Bases. protonated alcohols

Primary Strong acid Protonated alcohol Conjugate base... 

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. 

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. 

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. 

Compounds with a high HOMO and LUMO (Figure 5.5c) tend to be stable to selfreaction but are chemically reactive as Lewis bases and nucleophiles. The higher the HOMO, the more reactive. Carbanions, with HOMO near a, are the most powerful bases and nucleophiles, followed by amides and alkoxides. The neutral nitrogen (amines, heteroaromatics) and oxygen bases (water, alcohols, ethers, and carbonyls) will only react with relatively strong Lewis acids. Extensive tabulations of gas-phase basicities or proton affinities (i.e., —AG° of protonation) exist [109, 110]. These will be discussed in subsequent chapters. 

A good example of the use of the functional-group concept is for acid-base properties. Alcohols, ROH, are structurally related to water, HOH, in that both possess a hydroxyl function. We may then expect the chemistry of alcohols to be similar to that of water. In fact, both are weak acids because the OH group has a reactive proton that it can donate to a sufficiently strongly basic substance, written as B here ... 

Because of the presence of the nucleophilic oxygen and electrophilic proton, alcohols can act both as weak acids and as weak bases when dissolved in water (Following fig.). However, the equilibrium in both cases is virtually completely weighted to the unionised form. 

In the final step of the reaction, a proton is removed from the protonated alcohol by a base in the solution. This base is often a molecule of the solvent, water in this case. 

The first step of this reaction is an acid-base proton exchange between a hydroxide anion (pKa of water is approximately 16) and a primary alcohol (pKa is approximately 16) forming the illustrated alkoxide, B. Formation of water, the by-product, is not shown in this step. 

As proton donor coordination has an impact on the rate of carbonyl reduction, it is reasonable to expect that glycols should significantly accelerate the rate of carbonyl reduction. Seminal work by Hilmersson examined the correlation between the number of ethereal oxygens in a series of ethylene glycol-based proton donors and the rate of reduction of ketones.13 His work showed that coordinating alcohols enhance the rate of ketone reduction substantially and that the rate increase is proportional to the number of ethereal oxygens in the proton donor source.13 The mechanistic basis for this... 

For both carbon and hydrogen, a bond to oxygen is stronger than a bond to carbon. Yet we have no hesitation in breaking O-H bonds (of, say, carboxylic acids) with even the weakest of bases and we have spent much of the last chapter showing C-O bonds of protonated alcohols rupturing spontaneously What is going on ... 

Like water, many organic compounds that contain oxygen can act as bases and accept protons ethyl alcohol and ethyl ether, for example, form the oxonium ions I and II. For convenience, we shall often refer to a structure like I as a protonated alcohol and a structure like II as a protonated ether. 

The carbonium ion is formed by dissociation of the protonated alcohol this involves separation of a charged particle, R, from a neutral particle, H2O. It is obvious that this process requires much less energy than would formation of a carbonium ion from the alcohol itself, since the latter process involves separation of a positive particle from a negative particle. Viewed in another way, the carbonium ion (a Lewis acid) releases the weak base, water, much more readily than it... 

We notice that the carbonium ion combines with water to form not the alcohol but the protonated alcohol in a subsequent reaction this protonated alcohol releases a hydrogen ion to another base to form the alcohol. This sequence of reactions, we can see, is just the reverse of that proposed for the dehydration of alcohols (Sec. 5.20). In dehydration, the equilibria are shifted in favor of the alkene chiefly by the removal of the alkene from the reaction mixture by distillation in hydration, the equilibria are shifted in favor of the alcohol partly by the high concentration of water. 

We have seen that an alcohol, acting as a base, can accept a hydrogen ion to form the protonated alcohol, ROH. Let us now turn to reactions in which an alcohol, acting as an acid, loses a hydrogen ion to form the alkoxide ion, RO ". 

Since water is a much stronger base than alcohol, the addition of a small amount of water to an alcoholic solution of a weak acid will increase the dissociation of the latter considerably. The extent of the water effect is determined by the respective capacities of water and alcohol for taking up protons ... 

We have seen that water can behave both as an acid and as a base. An alcohol behaves similarly It can behave as an acid and donate a proton, or as a base and accept a proton. 

Protonation of the amino group makes it a weaker base and therefore a better leaving group, but it still is not nearly as good a leaving group as a protonated alcohol. Recall that protonated ethanol is more than 13p.Sra units more acidic than protonated ethylamine. 

So, unlike the leaving group of a protonated alcohol, the leaving group of a protonated amine cannot dissociate to form a carbocation or be replaced by a halide ion. Protonated amino groups also cannot be displaced by strongly basic nucleophiles such as HO because the base would react immediately with the acidic hydrogen, and protonation would convert it into a poor nucleophile. 

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