Chapter 15 Alcohols, Diols, and Thiols

Transcription

1 Chapter 15 Alcohols, Diols, and Thiols

2 15.1 Sources of Alcohols

3 Methanol Methanol is an industrial chemical end uses: solvent, antifreeze, fuel principal use: preparation of formaldehyde

4 Methanol Methanol is an industrial chemical end uses: solvent, antifreeze, fuel principal use: preparation of formaldehyde prepared by hydrogenation of carbon monoxide C C 3

5 Ethanol Ethanol is an industrial chemical Most ethanol comes from fermentation Synthetic ethanol is produced by hydration of ethylene Synthetic ethanol is denatured (made unfit for drinking) by adding methanol, benzene, pyridine, castor oil, gasoline, etc.

6 ther alcohols Isopropyl alcohol is prepared by hydration of propene. All alcohols with four carbons or fewer are readily available. Most alcohols with five or six carbons are readily available.

7 Sources of alcohols Reactions discussed in earlier chapters (Table 15.1) ydration of alkenes ydroboration-oxidation oxidation of alkenes ydrolysis of alkyl halides Syntheses using Grignard reagents organolithium reagents

8 Sources of alcohols New methods in Chapter 15 Reduction of aldehydes and ketones Reduction of carboxylic acids Reduction of esters Reaction of Grignard reagents with epoxides Diols by hydroxylation of alkenes

9 15.2 Preparation of Alcohols by Reduction of Aldehydes and Ketones

10 Reduction of Aldehydes Gives Primary Alcohols R R C C

11 Example: Catalytic ydrogenation C 3 C + 2 Pt, ethanol C 3 C 2 (92%)

12 Reduction of Ketones Gives Secondary Alcohols R R C C R' R'

13 Example: Catalytic ydrogenation + 2 Pt ethanol (93-95%) 95%)

14 Retrosynthetic Analysis R R C : C R R C : C R' R'

15 Metal ydride Reducing Agents Na + B Li + Al Sodium borohydride Lithium aluminum hydride act as hydride donors

16 Examples: Sodium Borohydride Aldehyde 2 N Ketone C NaB 4 methanol 2 N (82%) C 2 NaB 4 ethanol (84%)

17 Lithium aluminum hydride more reactive than sodium borohydride cannot use water, ethanol, methanol etc. as solvents diethyl ether is most commonly used solvent

18 Aldehyde Examples: Lithium Aluminum ydride Ketone C 3 (C 2 ) 5 C (C 6 5 ) 2 CCC 3 1. LiAl 4 diethyl ether LiAl 4 diethyl ether 2. 2 C 3 (C 2 ) 5 C 2 (86%) (C 6 5 ) 2 CCC 3 (84%)

19 Selectivity neither NaB 4 or LiAl 4 reduces isolated double bonds 1. LiAl 4 diethyl ether 2. 2 (90%)

20 15.3 Preparation of Alcohols By Reduction of Carboxylic Acids and Esters

21 Reduction of Carboxylic Acids Gives Primary Alcohols R R C C lithium aluminum hydride is only effective reducing agent

22 Example: Reduction of a Carboxylic Acid C 1. LiAl 4 diethyl ether 2. 2 C 2 (78%)

23 Reduction of Esters Gives Primary Alcohols Lithium aluminum hydride preferred for laboratory reductions Sodium borohydride reduction is too slow to be useful Catalytic hydrogenolysis used in industry but conditions difficult or dangerous to duplicate in the laboratory (special catalyst, high temperature, high pressure

24 Example: Reduction of a Carboxylic Acid CC 2 C 3 1. LiAl 4 diethyl ether 2. 2 C 2 + C 3 C 2 (90%)

25 15.4 Preparation of Alcohols From Epoxides

26 Reaction of Grignard Reagents with Epoxides R MgX R 2 C C 2 C 2 C 2 MgX 3 + RC 2 C 2

27 Example C 3 (C 2 ) 4 C 2 MgBr + 2 C C 2 1. diethyl ether C 3 (C 2 ) 4 C 2 C 2 C 2 (71%)

28 15.5 Preparation of Diols

29 Diols are prepared by... reactions used to prepare alcohols hydroxylation of alkenes

30 Example: reduction of a dialdehyde CC C 2 CC 2 C C 3 2 (100 atm) Ni, 125 C C 2 C 2 CC 2 C 2 C 3 3-Methyl-1,5-pentanediol (81-83%) 83%)

31 ydroxylation of Alkenes Gives Vicinal Diols vicinal diols have hydroxyl groups on adjacent carbons ethylene glycol (C 2 C 2 ) is most familiar example

32 smium Tetraoxide is Key Reagent syn addition of groups to each carbon of double bond C C C C C C s

33 Example C 3 (C 2 ) 7 C C 2 (73%) (C 3 ) 3 C s 4 (cat) C 3 (C 2 ) 7 CC 2 tert-butyl alcohol

34 Example (C 3 ) 3 C s 4 (cat) tert-butyl alcohol (62%)