Alcohols, Ethers,and Thiols by Dr.Alaa J. Mahrath

Transcription

1 Fermentation vats of wine grapes at the Beaulieu Vineyards, California. In this lecture we will studying the physical and chemical properties of alcohols and ethers, two classes of oxygen-containing organic compounds. We also study thiols, a class of sulfur containing organic compounds. Thiols are like alcohols in structure, except that they contain an -SH group rather than an -OH group. Ethanol is the fuel additive in E85 and E15, the alcohol in alcoholic beverages, and an important Laboratory and industrial solvent. Diethyl ether was the first inhalation anesthetic used in general surgery. It is also an important laboratory and industrial solvent. Ethanethiol, like other lowmolecular-weight thiols, has a stench. Traces of ethanethiol are added to natural gas so that gas leaks can be detected by the smell of the thiol What Are the Structures, Names, and Physical Properties of Alcohols? Examples : P a g e 1

2 1. Write the IUPAC name for each alcohol: Solution (a) The parent alkane is pentane. Number the parent chain from the direction that gives the lower number to the carbon bearing the -OH group. This alcohol is 4-methyl-2-pentanol. (b) The parent cycloalkane is cyclohexane. Number the atoms of the ring beginning with the carbon bearing the -OH group as carbon 1 and specify that the methyl and hydroxyl groups are trans to each other. This alcohol is trans-2-methylcyclohexanol. 2.Write the IUPAC name for each alcohol. We classify alcohols as primary (1 ), secondary (2 ), or tertiary (3 ), depending on the number of carbon groups bonded to the carbon bearing the -OH group. 3.Classify each alcohol as primary, secondary, or tertiary. 4. Classify each alcohol as primary, secondary, or tertiary. In the IUPAC system, a compound containing two hydroxyl groups is named as a diol, while containing three hydroxyl groups as a triol, and so on. In IUPAC names for diols, triols, and so on, for example, 1,2-ethanediol. P a g e 2

3 5.2. Physical Properties of Alcohols: The most important physical property of alcohols is the polarity of their -OH groups. Because of the large difference in electronegativity. Some applications of alcohols, Ascanio Sobrero ( ) discovered at 1847 that 1,2,3-propanetriol, more commonly called glycerin, reacts with nitric acid in the presence of sulfuric acid to give a pale yellow, oily liquid called nitroglycerin. Sobrero also discovered the explosive properties of this compound; when he heated a small quantity of it, it exploded, as in equation below. Nitroglycerin very soon became widely used for blasting in the construction of canals, tunnels, roads, and mines and, of course, for warfare. One problem with the use of nitroglycerin was soon P a g e 3

4 recognized: It was difficult to handle safely, and accidental explosions occurred all too frequently. This problem was solved by the Swedish chemist Alfred Nobel ( ), who discovered that a claylike substance called diatomaceous earth absorbs nitroglycerin so that it will not explode without a fuse. Nobel gave the name dynamite to this mixture of nitroglycerin, diatomaceous earth, and sodium carbonate. Surprising as it may seem, nitroglycerin is used in medicine to treat angina pectoris, the symptoms of which are sharp chest pains caused by reduced flow of blood in the coronary artery. Nitroglycerin, which is available in liquid form (diluted with alcohol to render it nonexplosive) tablet form, and paste form, relaxes the smooth muscles of blood vessels, causing dilation of the coronary artery. This dilation, in turn, allows more blood to reach the heart. When Nobel became ill with heart disease, his physicians advised him to take nitroglycerin to relieve his chest pains. He refused, saying he could not understand how the explosive could relieve chest pains. It took science more than 100 years to find the answer. We now know that nitric oxide, NO, derived from the nitro groups of nitroglycerin, relieves the pain. Alcohols are much more soluble in water than are hydrocarbons of similar molecular weight (Table5.1), because alcohol molecules interact by hydrogen bonding with water molecules. Methanol, ethanol, and 1- propanol are soluble in water in all proportions. As molecular weight increases, the water solubility of alcohols becomes more like that of hydrocarbons of similar molecular weight. Higher-molecular-weight alcohols are much less soluble in water because the size of the hydrocarbon portion of their molecules (which decreases water solubility) becomes so large relative to the size of the - OH group (which increases water solubility). 5.3.What Are the Characteristic Reactions of Alcohols? A. Acidity of Alcohols Alcohols have about the same pka values as water which means that aqueous solutions of alcohols have approximately the same ph as that of pure water. the acidity of phenols, another class of P a g e 4

5 compounds that contains an -OH group are weak and react with aqueous sodium hydroxide to form water-soluble salts. B.Acid-Catalyzed Dehydration of Alcohols Dehydration Elimination of a molecule of water from an alcohol. In the dehydration of an alcohol, OH is removed from one carbon and an H is removed from an adjacent carbon. When the acid-catalyzed dehydration of an alcohol yields isomeric alkenes we studied the acid-catalyzed hydration of alkenes to give alcohols. In this section, we study the acidcatalyzed dehydration of alcohols to give alkenes. In fact, hydration dehydration reactions are reversible. Alkene hydration and alcohol dehydration are competing reactions, and the following equilibrium exists: In accordance with Le Chatelier s principle, large amounts of water (in other words, using dilute aqueous acid) favor alcohol formation, whereas scarcity of water (using concentrated acid) or experimental conditions where water is removed (heating the reaction mixture above P a g e 5

6 100 C) favor alkene formation. Thus, depending on the experimental conditions, we can use the hydration dehydration equilibrium to prepare both alcohols and alkenes, each in high yields. Some example : In part (a), acid-catalyzed dehydration of 2-methyl-3-pentanol gives predominantly Compound A. Treatment of Compound A with water in the presence of sulfuric acid in part (b) gives Compound B. Propose structural formulas for Compounds A and B. Solution (a) Acid-catalyzed dehydration of 2-methyl-3-pentanol gives predominantly 2-methyl-2-pentene, an alkene with three substituents on its double bond: two methyl groups and one ethyl group. (b) Acid-catalyzed addition of water to this alkene gives 2-methyl- 2-pentanol in accord with Markovnikov s rule. Home Work Acid-catalyzed dehydration of 2-methylcyclohexanol gives predominantly Compound C (C 7 H 12 ). Treatment of Compound C with water in the presence of sulfuric acid gives Compound D (C 7 H 14 O). Propose structural formulas for Compounds C and D. C. Oxidation of Primary and Secondary Alcohols: P a g e 6

7 oxidation is either the loss of hydrogens or the gain of oxygens. Using this definition, conversion of a primary alcohol to an aldehyde is an oxidation reaction because the alcohol loses hydrogens. Conversion of an aldehyde to a carboxylic acid is also an oxidation reaction because the aldehyde gains an oxygen. General Equation for oxidation of alcohols. Secondary alcohols may be oxidized to ketones by using potassium dichromate as the oxidizing agent. Menthol, a secondary alcohol present in peppermint and other mint oils, is used in liqueurs, cigarettes, cough drops, perfumery, and nasal inhalers. Its oxidation product, menthone, is also used in perfumes and artificial flavors. Tertiary alcohols resist oxidation because the carbon bearing the -OH is bonded to three carbon atoms and, therefore, cannot form a carbon oxygen double bond. Some applications Breath-Alcohol Screening: Potassium dichromate oxidation of ethanol to acetic acid is the basis for the original breath-alcohol screening test used by law enforcement agencies to determine a person s blood alcohol content (BAC). The test is based on the difference in color between the dichromate ion (reddish orange) in the reagent and the chromium (III) ion (green) in the product. In its simplest form, breath-alcohol screening uses a sealed glass tube that contains a potassium dichromate sulfuric acid reagent impregnated on silica gel as shown in the figure 5.1. As breath containing ethanol vapor passes through the tube, reddish P a g e 7

8 orange dichromate ion is reduced to green chromium(iii) ion. To estimate the concentration of ethanol in the breath, one measures how far the green color extends along the length of the tube. When it reaches beyond the halfway point, the person is judged as having a sufficiently high blood alcohol content to warrant further, more precise testing. 5.4.What Are the Structures, Names, and Properties of Ethers? The functional group of an ether is an atom of oxygen bonded to two carbon atoms. Common names are derived by listing the alkyl groups bonded to oxygen in alphabetical order and adding the word ether. Cyclic Ethers : An ether in which the ether oxygen is one of the atoms of a ring. These ethers arealso known by their common names. Ethylene oxide is an important building block for the organic chemical industry. Ethylene oxide is a colorless, flammable gas with a boiling point of 11 C. ethylene oxide reacts with the amino (-NH 2 ) and sulfhydryl ( -SH ) groups present in biological materials. At sufficiently high concentrations, it reacts with enough molecules in cells to cause the death of microorganisms. This toxic property is the basis for ethylene oxide s use as a fumigant in foodstuffs and textiles and its use in hospitals to sterilize surgical instruments. Ethers and Anesthesia: Diethyl ether was easy to use and caused excellent muscle relaxation. Blood pressure, pulse rate, and respiration were usually only slightly affected. Diethyl ether s chief drawbacks are its irritating effect on the respiratory passages and its aftereffect of nausea. Among the inhalation anesthetics used today are several halogenated ethers, the most important of which are enflurane and isoflurane. This photo shows the first use of ether as an anesthetic in Dr. Robert John Collins was removing a tumor from the patient s neck. P a g e 8

9 C. Physical Properties Ethers are polar compounds in which oxygen bears a partial negative charge and each attached carbon bears a partial positive charge (Figure 5.2). the boiling points of ethers are close to those of hydrocarbons of similar molecular weight. Ethers are excellent solvents in which to carry out many organic reactions. The most important ether solvents are diethyl ether and tetrahydrofuran. The effect of hydrogen bonding on physical properties is illustrated dramatically by comparing the boiling points of ethanol (78C) and its constitutional isomer dimethyl ether (-24 C). The difference in boiling point between these two compounds is due to the presence in ethanol of a polar -OH group, which is capable of forming hydrogen bonds. This hydrogen bonding increases intermolecular associations, thereby giving ethanol a higher boiling point than dimethyl ether. 5.5.What Are the Structures, Names, and Properties of Thiols: The functional group of a thiol is an -SH (sulfhydryl) group bonded to a tetrahedral carbon atom. methanethiol, CH 3 SH, the simplest thiol. Physical Properties Because of the small difference in electronegativity between sulfur and hydrogen ( = 0.40), we classify the S-H bond as nonpolar covalent. Because of this lack of polarity, thiols show little association by hydrogen bonding. Consequently, they have lower boiling points and are less soluble in water and other polar solvents than are alcohols of similar molecular weight Table 5.2. P a g e 9

10 Reactions of Thiols Thiols are weak acids (pka = 10) that are comparable in strength to phenols. Thiols react with strong bases such as NaOH to form thiolate salts. The most common reaction of thiols in biological systems is their oxidation to disulfides, the functional group of which is a disulfide (-S-S-) bond. Thiols are readily oxidized to disulfides by molecular oxygen. This easy interconversion between thiols and disulfides is very important in protein chemistry. For more details please see the following References 1. Introduction to General Organic and Biochemistry ; 9 th Edition, Frederick A. Bettelheim and William H. Brown..et al,brooks / Cole. Canada General Organic and Biochemistry. 4 th Edition, Denniston K., The McGraw Hill Companies, 2013 P a g e 11