chemical structure: chemical bonding. polar bonds

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Chemical Bonding 3 POLAR BONDS

University of Lincoln presentation

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http://chemistryfm.blogs.lincoln.ac.uk/


Definitions…

• A HOMONUCLEAR BOND is a bond between two identical atoms

• A HETERONUCLEAR BOND is a bond between different atoms



Homonuclear & Heteronuclear bonds

Homonuclear bonds Hetronuclear bonds

Ethane (C2H6)Hydrazine (N2H4)

Hydrogen peroxide (H2O2)


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Determining Bond Energies

• Consider the 2 homonuclear diatomics H2 and F2

• The bond energy of H–F would be expected to be the mean of the bond energies of H–H and F–F

• Is this right?





Bond EnergiesBond Dissociation Energy (kJmol-1)

X Y X–X Y–Y ½ (X–X + Y–Y)

ExptlX–Y

H F 436 159 298 570*

H Cl 436 242 339 432*

H Br 436 193 315 366*

H I 436 151 294 298



Anomalous Bond EnergiesMolecule Expected

Bond Energy

(kJmol-1)

Measured Bond

Energy (kJmol-1)

E

H–F 298 570 272

H–Cl 339 432 93

H–Br 315 366 51

H–I 294 298 4



Why are some heteronuclear bonds much stronger than

expected?

?



SYMMETRICAL BONDSIn a HOMONUCLEAR diatomic molecule, the

electrons within the bond are shared equally between the two atoms – a symmetrical bond:

The electrons sit in molecular orbitals which lie EQUI-DISTANT from each atom

Energ

y

2s 2s

σ*(2s)

σ*(2s)

LiLi



ASYMMETRICAL BONDS

• In a HETERONUCLEAR diatomic molecule, the electrons within the bond are NOT always shared equally between the two atoms – an asymmetrical bond.

• In an assymetrical bond, the electrons sit closer to one atom than the other, leading to a POLAR BOND:

H–F–+

The electrons are sitting closer to the F atom



Why does this happen?



ElectronegativityPauling defined

ELECTRONEGATIVITY as:

“the power of an atom in a molecule to attract electrons

to itself”

This is an atomic property, but only applies when the atoms are in a bond



The higher the electronegativity, the stronger the ‘pulling’ power of the atom within a bond

O

3.4

F

4.0

N

3.0

C

2.6

Cl

3.2

H

2.2

Li

1.0

Na

0.9

K

0.8

Rb

0.8

Cs

0.8

Mg

1.3

Be

1.6

Ca

1.0

Sr

0.9

Ba

0.9

S

2.6

P

2.2

B

2.0

Si

1.9

Al(III)

1.6

Se

2.6

Br

3.0

As(III)

2.2

Ge(IV)

2.0

I

2.7

Te

2.1

Sb

2.1

Ga(III)

1.8

Sn(IV)

2.0

In(III)

1.8

At

2.2

Po

2.0

Bi

2.0

Pb(IV)

2.3

Tl(III)

2.0

Electronegativity



…When electrons are held tightly by an atom in a bond,

due to the high electronegativity of that atom, the bond is much

harder to break

So, why are some heteronuclear bonds much stronger than expected?



Examples of Polar Bonds

F H

Cl H

–

+

+

+-

+-

The slight charges on each end of the molecule lead to electrostatic attraction between adjacent molecules – HYDROGEN BONDING

OH

H



Definition…

• A HYDROGEN BOND is an interaction between a hydrogen atom attached to an electronegative atom, and an electronegative atom which possesses a lone pair of electrons

The strongest hydrogen bonds involve the first row elements F, O or N



HYDROGEN BONDING ()

H–F

H–F

H–F

H–F

H–F



Hydrogen bonding affects the physical properties of

molecules with polar bonds

150170190210230250270

NH3 PH3 AsH3 SbH3Molecule

Boi

ling

poi

nt (K)

NH3, H2O and HF all have anomalously HIGH boiling points, since extra energy is needed to break the hydrogen bonds

15

20

25

NH3 PH3 AsH3 SbH3Molecule

Δvap

H/k

J mol

-1

150

200

250

300

350

400

H2O H2S H2Se H2TeMolecule

Boili

ng p

oint

(K)

150

200

250

300

350

HF HCl HBr HIMolecule

Boi

ling

poi

nt (K)



Can Molecular Orbital Theory account for polar

bonds?

?



A quick recap…ATOMIC Orbitals

MOLECULAR Orbitals

H + H H2



F2

Electronic configuration of 9F is:1s2 2s2 2p5 (9 electrons)

The F atom needs 1 more electron to give it a full valence shell (8 outer electrons)– it does this by forming a single covalent bond (in this case with another F atom)

Hence, we know we have a single bond in F2: F–F

F F



BUT we know that the F–F molecule has 18 electrons (2 x 9)

How can we arrange 18 electrons in molecular orbitals and end up with only ONE bond?

SOLUTION:

•For every bonding orbital there must be an ‘anti-bonding orbital’

•An electron in a bonding orbital is cancelled out by an electron in an anti-

bonding orbital




http://positron.ps.uci.edu/~dkirkby/CPEP/antimatter/V10/AntimatterV10.pdf


Energ

y

2p 2p

σ*(2pZ)

σ (2pZ)

π*(2py)π*(2px)

π(2py)π(2px)

2s 2s

σ*(2s)

σ (2s)F F

Consider the MO diagram of F2



Homonuclear MO diagrams are symmetrical. Heteronuclear MOs are asymmetrical – the energies of equivalent atomic orbitals are DIFFERENT

Energ

y

2s

2s

σ*(2s)

σ*(2s)X Y

Heteronuclear Diatomic molecule MO



Energ

y

2s

2s

σ*(2s)

σ*(2s)Li H

Only valence orbitals shown. The 1s (H) and 2s (Li) overlap to form the and * molecular orbitals

LiH molecule



The 2pz(F) can overlap with the 1s(H). The orbitals that do not overlap form NON-BONDING MOs

Energ

y 1s

2p

σ*

σ

2s

H F

Non-bonding

Non-bonding

HF

HF



The 1s orbital on the H overlaps with the 2pz on the F to form a -bond. No overlap can occur between the 1s and the 2px or 2py, as

these are pointing in the wrong direction

1s 2pz

1s 2px

H F

H F

Z Z

x

Bonding

Anti- Bonding



HF

The electrons are sat closer to the F atomic orbitals than the H atomic orbitals. Therefore it is predicted that the H–F bond

would be POLAR

Energ

y 1s

2p

σ*

σ

2s

H F

Non-bonding

Non-bonding

HF

H–F+ -



Energ

y 2s

2p

σ*

σ

2s

Li F

Non-bonding

Non-bonding

LiF

Li–F+ -

LiF



Hence, the MO theory can predict POLAR

bonds



Summary



What you should know…• Difference between homonuclear and

heteronuclear bonds

• Explain why some heteronuclear bonds are harder than expected to break

• How the presence of hydrogen bonding in molecules affects some of their physical properties, like boiling points

• How to draw the MO diagram of a heteronuclear diatomic molecule, and understand how bonding, anti-bonding and non-bonding orbitals are formed

• Use the MO diagram to determine whether the bonding is likely to be polar



Definitions…

• Homonuclear bond• Heteronuclear bond• Polar bond• Hydrogen bond• Electronegativity


Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org

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