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15.7 Physical Properties of Esters

Learning Objectives

  1. Compare the boiling points of esters with alcohols of similar molar mass.
  2. 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 15.4 "Physical Properties of Some Esters" lists the physical properties of some common esters.

Note

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.

Table 15.4 Physical Properties of Some Esters

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

Concept Review Exercises

  1. Which compound has the higher boiling point—CH3CH2CH2CH2OH or CH3COOCH3? Explain.

  2. Which compound is more soluble in water—methyl butyrate or butyric acid? Explain.

Answers

  1. CH3CH2CH2CH2OH because there is intermolecular hydrogen bonding (There is no intermolecular hydrogen bonding in CH3COOCH3.)

  2. butyric acid because of hydrogen bonding with water

Key Takeaways

  • 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.

Exercises

  1. Which compound has the higher boiling point—CH3CH2CH2COOH or CH3CH2CH2COOCH3? Explain.

  2. Which compound is more soluble in water—methyl acetate or octyl acetate? Explain.

Answer

  1. CH3CH2CH2COOH because there is intermolecular hydrogen bonding (There is no intermolecular hydrogen bonding in CH3CH2COOCH3.)