BAB 2BAB 2

Orbital dan perannya pada Ikatan Kovalen

FROM LEWIS DIAGRAMS TO MOLECULAR SHAPE

VSEPR THEORYVSEPR THEORY

H. H.

H. H.

H H

IKATAN KOVALENIKATAN KOVALEN

Tumpang tindih orbital

Pembentukan Ikatan

Atom yang terpisah

atomsmovecloser

( Model tumpang tindih orbital)

QUESTIONQUESTION

…….. LET’S TRY IT FOR H.. LET’S TRY IT FOR H22OO

Can we predict the shapes of molecules simply bycombining the atomic orbitals available on each atom?

H O

H

:..

O

y

x

z

..2s

.

2p

.2p

oxygen = [He]2s22p4

OXYGENOXYGENORBITALSORBITALS

[ cartoon ]

2p Orbital saling tegak lurus (90o)

2p22p12p1

O

y

x

z

.

... ..

2s

2p

2p

2p

OXYGENOXYGENORBITALSORBITALS

oxygen = [He]2s22p4

unpaired

unpaired

O

y

x

z

H. .

.

H.

.. ..

Combining atomicorbitals to form H2O.

Incorrectlypredicts a90o angle.

2s

2p

2p

2p

1s

1s

oxygen = [He]2s22p4

hydrogen = 1s1

H

H

O

105o

The actual H-O-H anglein water (measured byelectron diffraction) is 105o

EXPERIMENTAL RESULTEXPERIMENTAL RESULT

This is not very good agreement with the atomic orbital model!

VSEPRVSEPR TheoryTheoryValenceShellElectronPairRepulsionBased on the simple idea that

groups of electrons repel each otherPredicts molecular shapes quite

well

A better result is predicted by VSEPR theory

consider the completedvalence shell to be aspherical volume aroundthe nucleus

try to minimize repulsionsby maximizing the distancebetween all pairs ofelectrons

electron pairs (4 pair)repel each other

in the final solution, theyshould all be equidistant

valenceshell

nucleus

TETRAHEDRAL

Basic Shapes of MoleculesBasic Shapes of Molecules

A B A

A

BA A

B A

A

Aor

A

BAA

A

B A

A

AA

A

A

B

AA A

A A

LINEAR

TRIGONAL PLANAR

TETRAHEDRAL

TRIGONAL BIPYRAMID OCTAHEDRAL

Bond angle = 180°

Bond Angles = 120°

Bond angles = 109° 28'

Bond angles = 120°, 90° Bond angles = 90°

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

OrbitalsOrbitals

The region of space around an atom in which an electron is likely to be found is an orbital.

The shape and size of the orbital are determined by a mathematical equation called a wave function.

OrbitalsOrbitals

When atoms combine to form molecules, they do so by combining the wave functions for the individual atomic orbitals.

We say that the orbitals “overlap.”The region of space defined by this

combination of orbitals is the molecular orbital.

Sigma BondsSigma BondsHead-on overlap of atomic

orbitalsElectron density is a symmetrical

cylinder around the bond axis

Atomic orbital combinations that give bonds:s s p p ps

Pi BondsPi BondsSide-on overlap of atomic orbitalsElectron density is above and below a

nodal plane on the internuclear axis

Atomic orbital combinations that give bonds:

p p p d

HYBRIDIZATIONHYBRIDIZATION

HOW ARE THE OBSERVED BOND ANGLES ACHIEVED?

“Vision is the art of seeing things invisible.”

Jonathan Swift

atomicorbitals

hybrid atomic orbitals

molecular orbitals

HYPOTHETICAL BONDING PROCESS

2s2px,2py,2pz

sp,sp2py,2pz , , ,n

These orbitals are for the atom - we can’t expect that they are suitable for the molecule.

WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ?WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ?

During bonding ….new orbitals form thatare more suitable for making bonds.

After bonding (overlap)we get a totallynew solution for thenew molecule.

overlap

NOTE. Formally LCAO theory and Molecular Orbital theory are two completely different approaches. You do not need to use hybid orbitals to derive the molecular orbitals, combinations of any type of function will do. Nevertheless, the abstraction presented above is quite useful, as we will see quite soon.

LCAO

O

tetrahedral geometry

2s

2p

sp3 hybrid orbitals

hybridization

109o28’

(cartoon)

FORMATION OF TETRAHEDRAL HYBRID ORBITALSFORMATION OF TETRAHEDRAL HYBRID ORBITALS

4 pair

sp3(1)

(1) (2) (3) (4)

sp3(3)

sp3(4)sp3(2)

New orbitals pointto the corners of atetrahedron.

FILLED VALENCESHELL

occurs when orbitalsare full and have finished bonding

X

unhybridized atom

2s

2pz

2py

2px

2s

2p

FORMATION OFFORMATION OFSPSP33 HYBRID ORBITALS HYBRID ORBITALS

X

sp3 hybridized atom

(1) (2) (3) (4)

FORMATION OFFORMATION OFSPSP33 HYBRID ORBITALS HYBRID ORBITALS

These orbital shapes arecartoons - actual shapesare shown on the nextslide. [animation]

sp3

SPSP33 HYBRID ORBITAL HYBRID ORBITAL

The hybrid orbitalhas more density in the bonding lobethan a p orbital andforms stronger bonds.

( cross section )

The shape shown is calculated fromquantum theory.To avoid confusion the

back lobe is omitted from the cartoons, already shown, and thefront lobe is elongatedto show its direction. omitted

Courtesy ofProfessor George Gerhold

… and its cartoon

2s 2p

add together, divide in four

sp3 hybrid orbitals(1) (2) (3) (4)

each new orbital is1/4 s + 3/4 p (25% s, 75% p)

S1P3 = SP3

( 1+3 ) = 4 parts total

ORIGIN OF THE SPORIGIN OF THE SP33 DESIGNATION DESIGNATION

hybridization

+

+

-

2s orbital

2p orbital

x

HYBRIDIZATION

x +-

sp3 hybrid orbital

HYBRIDIZATION

signs are mathematicalcoordinates, not electronic charge

RECALL:

ORIGIN OF THE SPORIGIN OF THE SP33 ORBITAL SHAPE ORBITAL SHAPE

[animation]

Ikatan padaIkatan pada

Alkana sp3

Alkena sp2

Alkuna sp

C

Carbon has 4 valence electrons, 2s22p2

.. ..

Carbon can form single, double or triple bonds

sp, sp2 and sp3 hybrid orbitals.

Let’s do sp3 first.

2p

2s

hybridize sp3

H

H

H

H

CC H

H

HH

H

H

C C

H HH

H H

H

Multiple Bonds and hybridization

Ethylene C CH

H

H

H

Each carbon is hybridized sp2 . The hydrogens are 1s. One of the double bonds is sp2 - sp2. The other one isp - p.

2p

2s

hybridize

2p

sp2

C C

Note that a double bond consists of a and a type bond

C C

H

H

H

H

C C

H

H

H

H

What about acetylene?

C CH HEach carbon atom is sp hybridized. The hydrogens areunhybridized, 1s orbitals.

2p

2s

hybridize

2p

sp

Note that a triple bond consists of a and 2 bonds. Thetwo bonds use unhybridized p orbitals.

CCH H

C C HH

COMPARISON OF SPCOMPARISON OF SPxx HYBRID ORBITALS HYBRID ORBITALS

more “p” character

more “s” character

sp3 sp2 sp

bigger“tail”

more electrondensity in thebonding lobe

BOND STRENGTHS - MULTIPLE BONDSBOND STRENGTHS - MULTIPLE BONDS

CC bond bond bond energy molecule bond type length per mole measured

Kcal (KJ)

C-C sp3-sp3 1.54 Å 88 (368) CH3- CH3

C=C sp2-sp2 1.34 Å 145 (607) CH2=CH2

C=C sp - sp 1.21 Å 198 (828) HC=CH= =

increasings-character

and p - p

and two p-p

Bond EnergyBond Energy

Molecular Distortions: Molecular Distortions: VSEPR Revisited VSEPR Revisited

Four situations:

1) electron pair repulsion

2) effect of electronegative atoms

3) double bond and electronegativity

4) steric repulsion

C

H

H HH

:

:....

symmetrical moleculeall repulsions are equal

perfect tetrahedral

all angles 109o28’

NH H:

....

H

.. anglebecomeslarger

larger repulsion

repulsionsmaller angle becomes

smaller

not all pairs are equivalent

the unshared pairs repel morestrongly than the bonded pairs

Electron Pair RepulsionElectron Pair Repulsion

Effect of double bond and electronegativity

C CHH

H H

C OH

H

C CH2

H

H

C CH2

Cl

Cl

C CH2

F

FC O

F

F

C OCl

Cl

C OH

H

117o

121.5o

121.5o

123o

125o

117o

114o

110o

116o

122o

116o

124.5o

126o

111o

108o

122o

Most pi bonds have a bond energy of

50 - 60 Kcal / mole( 210 - 250 Kj / mole )

MOLECULES WITH PI BONDSMOLECULES WITH PI BONDS

When the total energy of a multiple bond is given,you must subtract the energy of the pi bonds toobtain the sigma bond energy.

C=C 145 Kcal/mole C-C = 95 Kcal/mole

( 145 - 50 = 95 )bothbonds

thus:

TOTAL BOND ENERGY

Non Bonded Electrons (unshared pairs) do not significantly change their energyin going from an atom to a bonded molecule

MOLECULES WITH UNSHARED PAIRSMOLECULES WITH UNSHARED PAIRS

OC

H

H

H

H

2s2p

sp3 hybrids

hybridization C OHH

H

Hsp3 sp3 Metanol

2s2p

sp2

hybridization

2p

2p

C O

H

HC

HO

H

used for

bond

used for

bonds

sp2

sp2

2s2p

sp hybrids

hybridization

2p

2p

NCH

C NHsp sp

C CC

H

H

H

H

C C CH

H

H

Hallena

sp2

sp2sp

CH

HH H

endview

molecule has a twist in the center

Start with theLewis Diagram

Determine thegeometry ofeach atom

Use the correct hybid in eachcase

Assemble themolecule fromthe hybrids.

C N

H

H

H

H

H :

C = 4 pair = tetrahedralN = 4 pair = tetrahedral

C = sp3 N = sp3

VSEPR

C N

C NH

H

H

H

H

ASSEMBLY METHODASSEMBLY METHOD

..

Sample ProblemsSample Problems

Predict the hybridization, geometry, and bond angle for each atom in the following molecules:

Caution! You must start with a good Lewis structure!– NH2NH2

– CH3-CC-CHO

CH3 C

O

CH2

_