The physical, chemical and spectroscopic properties of alcohols are relative to it’s chemical structures. Alcohols are compounds of the general formula ROH, where R is any alkyl or substituted alkyl group. The hydroxyl group (OH groups) is the characteristic functional group of alcohols and is one of the most important functional groups of naturally occurring organic molecules. All carbohydrates and their derivatives, including nucleic acids, have hydroxyl groups. Some amino acids, most steroids, many terpenes, and plant pigments have hydroxyl groups. These substances serve many diverse purposes for the support and maintenance of life. One extreme example is the potent toxin tetrodotoxin, which is isolated from puffer fish and has obvious use for defense against predators. This compound has special biochemical interest, having six different hydroxylic functions arranged on a cagelike structure:
On the more practical side, vast quantities of simple alcohols —methanol, ethanol, 2-propanol, 1-butanol —and many ethers are made from petroleum-derived hydrocarbons. These alcohols are widely used as solvents and as intermediates for the synthesis of more complex substances.
The reactions involving the hydrogens of alcoholic OH groups are expected to be similar to those of water, HOH, the simplest hydroxylic compound. Alcohols, ROH, can be regarded in this respect as substitution products of water. However, with alcohols we shall be interested not only in reactions that proceed at the O-H bond but also with processes that result in cleavage of the C-O bond, or changes in the organic group R.
The simple ethers, ROR, do not have O-H bonds, and most of their reactions are limited to the substituent groups. The chemistry of ethers, therefore, is less varied than that of alcohols. This fact is turned to advantage in the widespread use of ethers as solvents for a variety of organic reactions, as we already have seen for Grignard reagents . Nonetheless, cyclic ethers with small rings show enhanced reactivity because of ring strain and, for this reason, are valuable intermediates in organic synthesis.
PHYSICAL PROPERTIES OF ALCOHOLS; HYDROGEN BONDING
Comparison of the physical properties of alcohols with those of hydrocarbons of comparable molecular weight shows several striking differences, especially for those with just a few carbons. Alcohols are substantially less volatile, have higher melting points, and greater water solubility than the corresponding hydrocarbons (see Table 15-1), although the differences become progessively smaller as molecular weight increases.
The reason for these differences in physical properties is related to the high polarity of the hydroxyl group which, when substituted on a hydrocarbon chain, confers a measure of polar character to the molecule. As a result, there is a significant attraction of one molecule for another that is particularly pronounced in the solid and liquid states. This polar character leads to association
Comparison of Physical Properties of Alcohols and Hydrocarbons
of alcohol molecules through the rather positive hydrogen of one hydroxyl group with a correspondingly negative oxygen of another hydroxyl group:
This type of association is called “hydrogen bonding,” and, although the strengths of such bonds are much less than those of most conventional chemical bonds, they are still significant (about 5 to 10 kcal per bond). Clearly then, the reason alcohols have higher boiling points than corresponding alkyl halides, ethers, or hydrocarbons is because, for the molecules to vaporize, additional energy is required to break the hydrogen bonds. Alternatively, association through hydrogen bonds may be regarded as effectively raising the molecular weight, thereby reducing volatility.
The water solubility of the lower-molecular-weight alcohols is pronounced and is understood readily as the result of hydrogen bonding with water molecules:
In methanol, the hydroxyl group accounts for almost half of the weight of the molecule, and it is not surprising that the substance is completely soluble in water. As the size of the hydrocarbon groups of alcohols increases, the hydroxyl group accounts for progressively less of the molecular weight, hence water solubility decreases (Figure 15-1). Indeed, the physical properties of higher-molecular-weight alcohols are very similar to those of the corresponding hydrocarbons (Table 15-1). The importance of hydrogen bonding in the solvation of ions was discussed in Section 8-7F.
SPECTROSCOPIC PROPERTIES OF ALCOHOLS
The hydrogen-oxygen bond of a hydroxyl group gives a characteristic absorption band in the infrared but, as we may expect, this absorption is considerably influenced by hydrogen bonding. For example, in the vapor state (in which there is essentially no hydrogen bonding), ethanol gives an infrared spectrum with a fairly sharp absorption band at 3700 cm-1, owing to a free or unassociated hydroxyl group (Figure 15-2a). In contrast, this band is barely visible at 3640 cm-1 in the spectrum of a 10% solution of ethanol in
carbon tetrachloride (Figure 15-2b). However, there is a relatively broad band around 3350 cm-1, which is characteristic of hydrogen-bonded hydroxyl groups. The shift in frequency of about 300 cm-1 arises because hydrogen bonding weakens the O-H bond; its absorption frequency then will be lower. The association band is broad because the hydroxyl groups are associated in aggregates of various sizes and shapes. This produces a variety of different kinds of hydrogen bonds and therefore a spectrum of closely spaced O-H absorption frequencies.
In very dilute solutions of alcohols in nonpolar solvents, hydrogen bonding is minimized. However, as the concentration is increased, more and more of the molecules become associated and the intensity of the infrared absorption band due to associated hydroxyl groups increases at the expense of the free-hydroxyl band. Furthermore, the frequency of the association band is a measure of the strength of the hydrogen bond. The lower the frequency relative to the position of the free hydroxyl group, the stronger is the hydrogen bond. The hydroxyl group in carboxylic acids (RCOaH) forms stronger hydrogen bonds than alcohols and accordingly absorbs at lower frequencies (lower by about 400 cm-1, see Table 9-2).
The infrared spectra of certain 1,2-diols (glycols) are interesting in that they show absorption due to intramolecular hydrogen bonding. These usually are fairly sharp bands in the region 3450 to 3570 cm-1, and, in contrast to bands due to intermolecular hydrogen bonding, they do not change in intensity with concentration. A typical example is afforded by m-l,2-cyclopentanediol:
Besides the O—H stretching vibrations of alcohols, there is a bending O—H vibration normally observed in the region 1410-1260 cm-1. There also is a strong C—O stretching vibration between 1210 cm”1 and 1050 cm-1. Both these bands are sensitive to structure as indicated below:
The influence of hydrogen bonding on the proton nmr spectra of alcohols has been discussed previously (Section 9-10E). You may recall that the chemical shift of the OH proton is variable and depends on the extent of association through hydrogen bonding; generally, the stronger the association, the lower the field strength required to induce resonance. Alcohols also undergo intermolecular OH proton exchange, and the rate of this exchange can influence the line-shape of the OH resonance, the chemical shift, and the incidence of spin-spin splitting, as discussed in more detail in Sections 9-10E and 9-101. Concerning the protons on carbon bearing the hydroxyl group, that is, they are deshielded by the electron-attracting oxygen atom and accordingly have chemical shifts some 2.5-3.0 ppm to lower fields than alkyl protons.
Perhaps you are curious as to why absorptions are observed in the infrared spectrum of alcohols that correspond both to free and hydrogen-borided hydroxyl groups, whereas only one OH resonance is observed in their proton nmr spectra. The explanation is that the lifetime of any molecule in either the free or the associated state is long enough to be detected by infrared absorption but much too short to be detected by nmr. Consequently, in the nmr one sees only the average OH resonance of the nonhydrogen-bonded and hydrogen-bonded species present. The situation here is very much like that observed for conformational equilibration.
The longest-wavelength ultraviolet absorption maxima of methanol and methoxymethane (dimethyl ether) are noted in Table 9-3. In each case the absorption maximum, which probably involves an n —» σ* transition, occurs about 184 nm, well below the cut-off of the commonly available spectrometers.
The mass spectra of alcohols may not always show strong molecular ions. The reason is that the M+ ions readily fragment by α cleavage. The fragment ions are relatively stable and are the gaseous counterparts of protonated aldehydes and ketones:
Ethers also fragment by a cleavage:
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