Hydroxyl protons

The phenomenon of intemiolecular exchange is very common. The loss of couplings to hydroxyl protons in all but the very purest etiianol samples was observed at a very early stage. Proton transfer reactions are still probably the most carellilly studied [14] class of intemiolecular exchange. 

The —OH proton of a primary alcohol RCH2OH is vicinal to two protons and its sig nal would be expected to be split into a triplet Under certain conditions signal splitting of alcohol protons is observed but usually it is not Figure 13 21 presents the NMR spec trum of benzyl alcohol showing the methylene and hydroxyl protons as singlets at 8 4 7 and 2 5 respectively (The aromatic protons also appear as a singlet but that is because they all accidentally have the same chemical shift and so cannot split each other)... 

The chemical shift of the hydroxyl proton is variable with a range of 8 0 5-5 depending on the solvent the temperature at which the spectrum is recorded and the concentration of the solution The alcohol proton shifts to lower field m more concen trated solutions... 

An easy way to verify that a particular signal belongs to a hydroxyl proton is to add D2O The hydroxyl proton is replaced by deuterium according to the equation... 

Deuterium does not give a signal under the conditions of H NMR spectroscopy Thus replacement of a hydroxyl proton by deuterium leads to the disappearance of the OH peak of the alcohol Protons bonded to nitrogen and sulfur also undergo exchange with... 

MHz H NMR spectrum of benzyl alcohol The hydroxyl proton and the methylene protons are vicinal but do not split each other because of the rapid intermolecular exchange of hydroxyl protons... 

Section 13 12 Splitting resulting from coupling to the O—H proton of alcohols is not normally observed because the hydroxyl proton undergoes rapid inter molecular exchange with other alcohol molecules which decouples it from other protons in the molecule... 

The chemical shift of the hydroxyl proton signal is variable depending on solvent temperature and concentration Its precise position is not particularly significant m struc ture determination Because the signals due to hydroxyl protons are not usually split by other protons m the molecule and are often rather broad they are often fairly easy to... 

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

NMR The H NMR signals for the hydroxyl protons of phenols are often broad and their chemical shift like their acidity lies between alcohols and carboxylic acids The range is 8 4-12 with the exact chemical shift depending on the concentration the solvent and the temperature The phenolic proton m the H NMR spectrum shown for p cresol for example appears at 8 5 1 (Figure 24 4)... 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

Localized positive charge in hydroxyl-protonated benzoic acid... 

Reduction of epoxide 21 with lithium aluminium hydride gave a crystalline branched-chain methyl heptoside derivative 24. The NMR spectra of compounds 21 and 24 were very similar. In the spectrum of compound 24 the disappearance of the two sharp doublets at r 6.80 and 7.45 (2 protons) and the appearance of a singlet at r 8.65 (3 protons) is consistent with the reductive cleavage of epoxide 21 to give a substance 24 with a methyl substituent. The multiplet at r 7.40-8.50 ( 5 protons ) was assigned to the four protons of the two methylene groups and the hydroxylic proton. 

The compounds were also run with D20 leaking into the ion source from a stainless steel reservoir. The resulting mass spectra show partial exchange of the hydroxyl protons with deuterium. From the shifts of the peaks in the mass spectra of the deuterated analogs, the number of exchangeable hydrogens retained in each fragment can be determined. 

When the H NMR spectrum of an alcohol is run in dimethyl sulfoxide fDMSO) solvent rather than in chloroform, exchange of the O—H proton is slow and spin-spin splitting is seen between the O-H proton and C-H protons on the adjacent carbon. What spin multiplicities would you expect for the hydroxyl protons in the following alcohols ... 

The 1H NMR spectra of the epimeric cyclohexanols in DMSO reveal that the hydroxyl proton in the axial alcohol shows a resonance absorption at a higher field than in the equatorial one, indicating that the conformational effect of the hydrogen bond influences the XH NMR chemical shifts128. 

Melt reaction mechanisms of tertiary aliphatic amine catalyzed phenolic-epoxy reactions were proposed to begin with a trialkylamine abstracting a phenolic hydroxyl proton to form an ion pair (Fig. 7.36). The ion pair was suggested to complex with an epoxy ring, which then dissociated to form a /1-hydroxycther and a regenerated trialkylamine.87... 

The intensities (heights) of the peaks are proportional to the numbers of protons that they represent. The three peaks in the ethanol spectrum, for example, have overall intensities in the ratio 3 2 1, which is what we would expect for the three methyl, two methylene, and one hydroxyl protons. 

The submitters purchased glacial acetic acid from Showa Denko K. K., Tokyo, Japan, and acetic anhydride from Riedel de Haen AG, Seelze-Hannover, Germany. A solution prepared from 4 volumes of glacial acetic acid, 1 volume of acetic anhydride, and a catalytic amount of p-toluenesulfonic acid was heated under reflux for 24 hours and distilled. The distillate, which contained 5% water and 4% acetic anhydride according to analysis of the proton magnetic resonance spectrum, was then used by the submitters. The water content was determined from the chemical shift of the hydroxyl proton. ... 

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