Example 7.6

Example . Phenols are also converted to ethers by this method. In this, phenol is used as the phenoxide moiety. The C-O bonds in ethers are polar and thus, ethers have a net dipole moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases: Formula CH (CH ) CH C H -O-C H CH (CH ) -OH n-Pentane Ethoxyethane Butan- -ol b.p./K . . The large difference in boiling points of alcohols and ethers is due to the presence of hydrogen bonding in alcohols. The miscibility of ethers with water resembles those of alcohols of the same molecular mass. Both ethoxyethane and butan- -ol are miscible to almost the same extent i.e., . and g per mL water, respectively while pentane is essentially immiscible with water. Can you explain this observation ? This is due to the fact that just like alcohols, oxygen of ether can also form hydrogen bonds with water molecule as shown: . Cleavage of C–O bond in ethers Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules. Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide. Ethers with two different alkyl groups are also cleaved in the same manner. The order of reactivity of hydrogen halides is as follows: HI > HBr > HCl. The cleavage of ethers takes place with concentrated HI or HBr at high temperature. . . Physical Properties . . Chemical Reactions The reaction of an ether with concentrated HI starts with protonation of ether molecule. Step : The reaction takes place with HBr or HI because these reagents are sufficiently acidic. Step : Iodide is a good nucleophile. It attacks the least substituted carbon of the oxonium ion formed in step and displaces an alcohol molecule by S N mechanism. Thus, in the cleavage of mixed ethers with two different alkyl groups, the alcohol and alkyl iodide formed, depend on the nature of alkyl groups. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide (S N reaction). When HI is in excess and the reaction is carried out at high temperature, ethanol reacts with another molecule of HI and is converted to ethyl iodide. Step :