9.7: Physical Properties of Esters
- Page ID
- 60626
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- Compare the solubilities of esters in water with the solubilities of comparable alkanes and alcohols in water.
Ester molecules are polar but have no hydrogen atom attached directly to an oxygen atom. They are therefore incapable of engaging in intermolecular hydrogen bonding with one another and thus have considerably lower boiling points than their isomeric carboxylic acids counterparts. Because ester molecules can engage in hydrogen bonding with water molecules, however, esters of low molar mass are somewhat soluble in water. Borderline solubility occurs in those molecules that have three to five carbon atoms. Table \(\PageIndex{1}\) lists the physical properties of some common esters.
Esters are common solvents. Ethyl acetate is used to extract organic solutes from aqueous solutions—for example, to remove caffeine from coffee. It also is used to remove nail polish and paint. Cellulose nitrate is dissolved in ethyl acetate and butyl acetate to form lacquers. The solvent evaporates as the lacquer “dries,” leaving a thin film on the surface. High boiling esters are used as softeners (plasticizers) for brittle plastics.
| Condensed Structural Formula | Name | Molar Mass | Melting Point (°C) | Boiling Point (°C) | Aroma |
|---|---|---|---|---|---|
| HCOOCH3 | methyl formate | 60 | −99 | 32 | |
| HCOOCH2CH3 | ethyl formate | 74 | −80 | 54 | rum |
| CH3COOCH3 | methyl acetate | 74 | −98 | 57 | |
| CH3COOCH2CH3 | ethyl acetate | 88 | −84 | 77 | |
| CH3CH2CH2COOCH3 | methyl butyrate | 102 | −85 | 102 | apple |
| CH3CH2CH2COOCH2CH3 | ethyl butyrate | 116 | −101 | 121 | pineapple |
| CH3COO(CH2)4CH3 | pentyl acetate | 130 | −71 | 148 | pear |
| CH3COOCH2CH2CH(CH3)2 | isopentyl acetate | 130 | −79 | 142 | banana |
| CH3COOCH2C6H5 | benzyl acetate | 150 | −51 | 215 | jasmine |
| CH3CH2CH2COO(CH2)4CH3 | pentyl butyrate | 158 | −73 | 185 | apricot |
| CH3COO(CH2)7CH3 | octyl acetate | 172 | −39 | 210 | orange |
Summary
Esters have polar bonds but do not engage in hydrogen bonding and are therefore intermediate in boiling points between the nonpolar alkanes and the alcohols, which engage in hydrogen bonding. Ester molecules can engage in hydrogen bonding with water, so esters of low molar mass are therefore somewhat soluble in water.