Boiling points of ethers

An extremely violent reaction is observed if the dianion is quenched above 0°C, with ether boiling off at an uncontrollable rate. The submitters observed that if the chiorotrimethyl si lane addition is performed at a lower temperature, the reaction temperature will remain below that of the boiling point of ether. A dry-ice bath made up of 80 20 (v/v) ethanol-water was used the checkers measured a bath temperature of -55 C and kept the reaction in the bath for 16 min before adding chlorotnmethylsilane. 

Why are the boiling points of ethers typically lower than those of isomeric alcohols ... 

Table 18.1 j Comparison of Boiling Points of Ethers and Hydrocarbons... 

FIGURE 19.2 The boiling points of ethers are lower than those of isomeric alcohols, because there is hydrogen bonding in alcohols but not in ethers. All the molecules represented here are unbranched. 

Melting and boiling points The boiling points of ethers are much lower than the boiling points of alcohols with the same number of carbon atoms. 

Ethers may be prepared by (1) dehydration of alcohols and (11) Williamson synthesis. The boiling points of ethers resemble those of alkanes while their solubility Is comparable to those of alcohols having same molecular mass. The C-O bond In ethers can be cleaved by hydrogen halides. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the Incoming group to ortho and para positions. 

Chlorides of sulfonic acids can be reduced either partially to sulfinic acids, or completely to thiols. Both reductions are accomplished in high yields with lithium aluminum hydride. An inverse addition technique at a temperature of —20° is used for the preparation of sulfinic acids, while the preparation of thiols is carried out at the boiling point of ether [69S]. 

Comparison of the Boiling Points of Ethers, Alkanes, and Alcohols of Similar MolecularWeights... 

The boiling points of ethers increase as their molar masses increase. 

Like other organic compounds, branching lowers the boiling points of ethers. 

The reaction starts easily the addition of a crystal of iodine is not necessary. To minimize the formation of hexadiene, the addition of ally I bromide must be slow enough to maintain the temperature of the flask below the boiling point of ether. Allyl bromide (Fluka "purum") even when stored in 1he dark in the presence of silver wool contains some high-boiling material and must be purified by distillation. Pure product (bp 71 °C) is used just after distillation. 

Following Arndt (1943), 1-methyl-l-nitroso urea is added to aqueous potassium hydroxide and ether at 5 °C in a flask fitted with a condenser set for distillation. On heating the mixture to the boiling point of ether, the yellow diazomethane is codistilled with the ether and collected in ice-cooled ether contained in another flask. On completion of the reaction, the distillate becomes colorless. 

Ethers are polar molecules. Compared to alcohols, ethers have very low boiling points mainly because they cannot form hydrogen bonds (intermolecular). The boiling points of ethers are close to alkanes of comparable weights. Ethers are not very reactive. 

However, ethers cannot function as hydrogen bond donors, and therefore, ethers cannot form hydrogen bonds with each other. As a result, the boiling points of ethers are significandy lower than their isomeric alcohols. 

This accounts for the fact that the boiling points of ethers are similar to the B.P. s of alkanes of approximately the same molecular weight. It also accounts for the high solubility of the smaller ethers in water. 

The molecular formula of simple alkoxyalkanes is C H2 +20, identical with that of the alkanols. However, because of the absence of hydrogen bonding, the boiling points of ethers are much lower than those of the corresponding isomeric alcohols (Table 9-1). The two smallest members of the series are water miscible, but ethers become less water soluble as the hydrocarbon residues increase in size. For example, methoxymethane is completely water soluble, whereas ethoxyethane forms only an approximately 10% aqueous solution. 

Table 9-1 Boiling Points of Ethers and the Isomeric 1-Alkanols ...

Explain why the boiling points of ethers are lower than those of the isomeric alcohols. Would you expect the relative water solubilities to differ in a similar way ... 

Problem 4. A solution oflg phenol in 50 mL of ether (density = 0.714) boiled at a temperature 0.63° higher than the boiling point of ether. The molal elevation constant of ether is 2.12°. Calculate the molecular weight ofphenoL... 

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