Alcohol proton transfer

Mechanism of esterification of carboxylic acids The esterification of carboxylic acids with alcohols is a kind of nncleophilic acyl snbstitntion. Protonation of the carbonyl ojq gen activates the carbonyl gronp towards nncleophilic addition of the alcohol. Proton transfer in the tetrahedral intermediate converts the hydrojq l group into - 0H2 group, which, being a better leaving group, is eliminated as neutml water molecule. The protonated ester so formed finally loses a proton to give the ester. 

Et30+[PF6] in bulk and in the presence of an alcohol. Proton transfer to monomer, followed by ring-opening and insertion into the chain (Scheme 12) initiates the reaction. 

Cyclic ethers can be prepared by the intramolecular Sj j2 reaction of a halogen-substituted alcohol such as a bromo alcohol. Proton transfer to a base such as sodium hydroxide gives a bromo alkoxide. If the solution is dilute, the alkoxide acts as a nucleophile, and an intramolecular reaction displaces a bromide ion. This process is shown below for 5-bromo-l-penten-l-ol. 

We saw m Chapter 1 especially m Table 1 7 that alcohols resemble water m respect to their Brpnsted acidity (ability to donate a proton/rom oxygen) They also resemble water m their Brpnsted basicity (ability to accept a proton on oxygen) Just as proton transfer... 

Furthermore a substance such as HCl that dissociates completely when dissolved m water also dissociates completely when dissolved m an alcohol Many important reactions of alco hols involve strong acids either as reactants or as catalysts In all these reachons the first step IS formation of an alkyloxonium ion by proton transfer from the acid to the alcohol... 

FIGURE 4 7 Potential energy diagram for proton transfer from hydrogen chio ride to tert butyl alcohol... 

Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another Thus the height of the energy barrier the activation energy for proton transfer must be quite low... 

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

The mechanism by which the Birch reduction of benzene takes place (Figure 118) IS analogous to the mechanism for the metal-ammonia reduction of alkynes It involves a sequence of four steps m which steps 1 and 3 are single electron transfers from the metal and steps 2 and 4 are proton transfers from the alcohol... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

Step 1 Proton transfer from the acid catalyst (sulfuric acid) to the oxygen of the alcohol to produce an alkyloxonmm ion... 

These reactions are usually performed in water or alcohols as solvents and the alkox ide ion intermediate is rapidly transformed to an alcohol by proton transfer The other involves acid catalysis Here the nucleophile is often... 

Step 1 The carboxylic acid is protonated on its carbonyl oxygen The proton donor shown in the equation for this step is an aUtyloxonium ion formed by proton transfer from the acid catalyst to the alcohol... 

Step 4 Proton transfer steps yield an alcohol and a carboxylate anion... 

One common kind of reaction involves proton transfer occurring as a rapid equilibrium preceding the rate-determining step, for example, in the reaction of an alcohol with hydrobromic acid to give an alkyl bromide ... 

This mechanism explains the observed formation of the more highly substituted alcohol from unsymmetrical alkenes (Markownikoff s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water ... 

The concerted nature of proton transfer contributes to its rapid rate. The energy cost of breaking the H—Cl bond is partially offset by the energy released in forming the new bond between the transfened proton and the oxygen of the alcohol. Thus, the activation energy is far- less than it would be for a hypothetical two-step process in which the H—Cl bond breaks first, followed by bond formation between FF and the alcohol. 

As pointed out in Chapter 4, the first step in the reaction is proton transfer to the alcohol from the hydrogen halide to yield an alkyloxonium ion. This is an acid-base reaction. 

Much information on proton transfers has been obtained by NMR chemical exchange studies. An example is the proton exchange between neopentyl alcohol and acetic acid in acetic acid as the solvent. The reaction is... 

Alcohols react with metal hydrides, MH, and with hydrogen halides, HX, but in very different ways. Proton transfer is involved in both reactions, but different molecules act as the proton donor and acceptor. 

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