CMY 117-Theme 5 Molecular Geometry PDF

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Summary

Summary theme 5 part 2...

Description

2021/04/16

Molecular Geometry Same components Same connections

But …….

∼ Same size

Non-superimposable mirror images … different spatial arrangements 1

CMY 117 Theme 5 Molecular structure

Molecular Geometry

Same components ... C10H14O

Consider ‘carvone’ : H3 C

CH3 C HC

Chiral ...

H2 C

Same properties ..

O C

 CH

CH2

C

O C H2 C

Caraway ..

CMY 117 Theme 9 Chemical bonding and Molecular structure

CH2

Different (3-D) arrangements of atoms in space… Geometries

C CH2

(S)-(+)-Carvone

CMY 117 Theme 5 Molecular structure



CH

C CH3

CH

Same connections (bonds)… Lewis structures

CH2

CH3

(R)-(-)-Carvone

Spearmint ..

Non-superimposable mirror images … Enantiomers

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Molecular Geometry Receptor sites for smell interact differently with the two enantiomers Receptor sites are also chiral … Most amino acids, proteins, sugars, etc. in our bodies are present in only one particular chiral form ….. Includes active sites for medicinal drugs … ⇒ The body may react differently to enantiomers of a medicinal drug … Most drugs are initially made as a mixture of enantiomers … refine to become ‘enantiopure’ 3

CMY 117 Theme 5 Molecular structure

Molecular Geometry “Thalidomide disaster” Prescribed as anti-nausea medication for pregnant mothers in late 1950 ‘s ….

(R)-Thalidomide

(S)-Thalidomide

10 000 children…. CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Molecular Geometry See Fig. 8.14 “A closer look” pp. 395-396

Bond ‘straws’ Atom ‘centres’ “Ball – and - stick”

Use molecular models to aid in ‘seeing’ the 3-D construction of molecules/polyatomic ions…

“Space - filling”

Terminal atoms…

5

CMY 117 Theme 5 Molecular structure

Molecular Geometry CH4 , CO2 , CFC’s , ..

CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Molecular Geometry So ,knowledge of molecular geometry (3 - D structures) is very important…. This can be determined, inter alia, by well-established, sophisticated (time-consuming and expensive!) analytical techniques, such as by X- ray diffraction, IR spectroscopy, UV-VIS spectroscopy, etc. Is there a simple, quick method of predicting likely structures (shapes) of simple molecules and polyatomic ions? Yes! …We use a simple (non-empirical) geometrical model…. 7

CMY 117 Theme 5 Molecular structure

Molecular Geometry Bonding occurs when there is an overall decrease in energy (stabilization) of the chemical system. This is influenced by (i) the arrangement of valence electrons pairs amongst the atoms involved … Octet rule … Formal charges …

(ii) minimizing the repulsion amongst the valence electron pairs involved … For molecules / ions with more than two atoms, the electron pairs around the central atom(s) arrange themselves as far apart as possible in space, to minimize their mutual repulsions (maximizes stability). CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Molecular Geometry Known as the

Valence Shell Electron Pair

VSEPR Theory

Repulsion

Keeping the electron pairs around the central atom(s) as far apart as possible in space, requires that Lewis Structures form our point of departure… (i) we know their relative positions …. (ii) we consider factors that may affect the repulsion…. 9

CMY 117 Theme 5 Molecular structure

Molecular Geometry Define (count) the regions or domains of electron density around the central atom /atom whose local geometry is of interest:

H C

Four domains

S

O H

H

N H

C

N

H Two domains

Three domains CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Molecular Geometry ‘Stylized’ electron

Occupies a larger region of space

Double /triple bond

density domain..

A

X

A

A

X

Lone pair

Two single bonds

Single bond

Single bond

X

Occupies a much larger region of space

X

Repulsion between

Repulsion between

<

single bond and single bond

<

X

single bond and lone pair Repulsion between Repulsion between single bond and multiple bond lone pair and lone pair

<

Molecular Geometry A

X

X X Position of domains in space … “electronic geometry” or “electron pair geometry” “electronic geometry” CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Repulsions

CMY 117 Theme 5 Molecular structure

Position of atoms in space … “molecular geometry” Position of hands in space … “hand geometry”

AX3 “arm geometry”

“hand geometry”

Position of in space … “arm geometry”

“molecular geometry” 12

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Molecular Geometry Lone pair

“electronic geometry” involves ALL the domains

A

“molecular geometry” involves ONLY the bonding domains

X X

AX2E1

13

CMY 117 Theme 5 Molecular structure

(i) Draw imaginary lines through the vertices ….

Molecular Geometry …balloons

There is a ‘natural’ orientation (geometry) for a number of balloons tied together, in order to maximize the space that each one can occupy…. Trigonal Planar

Linear 2

Tetrahedral

3

(ii) Connect…. (iii) Describe the resulting polygon….

Trigonal bipyramidal

Octahedral

5

6

4

‘equatorial’ …. ‘axial’ ….

Results in five e- domain arrangements of minimum energy seen in the great majority of simple molecules and polyatomic ions…. Tetrahedron CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

Trigonal bipyramid

Octahedron

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Molecular Geometry …’ procedure’… Draw 3 – D structure.

6.

Determine the molecular geometry.

5. 4.

Consider lone pairs.

3.

Determine the electronic geometry.

2.

Count the number of electron domains around central atom(s).

1.

Draw the correct Lewis structure. 15

CMY 117 Theme 5 Molecular structure

Molecular Geometry Bond polarities and polar molecules …. The difference in electronegativity (Δ Δχ) between the atoms in a bond causes an asymmetrical distribution of the shared electron density…. See figure 8.10 …. δδ 2.1 3.9 Direction (arrowhead)

A

+

A

X

A

X

Bond angle

Dipole moment (μ ) Resultant Dipole moment (μ )

X CMY 11

X

ure

towards greater e- density. Length indicates the magnitude of μ (related to Δχ)

STUDY examples on pp. 385- 386… (!!)

Non-zero resultant ⇒ molecule is polar “A closer look” p. 384

CMY 117 Theme 9 Chemical bonding and Molecular structure

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Molecular Geometry Linear geometry – 2 domains / regions:

AX2

- 2 bonding domains A

X

X

- 0 lone pairs

Lewis structure

Cℓ

Be

3 – D Lewis structure

Electronic geometry

Cℓ

180 °

Cℓ

180 °δ +

δ-

Be

Be

δ-

Cℓ

No net resultant, μ = 0 ∴ Non-polar

- Linear

Molecular geometry

CO2, HCN, C2H2, NO2+, I3-, ICℓ2- , SCN- , …

Cℓ

Cℓ

Be

17

- Linear

CMY 117 Theme 5 Molecular structure

Molecular Geometry

3

Trigonal planar geometry – 3 domains / regions: X

AX3

- 3 bonding domains Molecular geometry F

- 0 lone pairs

A

Electronic geometry Lewis structure B

F

120 °

- Trigonal Planar

F

B

- Trigonal Planar

F

B

F

BF3, SO3, Cℓ2CO, H2CO, NO3-, CO32- , NO2F , HCO2- , C2H4 ,… CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

F 120 °

No net resultant ∴ Non-polar δ-

F

F

δ-

120 °

3 – D Lewis structure

δ+

B

120 °

F

δ-

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Molecular Geometry

3

Trigonal planar geometry – 3 domains / regions:

H

2.1 H 116 °

C

122 °

2.5

O

O 3.5

C

H

H -

O

120 °

N

O

N

O

O CMY 117 Theme 5 Molecular structure

-

120 °

O

O 120 °

ALL the angles are equal, since resonance is present!!….

Molecular Geometry

19

3

Trigonal planar geometry – 3 domains / regions:

AX2E1

- 2 bonding domains Molecular geometry

- 1 lone pair

A

Angular / Bent Lewis structure

Electr

eometry O

O

O

SO2, NH2-, HOCℓ , …

O

>120 ° - Trigonal Planar

NO2, NO2-,

No dipole moments ∴ Non-polar

CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

O

O

O

O 116.8 °

O 3 – D Lewis structure

O 20

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Molecular Geometry

4

Tetrahedral geometry – 4 domains / regions: X

AX4

- 4 bonding domains Molecular geometry

- 0 lone pairs

A

H

Electronic geometry

- Tetrahedral

Lewis structure C

H C

H

109.5 °

H

H

H

H

H

H

109.5 °

- Tetrahedra

In the plane

NB: See Figure 8.13 !! loser look” p. 384

NH4+, CHBrCℓ, SO42- , CℓO4- , NF4+, …

Behind the plane

H

C

3 – D Lewis structure

H

21

In front of the plane

H

Molecular Geometry

4

Tetrahedral geometry – 4 domains / regions:

AX3E1

- 3 bonding domains

Molecular geometry

- 1 lone pair Electronic geometry

A

- Trigonal pyramidal

Lewis structure

N H

H

N H

109.5 °

H

H

- Tetrahedra

NF3, AsBr2Cℓ, H3O+, NO2- , … CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

Resultant dipole moment ∴ Polar δ +

H

δ-

H

N H

3 – D Lewis structure

H

107 ° δ + δ+

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Molecular Geometry

4

Tetrahedral geometry – 4 domains / regions:

AX2E

- 2 bonding domains Molecular geometry

- 2 lone pairs

- Angular / Bent

Electronic geometry Lewis structure

O

H

SeBrCℓ, H2S, SF2, HOCℓ…

CℓO2- ,

H

O H

109.5 °

H

- Tetrahedra PCℓO2- , Resultant dipole moment

O

3 – D Lewis structure

H

104.5° δ + δ+

∴ Polar

CMY 117 Theme 5 Molecular structure

δ-

23

H

Molecular Geometry

5

Trigonal bipyramidal geometry – 5 domains / regions: X

AX5

- 5 bonding domains A

Molecular geometry

- 0 lone pair Electronic geometry

F

X

X

Lewis structure

F F F

F

P F

F 120 °

120 ° - Trigonal bipyramidal Axial positions

AsF5, SbCℓ5 , SOF4, BrF5 , SnCℓ5- … CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

P

F

Equato positio

- Trigonal bipyramidal F

F

90 ° 90 °

δNo Non n -P -Polar

δδ+

F

F

P 120°

F

F

F δ-

δ-

3 – D Lewis structure δ-

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Molecular Geometry

5

Trigonal bipyramidal geometry – 5 domains / regions: X

- 4 bonding domains - 1 lone pair

AX4E1 A

Molecular geometry

- In equatorial position! Electronic geometry

F

X

- See-saw F

Lewis structure S

F F

F

F

S

120 °

- Trigonal bipyramidal

F

ICℓ4+ , AsCℓ4- , PBr4-, TeCℓ4-, IO2F2-, XeO2F2…

F

δ-

δ+ 90 °

F

S

> 180 °

F δ< 120 °

3 –D Lewis structure

F

Polar

δ-

F

25

δ-

Molecular Geometry

5

Trigonal bipyramidal geometry – 5 domains / regions: X

- 3 bonding domains - 2 lone pairs Molecular geometry - BOTH in equatorial positions!! F T - shaped Electronic geometry

AX3E2

X

A

F

Lewis structure I

F

F 120 °

I F

- Trigonal bipyramidal

F

CℓF3 , BrF3, SeO32-,

…

CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

> 180 ° 90 °

Polar

F

δ+

δ< 90 ° δ-

I

F

F

F

3 –D Lewis structure 26

δ-

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Molecular Geometry

5

Trigonal bipyramidal geometry – 5 domains / regions: X

- 2 bonding domains - 3 lone pairs - ALL in equatorial positions!!

AX2E3

Molecular geometry I

- Linear

X

Electronic geometry Lewis structure

-

I

I

I 120 °

I

I

- Trigonal bipyramidal

I

I

180 °

I

90 °

XeF2 , ICℓ2- , RnCℓ2 , …

-

3 –D Lewis structure

No Nonn- P olar

ructure

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6

Molecular Geometry Octahedral geometry – 6 domains / regions: X

- 6 bonding domains - 0 lone pairs

AX6

Molecular geometry

A

F

- Octahedral

X

Electronic geometry Lewis structure

S

F F

90 °

S F

F - Octah

SeF6 , PF6- ,PCℓ6- , SbF6- , IF6+ , SiF62- , XeO2F4… Non Non-P -P -Polar

CMY 117 Theme 9 Chemical bonding and Molecular structure

90 °

δ+ δ-

F

F

δ-

F F

F

δ-

F

F

F

90 °

δ-

S

F δ-

F

F

F F

δ-

3 –D Lewis structure

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6

Molecular Geometry Octahedral geometry – 6 domains / regions: X

- 5 bonding domains - 1 lone pair

AX5E1

Molecular geometry

A

F

Electronic geometry Lewis structure

Br

F F

90 °

Br F

F

δ-

F - Octah

δ-

SF5-, XeOF4 , IF5 , SbF5-, …

F

F

δ-

F

F

F

90 °

- Square pyramidal

F < 90 ° δ + Br F

δ-

F F

δ-

3 –D Lewis structure

Polar

29

6

Molecular Geometry Octahedral geometry – 6 domains / regions: - 4 bonding domains - 2 lone pairs - In opposite positions.

AX4E2 A

Molecular geometry - Square planar

Electronic geometry Lewis structure

F

F

90 °

Xe F

F

F

F

90 °

Xe F

δδ-

F

F - Octah

ICℓ4- , BrF4- , IF4- , … CMY 117 Theme 5 Molecular structure

CMY 117 Theme 9 Chemical bonding and Molecular structure

δ+ δ-

NonNon-P Polar

F

Xe 90 °

F F

δ-

3 –D Lewis structure 30

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Molecular Geometry Complex molecules: 180 °

Methyl cyanoacrylate ….

N C

H

C

(Bond order)

C

>C

C

(Atom size…group) Cℓ < H Br C O C >C (Atom size…period) 3

CMY 117 Theme 5 Molecular structure

Molecular Geometry (iii) Bond Energy …. “Bond

H

I

(g)

H

+

I

(g)

(g)

Require energy to break the bond…. endo...