15.7 Physical Properties of Esters

Learning Objectives

Compare the boiling points of esters with alcohols of similar molar mass.
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

Which compound has the higher boiling point: CH₃CH₂CH₂CH₂OH or CH₃COOCH₃? Explain.
Which compound is more soluble in water—methyl propanoate or butyric acid? Explain.

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Key Takeaways

Esters have polar bonds but do not engage in intermolecular hydrogen bonding and are therefore intermediate in boiling points between the nonpolar alkanes and the alcohols, which engage in intermolecular hydrogen bonding.
Ester molecules can engage in hydrogen bonding with water, so esters of low molar mass (3-5 C atoms) are somewhat soluble in water.

Exercises

Which compound has the higher boiling point: CH₃CH₂CH₂COOH or CH₃CH₂CH₂COOCH₃? Explain.
Which compound is more soluble in water—methyl acetate or octyl acetate? Explain.