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5.7: Physical Properties of Esters

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    338520

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    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 \(\PageIndex{1}\) 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.

    Table \(\PageIndex{1}\): Physical Properties of Some Esters
    Condensed Structural Formula Name Molar Mass Melting Point (°C) Boiling Point (°C) Aroma
    Screen Shot 2021-01-18 at 1.41.13 PM.png methyl methanoate 60 −99 32  
    Screen Shot 2021-01-18 at 2.46.28 PM.png ethyl methanoate 74 −80 54 rum
    Screen Shot 2021-01-18 at 2.21.19 PM.png methyl ethanoate 74 −98 57  
    Screen Shot 2021-01-18 at 1.42.48 PM.png ethyl ethanoate 88 −84 77  
    Screen Shot 2021-01-18 at 2.41.01 PM.png methyl butanoate 102 −85 102 apple
    Screen Shot 2021-01-18 at 2.50.51 PM.png ethyl butanoate 116 −101 121 pineapple
    Screen Shot 2021-01-18 at 2.54.20 PM.png pentyl ethanoate 130 −71 148 pear
    CH3COOCH2CH2CH(CH3)2 isopentyl ethanoate 130 −79 142 banana
    Screen Shot 2021-01-18 at 2.18.11 PM.png phenyl ethanoate 150 −51 215 jasmine
    CH3CH2CH2COO(CH2)4CH3

    pentyl butanoate

    		 158 −73 185 apricot
    CH3COO(CH2)7CH3 octyl ethanoate 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.

    Concept Review Exercises

    1. Which compound has the higher boiling point—  CH3-CH2-CH2-CH2-OH  or Screen Shot 2021-01-18 at 1.40.08 PM.png? Explain.

    Answers

    1. CH3-CH2-CH2-CH2-OH because there is intermolecular hydrogen bonding (There is no intermolecular hydrogen bonding in Screen Shot 2021-01-18 at 2.21.19 PM.png.)

    Exercises

    1. Which compound has the higher boiling point— CH3-CH2-CH2-COOH or Screen Shot 2021-01-18 at 2.41.01 PM.png? Explain.

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

    Answer

    1. CH3-CH2-CH2-COOH because there is intermolecular hydrogen bonding and forms a dimer (There is no intermolecular hydrogen bonding between ethers Screen Shot 2021-01-18 at 2.41.01 PM.png.)

    5.7: Physical Properties of Esters is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.