Unit 6 – Alcohols and Ethers

1. Unit 6 – Alcohols and Ethers • Types of Alcohols • Nomenclature • Properties of Alcohols • Synthesis of Alcohols • Reactions of Alcohols

2. Types of Alcohols • Alcohol: • organic compound containing one or more hydroxyl (OH) groups. • Alcohols are often classified by the type of carbinol carbon atom present • the carbon bonded to the OH group Methanol wood alcohol Ethanol drinking alcohol Isopropyl alcohol rubbing alcohol

3. Types of Alcohols • Primary alcohol: • the carbinol carbon is attached to one other carbon atom • Secondary alcohol: • the carbinol carbon is attached to two other carbon atoms • Tertiary alcohol: • the carbinol carbon is attached to three other carbon atoms

4. Types of Alcohols • Diol: • an alcohol with two OH groups • Glycol: • a vicinal diol • OH groups on adjacent carbons Ethylene glycol car antifreeze propylene glycol medicine, food

5. Types of Alcohols • Phenol: • a compound with a hydroxyl group bonded directly to an aromatic (benzene) ring • Thiols: • an organic compound with an SH group • sulfur analog of alcohols • also called mercaptans phenol 3-methyl-1-butanethiol

6. IR of Alcohols • Alcohols typically exhibit a strong, broad, rounded peak at about 3300 cm-1+ for the O-H bond. SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

7. IR of Alcohols • In the absence of hydrogen bonding, the O – H peak can be relatively sharpand hard to distinguish from an N – H peak. • The frequency for an O-H peak in the absence of hydrogen bonding is typically somewhat higher (~3600 cm-1). • The O-H peak for a carboxylic acid is much broader and typically spans the distance from ~2500 – 3500 cm-1.

8. IR of Alcohols acid O-H acid O-H alcohol O-H w/hydrogen bonding alcohol O-H w/o hydrogen bonding

9. Nomenclature • IUPAC Naming System for Alcohols: • Find the longest continuous chain that contains the carbinol carbon atom • Drop the “e” from the corresponding alkane name and add the suffix “ol” a substituted pentanol

10. Nomenclature • IUPAC Naming System for Alcohols: • Number the longest chain starting from the end closest to the OH group. • OH takes priority over double and triple bonds • OH group is assumed to be on C #1 on a cycloalkane ring • Name and number all substituents as with an alkane or alkene. Don’t forget R or S if appropriate. (2R, 3R)-3-methyl-2-pentanol

11. Nomenclature • To name an alcohol that contains a double bond: • use “ol” suffix after the name of the alkene (drop the last “e”) • number the chain to give the carbinol carbon the lowest possible number • place the position number for the C=C in front of the base name (or before the “en”) and the position number for the OH in front of the “ol” suffix 4-methyl-5-hexen-1-ol 4-methylhex-5-en-1-ol

12. Nomenclature • To name diols: • use suffix “diol” after the name of the alkane (keep the ending “e”) • indicate the position of each OH group • place position of each OH group in front of the base name or in front of the suffix “diol” 2, 3-pentanediol or pentane-2, 3-diol

13. Nomenclature • To name thiols: • Use the same rules for naming alcohols • use “thiol” suffix instead of “ol” suffix 2-pentanethiol 3-methyl-2-pentanethiol

14. Nomenclature • Common names are often used with phenols: 2-bromophenol or ortho-bromophenol phenol 3-nitrophenol or meta-nitrophenol 4-chlorophenol or para-chlorophenol ortho = 1,2 meta = 1,3 para = 1,4

15. Nomenclature • Common names are often used with phenols: 2-methylphenol ortho-cresol 4-methylphenol para-cresol Benzene-1,2-diol catechol Benzene-1,3-diol resorcinol Benzene-1,4-diol hydroquinone

16. Nomenclature Example: Name the following compounds:

17. Nomenclature Example: Draw the structures of the following compounds. 3-isopropyl-2-methyl-2-hexanol trans-2-bromocyclohexanol (E)-2-chloro-2-buten-1-ol

18. Properties of Alcohols • The physical properties of alcohols are strongly influenced by the presence of the hydrophilic (“water loving”) OH group. • The OH group is capable of forming hydrogen bonds with other alcohol groups or with water.

19. Properties of Alcohols • Due to hydrogen bonding, alcohols have significantly higher boiling points than alkanes with comparable molecular weights. • CH3CH2OH MW = 46 BP = 78oC • CH3CH2CH3 MW = 44 BP = - 42oC

20. Properties of Alcohols • BP increases as the amount of hydrogen bonding increases: • 1-propanol BP = 97oC • 1,2-propanediol BP = 188oC (propylene glycol) • 1,2,3-propanetriol BP = 290oC (Glycerol)

21. Physical Properties Alcohols with 1-3 carbons are soluble in water.

22. Properties of Alcohols • Solubility decreases as the size of the alkyl group increases. • 1-hexanol is less soluble than ethanol • Solubility increases as the alkyl group becomes more compact/spherical. • t-butyl alcohol is more soluble than 1-butanol. • Solubility increases with increasing number of OH groups.

23. Acidity of Alcohols and Phenols • The acidity of alcohols varies widely and can be expressed using the acid dissociation constant, Ka ROH + H2O RO- + H3O+ Ka = [RO-][H3O+] pKa = -log Ka [ROH] • Acidity increases as: • Ka increases • pKa decreases Alkoxide ion

24. Acidity of Alcohols and Phenols • Structural trends and acidity: • Acidity of H2O, CH3OH, and CH3CH2OH are similar. (pKa = 15.7, 15.5, and 15.9 respectively) • Acidity decreases as number of carbons in the R group increases. • CH3OH pKa = 15.5 • t-butyl alcohol pKa = 18.0

25. Acidity of Alcohols and Phenols • Structural trends and acidity: • Acidity increases with the addition of electron-withdrawing halogens pKa = 14.3 pKa = 12.2 pKa = 15.9

26. Acidity of Alcohols and Phenols • Structural trends and acidity: • Phenols are more acidic than water or alcohols pKa = 18.0 pKa = 10.0

27. Acidity of Alcohols and Phenols • Phenols are stronger acids than alcohols. • React readily with aqueous NaOH • Phenols are soluble in aqueous strong base solutions but are insoluble in aqueous NaHCO3. + Sodium phenoxide

28. Acidity of Alcohols and Phenols • Since alcohols are very weak acids, a very strong base is used to remove the acidic proton. • Unless they are water soluble, alcohols are not soluble in aqueous base solution.

29. Synthesis of Alcohols • Nucleophilic Substitution on an alkyl halide • SN2 conditions most useful • strong nucleophile • methyl > 1o > 2o • inversion of configuration

30. Synthesis of Alcohols • Acid Catalyzed Hydration (Hydrolysis) of Alkenes • Markovnikov product formed • Equilibrium process • Less useful synthetically than other methods • rearrangements often occur

31. Synthesis of Alcohols • Oxymercuration-Demercuration of Alkenes • Markovnikov product • Anti addition to double bond

32. Synthesis of Alcohols • Hydroboration-Oxidation of Alkenes • Anti-Markovnikov product • Syn addition to double bond

33. Synthesis of Alcohols • Syn Hydroxylation of Alkenes • Syn addition to double bond • Reagents: • OsO4, H2O2 • cold, dilute KMnO4, OH- or

34. Synthesis of Alcohols • Anti-Hydroxylation of Alkenes • epoxide intermediate • Anti addition to double bond • Common peroxyacids: • CH3CO3H • MCPBA