Geometria Molecular e Ligação Química · BCl3 has 1(3) + 3(7) = 24 valence electrons; B is the...

© 2012 Pearson Education, Inc.

Geometria Molecular e Ligação Química

Atorvastatina cálcica


Pontos importantes a serem aprendidos:

• As estruturas de Lewis não mostram o tamanho e a forma das moléculas.

• Qual é a relação entre as estruturas 2D e as 3D?

• Desenvolver o senso de geometria molecular e de como esta é governada pelos tipos de ligações químicas

Copyright © Cengage Learning. All rights reserved. 10 | 8

Geometria molecular é a forma geral de uma molécula, sendo descrita pelas posições relativas dos átomos que a constituem.

O que é geometria molecular?


Valence-shell electron-pair repulsion (VSEPR):

Prediz a geometria de moléculas e ions considerando que os pares eletrônicos da camada de valência são arranjados ao redor de cada átomo de modo que eles fiquem o mais afastados possível, minimizando a respulsão eletrônica

Teoria VSEPR


Dois pares: 180° (arranjo linear).

Três pares: 120° em um plano (trigonal planar).

Quatro pares: 109.5° (arranjo tetraédrico).


5 pares: 3 pares no plano a plane 120° e 2 pares a 90°perpendiculares ao plano (bipirâmide trigonal).

6 pares: 90° entre si (octaedro).


Modelos com balões de gás

MolecularGeometries

and Bonding© 2012 Pearson Education, Inc.

Domínios Eletrônicos

• Cada par de elétrons é um domínio eletrônico

• Ligações múltiplas entre dois átomos contam com um único domínio eletrônico.

• Quantos domínios há no átomo A?

MolecularGeometries


Geometrias

• Conta-se o número de domínios eletrônicos na estrutura de Lewis.

A geometria será aquela que corresponde ao número de

domínios eletrônicos

MolecularGeometries


Geometria Molecular

• Nem sempre a geometria molecular é exatamente aquela prevista pelos domínios eletrônicos.

• Os pares não ligados não fazem parte da geometria molecular: somente os átomos ligados devem ser contabilizados

MolecularGeometries


MolecularGeometries


Molecular Geometries

Dentro de cada domínio eletrônico, pode haver mais do que uma geometria molecular.


Exemplo de impacto dos pares eletrônicos isolados


Prevendo ângulos de ligação:

Valores padrões: 180°, 120°, 109.5°, quando o átomos central não apresentea pares isolados

O par isolado requer mais espaço (não contribuem para a formação da ligação química), logo estes ângulos padrões são alterados.


Ligações múltiplas requerem mais espaço que as ligações simples, portanto:

?


Exercício: preveja as geometrias das moléculas abaixo:

a. AsF3

b. PH4+

c. BCl3


No AsF3 há 1(5) + 3(7) = 26 elétrons de valência; As é o átomo central:

Há 4 regiões ao redor do As: 3 ligações e um par isolado.

AsF

F

F

Pirâmide Trigonal


B

Cl

Cl

Cl

BCl3 has 1(3) + 3(7) = 24 valence electrons;B is the central atom.The electron-dot formula is

There are three regions of electrons around B; all are bonding.The electron-pair arrangement is trigonal planar.All of these regions are bonding, so the molecular geometry is trigonal planar.


P

H

HH

H+

PH4+ há 1(5) + 4(1) – 1 = 8 elétrons de valência; P

é o átomo central atom.

Tetraédrica

?


Preveja as geometrias de:

a. ICl3b. ICl4-


I

Cl

Cl

Cl

ICl3 há 1(7) + 3(7) = 28 elétrons de valência. I é o átomo central:

Bipirâmide trigonal

T-shaped


-

I

Cl

Cl

Cl

Cl

ICl4- há 1(7) + 4(7) + 1 = 36 elétrons de valência e I o átomo central:

octaédrico

Quadrado “planar”

MolecularGeometries


Moléculas Maiores

Discute-se a geometria de um átomo em particular do que da molécula inteira.

MolecularGeometries


Sample Exercise 9.3 Predicting Bond Angles

Analyze We are given a Lewis structure and asked to determine two bond angles.Plan To predict a bond angle,we determine the number of electron domains surrounding the middle atom in the bond. The ideal angle corresponds to the electron-domain geometry around the atom. The angle will be compressed somewhat by nonbonding electrons or multiple bonds.Solve In H — O — C , the O atom has four electron domains (two bonding, two nonbonding). The electron-domain geometry around O is therefore tetrahedral, which gives an ideal angle of 109.5°. The H — O — C angle is compressed somewhat by the nonbonding pairs, so we expect this angle to be slightly less than 109.5° . To predict the O — C — C bond angle, we examine the middle atom in the angle. In the molecule, there are three atoms bonded to this C atom and no nonbonding pairs, and so it has three electron domains about it. The predicted electron-domain geometry is trigonal planar, resulting in an ideal bond angle of 120°. Because of the larger size of the C = C domain, the bond angle should be slightly greater than 120°.

Solution

Eyedrops for dry eyes usually contain a water-soluble polymer called poly(vinyl alcohol), whichis based on the unstable organic molecule vinyl alcohol:

Predict the approximate values for the H — O — C and O — C — C bond angles in vinyl alcohol.

MolecularGeometries


MolecularGeometries


Momento de dipolo

• Há dipolos de ligação:• Não necessariamente

que dizer que a molécula seja polar

MolecularGeometries


Polarity

Adição vetorial de dipolos de ligação

MolecularGeometries


Polaridade

© 2012 Pearson Education, Inc.Chemistry, The Central Science, 12th EditionTheodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward

Sample Exercise 9.4 Polarity of Molecules

Analyze We are given three molecular formulas and asked to predict whether the molecules are polar.Plan A molecule containing only two atoms is polar if the atoms differ in electronegativity. The polarity of a

molecule containing three or more atoms depends on both the molecular geometry and the individual bond polarities. Thus, we must draw a Lewis structure for each molecule containing three or more atoms and determine its molecular geometry.We then use electronegativity values to determine the direction of the bond dipoles. Finally, we see whether the bond dipoles cancel to give a nonpolar molecule or reinforce each other to give a polar one.

Solve (a) Chlorine is more electronegative than bromine. All diatomic molecules with polar bonds are polar

molecules. Consequently, BrCl is polar, with chlorine carrying the partial negative charge:

The measured dipole moment of BrCl is µ = 0.57 D.(b) Because oxygen is more electronegative than sulfur, SO2 has polar bonds. Three resonanceforms can be written:

For each of these, the VSEPR model predicts a bent molecular geometry. Because the molecule is bent, the bond

dipoles do not cancel, and the molecule is polar:

Solution

Predict whether these molecules are polar or nonpolar: (a) BrCl, (b) SO2, (c) SF6.

© 2012 Pearson Education, Inc.Chemistry, The Central Science, 12th EditionTheodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward

Sample Exercise 9.4 Polarity of Molecules

Experimentally, the dipole moment of SO2is µ = 1.63 D.

(c) Fluorine is more electronegative than sulfur, so the bond dipoles point toward fluorine. For clarity, only oneS — F dipole is shown. The six S — F bonds are arranged octahedrally around the central sulfur:

Because the octahedral molecular geometry is symmetrical, the bond dipoles cancel, and the molecule isnonpolar, meaning that µ = 0.

Practice ExerciseDetermine whether the following molecules are polar or nonpolar: (a) NF3, (b) BCl3.Answers: (a) polar because polar bonds are arranged in a trigonal-pyramidal geometry, (b) nonpolar becausepolar bonds are arranged in a trigonal-planar geometry

Continued


Uma ligação se forma quando:

• Há overlap de dois orbitais de dois átomos vizinhos.

• Há no máximo dois elétrons por orbital.• Quanto maior o overlap, mais forte é a ligação.

Teoria da Ligação de valência: ligação de covalente através da mecânica quântica.

MolecularGeometries


Overlap and Bonding

• We think of covalent bonds forming through the sharing of electrons by adjacent atoms.

• In such an approach this can only occur when orbitals on the two atoms overlap.

MolecularGeometries


Overlap e Ligação

• Efeito do aumento do overlap

• Efeito de overlap excessivo


Hibridização: usa-se uma combinação de orbitais atômicos (de um mesmo átomo) para descrever-se a ligação química)

O número de orbitais híbridos formados é sempre igual ao número de orbitais atômicos que são combinados.

MolecularGeometries


Orbitais Híbridos

• Considere berílio:

MolecularGeometries


Se energia é absorvida: 2s --> 2p

MolecularGeometries


Hybrid Orbitals

• Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals.– These sp hybrid orbitals have two lobes like a p orbital.– One of the lobes is larger and more rounded, as is the

s orbital.

MolecularGeometries


Hybrid Orbitals• These two degenerate orbitals would align

themselves 180 from each other.• This is consistent with the observed geometry of

beryllium compounds: linear.

MolecularGeometries


Hybrid Orbitals

• With hybrid orbitals, the orbital diagram for beryllium would look like this (Fig. 9.15).

• The sp orbitals are higher in energy than the 1s orbital, but lower than the 2p.

MolecularGeometries


Hybrid Orbitals

Using a similar model for boron leads to three degenerate sp2 orbitals.

MolecularGeometries


Hybrid Orbitals

With carbon, we get four degenerate sp3 orbitals.

MolecularGeometries


Hibridizações sp3d e sp3d2

• Examplos:

• PCl5• SF6

Quais são as geometrias?

MolecularGeometries

and Bonding© 2012 Pearson Education, Inc.52

• A hibridização maximiza a ligação:– Mais ligações = mais orbitais ocupados = maior

estabilidade

• Para um mesmo átomo, há diferentes hibridizações:– C = sp, sp2, sp3

MolecularGeometries


Carbon HybridizationsUnhybridized

2s 2p

sp hybridized

2sp

sp2 hybridized

2p

sp3 hybridized

2p

2sp2

2sp3

MolecularGeometries


Teoria da Ligação de Valência

• Hybridização é importante!!

• Há várias possibilidades para o overlap de orbitais:

MolecularGeometries


Sigma () Bonds

• Sigma bonds are characterized by– Head-to-head overlap.– Cylindrical symmetry of electron density about the

internuclear axis.

MolecularGeometries


Pi () Bonds

• Pi bonds are characterized by– Side-to-side overlap.– Electron density above and below the internuclear

axis.

MolecularGeometries


Ligações Delta

Os 4 lóbulos de orbitais d interagem frontalmente

MolecularGeometries


Single BondsSingle bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.

MolecularGeometries


Multiple Bonds

In a multiple bond, one of the bonds is a bond and the rest are bonds.

MolecularGeometries


Multiple Bonds

• In a molecule like formaldehyde (shown at left), an sp2 orbital on carbon overlaps in fashion with the corresponding orbital on the oxygen.

• The unhybridized p orbitals overlap in fashion.

MolecularGeometries


Multiple Bonds

In triple bonds, as in acetylene, two sp orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds.

MolecularGeometries


ResonanceThe organic molecule benzene has six bonds and a p orbital on each carbon atom.

MolecularGeometries


Resonance• In reality the electrons in benzene are not

localized, but delocalized.• The even distribution of the electrons in benzene

makes the molecule unusually stable.

MolecularGeometries


Teoria do Orbital Molecular (MO)• Aplica-se a equação de Schrödinger (ondulatória)

em uma molecule para se calcular um conjunto de orbitais moleculares .

• Os electrons pertencem à molécula como um todo, logo há delocalização

MolecularGeometries


LCAO• Combinação Linear de Orbitais Atômicos:

orbitais de átomos diferentes são somados ou subtraídos para originar uma nova função de onda.

• A combinação pode ser constrututiva ou destrutiva

MolecularGeometries


Orbitais Moleculares• bonding molecular orbital (ligante):

construtiva , – Característica?

• antibonding molecular orbital (anti-ligante): *, *– Característica?

MolecularGeometries


Interaction of 1s Orbitals

MolecularGeometries


Molecular Orbital Theory

• Electrons in bonding MOs are stabilizing.– lower energy than the atomic orbitals

• Electrons in antibonding MOs are destabilizing.– higher in energy than atomic orbitals– electron density located outside the

internuclear axis– electrons in antibonding orbitals cancel

stability gained by electrons in bonding orbitals

MolecularGeometries


1s 1s

*

hydrogen atomicorbital

hydrogen atomicorbital

Dihydrogen, H2 molecularorbitals

Since more electrons are in bonding orbitals than in antibonding orbitals,

there is a net bonding interaction.

MolecularGeometries


MO and Propriedades• Bond order (Ordem de ligação): diferença entreo

número de electrons ligantes e anti-ligantes.– Considerar sobmente os elétrons de valência

– Pode ser fracionário

– Maior ordem de ligação: maior força e menor comprimento.

– Se B.O. = 0, a ligação é instável se comparada com os átomos individuais

MolecularGeometries


H2

* Antibonding MOLUMO

bonding MOHOMO

MolecularGeometries


1s 1s

*

helium atomicorbital

helium atomicorbital

Dihelium, He2 molecular

orbitals

Since there are as many electrons in antibonding orbitals as in bonding orbitals,

there is no net bonding interaction.

BO = ½(2 − 2) = 0

MolecularGeometries


1s 1s

lithium atomicorbitals

lithium atomicorbitals

Dilithium, Li2 molecular

orbitals

Since more electrons are in bonding orbitals than in

antibonding orbitals, there is a net bonding interaction.

2s 2s

Any filled energy level will generate filled bonding and antibonding MOs;therefore, only need to

consider the valence shell.BO = ½(4 − 2) = 1

MolecularGeometries


bonding MOHOMO

* Antibonding MOLUMO

Li2

MolecularGeometries


Interaction of p Orbitals

MolecularGeometries


MO Theory

• The smaller p-block elements in the second period have a sizable interaction between the s and p orbitals.

• This flips the order of the and molecular orbitals in these elements.

MolecularGeometries


MolecularGeometries


Second-Row MO Diagrams

MolecularGeometries


Geometria Molecular e Ligação Química · BCl3 has 1(3) + 3(7) = 24 valence electrons; B is the...

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