Chemical Bonding and Molecular Structure (Chapter 9)

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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 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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Rules for making Lewis dot structures
1. Count no. of valence electrons (- dont
forget to include the charge on molecular ions!)
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 lt8 e-, make
multiple bonds to complete octets 5. Assign
formal charges fc Z - (BP/2) - (LP)
Indicate equivalent (RESONANCE) structures 6.
Structures with smaller formal charges are
preferred - consider non-octet alternatives
(esp. for 3rd, 4th row)

• OCTET RULE Bond Pairs Lone Pairs 4
• (except for H and atoms of 3rd and higher
  periods)

lone pairs at central atom in AXn (e-) -
8n/2
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Sulfur Dioxide, SO2
These equivalent structures are 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
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SO2 (2)
Alternate Lewis structure for SO2 uses 2 double
bonds
Sulfur does not obey OCTET rule BUT the formal
charge 0
This is better structure than OS-O- since it
reduces formal charge (rule 6). 3rd row S atom
can have 5 or 6 electron pairs
NB of central atom lone pairs (36 -82)/2
1 in both OS-O- and OSO structures
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Thiocyanate ion, (SCN)-
Which of three possible resonance structures is
most important?
ANSWER C gt A gt B
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MOLECULAR GEOMETRY
Molecule adopts the shape that minimizes the
electron pair repulsions.

• VSEPR
• Valence Shell Electron Pair Repulsion theory.
• Most important factor in determining geometry is
  relative repulsion between electron pairs.

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CAChe image
Example
Geometry
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Structure Determination by VSEPR
Ammonia, NH3

• There are 4 electron pairs at the corners of a
  tetrahedron.

The ELECTRON PAIR GEOMETRY is tetrahedral.
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VSEPR - ammonia
Ammonia, NH3

• Although the electron pair geometry is
  tetrahedral . . .

. . . the MOLECULAR GEOMETRY the positions of
the atoms is PYRAMIDAL.
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AXnEm notation

• a good way to distinguish between
• electron 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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VSEPR - water
Water, H2O
1. Draw electron dot structure

• 2. Count BPs and LPs 4
• 3. The 4 electron pairs are at the corners of a
  tetrahedron.

The electron pair geometry is TETRAHEDRAL.
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VSEPR - water (2)
Although the electron pair geometry is
TETRAHEDRAL . . .
. . . the molecular geometry is bent.
H2O - AX2E2 system - angular geometry
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VSEPR - formaldehyde
Formaldehyde, CH2O
1. Draw electron dot structure

• 2. Count BPs and LPs
• 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 density
• Electron 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.
H2CO at the C atom is an AX3 species
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VSEPR - Bond Angles
Methanol, CH3OH
H

• Define bond angles 1 and 2
• Angle 1 H-C-H ?
• Angle 2 H-O-C ?
• Answer

HCOH

H
Angle 1
Angle 2
109o because both the C and O atoms are
surrounded by 4 electron pairs.
AXnEm designation ? at C at O
AX4 tetrahedral
AX2E2 bent
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VSEPR - bond angles (2)
Acetonitrile, CH3CN

• Define bond angles 1 and 2

Angle 1 ?
109o
Angle 2 ?
180o
Why ? The CH3 carbon is surrounded by 4 bond
charges The CN carbon is surrounded by 2 bond
charges
AXnEm designation ? at CH3 carbon at CN carbon
AX4 tetrahedral
AX2 linear
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What aboutSTRUCTURES WITH CENTRAL ATOMS THAT
DO NOT OBEY THE OCTET RULE ?
PF5
BF3
SF4
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Geometry for non-octet species also obey VSEPR
rules
Consider boron trifluoride, BF3

• The B atom is surrounded by only 3 electron
  pairs.
• Bond angles are 120o

Molecular Geometry is planar trigonal BF3 is an
AX3 species
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Compounds with 5 or 6 Pairs Around the Central
Atom
AX5 system
AX6 system
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Sulfur Tetrafluoride, SF4
Number of valence e- 34 No. of S lone pairs
17 - 4 b.p. - 3x4 l.p.(F) 1 lone pair on
S
There are 5 (BP LP) e- pairs around the
S THEREFORE electron pair geometry ?
trigonal bipyramid
OR
AX4E system. Molecular geometry ?
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Sulfur Tetrafluoride, SF4 (2)
90
120

• Lone pair is in the equatorial position because
  it requires more room than a bond pair.

Molecular geometry of SF4 is see-saw
Q What is molecular geometry of SO2 ?
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Bonding with Hybrid Atomic Orbitals
- Carbon prefers to make 4 bonds as in CH4
But atomic carbon has an s2p2 configuration Why
can it make more than 2 bonds ?

• 4 C atom orbitals hybridize to form four
• equivalent sp3 hybrid atomic orbitals.

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Orbital Hybridization

• BONDS SHAPE HYBRID REMAIN e.g.

s2p2 ?
2 linear 2 x sp 2 ps C2H2
3 trigonal 3 x sp2 1 p C2H4 planar
4 tetrahedral 4 xsp3 CH4
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Multiple Bonds s and p Bonding in C2H4
C atom orbitals are COMBINED ( re-hybridized)
to form orbitals better suited for BONDING

• The extra p orbital electron on each C atom
  overlaps the p orbital on the neighboring atom to
  form the p bond.

• The 3 sp2 hybrid orbitals
• are used to make the C-C
• and two C-H ? bonds

6_C2H4-sg.mov
6_C2H4-pi.mov
6_C2H4.mov
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Consequences of Multiple Bonding
Restricted rotation around CC bond in 1-butene
CH2CH-CH2-CH3.

• See Butene.Map in ENER_MAP in CAChe models.

P. 475 - Photo-rotation about double bonds lets
us see !!
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Bond Properties

• What is the effect of bonding and structure on
  molecular properties ?

- bond order - bond length - bond strength -
bond polarity - MOLECULAR polarity
Buckyball in HIV-protease, see page 107
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Bond Order

• the number of bonds between a pair of atoms.

CH2CHCN Acrylonitrile
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Bond Order (2)

• Fractional bond orders occur in molecules with
  resonance structures.
• Consider NO2-

N-O bond order in NO2- 1.5
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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.
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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 pm OCO 132
pm C? O 119 pm
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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) HH 436 74 CC 347 154 CC 611 134 CºC
837 121 NºN 946 110
The GREATER the number of bonds (bond order) the
HIGHER the bond strength and the SHORTER the bond.
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Bond Strength (2)

• Bond Order Length Strength
• HOOH 1 149 pm 210 kJ/mol
• OO 2 121 498 kJ/mol

1.5 128 ?
303 kJ/mol
O3 (g) ? 3 O(g)
HOW TO CALCULATE ?
?Hrxn 3x?Hf(O) - ?Hf(O3) 3x249.2 - 142.7
605 kJ/mol 2 O-O bonds in O3 ? DE (O3)
605/2 302.5 kJ/mol
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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.

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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Bond Polarity (2)

• Due to the bond polarity, the HCl bond energy is
  GREATER than expected for a pure covalent bond.

BOND ENERGY pure bond 339 kJ/mol
calculated real bond 432 kJ/mol measured
Difference 92 kJ/mol. This difference is the
contribution of IONIC bonding It is proportional
to the difference in
ELECTRONEGATIVITY, c.
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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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Electronegativity, c
Figure 9.7

• 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).

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Bond Polarity
Which bond is more polar ? (has larger bond
DIPOLE) OH OF
c H 2.1 O F 3.5 4.0

• c(A) - c(B) 3.5 - 2.1
• Dc 1.4

3.5 - 4.0 0.5
??(O-H) gt ??(O-F) Therefore OH is more polar
than OF
Also note that polarity is reversed.
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Molecular Polarity

• Moleculessuch 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.

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Predicting molecular polarity

• A molecule will be polar ONLY if
• a) it contains polar bonds
• AND
• b) the molecule is NOT symmetric

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Molecular Polarity H2O
Water is polar because a) O-H bond is polar b)
water is non-symmetric
The dipole associated with polar H2O is the
basis for absorption of microwaves used in
cooking with a microwave oven
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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
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Molecular Polarity in NON-symmetric molecules
BF, BH bonds polar molecule is NOT
symmetric

• BF bonds are polar
• molecule is symmetric

Atom Chg. ? B ve 2.0 H ve
2.1 F -ve 4.0
B ve F -ve
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Fluorine-substituted Ethylene C2H2F2
CF bonds are MUCH more polar than CH bonds.
??(C-F) 1.5, ??(C-H) 0.4

• CIS isomer
• both CF bonds on same side
• ? molecule is POLAR.

• TRANS isomer
• both CF bonds on opposite side
• ? molecule is NOT POLAR.

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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 structures
• octet vs. non-octet
• resonance structures
• formal charges
• VSEPR - predicting shapes of molecules
• Bond properties
• electronegativity
• polarity, bond order, bond strength