Protonation of alcohols

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 Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

By protonation of alcohols The protonated alcohols give carbocations on decomposition. 

The situation here is analogous to that discussed previously for the splitting of the resonances of O—H protons of alcohols by protons on the a carbons (see Section 9-101). 

The methyl groups of dimethyl sulfoxide are also anisochronous in the presence of chiral lanthanide shift reagents, such as Eu(facam) or Eu(hfbc)3 (Fig. 31)51). The enantiotopic carbinol protons of alcohols RCH2OH are similarly rendered anisochronous by chiral shift reagents 52 . 

Like the O—H protons of alcohols, the N—H protons of amines absorb at chemical shifts that depend on the extent of hydrogen bonding. The solvent and the sample concentration influence hydrogen bonding and therefore the chemical shift. Typical N—H chemical shifts appear in the range 81 to 8 4. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

These observations are easily explained by another simple reaction mechanism, nucleophilic substitution of an alkoxide on silicon (12). In this case, the basic alkoxide oxygens tend to repel the nucleophile, OH, and the bulkier alkyl groups tend to crowd it. Therefore, more highly hydrolyzed silicons are more prone to attack. Because this mechanism would have a pentacoordinated silicon atom in the activated complex, hydrolysis of a polymer would be more sterically hindered than hydrolysis of a monomer. Reesterification would be much more difficult in alkaline solution than in acidic solution, because silanols are more acidic than the hydroxyl protons of alcohols and would be deprotonated and negatively charged at a pH lower than the point at which the nucleophile concentration becomes significant (ii). Thus, although hydrolysis in alkaline solution is slow, it still tends to be complete and irreversible, if extensive polymerization does not occur first. 

The pioneering studies in this field are done by Pracejus and coworkers in a series of papers published in the 1960s in which they studied the asymmetric addition/protonation of alcohols to ketenes [8]. They carried out a number of experiments to demonstrate the catalytic role of a tertiary amine on the nucleophilic... 

The reaction sites in alcohols, phenols, and ethers are the polar bonds (carbon-oxygen and oxygen-hydrogen) and the lone pairs of electrons on the oxygen. The unshared electron-pairs on alcohols and ethers make these compounds Lewis bases. Oxoniums ions, in which the oxygen has three bonds and is positive, result from the protonation of alcohols and ethers. Most reactions of alcohols involve the O-H bond, C-0 bond, or both. 

Under closer scrutiny, one can see that in the process of strong acid protonation of alcohol, another reaction can occur instead of the substitution attack on another alcohol. A carbonium ion is a highly reactive species. It can participate in a substitution reaction, as in ether formation, or undergo an elimination reaction to form an alkene. The latter reaction is most prominent in tertiary alcohols. 

The reaction in question is an acid-=catalyzed dehydration. The first step of this reaction is the protonation of alcohol by the acid, H2SO. Since the reaction... 

So now we can expand our chart of acid and base strengths to include the important classes of alcohols, phenols, and carboxylic acids. They conveniently, and memorably, have piCa values of about 0 for the protonation of alcohols, about 5 for the deprotonation of carboxylic acids, about 10 for the deprotonation of phenols, and about 15 for the deprotonation of alcohols. The equilibria above each piCa shows that at approximately that pH, the two species each form 50% of the mixture. You can see that carboxylic acids are weak acids, alkoxide ions (RO ) are strong bases, and that it will need a strong acid to protonate an alcohol. 

As an example, Equation 1 gives the results for the proton affinities (PA) of ethers and alcohols. The proton affinities, the heats of reaction released on protonation of alcohols and ethers, can be calculated from the electronegativity parameters, X12, being a measure of the inductive effect, and aa, representing the polarisability effect. ... 

Section 13.12 Splitting resulting from coupling to the O—H proton of alcohols is not... 

The NMR peak for the hydroxyl proton of alcohols can be found anywhere from d 0.5 to 8 5.4. Explain this variability. 

Strong acids produce carbocations from a variety of functional molecules. Protonation of alcohols, epoxides, carbonyl compounds, and alkenes does so. Lewis acids such as anhydrous aluminum chloride can combine with these substrates and can also remove halide ions from carbon to give carbocations. Diazotization of primary amines in acid solution is another source. 

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