CHAPTER 12: Unit 4. Physical Properties
| Alcohols: Alcohol molecules hydrogen-bond to each other and to water molecules. They thus have a higher than normal boiling points, and low molecular mass alcohols are soluble in water. |
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Because oxygen atom is more electronegative than Carbon and hydrogen, O-H bond in alcohol is polar and it exhibits intermolecular hydrogen bonding.
Table: Comparison of Boiling Points and Molar Alcohols can also engage in hydrogen bonding with water molecules (Figure 2.3 “Hydrogen Bonding between Methanol Molecules and Water Molecules”). T Thus, whereas the hydrocarbons are insoluble in water, alcohols with one to three carbon atoms are completely soluble. As the length of the chain increases, however, the solubility of alcohols in water decreases; the molecules become more like hydrocarbons and less like water. The alcohol 1-decanol (CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2OH) is essentially insoluble in water. We frequently find that the borderline of solubility in a family of organic compounds occurs at four or five carbon atoms. |
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As a result, alcohols have higher boiling points and melting points than hydrocarbon of comparable molecular weight and shapes.Alcohols are soluble in organic solvents.Low molecular weight alcohols (having fewer than six carbons) are soluble in water. High molecular weight alcohols (more than six carbons) are not soluble in water. Lower molar mass alcohols form hydrogen bonding with polar water molecules and thus get dissolved in water.
Homework ExerciseAnswer the following exercises without consulting tables in the text.Arrange these alcohols in order of increasing boiling point: ethanol, methanol, and 1-propanol.Which has the higher boiling point—butane or 1-propanol?Arrange these alcohols in order of increasing solubility in water: 1-butanol, methanol, and 1-octanol.Arrange these compounds in order of increasing solubility in water: 1-butanol, ethanol, and pentane.Answers1. methanol < ethanol < 1-propanol3. 1-octanol < 1-butanol < methanol |
| Higher molar mass alcohols have heavier nonpolar part (carbon chain) and thus they are difficultly soluble or insoluble in water. |
| Phenols: Phenols are generally low-melting solids; most are slightly soluble in water. The chemical reactions of phenols are significantly different from those of alcohols even though both types of compounds possess hydroxyl groups. Phenols are more resistant to oxidation and do not undergo dehydration. Phenols have acidic properties, whereas alcohols do not. Phenoxide ion ( negatively charged species after losing proton) is extremely stable due to multiple resonating structures. |
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| In sodium hydroxide solution, the salt, sodium phenoxide is formed. |
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| Ethers: Due to difference in electronegativity between C and O atoms, both C-O bonds are polar in ether. It has a net dipole. |
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| But ethers do not contain a hydrogen atom bonded to O atom. That is why, it doesn’t contain any Hydrogen bond. This gives ethers stronger intermolecular force than alkanes but weaker intermolecular forces than alcohols. |
| All ethers are soluble in organic solvents. Low molecular weight ethers are soluble in water because hydrogen atom of water makes hydrogen bonding
Thiols: Most obvious physical property of thiols is their distinctive foul order. Some are responsible for onion like odor of human sweat. The difference between alcohol and thiol is that thiols don’t have hydrogen bonding unlike alcohol since sulfur is nota strong electronegative atom. That gives thiols lower melting and boiling points than alcohols of similar size and shape.
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| Ethers: Ether molecules have no hydrogen atom on the oxygen atom (that is, no OH group). Therefore there is no intermolecular hydrogen bonding between ether molecules, and ethers therefore have quite low boiling points for a given molar mass. Indeed, ethers have boiling points about the same as those of alkanes of comparable molar mass and much lower than those of the corresponding alcohols.Comparison of Boiling Points of Alkanes, Alcohols, and EthersCondensed Structural FormulaNameMolar MassBoiling Point (°C)Intermolecular Hydrogen Bonding in Pure Liquid?CH3CH2CH3propane44–42noCH3OCH3dimethyl ether46–25noCH3CH2OHethyl alcohol4678yesCH3CH2CH2CH2CH3pentane7236noCH3CH2OCH2CH3diethyl ether7435noCH3CH2CH2CH2OHbutyl alcohol74117yesEther molecules do have an oxygen atom, however, and engage in hydrogen bonding with water molecules. Consequently, an ether has about the same solubility in water as the alcohol that is isomeric with it. For example, dimethyl ether and ethanol (both having the molecular formula C2H6O) are completely soluble in water, whereas diethyl ether and 1-butanol (both C4H10O) are barely soluble in water (8 g/100 mL of water). |
