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Intermolecular Forcesof Organic Molecules

Dr. C. Yau

Spring 2014

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Review of Organic MoleculesWhat are considered “organic compounds?”

They are compounds containing C.

What are hydrocarbons?

They are compounds containing only C & H.

In a previous lecture we talked about several of the classes of organic compounds:

• hydrocarbons

• alcohols

• carboxylic acids• amines

It would be wise to review what distinguish these classes of compounds from each other.

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Intermolecular Forces of Hydrocarbons

Hydrocarbons are nonpolar.

You should know why.

Intermolecular forces of hydrocarbons can only be London forces, but don’t underestimate them!

The larger the molecule, the stronger are the London forces (molecules become more polarizable).

What is the significance of this fact?

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Intermolecular Forces of Hydrocarbons

Hydrocarbons with1 to 5 carbons are gases,5 to 13 carbons are liquids, and>13 carbons are solids.

You should be able to explain why.What does the number of carbon atoms

have to do with the physical state of a hydrocarbon?

Think about how intermolecular forces affect the physical state of a substance.

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Oil Refinery: Distillation Towers

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Alcohols Contain –OH: They are not hydroxides!

They contain O-H and C-O bonds, which are polar. They have STRONG H-bonding (due to the O-H bonds), as well as dipole forces (due to the C-O bonds)

What is the predominant IMF for alcohols?

ANS. The predominant IMF is hydrogen bonding.

Why are alcohols not hydroxides?

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H

C

H

OH H

This is methyl alcohol (methanol), with the formula CH3OH.The formula contains “OH” so why is it not classified as a hydroxide? How can you tell from the formula that it is not a hydroxide?

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Carboxylic Acids

Formic acid HCOOH or HCO2H H C

O

O H..:

: :

What intermolecular forces do carboxylic acids exhibit?

STRONG H-bonding and dipole forces, and London forces.

Acetic acid CH3COOH or CH3CO2H

C

O

O H

CH

H

H ..:

: :

Predominant IMF is H-bonding.

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Amines Contain NCH3NH2

CH3CH2NH2

CH3NHCH3

CH3 NHH

CH3CH2 NHH

CH3 N HCH3

NH2

(CH3)2NH

..

..

..

What intermolecular forces do they exhibit?

Mostly STRONG H-bonding (due to N-H bonds)

Do ALL amines exhibit H-bonding?

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Decide for yourself:

Amines Contain NDo they ALL exhibit H-bonding?

NH2

NH2

NH2

CH3 N CH3

CH3

OH

OH

CHCH2NHCH3

OH

aniline cadaverine

..

trimethylamine adrenaline

One of the above does NOT exhibit H-bonding. Do you know which one, and why?

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NH2

NH2

NH2

CH3 N CH3

CH3

OH

OH

CHCH2NHCH3

OH

aniline cadaverine

..

trimethylamine adrenaline

Trimethylamine exhibit only dipole forces and NOT H-bonding because it does not contain any N-H bonds.

Note N-H bond here

also O-H bonds

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Acetone (fingernail polish remover)Acetone belongs in class called ketones.

You should have already memorized the structure of acetone.

It is a very common organic solvent.

CH3COCH3 or (CH3)2CO

CH3C CH3

O: :

Does it exhibit H-bonding?

NO, because it does not contain any O-H bonds.

It is polar and exhibits dipole forces (due to C=O).

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BenzeneBenzene is another very common organic

solvent.

What intermolecular forces does it exhibit?

(You should have memorized its structure.)

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NOTE: H-bonding does NOT refer to the actual N-H, O-H or H-F bonds themselves!

We say water has H-bonding because it contains O-H bonds,

but the actual H-bonding refers to the attraction between H of one molecule to the - of O in a DIFFERENT molecule.

HO

HH

O

H This is a H bond, not H-bonding.

THIS is the H-bonding.Note that it is BETWEEN two DIFFERENT molecules. It is NOT a covalent bond.

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Why ethyl alcohol dissolves well in water....

They form strong H-bonding between each other. How? Where?

C C

H

HH O

HH

H

HO

H

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H-bonding between ethyl alcohol and water

C C

H

HH O

HH

H

HO

H

HO

H

HO

H H-bonding here

H-bonding here

There is NO H-bonding here!

Why not? Be sure you know why not!