Physical properties of ethers

Ethers are colorless compounds with characteristic, relatively pleasant odors. They have lower boiling points (bp s) than alcohols with an equal number of carbon atoms. In fact, an ether has nearly the same bp as the corresponding hydrocarbon in which a —CH2— group replaces the ether s oxygen. The data in Table 8.1 illustrate these facts  

Because of their structures (no O—H bonds), ether molecules cannot form hydrogen bonds with one another. This is why they boil at much lower temperatures than their isomeric alcohols.  

PROBLEM 8.3 Write structures for each of the following isomers and arrange them in order of decreasing boiling point 3-methoxy-l-propanol, 1,2-dimethoxyethane, and 1,4-butanediol. 

C sht 2010 Cengage Learning. All Ri ts Reserved. May not be copied, seamed, or duplicated, in whole or in part. Due to dectranc limits, some tlikd party content be suppressed from the eBook andforeOHpter(s). 

Although ethers cannot form hydrogen bonds with one another, they do form hydrogen bonds with alcohols  

Physical properties Methyl Ethyl Methyl /m-butyl Isopropyl -Butyl Amyl -Hexyl 

Source data compiled from product literature cited in Rick, E. W., Industrial Solvents Handbook, Fourth Edition, Noyes Data Corporation, Park Ridge. NJ, 1991, and the author s personal files. 

Physical properties Propylene oxide 1,2-Butyl-ene oxide 1,4 Diox-ane 1,3 Diox-olane Furan 2-Methyl furan Tetra- hydro- furan Tetra- hydro- pyran Anisole 

Solubility solvent in water (g/IOO g) 40.5 5.9 Complete Complete 1 0.3 Complete 8 Trace 

Although ethers lack the polar hydroxyl group of alcohols, they are still strongly polar compounds. The dipole moment of an ether is the vector sum of two polar C—O bonds, with a substantial contribution from the two lone pairs of electrons. Table 14-1 compares the dipole moments of dimethyl ether, diethyl ether, and tetrahydrofuran (THE) with those of alkanes and alcohols of similar molecular weights. An ether such as THE provides a strongly polar solvent without the reactivity of a hydroxyl group. 

Diethyl ether was found to be a much safer anesthetic than chloroform. Like chloroform, ether is more soluble in fatty tissue than in water, so it passes quickly into the central nervous system and takes effect quickly. Ether is also volatile, making it easy to administer. But ether is much less toxic than chloroform because ether degrades to ethanol, which the body can oxidize. 

An ether inhaler used by William Morton in the first public demonstration of ether anesthesia on October 16, 1846, at Massachusetts General Hospital. 

TABLE 14-1 Comparison of the Boiling Points of Ethers, Alkanes, and Alcohols of Similar Molecular Weights 

Note The alcohols are hydrogen bonded, giving them much higher boiling points. The ethers have boiling points that are closer to those of alkanes with similar molecular weights. 

As we have seen before, alcohols have unusually high boiling points because of hydrogen bonding between —OH groups. 

TABLE 16.1 Physical Properties of Diethyl Ether, Pentane, and 1-Butanol 

Lacking —OH groups, ethers resemble alkanes in that dispersion forces are the major contributors to intermolecular attractions. Although ethers have significant dipole moments, the fact that their boiling points are closer to alkanes than to alcohols tells us that dipole-dipole attractive forces are minor contributors. 

On the other hand, ethers have a negatively polarized oxygen that can hydrogen bond to an —OH proton of water. 

Hydrogen bonding between diethyl ether and water 

Since the bond angle, R—O — R, is not equal to 180 °C in ethers, they show limited polarity. Increasing the size of the R — groups decreases the polarity of ethers. 

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

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

Since ethers are polar, they dissolve in water by forming hydrogen bonds. The solubility of ethers decrease when the molar masses increase. Ethers dissolve many polar and nonpolar substances very well. 

Name Formula Melting Point (°C) Boiling Point (°C) Density (g/mol) 

It is instructive to compare the physical properties of ethers with alkanes and alcohols. With respect to boiling point, ethers resemble alkanes more than alcohols. With respect to solubihty in water the reverse is true ethers resemble alcohols more than alkanes. Why  

Diethyl ether—often called ethyl ether or just ether —was used as a general anesthetic as early as 1842. Administered as a vapor, it acts as a depressant on the central nervous system, causing unconsciousness. However, its high flammability and volatility present hazards in the operating room. Ethers such as ethyl vinyl ether, divinyl ether, and methyl propyl ether have also seen use as anesthetics. All the low molecular weight ethers are potentially explosive when mixed with oxygen. 

Incorporating an oxygen atom into a three-membered ring requires its bond angle to be seriously distorted from the normal tetrahedral value. In ethylene oxide, for example, the bond angle at oxygen is 61.5°. 

Thus epoxides, like cyclopropanes, have significant angle strain. They tend to undergo reactions that open the three-membered ring by cleaving one of the carbon-oxygen bonds. 

The heats of combustion of 1,2-epoxybutane (2-ethyloxirane) and tetrahydrofuran have been measured one is 2499 kJ/mol the other is 2546 kJ/mol. Match the heats of combustion with the respective compounds. 

Hydrogen bonding between diethyl ether and water results from the attractive force between the negatively polarized oxygen of diethyl ether and the positively polarized hydrogen of water. The color ranges of the three electrostatic potential maps are the same. 

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Table 1.7 describes some physical properties of ethers. Like alcohols, ethers are polar molecules. Ethers, however, cannot form hydrogen bonds with themselves. The physical properties of ethers depend on these characteristics. 

In this section we examine some of the physical properties of ethers. 

Table 3 Physical properties of ether complexes with diorganoz-iuc compoimds...

TABLE 10.2.1 Summary of physical properties of ethers and halogenated ethers Molecular Molecular weight, MW Compound CAS no. formula g/mol m.p. °C b.p. °c Fugacity ratio, F at 25°C Density, p g/m at 20°C Molar volume, cmVmol Vm... 

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