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Carboxylic Derivatives - Physical Properties

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    1215
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    The important classes of organic compounds known as alcohols, phenols, ethers, amines and halides consist of alkyl and/or aryl groups bonded to hydroxyl, alkoxyl, amino and halo substituents respectively. If these same functional groups are attached to an acyl group (RCO–) their properties are substantially changed, and they are designated as carboxylic acid derivatives. Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy. Some examples of these functional derivatives were displayed earlier.

    The following table lists some representative derivatives and their boiling points. An aldehyde and ketone of equivalent molecular weight are also listed for comparison. Boiling points are given for 760 torr (atmospheric pressure), and those listed as a range are estimated from values obtained at lower pressures. The relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization. Similar hydrogen bonding occurs between molecules of 1º and 2º-amides (amides having at least one N–H bond), and the first three compounds in the table serve as hydrogen bonding examples.

    Table 1: Physical Properties of Some Carboxylic Acid Derivatives

    Formula

    IUPAC Name

    Molecular Weight

    Boiling Point

    Water Solubility

    CH3(CH2)2CO2H butanoic acid 88 164 ºC very soluble
    CH3(CH2)2CONH2  butanamide 87 216-220 ºC soluble
    CH3CH2CONHCH3 N-methylpropanamide 87 205 -210 ºC soluble
    CH3CON(CH3)2 N,N-dimethylethanamide 87 166 ºC very soluble
    HCON(CH3)CH2CH3 N-ethyl, N-methylmethanamide 87 170-180 ºC very soluble
    CH3(CH2)3CN pentanenitrile 83 141 ºC slightly soluble
    CH3CO2CHO ethanoic methanoic
    anhydride
    88 105-112 ºC reacts with water
    CH3CH2CO2CH3 methyl propanoate 88 80 ºC slightly soluble
    CH3CO2C2H5 ethyl ethanoate 88 77 ºC moderately soluble
    CH3CH2COCl propanoyl chloride 92.5 80 ºC reacts with water
    CH3(CH2)3CHO pentanal 86 103 ºC slightly soluble
    CH3(CH2)2COCH3 2-pentanone 86 102 ºC slightly soluble

    The last nine entries in the above table cannot function as hydrogen bond donors, so hydrogen bonded dimers and aggregates are not possible. The relatively high boiling points of equivalent 3º-amides and nitriles are probably due to the high polarity of these functions. Indeed, if hydrogen bonding is not present, the boiling points of comparable sized compounds correlate reasonably well with their dipole moments.

    Contributors

    William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry