Alcohols: hydrogen bonding volatility

The copolymers are insoluble in water unless they are neutralized to some extent with base. They are soluble, however, in various ratios of alcohol and water, suggesting appHcations where deUvery from hydroalcohoHc solutions (149) but subsequent insolubiUty in water is desired, such as in low volatile organic compound (VOC) hair-fixative formulations or tablet coatings. Unneutralized, their Ts are higher than expected, indicating interchain hydrogen bonding (150). 

Alcohols with low molar masses are liquids, and alcohols have much lower vapor pressures than do hydrocarbons with approximately the same molar mass. For example, ethanol is a liquid at room temperature, but butane, which has a higher molar mass than ethanol, is a gas. The relatively low volatility of alcohols is a sign of the strength of hydrogen bonds. The ability of alcohols to form hydrogen bonds also accounts for the solubility in water of alcohols with low molar mass. 

Ethers are more volatile than alcohols of the same molar mass because their molecules do not form hydrogen bonds to one another (Fig. 19.2). They are also less soluble in water because they have a lower ability to form hydrogen bonds to water molecules. Because ethers are not very reactive and have low molecular polarity, they are useful solvents for other organic compounds. However, ethers are flammable diethyl ether is easily ignited and must be used with great care. 

Ethers are not very reactive. They are more volatile than alcohols with similar molar masses because their molecules cannot form hydrogen bonds with one another. Crown ethers adopt shapes that can enclose ions and carry them into nonpolar solvents. 

Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable moleeular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotie solvents. 

In summary, large molecules tend to be more persistent than smaller ones, though this may be modified by the presence of functional groups, which cause polarization and hydrogen bonding. For an equivalent molecular size, hydrocarbons and ethers are more volatile than aldehydes, ketones, and esters, which are in turn more volatile than alcohols and acids. 

Coming back once more to the alcoholic solutions After the alcohol has evaporated, there are still attraction forces of the dipole-dipole or hydrogen bonding kind at work that slow down the evaporation of the perfume material. These originate in part from interactions between the molecules of the different materials contained in the perfume. Perfumers may use materials of low volatility in their perfumes intending thereby to slow down the evaporation of more volatile perfume components. This is the practice commonly referred to as fixation (Sturm and Mansfeld 1976 Jellinek 1978). 

Alcohols have relatively high boiling points and low volatility because of hydrogen bond formation through the -OH group. In this way, they are similar to water. 

More volatile than alcohols with the same molar mass because ethers do not form hydrogen bonds with each other... 

In the same way dimethyl ether is a very volatile liquid but has the same empirical formula as ethyl alcohol which boils at 78°C. This is accounted for by the formation of hydrogen bonds. 

Since ethers do not have a hydrogen atom bonded to the oxygen, they do not undergo hydrogen bonding between their molecules. Therefore, ethers are more volatile than their corresponding alcohols. 

Hydrogen bonding can exist in ethyl alcohol but not dimethyl ether. Therefore ethyl alcohol would be less volatile (higher boiling point). 

Both substances have the same molar mass. Ethyl alcohol contains a hydrogen atom directly bonded to an oxygen atom, however. Therefore, hydrogen bonding can exist in ethyl alcohol, whereas only weak dipole-dipole forces exist in dimethyl ether. Dimethyl ether is more volatile ethyl alcohol has a higher boiling point. 

When two hydrocarbons are to be separated, the larger the difference in the number of double bonds the better a polar solvent will work to change the volatility. For exanple, furfural will decrease the volatility of butenes conpared to butanes. Furfural (a cyclic alcohol) is used instead of water because the hydrocarbons are miscible with furfural. A more detailed analysis of solvent selection shows that hydrogen bonding is more inportant than polarity IBerg. 1969 DoherU/ et al.. 2008 Smith. 1963). 

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