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Ch13−p1 Chapter 13 Alcohols, Phenols, Ethers, and Thiols (Chapter 14, 1st Edition) In this chapter, we are going to look at molecules that contain single bonds to oxygen atoms or sulphur atoms. Structure of Alcohols and Thiols. In an alcohol, a hydroxyl group (-OH) replaces a hydrogen atom in an alkane. In a phenol, a hydroxyl group is attached to an aromatic ring. Thiols are similar to alcohols except that thiols contain a sulfhydryl (-SH) instead of a hydroxyl group. O H H O O H H methanol water phenol S S H H methanethiol thiophenol Classification of Alcohols. Recall how we classified the different carbon atoms in alkanes. Carbon atoms bonded to only one other carbon atom was designated as a 1° carbon atom. Carbon atoms bonded to two other carbon atoms were designated as 2° Ch13−p2 and carbon atoms bonded to three or four other carbon atoms were designated as 3° or 4° respectively. Alcohols can be classified in a similar manner. Alcohols in which the hydroxyl group is bonded to a 1° carbon is said to be a 1° alcohol. Similarly, alcohols bonded 2° and 3° carbon atoms are said to be 2° alcohols and 3° alcohols respectively. NOTE: The symbol R is used to represent any alkyl group. H R C R' OH R R' C OH R C OH H H R'' 1° alcohol 2° alcohol 3° alcohol e.g. OH OH OH OH OH OH Ch13−p3 Naming Alcohols. Step 1. Select the longest carbon chain containing the hydroxyl group, and derive the parent name by replacing the –e of the corresponding alkane with –ol. Step 2. Number the main chain beginning at the end nearer the hydroxyl group. Molecules containing more than one hydroxyl group are named as diols, triols, etc. Step 3. Name and number other substituents relative to the hydroxyl groups. Step 4. Cyclic alcohols are named as cycloalkanols. The carbon atom bearing the hydroxyl group is numbered as carbon 1. Number in the direction that gives the lowest series of numbers. e.g. Name the following molecules, ignoring any chiral centers. OH Ch13−p4 CH3 Cl OH OH Br Br OH OH Cl Phenols are a family of molecules that have a hydroxyl group bonded to a benzene ring. Ch13−p5 e.g. OH OH OH OH F 2-fluorophenol ortho-fluorophenol phenol 3-ethylphenol meta-ethylphenol Br 4-bromo-3-methylphenol Many disubstituted phenols have common names based on historical uses. Some are listed below: OH OH OH CH3 OH OH OH ortho-cresol catechol OH resorcinol hydroquinone Naming Thiols. Thiols are named by the same system used for alcohols, with the suffix –thiol used in place of –ol. The most obvious physical characteristic of thiols is their strong, disagreeable, odour. For example, the scent of skunks is caused primarily by the simple thiols, 3-methyl-1butanethiol and 2-butene-1-thiol. CH3 SH SH 3-methyl-1-butanethiol 2-butene-1-thiol Ch13−p6 e.g. Name the following thiols: SH SH F SH F SH Some Important Alcohols. Methanol. Methanol, also known as ‘wood alcohol’, is found in many solvents and paint removers. Methanol is also used in the productions of plastics and medicines and used as fuels. Methanol is highly toxic and, if ingested, can cause blindness and possibly death. O CH3OH methanol [O] H H formaldehyde Ch13−p7 Ethanol. The fermentation of grains, fruits, and starches has been known for millennia. The intoxicating effect of ethanol, for good or bad, is a part of our society. Commercially, ethanol is used as a solvent for perfumes, varnishes, and some medicines. 2-Propanol (isopropyl alcohol). Commonly referred to as ‘rubbing alcohol’, 2-propanol is used as an astringent and to sterilize medical equipment because it destroys bacteria. 1,2-Ethanediol (ethylene glycol). Often used as antifreeze in heating and cooling systems. It is also used as a solvent for paints, inks, and plastics. In the body, 1,2ethanediol is oxidized to oxalic acid, which can lead to death. O OH [O] HO OH HO O 1,2-ethanediol oxalic acid 1,2,3-Propanetriol (glycerol or glycerin). The presence of three hydroxyl groups attracts 1,2,3-propanetriol strongly to water. For this reason, it is used as a skin softener in skin lotions, cosmetics and shaving creams. It is also used as antifreeze. When mixed with a strong oxidizer, extremely explosive products are formed. Ch13−p8 Menthol. Menthol has a pleasant peppermint taste and odour and is used in candy, throat lozenges, and nasal inhalers. OH menthol Ethers. Ethers are molecules that contain an oxygen atom attached to two carbon groups (alkyl or aryl) by single bonds. O H O H water O H methanol dimethyl ether Naming Ethers. Simple ethers are named by their common names. The IUPAC names are used only when the ether is more complex. Step 1. When naming ethers by their common names, write the name of each alkyl or aryl group attached to the oxygen Ch13−p9 atom in alphabetical order followed by the word ether. Step 2. For more complex ethers, the ether is named as an alkoxy (alkyl + oxygen) substituent similar to alkyl groups. Step 3. Number the main chain beginning at the end nearest the alkoxy group and give the location of the alkoxy group on the main chain. e.g. O O diethyl ether (ethoxyethane) 3-ethoxypentane O O anisole (methoxybenzene) diphenyl ether (phenoxybenzene) Ch13−p10 Name the following molecules. O O O OH OH O F Ch13−p11 Cyclic Ethers. Carbon rings that contain one oxygen atom are known as cyclic ethers. Such compounds are generally known as heterocyclic compounds because one or more of the atoms in the ring are not carbon. Most heterocycles are given common names. Some cyclic ethers are listed below. 4 CH3 3 4 3 O 5 2 5 O O O 1 ethylene oxide 2 1 furan tetrahydrofuran (THF) 3-methylfuran CH3 4 3 O O pyran tetrahydropyran 2 O 1 4-methylpyran Six membered rings containing two oxygen atoms are known as dioxanes. The oxygen atoms are numbered according to their positions. 1 O 1 2 3 O O 2 O3 4 1,4-dioxane 1,3-dioxane Ch13−p12 Physical Properties of Alcohols, Phenols, and Ethers. Consider the molecules and their physical properties listed in Table 14.1. Observe how all the alcohols have much higher boiling points than any alkanes or ethers of comparable molar mass. Why is this? Alcohols, which contain the polar hydroxyl group, can form hydrogen bonds. Hydrogen bonds are very strong dipole-dipole interactions between hydrogen atoms bonded to small, strongly electronegative atoms (O, N, or F) and nonbonding pairs on other such electronegative atoms. Although hydrogen bonds are considered strong dipoledipole interactions, they are much weaker than ordinary covalent bonds. hydrogen bonds O H H O H O Alcohols have higher boiling points because higher temperatures are required to provide the energy to break the hydrogen bonds. Although ethers contain oxygen atoms, there is no hydrogen atom attached. Therefore, ethers cannot hydrogen bond with each other, but can hydrogen bond with water or alcohols. The ability to hydrogen bond with water is what makes alcohols and ethers with one to four carbon atoms very soluble in water. Ch13−p13 O H O H H H O H CH3 O H H O O H H H H O CH3 CH3 When there are more than four carbon atoms in the alkyl portion of an alcohol, the effect of the hydroxyl group is diminished. The hydrogen atoms of the alkyl group do not participate in hydrogen bonds and therefore these longer chain alcohols are not very soluble in water. nonpolar carbon chain OH OH soluble in water not soluble in water Ethers are more soluble in water than alkanes of comparable mass but are less soluble than alcohols. Ch13−p14 Reactions of Alcohols. Just as the alkanes underwent combustion reactions, so too can the alcohols. Alcohols react with oxygen to produce carbon dioxide, water, and heat. 2C3H8O 9O2 + 6CO2 8H2O + + heat We saw in Chapter 13 that alkenes undergo addition reactions with water and a catalyst to form alcohols. Alcohols can undergo the reverse reaction, called an elimination reaction, to form alkenes. Because an alcohol loses a molecule of water to form an alkene, this type of elimination is termed a dehydration. During the dehydration of an alcohol, a hydrogen atom and a hydroxyl group are removed from adjacent carbon atoms of the same alcohol. A double bond forms between the same two carbon atoms to produce the alkene product. This reaction is also catalyzed with an acid. e.g. H H3C OH H CH3 CH CH2 H C + C H2O heat H H H OH H heat + H2O Ch13−p15 The dehydration of a 2° or 3° alcohol can give two different products. Consider the dehydration of 2-butanol below: b a H H H + H2O + H2O A heat OH B Saytzeff’s rule states that the major product will be the alkene that is more substituted. In the example above, product A is a monosubstituted alkene while product B is a disubstituted alkene. Thus, product B would be the major product. e.g. Provide the product(s) for the following reactions. Where possible, indicate the major product. H heat OH OH H heat Ch13−p16 Alcohols can also dehydrate to form ethers. In this reaction, a hydrogen atom and a hydroxyl group are removed from different alcohol molecules. This reaction also requires an acid catalyst but is done at a lower temperature then dehydration to the alkene. H CH3 O H + HO CH3 CH3 heat O H2O + CH3 H2SO4 CH2 180° C CH2 CH3CH2OH H2SO4 CH3CH2OCH2CH3 140° C Oxidation of Alcohols. Recall from Chapter 6 that oxidation can be defined as a loss of hydrogen atoms or a gain of oxygen atoms. Conversely, reduction can be defined as a gain of hydrogen atoms or a loss of oxygen atoms. In organic chemistry we can list different functional groups at different levels of oxidation or reduction. Shown below are the different stages of oxidation for ethane. OH oxidation CH3 CH3 alkane O oxidation CH3 reduction O CH2 alcohol oxidation CH3 reduction C aldehyde H CH3 reduction C OH carboxylic acid Ch13−p17 Oxidation of 1° Alcohols. Primary alcohols are oxidized to produce aldehydes and water. Oxidation occurs by removing two atoms of hydrogen, one from the hydroxyl group and a one from the carbon atom that is bonded to the hydroxyl group. The oxidation requires an oxidizing agent such as O2, KMnO4, H2CrO4, or K2Cr2O7. The oxidizing agents are often written with the symbol [O]. H O O [O] R C R H C H + H2O aldehyde H 1° alcohol As shown above, aldehydes can oxidize further to give carboxylic acids. This requires the addition of oxygen. This step occurs readily making it difficult to isolate the aldehyde product. O O [O] R C aldehyde H R C OH carboxylic acid The toxicity of methanol has already been discussed. We can now look more closely as to why methanol is so toxic to humans. In the body, enzymes oxidize methanol to formaldehyde and then to formic acid. Blindness occurs because these products destroy the retina of the eye. As Ch13−p18 little as 30mL of formic acid can lower blood pH severely enough and lead to coma or death. O O [O] CH3 [O] OH H C H H OH formic acid formaldehyde methanol C Oxidation of 2° Alcohols. The oxidation of secondary alcohols is similar to that of primary alcohols except the product is a ketone and not an aldehyde. Oxidation occurs by removing two atoms of hydrogen, one from the hydroxyl group and a one from the carbon atom that is bonded to the hydroxyl group. H O O [O] R C R R C R + H2 O ketone H 2° alcohol Consider the tertiary alcohol below. Provide a reason why tertiary alcohols resist oxidation. H O [O] R C R R 3° alcohol NO REACTION Ch13−p19 Oxidation of Thiols. Oxidation of thiols occurs by removing a hydrogen atom from two different thiol molecules. The oxidized product is called a disulfide and is characterized by a sulfur-sulfur single bond. [O] R S H + H S R R S S R Provide the product(s) for the following reactions. OH [O] [O] OH H heat OH OH [O] + H2O Ch13−p20 OH H3C H heat CH3 OH H heat H OH + OH H CH3OH + CH3CH2OH [O] 2 CH3CH2SH heat heat Ch13−p21 Important Concepts from Chapter 13 • Classification of Alcohols - 1°, 2°, and 3° • Nomenclature of Alcohols • Nomenclature of Thiols • Nomenclature of Ethers • Physical Properties of Alcohols and Ethers • Reactions of Alcohols: Dehydration (alkene product) Dehydration (ether product) Oxidation • Reaction of Thiols: Oxidation (disulfides)