Carboxylic Derivatives - Physical Properties
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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.
|CH3(CH2)2CO2H||butanoic acid||88||164 ºC||very 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|
|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.
William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry