chap 1-1 intra and intermolecular forces

Instrumental techniquesPhar 6521

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Chromatography is a physical method of

separation in which the components to be

separated are distributed between two

phases (KD/P = Distribution/partition constant)

one of which is stationary (stationary phase)

while the other (the mobile phase) moves

through it in a definite direction.

The chromatographic process occurs due to

differences in the distribution constant of the

individual sample components.

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Chromatography

KD of Cpd A = [A]S / [A]M

KD = Distribution constant of compound A

[A]S = concentration of compound A in stationary phase

[A]M = concentration of compound A in mobile phase

For eg. TLC Chromatography compounds distributes itself b/n a liquid mobile phase and a solid

stationary phase The rate of migration for a chemical compound is determined by

how much of it distributes into the mobile and stationary phases

Case 1. A compound that distributes itself 100% into the mobile phase

will migrate at the same rate of the mobile phase

Case 2: On the other hand, a compound that distributes itself 100%in the stationary phase will not migrate at all 4

In most molecular substances, there are two types of attractive forces:

1. Intramolecular and 2. Intermolecular forces

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Chromatographic separation is achieved due to

Different compounds have different KD values

Distribution constant (KD) is affected by

the type of intermolecular forces that present in

molecules

force that hold atoms in a single molecule or a force of attraction within a molecule

e.g. covalent bond, ionic bond

H-Cl, Na+Cl

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Intramolecular forces

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an attraction between two or more separate

molecules.

are the result of attractions between positively and

negatively charged regions of separate molecules.

They are not as strong as intramolecular force

(chemical bonds).

Intermolecular forces

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These intermolecular forces as a group are referred to as van der Waals forces.

There are three types of intermolecular forces,

1. Dipole-dipole interactions2. Hydrogen bond3. London force/Dispersion force

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A very approximate strength order would be:

Bond type Relative strength

Ionic bonds 1000

Hydrogen bonds 100

Dipole-dipole 10

London forces 1

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Intermolecular Forces

They are, however, strong enough to control physical properties such as, solubility, boiling and melting points, vapor pressures, and viscosities.

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Dipole-Dipole Interactions

• Molecules that have permanent dipoles are attracted to each other.

√ The positive end of one is attracted to the negative end of the other and vice-versa.

√ These forces are only important when the molecules are close to each other.

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It occurs in polar compounds These work in a similar manner to ionic

interactions, but are weaker because only partial charges are involved.

An example of this can be seen in Acetone

Dipole-Dipole Interactions

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Dipole-Dipole Interactions

The more polar the molecule, the higher is its boiling point.

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Hydrogen BondingHydrogen bonding occurs when

Hydrogen is bonded to N, O, or F are unusually strong.

Hydrogen atom has a partial positive charge and can interact with another highly electronegative atom in an adjacent molecule (N, O, or F).

it is a special type of dipole-dipole force

The result is a dipolar molecule e.g H2O, NH3, HF 14

London/Dispersion Forces

While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.

It involve the attraction between temporarily induced dipoles 15

London/Dispersion Forces

At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side.

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London/Dispersion Forces

Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

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London/Dispersion Forces

London dispersion forces, or dispersion forces, are attractions between an instantaneous /temporary dipole and an induced dipole.

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London/Dispersion Forces

These forces are present in all molecules, whether they are polar or non-polar.

The tendency of an electron cloud to distort in this way is called polarizability.

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This polarization can be induced either by A polar molecule or A non-polar molecule (the repulsion

of negatively charged electron clouds in non-polar molecules)

London/Dispersion Forces

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Factors Affecting London Forces

The strength of dispersion forces tends to increase with increased molecular weight.

Larger atoms have larger electron clouds, which are easier to polarize.

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Factors Affecting London Forces

The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane).

This is due to the increased surface area in n-pentane.

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Which Have a Greater Effect:Dipole-Dipole Interactions or Dispersion Forces?

• If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.

• If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

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Ion-Dipole Interactions

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• A fourth type of force, ion-dipole interactions are an important force in solutions of ions.

• The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

Anthocyanins (also anthocyans) are

belong to a parent class of molecules called flavonoids cationic organic compound well water-soluble pigments due to ion-dipole interaction

Quaternary ammonium cationare salts of quaternary ammonium cations.Soluble in water

Polar Molecule

A Molecule with a Positive and Negative Side

Dipole Moment

• A Measure of Molecular Polarity

• A Non-polar Molecule will have a Zero Dipole Moment

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Molecular Polarity

Why is Polarity Important?

Many Properties Depend on Polarity

Melting and Boiling Point

Surface Tension, Viscosity

Reactivity

Solubility (e.g., will it dissolve in water)

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Requirements

A Polar Molecule Requires

Polar Bonds

A Molecular Shape that Separates the by

the partial Positive from the Negative Side

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Unequal sharing of e- in a bond called Polar Covalent Bond or Polar Bond

Partial Charge indicated by delta +/-

Polar Covalent Bonds

-+

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Polar Covalent Bonds

NON-polar covalent bonds

Bonds between identical atoms such as H-H, F-F involve equal sharing of e-

Polar covalent bonds

Bonds between different atoms involve unequal sharing of e-

Polar Covalent Bonds have a Partial Charge Separation

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Electronegativity

Electronegativity is the ability of an

atom to attract electrons in a bond.

Nonmetals bonded to N, O, F usually

have polar bonds

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Electronegativity Index of Some Elements

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Electronegativity Used to Determine Bond Polarity

EN < 0.45 Non-polar Bond

1.75 > EN > 0.45 Polar Bond

EN > 1.75 Ionic Bond Atoms

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Polar or Non-polar Bond?

O-H EN = 1.4 Polar Bond

C-H EN = 0.4 Non-polar Bond

C-O EN = 1.0 Polar Bond

H-Cl EN = 0.8 Polar Bond

Polar Molecules

The molecule is usually polar

If all atoms attached to central atom

are not the same

Or if central atom has 1 or more lone

pairs of electrons

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Examples

Polar compounds

HCN, H2O, CHCl3, CH2Cl 2

Non-polar

CO2, CCl4

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In Conclusion

Polarity Determined from

Polar Bonds ( EN > 0.45)

Molecular Shape with + & - sides

N and O in Molecules often lead to

Polar Molecules or Regions

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In Conclusion

Polarity will be important

In Determining Intermolecular Forces

Vapor Pressure, Boiling and Melting Points

Surface Tension, Viscosity

Solubility

Reactivity (Organic Chemistry)

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Intermolecular Forces Affect Many Physical Properties

The strength of the attractions between particles can greatly affect the properties of a substance or solution.

Solubility

Defines as the amount of a solute that will dissolve in a specific solvent at given condition

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Degree of solubility (types of saturation)Saturated solution: A solution with solute that dissolves

until it is unable to dissolve anymore, leaving the undissolved substances at the bottom.

Unsaturated solution: A solution (with less solute than the saturated solution) that completely dissolves, leaving no remaining substances.

Supersaturated solution: solution (with more solute than the saturated solution) that contains more undissolvedsolute than the saturated solution because of its tendency to crystallize and precipitate

Factors that affects solubility

The nature of solute and solvent Temperature Pressure (only applicable to gases)

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The nature of solute and solvent When two substances are similar they can dissolve in

each other

Polar solutes dissolve in polar solventNon-polar solutes tend to dissolve in non-polar solvent

“Like dissolve like” two liquids dissolve in each other b/c the molecules

are alike in polarity

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Note: solvents are grouped either polar or non-polar solvent

Polar Solvent: a liquid made up of polar moleculesNon-polar Solvent: a liquid made up of non-polar molecules

The nature of solute and solvent

Ionic compounds are made up of charged ionssimilar to polar compounds e.g. NaCl

Ionic compounds are more soluble in polarsolvent than in a non-polar solvent ion-dipole interaction

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The nature of solute and solvent

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Temperature

Solubility of solids in liquids The solubility of a solid increases as temp increases

Solubility of gases in liquid are affected by temperature Opposite to the solubility of solids in liquidsAs the temperature increases, the solubility of a GAS

in a liquid decreases

WHY ? As the temperature increases, the kinetic energy of

the solute gas increases and the gas can escape

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Pressure when the pressure is increased over the

solvent, the solubility of gas is increased

WHY ? Pressure increases as gas molecules strike the

surface to enter the solution increased

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Factors of Dissolving

Rate of which a solid solute dissolves in a solutiondepends on three factors Surface area: speed up the solubility by

increasing surface area Stirring: increases contact b/n solvent and

solute Temperature: kinetic energy increase

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