chemical bonding and molecul
TRANSCRIPT
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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-octet
resonance structures
formal charges
• VSEPR - predicting shapes of molecules• Bond properties
electronegativity
polarity, bond order, bond strength
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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 atoms
3. Complete octets by using rest of e- as lone pairs (LP)
4. For atoms with <8 e-, make multiple bonds to complete octets
5. Assign formal charges : fc = Z - (#BP/2) - (#LP)I ndicate equivalent (RESONANCE) structures
6. Structures with smaller formal charges are preferred
- consider non-octet alternatives (esp. for 3rd, 4th row)
—
2 for # 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
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20 Oct 97 Bonding and structure (2) 3
Sulfur Dioxide, SO2
•
• O OS••
••
••
••••
•
•
bring in
left pair
OR bring in
right 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
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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 = OSulfur does not obey OCTET rule
BUT 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
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20 Oct 97 Bonding and structure (2) 5
A. S=C=N
Thiocyanate ion, (SCN)-
Which of three possible resonance structures
is most important?
-0.52
-0.32-0.16
Calculated partial charges
ANSWER:
C > A > B
C. S-C N
B. S=C - N
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20 Oct 97 Bonding and structure (2) 6
VSEPR • Valence Shell Electron Pair
R epulsion 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
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20 Oct 97 Bonding and structure (2) 7
H
H H
H
tetrahedral
109o
C 4
F
120o
planar trigonal F F
B 3
Geometry Example
No. of e- Pairs Around Central Atom
180o linear 2 F
— Be
— F
CAChe
image
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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 electrons
in tetrahedral position
H
H H
N H
••
H
H
N
The ELECTRON PAIR GEOMETRY is tetrahedral.
Ammonia, NH3
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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 electrons
in tetrahedral position
H
H H
N
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20 Oct 97 Bonding and structure (2) 10
AXnEm notation
•
a good way to distinguish betweenelectron pair and molecular geometries
is the AXnEm notation
where:
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)
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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
H
O The electron pair geometry is
TETRAHEDRAL.
H - O - H••
••
VSEPR - water
Water, H2O
1. Draw electron dot structure
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20 Oct 97 Bonding and structure (2) 12
H
H
O
. . . the molecular geometry is bent.
VSEPR - water (2)
Although the electron
pair geometry isTETRAHEDRAL . . .
H - O - H••
••
H2O - AX2E2 system - angular geometry
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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 aplanar triangle.
Both the electron pair geometry and the
molecular geometry are PLANAR TRIGONAL 120o bond angles.
• •
C HH
• • O
VSEPR - formaldehyde Formaldehyde, CH2O
1. Draw electron dot structure
• • •
CHH
• O
H2CO at the C atom is an AX3 species
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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
H
Angle 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
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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 = ?
H1
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
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20 Oct 97 Bonding and structure (2) 16
What about:
STRUCTURES WITH
CENTRAL ATOMSTHAT DO NOT OBEY
THE OCTET RULE ?
BF3
SF4
PF5
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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
FB
••
••
•
•
•
•
•
•
•
•
Molecular Geometry is
planar trigonalBF3 is an AX3 species
Consider boron trifluoride, BF3
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20 Oct 97 Bonding and structure (2) 18
FF
F
FF
Trigonal bipyramid
120
90
P 5 electron pairs
F
F F
Octahedron90 F
FF 90S6 electron pairs
Compounds with 5 or 6 Pairs
Around the Central Atom 6_VSEPR.mov
AX5 system
AX6 system
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20 Oct 97 Bonding and structure (2) 19
There are 5 (BP + LP)
e- pairs around the S
THEREFORE:
electron pair geometry ?
F F
F
F
• • • • • •
• • • •
• •
•
• •
••
•• ••
••
•• •• ••
••
S
Sulfur Tetrafluoride, SF4
F
FF
F• •
S
Number of valence e- = 34
No. 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
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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.
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20 Oct 97 Bonding and structure (2) 21
Bonding with Hybrid Atomic Orbitals
4 C atom orbitals hybridize to form four
equivalent sp3 hybrid atomic orbitals.
6_CH4.mov
But atomic carbon has an s2p2 configuration
Why can it make more than 2 bonds ?
- Carbon prefers to make 4 bonds as in CH4
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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
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20 Oct 97 Bonding and structure (2) 23
Multiple Bonds s and p Bonding in C2H4
• The extra p orbitalelectron on each C atomoverlaps the p orbital onthe neighboring atom to
form the p bond.
porbital
3 sp2
hybrid
orbitals
2p 2s
C atom orbitals are COMBINED
(= re-hybridized) to form orbitals
better suited for BONDING
• The 3 sp2 hybrid orbitals
are used to make the C-C
and two C-H s bonds6_C2H4-sg.mov
6_C2H4.mov
H HC
H H
sp2 120 C6_C2H4-pi.mov
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20 Oct 97 Bonding and structure (2) 24
Consequences of Multiple Bonding
Restricted rotation around C=C bond in
1-butene = CH2=CH-CH2-CH3.
27
233
E ( k J / m o
l )
-180 0 180
C-C=C angle (o)
P. 475 - Photo-rotation
about double bonds
lets us see !!
See Butene.Map in ENER_MAP in CAChe models.
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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
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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.
single
BO = 1
1 s
triple, BO = 31 s and 2 p
double, BO = 2
1 s and 1 p
CH2CHCN
Acrylonitrile
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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
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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 bondedatoms.
745 kJ
414 kJ 123 pm
110 pm
Formaldehye
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20 Oct 97 Bonding and structure (2) 29
Bond Length
- depends on size of bonded atoms:
Molecule R(H-X)H- F 104 pm
H- Cl 131 pm
H- I 165 pm
- depends on bond order .
Molecule R(C-O)
CH3C- OH 141 pmO=C=O 132 pm
C O 119 pm
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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 611 134
CC 837 121
NN 946 110
The GREATER the number of bonds (bond order)
the HIGHER the bond strength and the SHORTER the bond.
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20 Oct 97 Bonding and structure (2) 31
Bond Strength (2)
Bond Order Length StrengthHO — 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
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20 Oct 97 Bonding and structure (2) 32
Bond Polarity
HCl is POLAR because it has a positiveend 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.)
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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 calculatedreal bond 432 kJ/mol measured
ELECTRONEGATIVITY, c.
Difference 92 kJ/mol.
This difference is the contribution of IONIC bonding
It is proportional to the difference in
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20 Oct 97 Bonding and structure (2) 34
Electronegativity, c
c is a measure of the ability of an atom in a
molecule to attract electrons to itself.
Concept proposed by
Linus Pauling (1901-94)
Nobel prizes:
Chemistry (54), Peace (63)
See p. 425; 008vd3.mov (CD)
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20 Oct 97 Bonding and structure (2) 35
• F has maximum c.
• Atom with lowest c is the center atom in most
molecules.
• Relative values of c 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
C
NO
SP
Si
Electronegativity, c
Figure 9.7
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20 Oct 97 Bonding and structure (2) 36
Bond Polarity
c(A) - c(B) 3.5 - 2.1
c 1.4
+ -+-O—FO—H
c H
2.1
O F
3.5 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
c(O-H) > c(O-F)Therefore OH is more polar than OF
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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
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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
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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 polar
b) water is non-symmetric
The dipole associated with polar H2Ois the basis for absorption of microwaves
used in cooking with a microwave oven
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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 whywater attaches to CO2
CO2 + H2O H2CO3
-0.73 +1.46 -0.73
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20 Oct 97 Bonding and structure (2) 41
B — F, B — H bonds polar molecule is NOT symmetric
B — F bonds are polar molecule is symmetric
Molecular Polarity inNON-symmetric molecules
F
F F
B
B +ve
F -ve
H
F F
B
Atom Chg. c B +ve 2.0
H +ve 2.1
F -ve 4.0
BF3 is NOT polar HBF2 is polar
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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(C-F) = 1.5, c(C-H) = 0.4
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Chemical Bonding
and
Molecular Structure(Chapter 9)
• Ionic vs. covalent bonding
• Molecular orbitals and the covalent bond (Ch. 10)
• Valence electron Lewis dot structuresoctet vs. non-octet
resonance structures
formal charges
•
VSEPR - predicting shapes of molecules• Bond properties
electronegativity
polarity, bond order, bond strength