond Polarity
HCl is POLAR because it has a positive end and a negative end (partly ionic).
Polarity arises because Cl has a greater share of the bonding electrons than H.
Cl
-+
•••H••
••
Calculated charge by CAChe:
H (red) is +ve (+0.20 e-)
Cl (yellow) is -ve (-0.20 e-).
(See PARTCHRG folder in MODELS.)
20 Oct 97 Bonding and structure (2) 33
• Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond.
Cl
-+
•••H••
••
Bond Polarity (2)
BOND ENERGY
“pure” bond 339 kJ/mol calculated
real bond 432 kJ/mol measured
ELECTRONEGATIVITY, .
Difference 92 kJ/mol.
This difference is the contribution of IONIC bondingIt is proportional to the difference in
20 Oct 97 Bonding and structure (2) 34
Electronegativity,
is a measure of the ability of an atom in a molecule to attract electrons to itself.
Concept proposed byLinus Pauling (1901-94)Nobel prizes:Chemistry (54), Peace (63)
See p. 425; 008vd3.mov (CD)
20 Oct 97 Bonding and structure (2) 35
• F has maximum .
• Atom with lowest is the center atom in most molecules.
• Relative values of determines BOND POLARITY (and point of attack on a molecule).
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180
0.5
1
1.5
2
2.5
3
3.5
4
H
FCl
CN
O
SP
Si
Electronegativity,
Figure 9.7
20 Oct 97 Bonding and structure (2) 36
Bond Polarity
(A) - (B) 3.5 - 2.1
1.4
+ -+-O—FO—H
H 2.1O F3.5 4.0
Also note that polarity is “reversed.”
Which bond is more polar ? (has larger bond DIPOLE)
O—H O—F
3.5 - 4.0 0.5
(O-H) > (O-F) Therefore OH is more polar than OF
20 Oct 97 Bonding and structure (2) 37
Molecular Polarity
• Molecules—such as HCl and H2O—
can be POLAR (or dipolar).
• They have a DIPOLE MOMENT.
• Polar molecules turn to align their dipole with an electric field.
POSITIVE
NEGATIVE
H—Cl
POSITIVE
NEGATIVE
H—Cl
20 Oct 97 Bonding and structure (2) 38
Predicting molecular polarity
A molecule will be polar ONLY if
a) it contains polar bonds
AND
b) the molecule is NOT “symmetric”
Symmetric molecules
20 Oct 97 Bonding and structure (2) 39
H
HH H
O
••
••
O
+polar
Molecular Polarity: H2O
Water is polar because:
a) O-H bond is polarb) water is non-symmetric
The dipole associated with polar H2O is the basis for absorption of microwaves used in cooking with a microwave oven
20 Oct 97 Bonding and structure (2) 40
Carbon Dioxide
• CO2 is NOT polar even though the CO bonds are polar.
• Because CO2 is symmetrical the BOND polarity cancels
The positive C atom is why water attaches to CO2
CO2 + H2O H2CO3
-0.73 +1.46 -0.73
20 Oct 97 Bonding and structure (2) 41
B—F, B—H bonds polarmolecule is NOT symmetric
B—F bonds are polarmolecule is symmetric
Molecular Polarity in NON-symmetric molecules
F
F FB
B +ve F -ve
H
F FB
Atom Chg. B +ve 2.0H +ve 2.1F -ve 4.0
BF3 is NOT polar HBF2 is polar
20 Oct 97 Bonding and structure (2) 42
Fluorine-substituted Ethylene: C2H2F2
CIS isomer
• both C—F bonds on same side
molecule is POLAR.
C—F bonds are MUCH more polar than C—H bonds.
TRANS isomer
• both C—F bonds on opposite side
molecule is NOT POLAR.
(C-F) = 1.5, (C-H) = 0.4
20 Oct 97 Bonding and structure (2) 43
Chemical Bonding and
Molecular Structure (Chapter 9)
• Ionic vs. covalent bonding• Molecular orbitals and the covalent bond (Ch. 10)• Valence electron Lewis dot structures
octet vs. non-octetresonance structuresformal charges
• VSEPR - predicting shapes of molecules• Bond properties
electronegativitypolarity, bond order, bond strength