Organic Chemistry
Physical Properties
Physical Properties
If the functional groups are the same, the length of the carbon chain tells us
which organic compound will have higher melting and boiling points.
Longer carbon chain means higher melting and boiling points
Reason: increasing the length of the non-polar chain
increases the molar mass of the chain, which
increases the strength of the dispersion forces between the molecules.
Example: hexan-1-ol has higher melting and boiling points than ethanol
Name
Structural formula
Melting
point (°C)
Boiling
point (°C)
ethanol
HO – CH2 – CH3
-114
78.4
hexan-1-ol
HO – CH2 – CH2 – CH2 – CH2 – CH2 – CH3
-46.7
158
Physical Properties
If the lengths of the carbon chain are similar, the polarity of the functional groups
tells us which organic compound will have higher melting and boiling points.
More polar groups means higher melting and boiling points
Reason: more polar groups means
stronger dipole-dipole forces between molecules
Example: pentan-1-ol has higher melting and boiling points than pentanal
Explanation: the O-H bond is more polar than the C=O bond. The pentan-1-ol will
display hydrogen bonding between its molecules, pentanal will not.
Name
Structural formula
Melting
point (°C)
Boiling
point (°C)
O
pentanal
pentan-1-ol
H – C – CH2 – CH2 – CH2 – CH3
HO – CH2 – CH2 – CH2 – CH2 – CH3
-60.0
102
-77.6
138
Physical Properties
An isomeric acid of an ester is a carboxylic acid that has the same number of each
element as the ester. (They have the same molecular formula.)
Esters have lower boiling points than isomeric acids.
Reason: The ester and its isomeric acid have the same length of non-polar carbon
chain, so the boiling point depends on the polarity of the groups.
Carboxylic acids contain a highly polar O-H bond so the molecules will display
hydrogen bonding. The secondary interactions in the ester will be weaker.
Example: pentanoic acid has a higher boiling point than ethyl propanoate
(both have molecular formula C5H10O2)
Name
Structural formula
O
ethyl propanoate
CH3 – CH2 – O – C – CH2 – CH3
pentanoic acid
O
HO – C – CH2 – CH2 – CH2 – CH3
Boiling point (°C)
99.1
186
Physical Properties
In general, compounds with similar polarity dissolve each other (like dissolves like).
Water is highly polar and displays hydrogen bonding between its molecules.
Compounds with more polar groups and lower molar mass
will be more soluble in water.
Example: ethanol is more soluble in water than propanol
(ethanol has a lower molar mass)
Example: ethanol is more soluble in water than ethanal
(a hydroxyl group is more polar than an aldehyde group)
Example: butan-2,3-diol is more soluble than butan-2-ol
(butan-2,3-diol has two hydroxyl groups, butan-2-ol only has one)