Physical properties of alcohols

Alcohols follow trends similar to Hydrocarbons, but alcohols hydrogen bond, giving them considerably higher boiling points and water solubilities than similar-weight tiydrocarbons. 

Alcohols are more soluble in water than alkanes and alkenes. The hydroxyl group increases polarity and allows for hydrogen bonding with water. The longer the carbon chain, the less soluble the alcohol. 

The important physical properties of organic compounds include melting and boiling points and solubility and density. Whenever you compare physical properties of different compounds, stick to compounds with similar molecular weights. 

Impurities increase the boiling point and reduce the freezing points of materials. 

Alcohols with three or fewer carbons atoms are miscible in water in all proportions. The solubility of alcohols in water decreases with increasing number of carbon atoms, so that alcohols with more than six carbon atoms are nearly insoluble. As the number of carbon atoms increases, the solubility in nonpolar solvents increases. 

Alcohols tend to be denser than hydrocarbons with comparable carbon content. 

The data in Table 7.2 on ethanol is for pure undenatured ethanol. Most ethanol used for industrial applications contain a denaturant substance that renders the ethanol unfit for beverage use. The United States Codes of Federal Regulations authorizes more than fifty types of denatured ethanol for industrial 

Physical properties Methanol Ethanol -Propyl alcohol Isopropyl alcohol w-Butanol sec- Butanol Iso- butanol tert- Butanol -Amyl alcohol Isoamyl alcohol  

Solubility solvent Complete Complete Complete Complete 7.7 15.4 8.7 Complete 1.7 2 

Solubility water in Complete Complete Complete Complete 20.1 65.1 15 Complete 9.2 — 

Source data compiled from product literature of BASF Corporation, Eastman Chemical Company, Exxon Chemical Company, Hoechst Celanese Company, QO Chemicals Inc., Shell Chemical Company, and Union Carbide Corporation. 

Alcohol molecules are formed by nonpolar alkyl (R —) and polar hydroxyl (— OH) groups. These two groups play an important part in determining the physical properties of alcohols. 

For example, in methanol, the hydroxyl group has a strong influence. Methanol and water are miscible in all proportions. Methanol is polar and similar to water. 

The polarity of alcohols decreases as the length of the hydrocarbon chain in the molecule increases. Consequently, their solubilities in water decrease. At room temperature, the solubilities of n-butanol, n-pentanol and n-octanol in 100 ml of water are 8.3 g, 2.4 g and 0.05 g respectively. 

Branching of alcohols increases solubility. For example, the solubility of n-butyl alcohol is 8.3 g per 100 ml of H20 but tert-butyl alcohol is completely soluble in water. 

The polarity of alcohols increases as the number of —OH groups increases. Ethylene glycol, C2H4(OH)2 and glycerol, C3H5(OH)3 are soluble in water in any proportion. Also, longer chained alcohols such as 1,4-butandiol, 2,3-butandiol and the three isomers of pentandiol are highly soluble in water. 

Most of the common alcohols, up to about 11 or 12 carbon atoms, are liquids at room temperature. Methanol and ethanol are free-flowing volatile liquids with characteristic fruity odors. The higher alcohols (the butanols through the decanols) are somewhat viscous, and some of the highly branched isomers are solids at room temperature. These higher alcohols have heavier but still fraity odors. 1-Propanol and 2-propanol fall in the middle, with a barely noticeable viscosity and a characteristic odor often associated with a physician s office. Table 10-2 lists the physical properties of some common alcohols. 

Because we often deal with liquid alcohols, we forget how surprising it should be that the lower molecular weight alcohols are liquids. For example, ethyl alcohol and propane have similar molecular weights, yet their boiling points differ by about 120°C. Dimethyl ether has an intermediate boiling point. 

Such a large difference in boiling points suggests that ethanol molecules are attracted to each other much more strongly than propane molecules. Two important intermolecular forces are responsible hydrogen bonding and dipole-dipole attractions (Section 2-10). 

Dipole-dipole attractions also contribute to the relatively high boiling points of alcohols and ethers. The polarized C—O and H—O bonds and the nonbonding electrons add to produce a dipole moment of 1.69 D in ethanol, compared with a dipole moment of only 0.08 D in propane. In liquid ethanol, the positive and negative ends of these dipoles align to produce attractive interactions. 

The boiling point of dimethyl ether is -25°C, about 17° higher than that of propane, but still 103° lower than that of ethanol. Hydrogen bonds are clearly much stronger in-termolecular attractions than dipole-dipole attractions. 

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Physical Properties of Alcohols and Alkyl Halides Intermolecular Forces... 

PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES INTERMOLECULAR FORCES... 

Table 1.6 lists some common physical properties of alcohols. As you learned earlier in this chapter, alcohols are polar molecules that experience hydrogen bonding. The physical properties of alcohols depend on these characteristics. 

Structure and Classification of Alcohols 425 10-3 Nomenclature of Alcohols and Phenols 427 10-4 Physical Properties of Alcohols 430 10-5 Commercially Important Alcohols 433 10-6 Acidity of Alcohols and Phenols 435 10-7 Synthesis of Alcohols Introduction and Review 438 Summary Previous Alcohol Syntheses 438 10-8 Organometallic Reagents for Alcohol Synthesis 440 10-9 Addition of Organometallic Reagents to Carbonyl Compounds 443... 

Alcohols are synthesized by a wide variety of methods, and the hydroxyl group may be converted to most other functional groups. For these reasons, alcohols are versatile synthetic intermediates. In this chapter, we discuss the physical properties of alcohols and summarize the methods used to synthesize them. In Chapter 11 (Reactions of Alcohols), we continue our study of the central role that alcohols play in organic chemistry as reagents, solvents, and synthetic intermediates. 

TABLE 11.2.1 Summary of physical properties of alcohols Compound CAS no. Molecular formula Molecular weight, MW g/mol m.p. °C b.p. °C Fugacity ratio, F at 25°C Molar volume, V, cmVmol ... 

How these factors affect the physical properties of alcohols, ethers, and epoxides is summarized in Table 9.1. 

The physical properties of alcohols lay somewhere between the physical properties of hydrocarbons and water depending upon the R — and —OH groups that make up the alcohol molecule. The hydrocarbon properties become more significant as the number of carbon atoms in the molecule increases. Similarities to water become more obvious in smaller molecules. 

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