Alcohol protonated

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... 

Water adds to alkenes to yield alcohols, a process called hydration. The reaction takes place on treatment of the alkene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROH2+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). 

Loss of a proton from the protonated alcohol intermediate then gives the neutral alcohol product. 

Spontaneous dissociation of the protonated alcohol occurs in a slow, rate-limiting step to yield a carbocation intermediate plus water. 

V Two electrons from the oxygen atom bond to hT, yielding a protonated alcohol intermediate. 

A detailed mechanism of Goldschmidt s process has not been given two reaction paths are possible either proton transfer to the acid with the formation of RC(OH) (in which case the slow step would be an Aac2 Ingold mechanism) or nucleophilic attack of the carbonyl group of the acid on the protonated alcohol. The second mechanism would require an alkyl scission (A l). In more recent studies2501, it has been shown that scission in most cases is of the acyl type and particularly in the examples studied by Goldschmidt. 

Clearly the rate-determining step is the elimination of water from the protonated alcohol to form a carbonium ion, which loses a proton to give the readily oxidised olefin. 

The mechanism is an El reaction in which the substrate is a protonated alcohol or an alkyloxonium ion). 

The protonated alcohol loses a molecule of water to become a carbocation. This step is slow and rate determining... 

Primary Strong acid Protonated alcohol Conjugate base... 

The carbocation reacts with a molucule of water to form a protonated alcohol. 

Alcohols are nucleophiles => they can react with protonated alcohols to afford ethers. 

At a high enough temperature, and in the absence of a good nucleophile, protonated alcohols are capable of undergoing El or E2 reactions. 

The presence of ionic species is demonstrated by the conductivity of the solutions. It is a strongly acidic solvent that protonates alcohols, ethers, and acetic acid. These substances are not normally bases, but they have an unshared pair of electrons that can function as a proton acceptors. 

No intermediate tungsten complexes were observed in this reaction. The alcohol, sec-phenethylalcohol, is consumed at a rate which is much faster than that of its formation. It was shown separately to be converted to ethylbenzene (Eq. (23)) by HOTf and [Cp(CO)3WH]. This reaction presumably proceeds through loss of water from the protonated alcohol, followed by hydride transfer from [Cp(CO)3WH] to give ethylbenzene. 

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

The carbocatlon then undergoes rapid nucleophilic attack by a water molecule to give a protonated alcohol molecule (an alcohol with a hydrogen ion attached). 

The formation of ether is a nucleophilic bimolecular reaction (S 2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below ... 

Consider the conditions. If the reaction requires a catalyst, e.g. acid or base, it is almost certain that this needs to be used in the first step. For example, acid may protonate an electronegative atom, making a more reactive species. Thus, a protonated carbonyl becomes a better electrophile, and a protonated alcohol now has a better leaving group. Base may remove an acidic proton to generate a better nucleophile, although in some reactions it may itself act as the nucleophile. 

Differences in reaction rates depend on the relative abilities of the protonated alcohols to lose H,0 to form R The stability of R affects the AW for forming the incipient R" in the transition state and determines the overall rate. 

The protonated alcohol loses water readily to form the benzyl carbocation, which is stabilized by charge dispersion to the ring. 

An alcohol can behave like an acid and donate a proton. However, alcohols are much weaker organic acids, with pATg values close to 16. Alcohol may also behave as a base e.g., ethanol is protonated by sulphuric acid and gives ethyloxonium ion (C2H50H2 ). A protonated alcohol (pATg = —2.4) is a strong acid. 

Reactions involving esters and secondary or tertiary alcohols proceed by a different mechanism as demonstrated by the experiments using lsO-enriched alcohols. This mechanism entails essentially that the ester act as the nucleophile on a nascent carbenium ion produced from the protonated alcohol (Scheme 1). The initial association is described as a proton-bound cluster. 

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