Chemistry
Topic 9: Derivatives of Carboxylic Acids
Physical Properties
Boiling Points
The small esters have boiling points which are similar to those of aldehydes and ketones with a
similar RMM.
Like aldehydes and ketones, they are polar molecules and so have dipole-dipole interactions –
because of polar C=O group as well as Van der Waals dispersion forces. However, they don’t form
hydrogen bonds (as there is no hydrogen bonded to an oxygen) and so their boiling points aren’t
anything like as high as an acid with the same number of carbon atoms.
Molecule Type Boiling point
(oC) An alkyl chain in ester increases – boiling point
CH3COOCH2CH3 Ester 77.1 increases as increasing number of electrons
CH3CH2CH2COOH Carboxylic 164 causes Van der Waals forces of attraction
Acid between molecules.
Solubility in water
The small esters are fairly soluble in water, but solubility falls with chain length.
Ester Structural Formula Solubility (g per 100g of water)
Ethyl methanoate HCOOCH2CH3 10.5
Ethyl ethanoate CH3COOCH2CH3 8.7
Ethyl Propanoate CH3CH2COOCH2CH3 1.7
In a similar way to aldehydes and ketones, esters can’t hydrogen bond with themselves, they can
hydrogen bond with water molecules.
One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one
of the lone pairs on one of the oxygen atoms in an ester for a hydrogen bond to be formed.
There will also, of course, be Van der Waals dispersion forces and dipole-dipole attractions between
the ester and the water molecules.
Forming these attractions releases energy. This helps to supply the energy needed to separate water
molecule from water molecule and ester molecule from ester molecule before they can mix
together.
As chain lengths increase, the hydrocarbon parts of the ester molecules start to get in the way.
Longer chains force themselves between water molecules, breaking the relatively strong hydrogen
bonds between water molecules without replacing them by anything as strong (Van der Waals). This
makes the process energetically less profitable and so solubility decreases.
Ester
Topic 9: Derivatives of Carboxylic Acids
Physical Properties
Boiling Points
The small esters have boiling points which are similar to those of aldehydes and ketones with a
similar RMM.
Like aldehydes and ketones, they are polar molecules and so have dipole-dipole interactions –
because of polar C=O group as well as Van der Waals dispersion forces. However, they don’t form
hydrogen bonds (as there is no hydrogen bonded to an oxygen) and so their boiling points aren’t
anything like as high as an acid with the same number of carbon atoms.
Molecule Type Boiling point
(oC) An alkyl chain in ester increases – boiling point
CH3COOCH2CH3 Ester 77.1 increases as increasing number of electrons
CH3CH2CH2COOH Carboxylic 164 causes Van der Waals forces of attraction
Acid between molecules.
Solubility in water
The small esters are fairly soluble in water, but solubility falls with chain length.
Ester Structural Formula Solubility (g per 100g of water)
Ethyl methanoate HCOOCH2CH3 10.5
Ethyl ethanoate CH3COOCH2CH3 8.7
Ethyl Propanoate CH3CH2COOCH2CH3 1.7
In a similar way to aldehydes and ketones, esters can’t hydrogen bond with themselves, they can
hydrogen bond with water molecules.
One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one
of the lone pairs on one of the oxygen atoms in an ester for a hydrogen bond to be formed.
There will also, of course, be Van der Waals dispersion forces and dipole-dipole attractions between
the ester and the water molecules.
Forming these attractions releases energy. This helps to supply the energy needed to separate water
molecule from water molecule and ester molecule from ester molecule before they can mix
together.
As chain lengths increase, the hydrocarbon parts of the ester molecules start to get in the way.
Longer chains force themselves between water molecules, breaking the relatively strong hydrogen
bonds between water molecules without replacing them by anything as strong (Van der Waals). This
makes the process energetically less profitable and so solubility decreases.
Ester
