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 

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  

Exercise 15-3 What type of infrared absorption bands due to hydroxyl groups would you expect for frans-cyclobutane-1,2-diol and butane-1,2-diol (a) in very dilute solution, (b) in moderately concentrated solution, and (c) as pure liquids Give your reasoning. 

Perhaps you are curious as to why absorptions are observed in the infrared spectrum of alcohols that correspond both to free and bydrogen-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. Consequendy, 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 (Section 9-IOC). 

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 —— cr transition, occurs about 184 nm, well below the cut-off of the commonly available spectrometers. 

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Tables 3-2 and 3-3 summarize the infrared and proton-NMR (nuclear magnetic resonance) spectroscopic properties of alcohols and ethers. In the proton NMR, the oxygen atom is deshielding. Phenols and alcohols rapidly exchange protons so their NMR spectra are solvent dependant. The alcohol and ether groups don t have any characteristics absorptions in UV-vis spectra.

The spectroscopic properties of ethers might be anticipated on the basis of what you already know about the spectroscopic properties of alcohols and alkanes. Thus, the C-0 stretching vibrations in the region 1200-l(X)0cm (as seen in alcohols) would be anticipated in the IR. Again, as in alcohols, without unsaturation, there cannot be an n it transition in the UV or VIS and thus, with the C-O-C as the chromophore, only an n a transition is allowed, and there is no absorption above 200 nm. However, variously substituted methylated phenols (i.e., methoxy aryl ethers, ArOCHs) are common plant constituents, absorb strongly in the UV and VIS, and frequently contribute to coloring matter in plants. 

As each functional group is discussed in future chapters, the spectroscopic properties of that group will be described. For the present, we ll point out some distinguishing features of the hydrocarbon functional groups already studied and briefly preview some other common functional groups. We should also point out, however, that in addition to interpreting absorptions that ore present in an IR spectrum, it s also possible to get structural information by noticing which absorptions are not present. If the spectrum of a compound has no absorptions at 3300 and 2150 cm-1, the compound is not a terminal alkyne if the spectrum has no absorption near 3400 cm -, the compound Is not an alcohol and so on. 

Diselenophosphate complexes are prepared from the interaction of metal salts and complexes with appropriate diselenophosphoric acid or its salt. The acids are obtained from the reaction of phosphorus(V) selenide with alcohols 229). The preparation of phosphorus(V) selenide and its reactions with alcohols 229) and amines 22°) have been described and a variety of complexes reported (Table 4). The biological activity of these compounds does not seem to have described but the exercise of extreme caution when handling these materials is recommended. Zingaro and his coworkers 229-232) thoroughly characterized the thermal and spectroscopic properties of a number of compounds. 

The spectroscopic properties of ethers are unexceptional. Like alcohols, they have no electronic absorption beyond 185 nm the important infrared bands are the C—O stretching vibrations in the region 1000-1230 cm-1 their proton nmr spectra show deshielding of the alpha hydrogens by the ether oxygen (6Hcaoc 3.4 ppm). The mass spectra of ethers and alcohols are very... 

For simple carbonyl compounds, the equilibrium between an aldehyde or a ketone and its corresponding enol is usually so shifted towards the keto form that the amount of enol at equilibrium can neither be measured nor detected by spectroscopy. Nevertheless, as recently emphasised by Hart (1979), this does not mean that the enol cannot exist free, not in equilibrium with ketones and aldehydes. Several examples of kinetically stable enols in the gas phase or in aprotic solvents have been reported. Broadly speaking, it appears that enols have relatively large life-times when they are prepared in proton-free media [e.g. the half-life of acetone enol was reported to be 14 s in acetonitrile (Laroff and Fischer, 1973 Blank et al., 1975) and 200 s in the gas phase (MacMillan et al., 1964)]. These life-times are related to an enhanced intramolecular rearrangement, indicated by the very high energies of activation (85 kcal mol-1 for acetaldehyde-vinyl alcohol tautomerization) which have been calculated (Bouma et al., 1977 Klopman and Andreozzi, 1979) It has therefore been possible to determine most of the spectroscopic properties of simple enols [ H nmr,l3C nmr (CIDNP technique), IR and microwave spectra of vinyl alcohol... 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

The results established in the preceding paragraph may be verified by comparing the thermodynamic properties and the spectroscopic properties of associated solutions. Let us consider, for example, a solution of ethanol in carbon tetrachloride. The valency vibration of the OH group gives rise to two distinct infra-red absorption bands depending upon whether the OH group is in a monomer or in an associated complex. The fraction of molecules of alcohol which remain in the monomeric state can therefore be determined from measurements... 

Compound 110, readily prepared from 111 and acrylic acid at 135°, underwent the same sequence of reactions used in the model series, yielding exclusively 112 in which it is inferred that the hydrogen at C-4 is trans to the bridging group. The ketone was converted into the alcohol 113 and thence to the mesylate 114 which in turn was transformed to the alkene 115 with potassium tertiary butoxide in DMSO. Only under these conditions was 115 obtained in high yield uncontaminated with rearranged products. Functionality at C-5 was introduced by oxidation with selenium dioxide in glacial acetic acid. Acetate 116 so obtained was hydrolysed to the alcohol 117 and oxidized to the racemic ketone 118. One of the enantiomers of 118 had already been prepared from annotinine (1) and comparison of the spectroscopic properties of 118 with the naturally derived sample established the identity of the two systems. 

As expected, the values of X max are similar when comparing cyanidin-3-0-glycosides dissolved in acidic alcohols to cyanidin in the same solvent. A striking difference in the spectroscopic properties of cyanidin-3-0-glycosides exists when aqueous to organic solvents are compared. 

Ten new metabolites (507—516), isolated from Pseudocyphellaria species by Corbett, Wilkins and co-workers (57), provide the first examples of this new group of triterpenoids. The structures of these metabolites followed primarily from a detailed comparison of the H-n.m. r. spectra with those of other triterpenoid groups. Corbett and coworkers had previously synthesized a number of the parent triterpanes, 18a-oleanane (5 ), 17aH-hopane (55), 17aH-moretane (55) and 14a-taraxerane (55), by methods similar to those exemplified for stictane (Scheme 66). Thus oxidation of the alcohol (516) gave the diketone (517) which upon Wolff-Kishner reduction yielded the parent stictane (518). The physical and spectroscopic properties of this triterpane confirmed the novelty of this group of compounds. 

Carboxylic acids (RCO2H) react with alcohols (R OEI) in the presence of an acid catalyst. The reaction product of propanoic acid with methanol has the following spectroscopic properties. Propose a structure. 

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