Molecular Geometry and Bonding Theories. Two Simple Theories of Covalent Bonding Valence Shell Electron Pair Repulsion Theory VSEPR R. J. Gillespie

Molecular Geometry and Bonding Theories

Two Simple Theories of Covalent Bonding Valence Shell Electron Pair Repulsion

TheoryVSEPRR. J. Gillespie - 1950’s

Valence Bond TheoryHybridized orbitalsL. Pauling - 1930’s & 40’s

Stereochemistry

Study of the 3 dimensional shapes of molecules TWO MODELS VSEPR Theory Valence Bond Theory

Some questions to examine:Why are we interested in shapes?

What role does molecular shape play in life?

How do we determine molecular shapes?

How do we predict molecular shapes?

Determining Molecular Structure

Draw the Lewis dot structureidentify central atom

Count # of regions of high electron density on central atom

VSEPR tells the geometry around central atom

Determining Molecular Structure Identify lone pair effect on ideal

molecular geometry Repeat procedure for more than one

central atom Determine polarity from entire molecular

geometryelectronegativity differences

VSEPR Theory

regions of high electron density around the central atom go as far apart as possible to minimize repulsions

five basic shapesbased on # of regions of high electron density

several modifications of these five basic shapes will also be examined

VSEPR Theory

Two regions of high electron density

VSEPR Theory

Three regions of high electron density

VSEPR TheoryFour regions of high electron density

VSEPR Theory

Five regions of high electron density

VSEPR Theory

Six regions of high electron density

VSEPR Theory

electronic geometry (electron domain)determined by the locations of regions of high

electron density around the central atom(s) molecular geometry

determined by the arrangement of atoms around the central atom(s)

electron pairs are not used in the molecular geometry determination

VSEPR Theory CH

4 - methane

electronic geometrytetrahedral

molecular geometry tetrahedral

bond angles = 109.5o

VSEPR Theory

H2O - water

electronic geometry tetrahedral

molecular geometrybent or angular

bond angle = 104.50

VSEPR Theory

lone pairs of electrons (unshared pairs) require more volume than shared pairs

there is an ordering of repulsions of electrons around central atom

VSEPR Theory

1 lone pair to lone pair repulsion is strongest2 lone pair to bonding pair repulsion is

intermediate3 bonding pair to bonding pair repulsion is

weakest mnemonic for repulsion strengths

lp/lp > lp/bp > bp/bp lone pair to lone pair repulsion is why bond

angles in water are less than 109.50

Valence Bond Theory covalent bonds are formed by overlap of

atomic orbitals atomic orbitals on the central atom can mix

and exchange their character - hybridization

common hybridspink flowers, mules, corn, grass

Valence Bond Theory hybridized orbitals describe same shapes

as VSEPR Name of orbital Shape of orbital

sp3 tetrahedral

sp2 trigonal planar

sp linear

sp3d trigonal bipyramidal

sp3d2 octahedral

Hybrid Orbitals

Electronic Structures Lewis Formulas

1s 2s 2p

Be 3s 3p

Cl [Ne]

Be ··

····

··

.Cl

Hybrid Orbitals

Dot Formula Electronic Geometry

180o - linear

BeCl Cl··

····

····

··BeCl Cl

····

····

····

····

Hybrid Orbitals

VSEPR Polarity

bondspolar very

3.5 51. 3.5 ativitiesElectroneg

Cl - Be- Cl

0.22.0

180o-linear

BeCl Cl····

Hybrid Orbitals

VSEPR Polarity

moleculenonpolar

symmetric are dipoles bond

Cl---Be---Cl

180o-linear

BeCl Cl····

Hybrid Orbitals

Molecular Geometry

same as electronic geometrysymmetrical & nonpolar

Hybrid Orbitals

Valence Bond Theory (Hybridization)

1s 2s 2p 1s sp hyb 2p

Be 3s 3p

Cl [Ne]

Hybrid Orbitals Linear

Hybrid Orbitals

examples BF

3, BCl

3

all are trigonal planar, nonpolar molecules

Hybrid Orbitals


1s 2s 2p

B 3s 3p

Cl [Ne]

B·· .

Cl··

····

.

Hybrid Orbitals

Dot Formula Electronic Geometry··

B

Cl

Cl Cl··

····

··

···· ··

····

·· ··

B··

··

··

120-trigonal planar

Hybrid Orbitals

VSEPR Polarity

120-trigonal planar

BClCl

Clbondspolar ery v

3.0 1.5 ativitiesElectroneg

Cl - B

1.5

Hybrid Orbitals

Molecular Geometry

Hybrid Orbitals

Valence Bond Theory (Hybridization)

1s 2s 2p 1s sp2 hybrid

B 5s 5p

Cl [Ne]

Hybrid Orbitals Trigonal Planar

Hybrid Orbitals

examples CH

4, CF

4, CCl

4,

SiH

4,

SiF

4

all are tetrahedral, nonpolar moleculesas long as they have the same 4 substituents

Hybrid Orbitals


2s 2p

C [He] C..

. .

Hybrid Orbitals


2s 2p

C [He] 1s

H

C..

. .

H .

Hybrid Orbitals


C

H

H

H H......

..C

....

..

..

tetrahedral109.5o bond angles

Hybrid Orbitals

VSEPR Polarity

CH H

H

H

tetrahedralbondspolar slightly


H- C

0.4

Hybrid Orbitals

Molecular Geometry

Hybrid Orbitals

Valence Bond 2s 2p four sp3 hybrid orbitals

C [He] C [He] 1s H

Hybrid Orbitals Tetrahedron

Hybrid Orbitals

Examples PF5, AsF5, PCl5, etc.

All are trigonal bipyramidal, nonpolar molecules.

Hybrid Orbitals


4s 4p

As [Ar] 3d10 2s 2p

F [He]

As··

...

F···· .··

Hybrid Orbitals


··

As

F

F

FF

F

··

··

··

····

··

····

····

····

··

··

··

··

··

·· ··trigonal bipyramidal

As

··

··

······

Hybrid Orbitals

VSEPR Polarity

trigonal bipyramid

AsF

F

F

F

F

··

··

··

··

·· ··

··

··

····

··

····

····

bondspolar ry ve


F- As

1.9

Hybrid Orbitals

VSEPR Polarity

trigonal bipyramid

AsF

F

F

F

F

··

··

··

··

·· ··

··

··

····

··

····

····

symmetric dipoles cancel nonpolar molecule

AsF

F

F

F

F

··

··

··

··

·· ··

··

··

····

··

····

····

Hybrid Orbitals

Molecular Geometry

Hybrid Orbitals

Valence Bond (Hybridization) 4s 4p 4d

As [Ar] 3d10

five sp3 d hybrids

Hybrid Orbitals Trigonal Bipyramidal Molecules

Valence Bond (Hybridization)

Hybrid Orbitals Trigonal Bipyramid Molecules


Variations of Trigonal Bipyramidal Shape If lone pairs are incorporated into the

trigonal bipyramidal structure, there are three possible new shapes.

One lone pair - seesaw shape Two lone pairs - T-shape Three lone pairs - linear

Hybrid Orbitals

Example SF6, SeF6, SCl6, etc.

These are octahedral and nonpolar molecules. if all 6 substituents are the same

Hybrid Orbitals

Electronic Structures Lewis Formulas 4s 4p

Se [Ar] 3d10 2s 2p

F [He] ··F·· .··

·· Se·· ..

Hybrid Orbitals


Se

F

F

F

F

F

F

····

··

······

··

····

··

··

··

··

··

··

··

··

····

··

······

··

octahedral

Se

··

··

····

·· ··

Hybrid Orbitals

VSEPR Polarity

octahedral

SeF

F

F

F

F

F

bondspolar ery v


F- Se

1.6

Hybrid Orbitals

VSEPR Polarity

octahedral

SeF

F

F

F

F

F

symmetric dipoles cancel nonpolar molecule

SeF

F

F

F

F

F

Hybrid Orbitals

Molecular Geometry

Hybrid Orbitals Octahedral Molecules


4s 4p 4d

Se [Ar] 3d10

six sp3 d2 hybrids

Hybrid Orbitals Octahedral Molecules Valence Bond (Hybridization)

AB6- No Lone Pairs - Octahedral Molecules

Variations of Octahedral Shape

If lone pairs are incorporated into the octahedral structure, there are two possible new shapes.

One lone pair - square pyramidal Two lone pairs - square planar

Compounds Containing Double Bonds Ethene or ethylene is the simplest double

bond containing organic compound. Lewis Dot Formula for ethene C2H4

N = 2(8) + 4(2) = 24 A = 2(4) + 4(1) = 12 S = 12

Compounds Containing Double Bonds Lewis Dot Formula for ethene C2H4

CC

H

HH

H

C CH

H

H

H····

·· ·· ··

··or

Compounds Containing Double Bonds VSEPR

suggests C atoms at center of two equilateral triangles

C C

H

H

H

H

Compounds Containing Double Bonds Valence Bond Theory

sp2 hybrids for the C atoms

1 electron remains in an unhybridized p orbital

2s 2p three sp2 hybrids 2p

C

Compounds Containing Double Bonds

C atoms are sp2 hybridized with single electrons in each of the lobes

Top view


The unhybridized 2p orbital on the C atom lies perpendicular to the planar sp2 lobes.Side view


C=C double bond results from head-on overlap of two sp2 hybrid orbitals


The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a bond.

Compounds Containing Double Bonds The other portion of the double bond, resulting

from the side-on overlap of the p orbitals, is designated as a bond.

Compounds Containing Double Bonds The final result is a bond that looks like this.

Compounds Containing Triple Bonds Ethyne or acetylene is the simplest triple

bond containing organic compound. Lewis Dot Formula for ethyne C2H2

N = 2(8) + 2(2) = 20 A = 2(4) + 2(1) =10 S = 10

Compounds Containing Triple Bonds Lewis Dot Formula for ethyne C2H2

C C HHCH HC·· ·· ···· ·· or

Compounds Containing Triple Bonds VSEPR

suggests C and H atoms are 180o apart.

H C C H

Compounds Containing Triple Bonds Valence Bond Theory

sp hybrids for the C atoms

2 electrons remain in unhybridized p orbitals

Compounds Containing Triple Bonds Valence Bond Theory

sp hybrids for the C atoms

2 electrons remain in unhybridized p orbitals

2s 2p two sp hybrids 2p

C

Compounds Containing Triple Bonds

triple bond results from head-on overlap of two sp hybrid orbitals

C C

Compounds Containing Triple Bonds Note thatthis gives us one bond and two

bonds.

Compounds Containing Triple Bonds The final result is a bond that looks like this.

Summary of Electronic & Molecular Geometries

Synthesis Question 1 The basic shapes that we have discussed are present in

essentially all molecules. Shown below is the chemical structure of vitamin B6 phosphate. What is the shape and hybridization of each of the indicated atoms in vitamin B6 phosphate?

N+

H

CH3

OH

COH

CH2

OP

O

O

O

1

2

4

5

3

Synthesis Question 1trigonal planar sp2

bent or angular sp3

tetrahedral sp3

trigonal planar sp2

trigonal planar sp2

N+

H

CH3

OH

COH

CH2

OP

O

O

O

1

2

4

5

3

Group Question 1 Shown below is the structure of penicillin-G.

What is the shape and hybridization of each of the indicated atoms in penicillin-G?

CH

C N

CH S

CH

C

CH3

CH3

OH

OO

NHO

CH2CCC

CC C

HH

H

HH

1

23 4

5 6

7

8910

Synthesis Question 2 As we all know, in the wintertime we are more

likely to get shocked when we walk across carpet and touch the door knob. Here is another wintertime experiment to perform. Turn on a water faucet until you have a continuous but small stream of water coming from the faucet. Brush your hair vigorously then hold the brush near the stream of water. You will notice that the stream bends towards the brush. Why does the water bend?

Synthesis Question 2

Since water is a highly polar molecule, it is attracted by the electromagnetic field generated by the hair brush. This causes the stream to bend.

Group Question 2 On a recent “infomercial” it was claimed

that placing a small horseshoe magnet over the fuel intake line to your car’s carburetor would increase fuel mileage by 50%. The reason given for the mileage increase was that “the magnet aligned the molecules causing them to burn more efficiently.” Will this work? Should you buy this product?

Documents

Molecular Geometry and Bonding Theories. Two Simple Theories of Covalent Bonding Valence Shell Electron Pair Repulsion Theory VSEPR R. J. Gillespie