THE SHAPES OF MOLECULES

ELECTRON PAIR REPULSION THEORY.

•Molecular shape depends on electron pairs around a central atom.

•What are the rules that determine the shapes of simple molecules ?

Starter: What is a lone pair of electrons?

What is a charge cloud?

Electron charge clouds

• Repel each other – why ?

• Shape of the charge clouds affect how much

• So the greatest angles are between lone pairs of electrons…

A BETTER THEORYA BETTER THEORY

V alenceS hellE lectronP airR epulsion

The new orbitals point tothe corners of a tetrahedron.

Bond angles are 109o28’’

NEW BONDING ORBITALSNEW BONDING ORBITALS

Each bonding situationcan be treated in asimilar fashion ……..

VALENCE SHELL ELECTRON PAIR REPULSION

VSEPR THEORY

4 pair tetrahedral 109o28’ sp3 ( pyrimidal, angular )

3 pair trigonal planar 120o sp2

2 pair linear 180o sp

pairs geometry angles hybridization

For most molecules, these predictions are correct to within a few degrees (± 5o).

6 pair octahedral 90o d2sp3

5 pair trigonal bipyramid 120o, 90o dsp3

ORGAN IC

INTRODUCTIONA) Lewis structures do not indicate the three dimensional shape of a molecule. They do not show the arrangement space of the atoms, what we call the molecular geometry or molecular structure.

B) Molecules have definite shapes and the shape of a molecule controls some of its chemical and physical properties.

II. Valence Shell Electron Pair Repulsion Theory - VSEPR - predicts the shapes of a number of molecules and polyatomic ions.

A) Assumptions of VSEPR Theory

1) Electron pairs in the valence shell of an atom tend to orient themselves so that the total energy is minimized. This means that: the electrons will approach the nucleus so that we can predict the shapes according to the order of repulsion:

2) bbbbbbbbbbbbbbpositions as far away from each other as possible to minimize _______________ .

ORDER OF REPULSION BETWEEN

VARIOUS ELECTRON PAIRS • bp –bp LESS THAN bp-lp LESS THAN lp-lp

WHERE ‘bp’ MEANS BONDING OR

BOND PAIR AND ‘lp’MEANS LONE PAIR

2) Because lone pairs of electrons are spread out more broadly than bond pairs, repulsions are greatest between two lone pairs, intermediate between a lone pair and a bond pair, and weakest between two bonding pairs of electrons.3) Repulsive forces decrease rapidly with increasing interpair angle - greatest at 90o, much weaker at 120o, and very weak at 180o.

B) What are the ideal arrangements of electron pairs to minimize repulsions?

1) We need to identify the number of regions of high electron density, i.e. the number of stereoactive sets of electrons. One stereoactive set can be a single, double or triple bond.

2) If only single bonds, the number of stereoactive sets is equal to the bond pairs plus the lone pairs of electrons around the central atom.

TETRAHEDRAL GEOMETRY

4 pair tetrahedral 109o28’ sp3

VARIATIONS ON TETRAHEDRALVARIATIONS ON TETRAHEDRAL

TETRAHEDRAL

PYRIMIDAL

ANGULAR

C

N

O

..

....

H

H

HH

H

HH

H

H

Distinctions are oftenmade in general chemistry.

Although the molecules have different shapes, the orbitals use the same tetrahedral arrangement (includes pairs).

Usually we won’t bother.

HO

H

HC

HH

H

HN

HH

4 pair = tetrahedral ( 109o28’)

109o28’

107o

105o

HC

FH

F

CH3N

CH3CH3

ClC

HH

H

tetrahedral

pyrimidal

bent

108o

112o

108o

110o

108o

HC

CH3H

CH3112o

106o

HS

H

92o

HP

HH

93o

Period Three

Perfect Symmetry

EXPERIMENTAL RESULTS

NH H:

....

H

..

EXPERIMENTAL MEASUREMENT OF ANGLESEXPERIMENTAL MEASUREMENT OF ANGLES

Atoms have dense nuclei,the H-N-H angle is easy tomeasure.

Electron pairs are diffuse. It is difficult to measure theangle between an electron and an atom or another pair.

107o

?

Most experimental methods(X-ray and electron diffraction)rely on locating the atomswhich have massive nucleiand are easy to find.

Other angles are generally calculated.

DEVIATIONS FROM THE IDEAL TETRAHEDRAL ANGLES

C

H

H HH

:

:....

symmetrical moleculeall repulsions are equal

perfect tetrahedralall angles 109o28”

NH H:

....

H

.. anglebecomeslarger

larger repulsion

smallerrepulsion angle becomes

smaller

not all pairs are equivalent

the unshared pairs repel morestrongly than the bonded pairs

UNEQUAL REPULSIONS BETWEEN PAIRSUNEQUAL REPULSIONS BETWEEN PAIRS

107o

H-N-H angle reduced to 107o

109o28”

C

CH3

HCH3

H

:....

STERIC REPULSIONSSTERIC REPULSIONS

..

“Steric Repulsion”

HC

CH3H

CH3112o

106o

The CH3 groups areso large that they pushagainst each other inspace, opening the angle.

PERIOD THREE ELEMENTS ARE NOT PERIOD THREE ELEMENTS ARE NOT AS LIKELY TO BECOME TETRAHEDRALAS LIKELY TO BECOME TETRAHEDRAL

NP

..

....

..PERIOD 2

PERIOD 3

largerrepulsions

smallerrepulsions

Not as much is gained by adjusting to ideal angles.The unshared pair on P does take a lot more space.

Electrons are more diffuseand further from the nucleus.

HP

HH

93o

HN

HH 107o

TRIGONAL PLANAR GEOMETRY

3 pair trigonal planar 120o sp2

3 pair trigonal planar ( 120o)

F BF

F

HCH

H

+120o120o

( incomplete octet )

carbocation

all repulsions equal3 equivalent bonds

not a stable molecule,but an “intermediate”ion - it reacts quickly

EXPERIMENTAL RESULTS

3 pair trigonal planar ( 120o)

( pi bonded )

C CHH

H H117o

121.5o

C CH2

H

H

121.5o

117o

C CH2

Cl

Cl

123o

114o

C CH2

F

F

125o

110o

C OH

H116o

122o

C OH

H116o

122o

C OCl

Cl

124.5o

111o

C OF

F

126o

108o

double bonds countas only one pair

EXPERIMENTAL RESULTS

DEVIATIONS FROM THE IDEAL TRIGONAL PLANAR ANGLES

C ........

larger repulsionone pair : two pairs

smaller repulsionone pair : one pair

> 120o

< 120o

In alkenes the C=C H angle is typically larger-than the H C H angle. - -

UNEQUAL REPULSIONSUNEQUAL REPULSIONS

C

H

H

UNEQUAL REPULSIONSUNEQUAL REPULSIONS

more repulsion

less repulsion

Electrons in C-H bondsare shared nearly equally.

Electrons in a C-Cl bond are closer to chlorine and furtherfrom each other.

POLAR BOND

The electronegative Cldraws electrons closerto that end of the bond.

NONPOLAR BOND

Electrons in C-H bondsare closer to carbon(near center of bond) than in the case below.

C ........

H

H

r

.

.C ....

.

.Cl

Cl

polar

R

Fluorines cause even smaller angles.

C CH2

H

H

121.5o

117o

C CH2

Cl

Cl

123o

114o

C CH2

F

F

125o

110o

LINEAR GEOMETRY

2 pair linear 180o sp

2 pair linear ( 180o)

CH3 Mg CH3

H C NH C C H

CH3 C C CH3

CH2 C CH2

CH2 C O

both 180o

incomplete octet pi bonded

all 180o

O C O

H Be H

triple bonds countas only one pair

EXPERIMENTAL RESULTS

There are no deviations from the ideal 180o when observing linear molecules!

EXAMPLES

SOME MOLECULESSOME MOLECULESPREDICT ANGLES ( assume no distortions occur )

amide

azo compound

Remember this functional group?

Not a functional group you have to know right now.

C C

O

N

H

H

H H

H

....

:

N N CC

H

H

H H

H

H

.. ..