Chapter 10. Structure and Synthesis of Alcohols

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1 KOT 222 Organic Chemistry II Chapter 10 Structure and Synthesis of Alcohols 1

2 What are alcohols?? Organic compounds containing hydroxyl group, -OH. Examples: CH 3 -OH methanol CH 3 CH 2 -OH ethanol H 3 C CH 3 CH OH 2-propanol or Isopropyl alcohol 2

3 Structure of Alcohols R-O-H H-O-H Large alkyl group (methyl) increase the C-O-H bond angle. C-O bond is longer due to larger covalent radius of carbon. 3

4 Classification of Alcohols According to the type of carbinol carbon. 4

5 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, starting from the end closest to the -OH group. Number and name all substituents. 5 4 CH 2 I CH 2 CH 3 CH 2 CH CH OH CH CH 3 CH 3 3-(iodomethyl)-2-isopropylpentan-1-ol 5

6 For cyclic alcohols: Using prefix cyclo- Hydroxyl grop is assumed on C1. For unsaturated alcohols: Hydroxyl group takes precedence. Assign lowest possible number to the carbinol carbon. Use alkene or alkyne name H Br OH H Trans-2-bromocyclohexan-1-ol HO 2 1 CH H Cl (E)-4-chlorobut-3-en-2-ol 6

7 The functional group of alcohols, -OH is not at the highest-priority!!! Naming Priority Acids Esters Aldehydes Ketones Alcohols Amines Alkenes Alkynes Alkanes Ethers Halides 7

8 When OH is not the highest-priority functional group. -OH is named as a hydroxy substituent O CH 2 OH OH O 4 3 CH 3 CH 2 CH 2 1 C OH 2-hydroxymethylcyclohexanone 3-hydroxybutanoic acid HO H CH 2 CH 2 OH H trans-3-(2-hydroxyethyl)cyclopentanol 8

9 Naming Diols Diols = alcohols with two OH groups. Two numbers are needed to locate the two -OH groups. Use -diol as suffix instead of -ol. OH OH OH OH Hexane-2,5-diol Butane-2,3-diol 9

10 Glycols: Generally means a 1,2-diol, or vicinal diol. Synthesized by the hydroxylation of alkenes: CH 2 CH 2 CH 2 CH CH 3 OH OH ethane-1,2-diol ethylene glycol Common names for glycols use the name of the alkene from which they were made. OH OH propane-1,2-diol propylene glycol 10

11 Naming Phenols -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 OH CH 3 Cl OH 3-chlorophenol meta-chlorophenol 2-methylphenol ortho-cresol benzene-1,3-diol resorcinol 11

12 Physical Properties of Alcohols Unusually high boiling points due to hydrogen bonding (together with large dipole moment) between molecules. Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases 12

13 Boiling Points of Alcohols Ethanol : large dipole moment & hydrogen bonding. Dimethyl ether : large dipole moment Propane : weak Van der Waals force 13

14 Solubility of Alcohols R-OH Solubility decreases as the size of the alkyl group increases. 14

15 Acidity of Alcohols pk a range: (water: 15.7) Acidity decreases as substitution on the alkyl group increases. Halogens increase the acidity. Phenol is much more acidic than openedchain alcohols and is 100 million times more acidic than cyclohexanol. 15

16 Formation of Alkoxide Ions Alcohols react with sodium and potassium metal to form alkoxides. An oxidation-reduction reaction. 16

17 R OH + Na R O - + Na + 1/2 H 2 (g) 1 o /2 o alcohol R 3 C OH + K R 3 C O - + K + 1/2 H 2 (g) 3 o alcohol THF R OH + NaH R O - + Na + H 2 (g) OH + NaOH O -+ Na + H 2 O 17

18 Synthesis of Alcohols - Review Nucleophilic substitution on alkyl halides: From alkenes: 1. Acid-catalyzed hydration: 18

19 2. Oxymerculation-demerculation: 3. Hydroboration-oxidation 19

20 4. Hydroxylation: Synthesis of glycols Syn hydroxylation of alkenes osmium tetroxide, hydrogen peroxide cold, dilute, basic potassium permanganate. Anti hydroxylation of alkenes peroxyacids, hydrolysis C C R O C OH OOH, H 3 O + C C OH 20

21 Addition of acetylides to carbonyl compounds R R C C + C O R R C C C O R H 3 O + R C C C OH R' R' R' acetylide ketone/aldehyde alkoxide acetylenic alcohol 21

22 Organometallic Reagents for Alcohol Synthesis What are organometallic reagents? compounds contain covalent bonds between carbon atoms and metal atoms. δ- δ+ C - M An example is sodium acetylide. 22

23 Organometallic compound sodium acetylide Nucleophilic carbon can attack a partially positive carbon: 23

24 How about alkyl and alkenyl groups? Can be made into 1. Grignard Reagents 2. Organolithium reagents 24

25 1. Grignard Reagents Organomagnesium halides. Formula R-Mg-X (reacts like R: -+ MgX). Stabilized by anhydrous ether. From the reaction between alkyl halides and magnesium metal. reactivity for alkyl halides is R-I > R-Br > R-Cl >> R-F May be formed from any halide: - primary - vinyl - secondary - aryl - tertiary 25

26 Formation of Grignard reagents: 26

27 2. 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. R X + 2 Li Li + - X + R Li 27

28 Addition of Organometallic Reagents to Carbonyl Compounds δ- δ+ R Mg - X δ- δ+ R Li Organometallics are strong nucleophiles and strong bases. Attack the C=O group: nucleophilic addition. 28

29 Formation of 1 0 Alcohols Reaction of a Grignard with formaldehyde will produce a primary alcohol after protonation. magnesium alkoxide salt H CH 3 CH 2 CH 2 CH 2 MgBr + C H butylmagnesium bromide O 1. ether 2. H 3 O + CH 3 CH 2 CH 2 CH 2 C OH H 1-pentanol H 29

30 Formation of 2 0 Alcohols Addition of a Grignard reagent to an aldehyde followed by protonation will produce a secondary alcohol. 30

31 Formation of 3 0 Alcohols by addition of a Grignard to a ketone followed by protonation with dilute acid. O C CH3 + CH 3 CH 2 MgBr O C CH2 CH 3 MgBr + + CH 3 MgBr O C CH 3 CH2 CH 3 1. ether 2. H 3 O + CH 3 C OH CH 2 CH 3 31

32 Grignard Reactions with Acid Chlorides and Esters Use two equivalents of Grignard reagents. The product is a tertiary alcohol with two identical alkyl groups. 32

33 Addition of first equivalent of Grignard reagent: 33

34 Addition of second equivalent of Grignard reagent: ketone alkoxide tertiary alcohol The final product is a tertiary alcohol. One of the alkyl groups (R ) is from acid chloride or ester and the other two (R) are from Grignard reagent. 34

35 Grignard Reagents with Ethylene Oxide Grignard reagents do not react with ethers but they do react with epoxides and open them to form alcohols. The ring strain present in the epoxide is relieved by the opening. 35

36 Summary of Grignard Reactions 36

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38 Limitations of Organometallic Reactions Grignard and organolithium are strong nucleophiles and bases They can react with acidic or electrophilic compounds. 1. Grignard (and organolithium) reagents can be protonated by water and destroyed, become alkanes. other acidic protons: O-H, N-H, S-H, -C C-H. R-X + Mg R-Mg-X + H-O-H R-H + XMgOH H-O-H R-X + 2 Li RLi + LiX R-H + LiOH A reaction to convert (reduce) an alkyl halide to an alkane 38

39 2. No other electrophilic multiple bonds, like C=N, C N, S=O, or N=O in the solvent or in the Grignard reagent itself. MgBr O CH 3 CCH 3, H 3 O + X OH O 1. CH 3 CH 2 MgBr 2. H 3 O + X HO CH 2 CH 3 OH OH 39

40 Coupling Reactions Formation of carbon carbon bond. Use an organocopper reagent, lithium dialkylcuprate (Gilman reagent). 40

41 Reduction of Carbonyl Groups Carbonyl compounds can be reduced into alcohols. Reduction of aldehyde yields 1º alcohol. Reduction of ketone yields 2º alcohol. Reducing agents: Sodium borohydride, NaBH 4 Lithium aluminum hydride, LiAlH 4 Raney nickel 41

42 Uses of sodium borohydride: Hydride ion, H -, 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. O O CH 2 C OCH 3 NaBH 4 HO O CH 2 C OCH 3 H 42

43 Uses of lithium aluminium hydride: Stronger reducing agent. Dangerous!!! Reacts explosively with water and alcohols. Reduces: Ketone 2 o alcohol aldehyde carboxylic acid ester 1 o alcohol O O CH 2 C OCH 3 1. LiAlH 4 CH 2. H 3 O + 2 CH 2 OH H HO 43

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45 Uses of Raney nickel ( catalytic hydrogenation) Raney nickel: an effective catalyst for the hydrogenation of aldehydes and ketones to alcohols. Alkene double bonds will be reduced as well. 45

46 Thiols (Mercaptans) Sulfur analogues of alcohols: R-SH R-OH Functional group, -SH is called sulfhydryl group. Naming by using the suffix -thiol. CH 3 SH CH 3 CH CHCH 2 SH HS CH 2 CH 2 OH methanethiol methyl mercaptan 2-butane-1-thiol 2-marcaptoethanol Able to complex with heavy metals: arsenic, mercury. Stink!!! 46

47 Acidity of thiols: More acidic than alcohols because: S-H bonds are weaker than O-H bonds. Negative charge can be delocalized over a larger region in thiolate ion as compared to alkoxide ion. CH 3 CH 2 OH + - OH CH 3 CH 2 O - + H 2 O ethanol, pk a = 15.9 CH 3 CH 2 SH + - OH CH 3 CH 2 S - + H 2 O ethanethiol, pk a =

48 Synthesis of thiols: By S N 2 reactions of sodium hydrosulfide with unhindered alkyl halides. Thiols are easily oxidized (mild) into a dimer called disulfide. 48

49 Strong oxidation converts thiols to sulfonic acids, R-SO 3 H. O O O R S H KMnO 4 or HNO 3 R S O vigorous oxidation (boil) O H R S O O sulfonic acid H R S O O 2+ H O SH HNO 3 (boil) S O OH benzenethiol benzenesulfonic acid 49

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