1. Unit 1 – Organic Chemistry
L.O. #3 The properties and characteristics of organic compounds

2. Intermolecular forces
The physical and chemical properties are determined by the bonds and forces between atoms and molecules.
Intramolecular forces occur between atoms in molecules and determine a substance’s chemical properties.
Intermolecular forces occur between molecules and affect a substance’s physical properties.

3. Intermolecular forces
The physical properties of organic compounds are affected by the intermolecular forces of attraction and repulsion between molecules
Larger forces of intermolecular attraction result in high boiling & melting points.
Weak intermolecular forces result in low boiling & melting points producing gas states.

4. Intermolecular forces
The foundation for these intermolecular forces lies in the types of bonds that form between atoms in molecules.
The bonding continuum ranges from non-polar to ionic, however organic molecules are predominantly covalent molecules and lie in the polar to non-polar range.
Polar and non-polar regions affect solubility

5. Intermolecular forces
The polarity of a bond is determined by the electronegativity difference.
Electronegativity Difference (DE)
Non-polar
Polar
Ionic
3.3
1.7
0.4
Organic compounds

6. Intermolecular forces
The intermolecular forces are attractive and repulsive forces between molecules determined by factors such as:
The bond types within the molecules.
Polar vs. Non-polar
The size of the molecules.
The shape of the molecules.

7. Intermolecular forces
Johannes van der Waals ( ) studied these forces and they are often referred to by his name.
We will focus on the three main types:
Dispersion (London) forces
Dipole-dipole forces
Hydrogen bonding

8. Intermolecular forces
Dispersion (London) forces
The constant vibration of electrons in covalent bonds generates temporary regions of charge distribution and polar regions which result in weak forces of attraction between all covalent molecules.
The overall strength of these forces is proportional to;
The number of electrons in the molecule
The size and shape of the molecule

9. Intermolecular forces
Dipole-Dipole Forces
Polar molecules orient themselves so that electrostatic attraction occurs between the different charged regions of the molecules.
These forces are generally stronger than the dispersion (London) forces.
Their strength depends upon the;
Nature (i.e. DE difference) of the polar bonds
Number and placement of the polar bonds
Size of the molecules

10. Intermolecular forces
Hydrogen bonding
A strong form of dipole-dipole attraction between a hydrogen atom and oxygen, nitrogen or fluorine in a polar covalent molecule.
The high electronegative difference results in a strong polar bond and distinct regions of charge distribution.
Hydrogen bonding is responsible for water’s unique characteristics and influences organic compounds’ solubility in water.

11. Assessing intermolecular forces
Can molecules form hydrogen bonds?
If so:
They have higher boiling and melting points than similar molecules that cannot form hydrogen bonds
They form hydrogen bonds with water and thereby have greater solubility in water

12. Assessing intermolecular forces
Are they polar?
Polar molecules tend to have higher boiling & melting points than similar non-polar ones. Hydrogen bonding capability increases the trend further.
A large non-polar region (hydrocarbon) in a molecule reduces the affect of the smaller polar regions and are less soluble than smaller hydrocarbon polar molecules.

13. Assessing intermolecular forces
How strong are the dispersion forces?
Large hydrocarbon chains have stronger dispersion forces.
The boiling and melting points tend to be proportional to the number of carbons in the hydrocarbon.
Large hydrocarbons have high boiling and melting points.
Small hydrocarbons tend to be gases at SATP.

14. Hydrocarbons (CnH2n+2) Physical Properties
Polarity of functional group
The C-H bond is non-polar so most hydrocarbons are non-polar
The addition of halides results in polar regions.
The predominant intermolecular force in dispersion forces
Hydrogen bonding
None
Solubility in Water
Very poor solubility in water due to the lack of polar and hydrogen bonds
Melting & Boiling Points
The melting and boiling points tend to be very low
The boiling points increase as the length of the parent chain increases
1-5 C’s  SATP
5-20 C’s  liquids (some very
20 + C’s  SATP

15. Hydrocarbons (CnH2n+2) Additional Characteristics
As the number of carbons in the parent chain increase:
The state of matter at SATP moves from gas to liquid to solid
The melting and boiling points increase
The density increases
Popular fuel source (Methane, propane, octane, etc.)
Source for plastics

16. Alcohols (R-OH) Physical Properties
Polarity of functional group
OH is very polar
As the parent chain grows, its non-polar nature become more influential
Small alcohols are more polar than larger ones
Hydrogen bonding
Alcohols experience hydrogen bonding with other alcohols and water
Solubility in Water
The capacity to form hydrogen bonds make alcohols extremely soluble in water.
The solubility decreases as the number of carbons increase
Melting & Boiling Points
Due to the strength of hydrogen bonds most alcohols have higher melting & boiling points than similar alkanes.
Most alcohols are liquids at SATP

17. Alcohols Additional Characteristics
Alcohols are extremely flammable
Most alcohols are poisonous
Methanol can cause blindness or death.
Ethanol is consumed in moderate quantities. Excessive consumption may cause blindness or death.
Yeast ferment sugars to produce ethanol and carbon dioxide
Antifreeze, cosmetics, foods, medical preparations, solvents, antiseptics, etc.
Biological compounds – cholesterol, retinol, etc.

18. Ethers (R-O-R2) Physical Properties
Polarity of functional group
The C-O bond is less polar than the O-H
Ethers are less polar than alcohols
The bent shape around the O aids in the dipole-dipole force strength
Hydrogen bonding
There is no hydrogen bonding in ethers
Ethers can accept hydrogen bonds from water
Solubility in Water
Ethers are usually soluble in water
The solubility decreases as the size of the alkyl groups grows
Melting & Boiling Points
Much lower than comparable alcohols
Higher than similar hydrocarbons

19. Ethers Additional Characteristics
Ethers are extremely flammable
Have been used as anaesthetics
“ether” – ethoxyethane
More recent anaesthetic (1-methoxypropane) has fewer side effects.

20. Amines (R-NH2) Physical Properties
Polarity of functional group
C-N and N-H bonds are polar
Amines are usually polar
Hydrogen bonding
The presence of one or more N-H bonds allows hydrogen bonding
Solubility in Water
Low molecular weight amines are soluble in water (<4 C)
The solubility decreases as the number of carbons increase
Melting & Boiling Points
The boiling points of primary and secondary amines (N-H bonds present) are greater than tertiary amines (no N-H bonds)

21. Amines Additional Characteristics
Widely found in nature. They are often toxic. Many medical applications.
Low molecular weight amines have a “fishy” smell while others are responsible for odours associated with decay.
Amines act as weak bases. Adding an acid produces a salt. So, add lemon to fish to remove fishy odour.

22. Aldehydes (R-CO) & Ketones Physical Properties
Polarity of functional group
C=O is polar
Hydrogen bonding
No O-H bonds so no hydrogen bonding between aldehydes and ketones
The O atom can accept hydrogen bonds from water
Solubility in Water
Low molecular weight aldehydes and ketones have high solubility
The solubility decreases as the number of carbons increase
Melting & Boiling Points
Lower than comparable alcohols
Higher than corresponding alkanes

23. Aldehydes & Ketones Additional Characteristics
Aldehydes have a strong pungent odour while ketones smell sweet. As the size of the aldehyde increases the odour becomes more pleasant (i.e. cinnamon)
Odours in nature may be used to communicate between insects - pheromones
Since they are polar, they are used as polar solvents. The non-polar alkyl chain permits their use as non-polar solvents too.
Antiseptics, disinfectants (Formaldehyde), resins, dyes, preservatives (Aldehyde)
Hormones in biology – testosterone, progesterone, cortisone
Solvents, medication

24. Carboxylic acids (R-COOH) Physical Properties
Polarity of functional group
Polar due to C=O and O-H bonds
Hydrogen bonding
Strong hydrogen bonding
Solubility in Water
Low molecular masses are very soluble (<4C – miscible in water)
The solubility decreases as the number of C’s increase
Melting & Boiling Points
The melting and boiling points for carboxylic acids are very high due to the hydrogen bonding.
Higher than corresponding alcohols

25. Carboxylic acids Additional Characteristics
Carboxylic acids often have unpleasant odours
butanoic acid smells like stale sweat
Sour taste in many foods & plants – vinegar, spoiled wine, rhubarb, citrus fruits
By-product of anaerobic respiration in muscles – lactic acid
Industrial processes – dyes, tanning, rubber recycling, etc.
Food preservatives, Vitamin C, ASA
The OH does not behave like a base. The two O’s have high electronegativities and carry the H’s electron allowing the H ion to dissociate.

26. Boiling & Melting Points
CH3CH2CH2COH =
b.p oC m.p oC
Carboxylic acid
High
b.p oC m.p oC
Alcohol
CH3CH2CH2CH2OH
CH3CH2CCH3 =
b.p oC m.p oC
Ketone
CH3CH2CH2CH =
b.p oC m.p oC
Aldehyde
b.p oC m.p oC
Alkane
CH3CH2CH2CH3
Low

27. Esters (R-COO-R2) Physical Properties
Polarity of functional group
Usually polar due to C=O and C-O bonds
Polarity is reduced with increased number of C atoms
Hydrogen bonding
No O-H bonds, there fore they don’t form hydrogen bonds with other esters
Solubility in Water
The C=O bond can accept hydrogen bonds from water, therefore low molecular mass esters are soluble
Esters with chains greater than 3 or 4 are not soluble
Melting & Boiling Points
The melting and boiling points for esters are low due to the lack of hydrogen bonding.
They are usually volatile liquids at SATP

28. Esters Additional Characteristics
Esters often have pleasant odours
Low molecular mass esters have very pleasant odours and are used in soaps, perfumes, cosmetics, air fresheners, etc.
Natural esters are responsible for the scents in flowers & fruits. (Pg 64 Table 2)
Food – lipids and dietary fats are esters (triglycerides)

29. Amides (R-CONHR2) Physical Properties
Polarity of functional group
Polar due to C=O,C-N and N-H bonds
Similar physical properties to carboxylic acids
Hydrogen bonding
Since primary amides have two N-H bonds they have stronger hydrogen bonding than carboxylic acids
Secondary amides also experience hydrogen bonding
Solubility in Water
Amides are soluble in water
The solubility decreases as the non-polar alkyl chain increases in size
Melting & Boiling Points
Primary amides have much higher melting and boiling points than carboxylic acids
Many primary amides are solids at SATP

30. Amides Additional Characteristics
Amides such as acetaminophen are analgesics (pain killers)
Urea is a common amide found in urine and fertilizers