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  Reactions of Alcohols Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Reactions of Alcohols

Hydroxyl group, -OH is the functional group of the alcohols. Different from the alkyl halides, this group has the two reactive covalent bonds, the O-H bond and the C-O bond. The electronegativity of oxygen is considerably greater than that of hydrogen and carbon. Subsequently, the covalent bonds of this functional group are polarized so that oxygen is electron rich and both carbon and hydrogen are electrophilic, as displayed in the diagram on the right. certainly, the dipolar nature of the O-H bond is that type of  alcohols are much stronger acids than alkanes (by roughly 1030 times), and almost that much stronger than ethers. In an alcohol molecule the most reactive site, is the hydroxyl group, in spite of the fact that the O-H bond strength is considerably greater than that of the C-C, C-H and C-O bonds, representing again the variation between chemical and thermodynamic stability.

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Substitution of the Hydroxyl Hydrogen

Due to its improved acidity, the hydrogen atom on the hydroxyl group is rather simply replaced by other substituents. A simple instance is the facile reaction of simple alcohols with sodium (and sodium hydride), as depicted in the first equation below. Other such type of substitution reaction is the isotopic exchange that takes place on mixing an alcohol with deuterium oxide (heavy water). This replace, which is catalyzed by acid or base, is very rapid under normal conditions, because in most experimental systems it is hard to avoid traces of such type of catalysts.

2 R-O-H   +   2 Na   →   2 R-O(-)Na(+)   +   H2

R-O-H   +   D2O  ↔   R-O-D   +   D-O-H

The technique by which many substitution reactions of this type take place is simple. The oxygen atom of an alcohol is nucleophilic and so by electrophiles is prone to attack. The resultant "onium" intermediate then loses a proton to a base giving the substitution product. If a strong electrophile is not exist, the nucleophilicity of the oxygen may be improved by conversion to its conjugate base (an alkoxide). That powerful nucleophile then attacks the weak electrophile. These two differences of the substitution mechanism are demonstrated in the diagram.

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The preparation of tert-butyl hypochlorite from tert-butyl alcohol is an instance of electrophilic halogenation of oxygen but this reaction is controlled to 3º-alcohols due to 1º and 2º-hypochlorites lose HCl to give ketones and aldehydes. In the equation shown below the electrophile might be regarded as Cl(+).

(CH3)3C-O-H   +   Cl2   +   NaOH  →  (CH3)3C-O-Cl   +   NaCl  +   H2O

The hydroxyl group's alkyl substitution leads to ethers. That reaction provides examples of both weak electrophilic substitution (second equation) and strong electrophilic substitution (first equation below). The latter SN2 reaction is considered as the Williamson Ether Synthesis, and is usually used only with 1º-alkyl halide reactants since the strong alkoxide base leads to E2 elimination of 2º and 3º-alkyl halides.

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One of the most significant substitution reactions at oxygen is ester formation resultant from the reaction of alcohols with electrophilic derivatives of sulfonic and carboxylic acids. The following examples shows the common formulas of these reagents and their ester results, wherein the R'-O- group stands for the alcohol moiety. The electrophilic atom in acid chlorides and anhydrides is colored red in the diagram.

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