Organic Chemistry, 7th Edition L. G. Wade, Jr. Chapter 10 Structure and Synthesis of Alcohols Copyright © 2010 Pearson Education, Inc. Structure of Water and Methanol • Oxygen is sp3 hybridized and tetrahedral. • The H—O—H angle in water is 104.5°. • The C—O—H angle in methyl alcohol is 108.9°. Chapter 10 2 Examples of Classifications OH CH3 CH3 CH3 CH CH2OH * Primary alcohol CH CH2CH3 * Secondary alcohol CH3 CH3 C* OH Tertiary alcohol CH3 Chapter 10 3 IUPAC Nomenclature • Find the longest carbon chain containing the carbon with the —OH group. • Drop the -e from the alkane name, add -ol. • Number the chain giving the —OH group the lowest number possible. • Number and name all substituents and write them in alphabetical order. Chapter 10 4 Examples of Nomenclature OH CH3 CH3 3 CH3 CH CH2OH 2 1 1 2-methyl-1-propanol 2-methylpropan-1-ol 2 CH3 CH3 1 C OH CH3 CH CH2CH3 2 3 4 2-butanol butan-2-ol 2-methyl-2-propanol 2-methylpropan-2-ol Chapter 10 5 Alkenols (Enols) • Hydroxyl group takes precedence. Assign the carbon with the —OH the lowest number. • End the name in –ol, but also specify that there is a double bond by using the ending –ene before -ol OH CH2 5 CHCH2CHCH3 4 3 2 1 4-penten-2-ol pent-4-ene-2-ol Chapter 10 6 Naming Priority Highest ranking Lowest ranking 1. Acids 2. Esters 3. Aldehydes 4. Ketones 5. Alcohols 6. Amines 7. Alkenes 8. Alkynes 9. Alkanes 10. Ethers 11. Halides Chapter 10 7 Hydroxy Substituent • When —OH is part of a higher priority class of compound, it is named as hydroxy. carboxylic acid OH CH2CH2CH2COOH 4 3 2 1 4-hydroxybutanoic acid also known as g-hydroxybutyric acid (GHB) Chapter 10 8 Common Names • Alcohol can be named as alkyl alcohol. • Useful only for small alkyl groups. OH CH3 CH3 CH CH2OH isobutyl alcohol CH3 CH CH2CH3 sec-butyl alcohol Chapter 10 9 Naming Diols • Two numbers are needed to locate the two —OH groups. • Use -diol as suffix instead of -ol. 1 2 3 4 5 6 hexane-1,6- diol Chapter 10 10 Glycols • 1, 2-diols (vicinal diols) are called glycols. • Common names for glycols use the name of the alkene from which they were made. ethane-1,2- diol ethylene glycol propane-1,2- diol propylene glycol Chapter 10 11 Phenol Nomenclature • —OH group is assumed to be on carbon 1. • For common names of disubstituted phenols, use ortho- for 1,2; meta- for 1,3; and para- for 1,4. • Methyl phenols are cresols. OH OH H3C Cl 3-chlorophenol (meta-chlorophenol) 4-methylphenol (para-cresol) Chapter 10 12 Solved Problem 1 Give the systematic (IUPAC) name for the following alcohol. Solution The longest chain contains six carbon atoms, but it does not contain the carbon bonded to the hydroxyl group. The longest chain containing the carbon bonded to the —OH group is the one outlined by the green box, containing five carbon atoms. This chain is numbered from right to left in order to give the hydroxyl-bearing carbon atom the lowest possible number. The correct name for this compound is 3-(iodomethyl)-2-isopropylpentan-1-ol. Chapter 10 13 Boiling Points of alcohols • Alcohols have higher boiling points than ethers and alkanes because alcohols can form hydrogen bonds. • The stronger interaction between alcohol molecules will require more energy to break them resulting in a higher boiling point. Chapter 10 14 Solubility in Water Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases. Chapter 10 15 Table of Ka Values Chapter 10 16 Formation of Alkoxide Ions • Ethanol reacts with sodium metal to form sodium ethoxide (NaOCH2CH3), a strong base commonly used for elimination reactions. • More hindered alcohols like 2-propanol or tert-butanol react faster with potassium than with sodium. Chapter 10 17 Formation of Phenoxide Ion The aromatic alcohol phenol is more acidic than aliphatic alcohols due to the ability of aromatic rings to delocalize the negative charge of the oxygen within the carbons of the ring. Chapter 10 18 Charge Delocalization on the Phenoxide Ion • The negative charge of the oxygen can be delocalized over four atoms of the phenoxide ion. • There are three other resonance structures that can localize the charge in three different carbons of the ring. • The true structure is a hybrid between the four resonance forms. Chapter 10 19 Grignard Reagents • • • • Formula R—Mg—X (reacts like R:- +MgX). Ethers are used as solvents to stabilize the complex. Iodides are most reactive. May be formed from any halide. Chapter 10 20 Reactions with Grignards Br + Mg ether Cl CH3CHCH2CH3 + Mg ether Chapter 10 MgBr MgCl CH3CHCH2CH3 21 Organolithium Reagents • Formula R—Li (reacts like R:- +Li) • Can be produced from alkyl, vinyl, or aryl halides, just like Grignard reagents. • Ether not necessary, wide variety of solvents can be used. Chapter 10 22 Reaction with Carbonyl Chapter 10 23 Formation of Primary Alcohols Using Grignard Reagents • Reaction of a Grignard with formaldehyde will produce a primary alcohol after protonation. Chapter 10 24 Synthesis of 2º Alcohols • Addition of a Grignard reagent to an aldehyde followed by protonation will produce a secondary alcohol. Chapter 10 25 Synthesis of 3º Alcohols • Tertiary alcohols can be easily obtained by addition of a Grignard to a ketone followed by protonation with dilute acid. Chapter 10 26 Solved Problem 2 Show how you would synthesize the following alcohol from compounds containing no more than five carbon atoms. Solution This is a tertiary alcohol; any one of the three alkyl groups might be added in the form of a Grignard reagent. We can propose three combinations of Grignard reagents with ketones: Chapter 10 27 Solved Problem 2 (Continued) Solution (Continued) Any of these three syntheses would probably work, but only the third begins with fragments containing no more than five carbon atoms. The other two syntheses would require further steps to generate the ketones from compounds containing no more than five carbon atoms. Chapter 10 28 Reaction of Grignards with Carboxylic Acid Derivatives Chapter 10 29 Mechanism Step 1: Grignard attacks the carbonyl forming the tetrahedral intermediate. CH3 H3C R MgBr C O R C O Cl MgBr Cl Step 2: The tetrahedral intermediate will reform the carbonyl and form a ketone intermediate. CH3 R C O CH3 R C MgBr Cl Chapter 10 + MgBrCl O 30 Mechanism continued Step 3: A second molecule of Grignard attacks the carbonyl of the ketone. CH3 CH3 R MgBr + R C R C O O MgBr R Step 4: Protonation of the alkoxide to form the alcohol as the product. CH3 R C O HOH MgBr CH3 R C OH R R Chapter 10 31 Addition to Ethylene Oxide • Grignard and lithium reagents will attack epoxides (also called oxiranes) and open them to form alcohols. • This reaction is favored because the ring strain present in the epoxide is relieved by the opening. • The reaction is commonly used to extend the length of the carbon chain by two carbons. Chapter 10 32 Limitations of Grignard • Grignards are good nucleophiles but in the presence of acidic protons it will acts as a strong base. • No water or other acidic protons like O—H, N—H, S—H, or terminal alkynes. • No other electrophilic multiple bonds, like C═N, CN, S═O, or N═O. Chapter 10 33 Reduction of Carbonyl • Reduction of aldehyde yields 1º alcohol. • Reduction of ketone yields 2º alcohol. • Reagents: Sodium borohydride, NaBH4 Lithium aluminum hydride, LiAlH4 Raney nickel Chapter 10 34 Sodium Borohydride • NaBH4 is a source of hydrides (H-) • Hydride attacks the carbonyl carbon, forming an alkoxide ion. • Then the alkoxide ion is protonated by dilute acid. • Only reacts with carbonyl of aldehyde or ketone, not with carbonyls of esters or carboxylic acids. Chapter 10 35 Mechanism of Hydride Reduction • The hydride attacks the carbonyl of the aldehyde or the ketone. • A tetrahedral intermediate forms. • Protonation of the intermediate forms the alcohols. Chapter 10 36 Lithium Aluminum Hydride • LiAlH4 is source of hydrides (H-) • Stronger reducing agent than sodium borohydride, but dangerous to work with. • Reduces ketones and aldehydes into the corresponding alcohol. • Converts esters and carboxylic acids to 1º alcohols. Chapter 10 37 Reduction with LiAlH4 • The LiAlH4 (or LAH) will add two hydrides to the ester to form the primary alkyl halide. • The mechanism is similar to the attack of Grignards on esters. Chapter 10 38 Reducing Agents • NaBH4 can reduce aldehydes and ketones but not esters and carboxylic acids. • LiAlH4 is a stronger reducing agent and will reduce all carbonyls. Chapter 10 39 Catalytic Hydrogenation • Raney nickel is a hydrogen rich nickel powder that is more reactive than Pd or Pt catalysts. • This reaction is not commonly used because it will also reduce double and triple bonds that may be present in the molecule. • Hydride reagents are more selective so they are used more frequently for carbonyl reductions. Chapter 10 40 Thiols (Mercaptans) • Sulfur analogues of alcohols are called thiols. • The —SH group is called a mercapto group. • Named by adding the suffix -thiol to the alkane name. • They are commonly made by an SN2 reaction so primary alkyl halides work better. Chapter 10 41 Synthesis of Thiols • The thiolate will attack the carbon displacing the halide. • This is an SN2 reaction so methyl halides will react faster than primary alkyl halides. • To prevent dialylation use a large excess of sodium hydrosulfide with the alkyl halide. Chapter 10 42 A better way into thiols Thiol Oxidation Thiols can be oxidized to form disulfides. The disulfide bond can be reduced back to the thiols with a reducing agent. Chapter 10 44 Reactions Reactions Oxidation States of Carbons Chapter 11 47 Oxidation States of Carbons Chapter 11 48 Oxidation of 2° Alcohols • 2° alcohol becomes a ketone. • Oxidizing agent is Na2Cr2O7/H2SO4. • Active reagent probably is H2CrO4. • Color change is orange to greenishblue. Chapter 11 49 Oxidation Mechanism Chapter 11 50 Oxidation of 1° Alcohols to Carboxylic Acids • Chromic acid reagent oxidizes primary alcohols to carboxylic acids. • The oxidizing agent is too strong to stop at the aldehyde. Chapter 11 51 Pyridinium Chlorochromate (PCC) • PCC is a complex of chromium trioxide, pyridine, and HCl. • Oxidizes primary alcohols to aldehydes. • Oxidizes secondary alcohols to ketones. Chapter 11 52 3° Alcohols Cannot Be Oxidized • Carbon does not have hydrogen, so oxidation is difficult and involves the breakage of a C—C bond. • Chromic acid test is for primary and secondary alcohols because tertiary alcohols do not react. Chapter 11 53 Example of the Swern Oxidation Chapter 11 54 Swern Oxidation Chapter 11 55 Solved Problem 1 Suggest the most appropriate method for each of the following laboratory syntheses. (a) cyclopentanol ––––––> cyclopentanone Solution Many reagents are available to oxidize a simple secondary alcohol to a ketone. For a laboratory synthesis, however, dehydrogenation is not practical, and cost is not as large a factor as it would be in industry. Most labs would have chromium trioxide or sodium dichromate available, and the chromic acid oxidation would be simple. PCC and the Swern oxidation would also work, although these reagents are more complicated to prepare and use. Chapter 11 56 Solved Problem 1 (Continued) Suggest the most appropriate method for each of the following laboratory syntheses. (b) 2-octen-l-ol ––––––> 2-octenal (structure below) Solution This synthesis requires more finesse. The aldehyde is easily over-oxidized to a carboxylic acid, and the double bond reacts with oxidants such as KMnO4. Our choices are limited to PCC or the Swern oxidation. Chapter 11 57 Enzymatic Oxidation Alcohol dehydrogenase catalyzes an oxidation: the removal of two hydrogen atoms from an alcohol molecule. The oxidizing agent is called nicotinamide adenine dinucleotide (NAD+). Chapter 11 58 Alcohol as a Nucleophile H C O R X • ROH is a weak nucleophile. • RO- is a strong nucleophile. • New O—C bond forms; O—H bond breaks. Chapter 11 59 Alkoxide Ions: Williamson Ether Synthesis • Ethers can be synthesized by the reaction of alkoxide ions with primary alkyl halides in what is known as the Williamson ether synthesis. • This is an SN2 displacement reaction and as such, works better with primary alkyl halides to facilitate back-side attack. • If a secondary or tertiary alkyl halide is used, the alkoxide will act as a base and an elimination will take place. Chapter 11 60 Substitution and Elimination Reactions Using Tosylates Chapter 11 61 SN2 Reactions with Tosylates • The reaction shows the SN2 displacement of the tosylate ion (-OTs) from (S)-2-butyl tosylate with inversion of configuration. • The tosylate ion is a particularly stable anion, with its negative charge delocalized over three oxygen atoms. Chapter 11 62 Summary of Tosylate Reactions Chapter 11 63 Reduction of Alcohols • Dehydrate with concentrated H2SO4, then add H2. • Make a tosylate, then reduce it with LiAlH4. OH CH3CHCH3 H2SO4 alcohol OH CH3CHCH3 alcohol CH2 CHCH3 alkene TsCl OTs CH3CHCH3 tosylate Chapter 11 H2 Pt LiAlH4 CH3CH2CH3 alkane CH3CH2CH3 alkane 64 Reaction of Alcohols with Acids • The hydroxyl group is protonated by an acid to convert it into a good leaving group (H2O). • Once the alcohol is protonated a substitution or elimination reaction can take place. Chapter 11 65 Reaction with HBr • • • • –OH of alcohol is protonated. –OH2+ is good leaving group. 3° and 2° alcohols react with Br- via SN1. 1° alcohols react via SN2. + R O H H3O H R O H Chapter 11 - Br R Br 66 Reaction with HCl • Chloride is a weaker nucleophile than bromide. • Add ZnCl2, which bonds strongly with –OH, to promote the reaction. • The chloride product is insoluble. • Lucas test: ZnCl2 in concentrated HCl: 1° alcohols react slowly or not at all. 2 alcohols react in 1-5 minutes. 3 alcohols react in less than 1 minute. Chapter 11 67 SN2 Reaction with the Lucas Reagent • Primary alcohols react with the Lucas reagent (HCl and ZnCl2) by the SN2 mechanism. • Reaction is very slow. The reaction can take from several minutes to several days. Chapter 11 68 SN1 Reaction with the Lucas Reagent Secondary and tertiary alcohols react with the Lucas reagent (HCl and ZnCl2) by the SN1 mechanism. Chapter 11 69 Solved Problem 2 When 3-methyl-2-butanol is treated with concentrated HBr, the major product is 2-bromo-2methylbutane. Propose a mechanism for the formation of this product. Solution The alcohol is protonated by the strong acid. This protonated secondary alcohol loses water to form a secondary carbocation. Chapter 11 70 Solved Problem 2 (Continued) Solution (Continued) A hydride shift transforms the secondary carbocation into a more stable tertiary cation. Attack by bromide leads to the observed product. Chapter 11 71 Reactions with Phosphorus Halides • • • • Good yields with 1° and 2° alcohols. PCl3 for alkyl chlorides (but SOCl2 better). PBr3 for alkyl bromides. P and I2 for alkyl iodides (PI3 not stable). Chapter 11 72 Mechanism with PBr3 • Oxygen attacks the phosphorus, displacing one of the halides. • Br- attacks back-side (SN2). Chapter 11 73 Reaction of Alcohols with Thionyl Chloride • Thionyl chloride (SOCl2) can be used to convert alcohols into the corresponding alkyl chloride in a simple reaction that produces gaseous HCl and SO2. Chapter 11 74 Mechanism of Thionyl Chloride Reaction Chapter 11 75 Appel reaction Also CBr4, or Br2 Appel reaction Mechanism Dehydration of Cyclohexanol • The dehydration of cyclohexanol with H2SO4 has three steps: Protonation of the hydroxide, loss of water, and deprotonation. • Alcohol dehydration generally takes place through the E1 mechanism. Rearrangements are possible. • The rate of the reaction follows the same rate as the ease of formation of carbocations: 3o > 2o > 1o. Chapter 11 78 Energy Diagram, E1 Chapter 11 79 Pinacol Rearrangement • In the pinacol rearrangement, a vicinal diol converts to the ketone (pinacolone) under acidic conditions and heat. • The reaction is classified as a dehydration since a water molecule is eliminated from the starting material. Chapter 11 80 Mechanism of the Pinacol Rearrangement • The first step of the rearrangement is the protonation and loss of a water molecule to produce a carbocation. Chapter 11 81 Mechanism of the Pinacol Rearrangement (Continued) • There is a methyl shift to form a resonancestabilized carbocation, which upon deprotonation by water, yields the pinacolone product. Chapter 11 82 Periodic Cleavage of Glycols • Glycols can be oxidatively cleaved by periodic acid (HIO4) to form the corresponding ketones and aldehydes. • This cleavage can be combined with the hydroxylation of alkenes by osmium tetroxide or cold potassium permanganate to form the glycol and the cleavage of the glycol with periodic acid. • Same products formed as from ozonolysis of the corresponding alkene. Chapter 11 83 Periodic Acid Cleavage • Periodic acid cleaves vicinal diols to give two carbonyl compounds. • Separation and identification of the products determine the size of the ring. Chapter 23 84 => Fischer Esterification • Reaction of an alcohol and a carboxylic acid produces an ester. • Sulfuric acid is a catalyst. • The reaction is an equilibrium between starting materials and products, and for this reason, the Fischer esterification is seldom used to prepare esters. Chapter 11 85 Reaction of Alcohols with Acyl Chlorides • The esterification reaction achieves better results by reacting the alcohol with an acyl chloride. • The reaction is exothermic and produces the corresponding ester in high yields with only HCl as a by-product. Chapter 11 86 Nitrate Esters • The best known nitrate ester is nitroglycerine, whose systematic name is glyceryl trinitrate. • Glyceryl nitrate results from the reaction of glycerol (1,2,3-propanetriol) with three molecules of nitric acid. Chapter 11 87 Phosphate Esters Chapter 11 88 Phosphate Esters in DNA Chapter 11 89
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