chemical bonding and van der waals

8/3/2019 Chemical Bonding and Van Der Waals

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ElectronegativityElectronegativity

Electronegativity is a measure of the relativeElectronegativity is a measure of the relative

ability of an elements atoms to attract theability of an elements atoms to attract the

shared electrons in a chemical bond.shared electrons in a chemical bond. Higher electronegativities mean a greaterHigher electronegativities mean a greater

attraction for the electrons.attraction for the electrons.

Fluorine is the highest with a value of 4.0Fluorine is the highest with a value of 4.0


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Polarity

The difference of electronegativity between

two bonding atoms can be measured by

subtracting the smaller number from thelarger number.

The difference in the two electronegativities

determines the nature of the bond


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Polarity

Bonds that are sharing electrons UNEQUALLY

between two atoms are called POLAR

COVALENT BONDS If the atoms are identical (equal

electronegativity), the bond will not be polar.

This is called NON-POLAR COVALENT BONDS


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Polar covalent bonds have a positive pole

and a negative pole so they are also referred

to as bond dipoles

Polar covalent bonds have an electronegativity

difference between 0 and 1.7

Ionic bonds have an electronegativity

difference between 1.7 and 3.3


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Ionic Bonds

Ionic bonds form from the electrostatic

attraction between oppositely charged ions

Atoms become ionic by losing or gainingelectrons from the atom it is bonding with

Remember that an atom will lose its

electrons to fill its outer level if its valence

level is less than half full, as it is with metals


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Electron Exchange and Ionic BondFormation


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Van der Waals Forces

Johannes Diderik van der Waals


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Polarity

Separation of charge

An asymmetrical difference in electronegativity

along a bond or in a molecule


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Circle the polar molecules.

Label H+ and H-

O

HH

N

H

HH

CCO O

Al

Cl

Cl

Cl

Cl

Cl

Cl

Cl

S

HH

H

H

H

HH

H


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Van der Waals

Forces

Small, weak interactions between molecules


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Van der Waals

Forces

Intermolecular: between molecules (not a bond)

Intramolecular: bonds within molecules (stronger)


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What is

being attracted?H+ attracted to H-

electrostatic attraction

e- s of one atom to another atoms nucleus

e-

e-+ +


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Evidence of VDW Force

s?Non-polar molecules can form gases,

liquids and solids.

Ex: CO2

CO O

CO O

CO O

CO O

CO O

CO O


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3Types of Van der Waals Forces

1) dipole-dipole

2) dipole-induced dipole

3) dispersion


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Dipole-Dipo

le

Two polarmolecules align so that H+ and H-

are matched (electrostatic attraction)

Ex: ethane (C2H6) vs. fluromethane (CH3F)


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Fluoromethane (CH3F) boiling point = 194.7 K

H H

H C F H C F

H H

polar or non-polar?

H-H-H

H

Ethane (C2H6) boiling point = 184.5 K

H H H H

H C C H H C C H

H H H H

polar or non-polar?

Dipole-Dipole

NOT Dipole-Dipole


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Try This:Draw two KBr molecules and draw their

dipole-dipole interactions with a

dashed line.

BrBr

K K


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What does to induce mean?To cause or bring about

Ex:

Induced vomiting

Induced labor

Induced coma


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Dipole-Induced Dipole

A dipole can induce (cause)

a temporary dipole to form in a

non-polar molecule

The molecules then line up

to match H+ and H- charges


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Example

H ClH+ H Are-e- e-

e-

e-

e-e-

e- e-

e-

e

-

e

-

e-

e-e-e-

e-e-

A DIPOLE

(its polar)

non-polarINDUCED

DIPOLE

HH

Dipole Induced Dipole

(weak and short-lived)


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Dispersion ForcesA temporary dipole forms in a

non-polar molecule

which leads to

a temporary dipole to form inANOTHERnon-polar molecule

Dispersion is the ONLY intermolecularattraction that occurs between non-polarmolecules


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Dispersion Forces

Cl-Cle-e-

e-

e-e-

e-

e- e-

e-e-

e-e-

e-e-e-

e-e-

e-

non-polarINDUCED

DIPOLE

HH

TEMPORARY

DIPOLEnon-polar

Cl-Cle-e- e-e-

e-e-e

- e-e-

e-

e

-e-e-

e-e-

e

-

e

-HH

Dispersion

(weakest and very short-lived)


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ReviewDipole Dipole

between two polar molecules

Dipole Induced Dipole

b/w a polar & a non-polar molecule

Dispersionbetween two non-polar molecules


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Hydrogen BondingSTRONGEST Intermolecular Force!!

A special type of dipole-dipole attraction

Bonds form due to the polarity of water

Draw 3H2

O molecules in your notes

Ice Liquid


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Hydrogen Bonding cont

Hydrogen bonds keep water in the liquid

phase over a wider range of

temperatures than is found for any othermolecule of its size


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Selected parameters for testing

Molecular weight known relationship

between poor permeability and high molecular

weight.Lipophilicity (ratio of octanol solubility to

water solubility) measured through LogP.

Number of hydrogen bond donors and

acceptors High numbers may impairpermeability across membrane bilayer


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The rule of five - formulation

There are more than 5 H-bond donors.

The molecular weight is over 500.

T

he LogP is over 5.There are more than 10 H-bond

acceptors.

Poor absorption or permeation are

more likely when:


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Partition coefficient Definition

The ratio of the equilibrium

concentrations of a dissolved

substance in a two-phase systemcontaining two largely immiscible

solvents (water and n-octanol)

.oct

water

C

CP!


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Partition coefficient (cont.)

Since the differences areusually on a very large scale,

Log10(P) is used.

O

H

1-

octanolO

H H

water


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chemical bonding and van der waals

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