1.2 Van Der Waals

8/6/2019 1.2 Van Der Waals

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Interacciones molecularesInteracciones moleculares

Interacciones de Van der WaalsInteracciones de Van der Waals


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MOMENTOS DIPOLARES

X Y

Gy>Gx

Q Q!q . rq dif. de cargasq dif. de cargas

r distancia entrer distancia entre

cargascargas

LaLa separacinseparacin dede

cargascargas parcialesparcialescreacrea unun dipolodipolo

elctricoelctricoH+ H-


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3

Electric dipole moments: polyatomic molecules


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DIPOLO TOTALDIPOLO TOTAL

EN NHEN NH33

DIPOLO EN ENLACE

MOLCULAS POLARES Y APOLARES

DIPOLO TOTALDIPOLO TOTAL

EN HEN H22OO

NO HAY DIPOLOTOTAL EN CH4

AUNQUE EXISTEN

DIPOLOS EN ENLACES


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SLIDOS MOLECULARES

FUERZAS INTERMOLECULARES

+ +

+

+

- -

--

ENLACES DE VAN DER WAALS

Entre molculasEntre molculasdiscretas (dipolosdiscretas (dipolos

inducidos)inducidos)

Actan a largaActan a larga

distanciadistanciaNo son dirigidasNo son dirigidas

DbilesDbiles

Entre molculasEntre molculasdiscretas (dipolosdiscretas (dipolos

inducidos)inducidos)

Actan a largaActan a larga

distanciadistanciaNo son dirigidasNo son dirigidas

DbilesDbiles


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VdW (dbil)VdW (dbil)

1010--100 Kj.mol100 Kj.mol

--11

Covalente (fuerte)Covalente (fuerte)

5050--1000 Kj.mol1000 Kj.mol--11

ENLACES DE VAN DER WAALS


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Fuerzas Moleculares de Van der Waals (3)Fuerzas Moleculares de Van der Waals (3)

ElEl momento dipolar fugazmomento dipolar fugaz (instantneo, transitorio) se(instantneo, transitorio) se

origina por distribuciones asimtricas de las nubesorigina por distribuciones asimtricas de las nubes

electrnicas alrededor de los ncleos en molculas noelectrnicas alrededor de los ncleos en molculas no

polares.polares.

La molculas no polares (incluso tomos con capasLa molculas no polares (incluso tomos con capas

electrnicas cerradas (Ar)), responden a camposelectrnicas cerradas (Ar)), responden a campos

elctricos (momentos dipolares) de molculas vecinas,elctricos (momentos dipolares) de molculas vecinas,dandodando dipolos inducidos.dipolos inducidos.

+ - + - + - -

+H-H+

-

-

-

-

--

-


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Uniones Intermoleculares de Van der Waals (1)Uniones Intermoleculares de Van der Waals (1)

LosLos dipolosdipolos inducidosinducidos sese formanforman igualmenteigualmente enenrespuestarespuesta aa loslos dipolosdipolos permanentespermanentes yy aa loslos dipolosdipolostransitoriostransitorios..

LosLos movimientosmovimientos molecularesmoleculares reorientanreorientan loslos dipolosdipolos

enen ambasambas molculasmolculas..

LasLas unionesuniones intermolecularesintermoleculares dede VanVan der der WaalsWaalsexplicanexplican lala condensacincondensacin deldel HH22 yy deldel ArAr yy elel estadoestadolquidolquido deldel bencenobenceno aa temperaturatemperatura ambienteambiente..

+-+ - + - +-


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Dipolo inducidoDipolo inducido

El dipolo permanente de laEl dipolo permanente de la

molcula de agua produce un dipolomolcula de agua produce un dipolo

inducido (de menorinducido (de menorHH de carga)de carga) enenuna molcula contigua (un grupouna molcula contigua (un grupo

=CH=CH22 en este caso).en este caso).

Una molcula o un grupo apolarUna molcula o un grupo apolar

(como el =CH(como el =CH22) produce dipolos) produce dipolos

espontaneamente.espontaneamente.

_

HH ++ HH ++

HH --

HH O

_HH

C


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Interacciones electrostticas

C CO O

+

++----

----

----

---

---

OHH

OHH

HH++ HH--

Entre dipolos permanentesEntre dipolos permanentes

(molculas polares)(molculas polares)

Entre dipolos instantneosEntre dipolos instantneos

(ej.: Gases nobles)(ej.: Gases nobles)

Entre dipolos inducidosEntre dipolos inducidos

(ej.: molculas apolares en agua)(ej.: molculas apolares en agua)

H+ H-

H+H-

H+ H-

H+ H-


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Intermolecular Forcesvan der Waals Forces

van der Waals forces are also known as London forces.

They are weak interactions caused by momentary changes in electron

density in a molecule.

They are the only attractive forces present in nonpolar compounds.

Even though CH4 has no netdipole, at any one instant its

electron density may not be

completely symmetrical,

resulting in a temporary dipole.

This can induce a temporary

dipole in another molecule. The

weak interaction of these

temporary dipoles

constituents van der Waals

forces.

Introduction to Organic Molecules and Functional Groups


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All compounds exhibit van der Waals forces.

The surface area of a molecule determines the strength of the van der Waals

interactions between molecules. The larger the surface area, the larger the

attractive force between two molecules, and the stronger the intermolecular

forces.




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van der Waals forces are also affected by polarizability.

Polarizability is a measure of how the electron cloud around an atom respondsto changes in its electronic environment.

Larger atoms, like iodine,

which have more loosely held

valence electrons, are morepolarizable than smaller atoms

like fluorine, which have more

tightly held electrons. Thus,

two F2 molecules have little

attractive force between them

since the electrons are tightlyheld and temporary dipoles are

difficult to induce.




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Interactions between dipoles

62

0

2

2

2

1

)4(3

2

kTrV

TI

QQ!

E E 1/r6 => van der Waals interactionE E 1/T => greater thermal motion overcomes themutual orientating effects of the dipoles at higher T

V!

2Q12Q2

2

3(4TI0)2kTr

6


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Modelling the total interaction

The sum of the repulsive interaction with n = 12 andthe attractive interaction given by:

is called the Lennard-Jones (12,6)-potential. It is

normally written in the form:

6r

CV !

!

612

4rr

VWW

I


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Modelling the total interaction

The two parameters areI (epsilon), the depth of

the well, and W, the

separation at which V =

0.

The Lennard-Jones potential models theattractive component by a contribution that isproportional to 1/r6, and a repulsive componentby a contribution proportional to 1/r12


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Intermolecular ForcesDipole-Dipole Interactions

Dipoledipole interactions are the attractive forces between the permanentdipoles of two polar molecules.

Consider acetone (below). The dipoles in adjacent molecules align so that

the partial positive and partial negative charges are in close proximity.

These attractive forces caused by permanent dipoles are much stronger than

weak van der Waals forces.



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Hydrogen bonding typically occurs when a hydrogen atom bonded

to O, N, or F, is electrostatically attracted to a lone pair of

electrons on an O, N, or F atom in another molecule.

Intermolecular ForcesHydrogen Bonding



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Note: as the polarity of an organic molecule increases, so does thestrength of its intermolecular forces.

Intermolecular ForcesHydrogen Bonding



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LasLas fuerzasfuerzas dede vanvan derder WaalsWaals generangeneran interaccionesinteracciones

molecularesmoleculares queque nono perturbanperturban lala reactividadreactividad qumicaqumica

dede laslas molculasmolculas involucradasinvolucradas..

ReconocidasReconocidas enen elel siglosiglo XIXXIX comocomo responsablesresponsables dede laslasdesviacionesdesviaciones deldel comportamientocomportamiento idealideal dede loslos gasesgases

realesreales (( PP == [nRT/V[nRT/V -- nb]nb] -- [n/V][n/V]22 )) yy dede lala cohesincohesin dede

loslos gasesgases elctricamenteelctricamente neutrosneutros (como(como elel Argn)Argn)..

EstasEstas cargascargas elctricaselctricas interaccionaninteraccionan entreentre ss yy sonson

responsablesresponsables dede lala cohesincohesin dede lala materia,materia,

especialmenteespecialmente enen elel estadoestado lquidolquido yy enen loslos sistemassistemas

biolgicosbiolgicos..


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Uniones intermolecularesUniones intermoleculares

en agua a 30en agua a 30 rrCC

InteraccinInteraccin TipoTipo kJ/molkJ/mol

In/dipoloIn/dipolo NaNa+...

+...

HH22OO 60602 Dipolos permanentes2 Dipolos permanentes HH22OO

......HH22O (estructura del HO (estructura del H22O)O) 2020

2 Dipolos permanentes2 Dipolos permanentes =CO=CO......HN= (unin peptdica)HN= (unin peptdica) 1515

Dipolos: permante e inducidoDipolos: permante e inducido HH22OO......CHCH22== 1010

2 Dipolos inducidos2 Dipolos inducidos =H=H22CC......CHCH22= (= (London)London) 44

2 Dipolos inducidos2 Dipolos inducidos ArAr......Ar (London)Ar (London) 44

1.2 Van Der Waals

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