1. Alcohols and Thiols
Unit 11

2. Structure - Alcohols
The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbon
bond angles about the hydroxyl oxygen atom are approximately 109.5°
Oxygen is sp3 hybridized
two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen
the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons

3. Nomenclature-Alcohols
IUPAC names
the parent chain is the longest chain that contains the OH group
number the parent chain to give the OH group the lowest possible number
change the suffix -e to -ol
Common names
name the alkyl group bonded to oxygen followed by the word alcohol

4. Nomenclature-Alcohols
Examples

5. Nomenclature of Alcohols
Compounds containing more than one OH group are named diols, triols, etc. C H 2 C H 3 2 C H 2 O H O H H O O H H O H O O H
1,2-Ethanediol
(Ethylene glycol)
1,2-Propanediol
(Propylene glycol)
1,2,3-Propanetriol
(Glycerol, Glycerine)

6. Nomenclature of Alcohols
Unsaturated alcohols
show the double bond by changing the infix from -an- to -en-
show the the OH group by the suffix -ol
number the chain to give OH the lower number

7. Physical Properties Alcohols are polar compounds
they interact with themselves and with other polar compounds by dipole-dipole interactions
Dipole-dipole interaction: the attraction between the positive end of one dipole and the negative end of another

8. Physical Properties
Hydrogen bonding: when the positive end of one dipole is an H bonded to F, O, or N (atoms of high electronegativity) and the other end is F, O, or N
the strength of hydrogen bonding in water is approximately 21 kJ (5 kcal)/mol
hydrogen bonds are considerably weaker than covalent bonds
nonetheless, they can have a significant effect on physical properties

9. Hydrogen Bonding

10. Physical Properties
Ethanol and dimethyl ether are constitutional isomers.
Their boiling points are dramatically different
ethanol forms intermolecular hydrogen bonds which increase attractive forces between its molecules resulting in a higher boiling point
there is no comparable attractive force between molecules of dimethyl ether C H 3 2 O C H 3 O
Ethanol
Dimethyl ether
bp 78° C
bp -24°C

11. Physical Properties
In relation to alkanes of comparable size and molecular weight, alcohols
have higher boiling points
are more soluble in water
The presence of additional -OH groups in a molecule further increases solubility in water and boiling point

12. Physical Properties

13. Acidity of Alcohols

14. Acidity of Alcohols
In dilute aqueous solution, alcohols are weakly acidic
Acidity depends primarily on the degree of stabilization and solvation of the alkoxide ion
the negatively charged oxygens of methanol and ethanol are about as accessible as hydroxide ion for solvation; these alcohol are about as acidic as water
as the bulk of the alkyl group increases, the ability of water to solvate the alkoxide decreases, the acidity of the alcohol decreases, and the basicity of the alkoxide ion increases

15. Reaction with Metals
Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides
Alcohols are also converted to metal alkoxides by reaction with bases stronger than the alkoxide ion
one such base is sodium hydride

16. Reaction with HX 3° alcohols react very rapidly with HCl, HBr, and HI
low-molecular-weight 1° and 2° alcohols are unreactive under these conditions
1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides

17. Reaction with HX
with HBr and HI, 2° alcohols generally give some rearranged product
1° alcohols with extensive -branching give large amounts of rearranged product

18. Reaction with HX Based on
the relative ease of reaction of alcohols with HX
(3° > 2° > 1°) and
the occurrence of rearrangements,
Chemists propose that reaction of 2° and 3° alcohols with HX
occurs by an SN1 mechanism, and
involves a carbocation intermediate

19. Alkyl Sulfonates Sulfonyl chlorides are derived from sulfonic acids
sulfonic acids, like sulfuric acid, are strong acids O O O R - S C l R - S O H R - S O O O O A
sulfonyl
chloride
A sulfonic acid
(a very strong acid)
A sulfonate anion
(a very weak base and
stable anion; a very
good leaving group

20. Alkyl Sulfonates
A commonly used sulfonyl chloride is p-toluenesulfonyl chloride (Ts-Cl)

21. Alkyl Sulfonates
Another commonly used sulfonyl chloride is methanesulfonyl chloride (Ms-Cl)

22. Alkyl Sulfonates
Sulfonate anions are very weak bases (the conjugate base of a strong acid) and are very good leaving groups for SN2 reactions
Conversion of an alcohol to a sulfonate ester converts HOH, a very poor leaving group, into a sulfonic ester, a very good leaving group

23. Dehydration of ROH
An alcohol can be converted to an alkene by acid-catalyzed dehydration (a type of -elimination)
1° alcohols must be heated at high temperature in the presence of an acid catalyst, such as H2SO4 or H3PO4
2° alcohols undergo dehydration at somewhat lower temperatures
3° alcohols often require temperatures at or slightly above room temperature

24. Dehydration of ROH H S O C H O C H = + O 180°C O H H S O + H O 140° C2 S O 4 C H 3 2 O C H 2 = + O
180°C O H H 2 S O 4 + H 2 O
140° C
Cyclohexanol
Cyclohexene C H 3 C H 3 H 2 S O 4 C H 3 O C H 3 = 2 + H 2 O
50° C C H 3
2-Methyl-2-propanol (
tert-
Butyl alcohol)
2-Methylpropene
(Isobutylene)

25. Dehydration of ROH
where isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond (the more stable alkene) usually predominates (Zaitsev rule)

26. Dehydration of ROH
Dehydration of 1° and 2° alcohols is often accompanied by rearrangement
acid-catalyzed dehydration of 1-butanol gives a mixture of three alkenes H 2 S O 4 + O H °C
3,3-Dimethyl-
2-butanol
2,3-Dimethyl-
2-butene
(80%)
2,3-Dimethyl-
1-butene
(20%)

27. Dehydration of ROH Based on evidence of
ease of dehydration (3° > 2° > 1°)
prevalence of rearrangements
Chemists propose a three-step mechanism for the dehydration of 2° and 3° alcohols
because this mechanism involves formation of a carbocation intermediate in the rate-determining step, it is classified as E1

28. Dehydration of ROH
Acid-catalyzed alcohol dehydration and alkene hydration are competing processes
Principle of microscopic reversibility: the sequence of transition states and reactive intermediates in the mechanism of a reversible reaction must be the same, but in reverse order, for the reverse reaction as for the forward reaction

29. Oxidation: 1° ROH
Oxidation of a primary alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions
to an aldehyde is a two-electron oxidation
to a carboxylic acid is a four-electron oxidation

30. Oxidation of ROH
A common oxidizing agent for this purpose is chromic acid, prepared by dissolving chromium(VI) oxide or potassium dichromate in aqueous sulfuric acid H 2 S O 4 C r O 3 + H 2 O H 2 C r O 4
Chromium(VI)
oxide
Chromic acid H 2 S O 4 H 2 O K 2 C r O 7 H 2 C r O 7 2 H C r O 4
Potassium
dichromate
Chromic acid

31. Oxidation: 1° ROH Oxidation of 1-octanol gives octanoic acid
the aldehyde intermediate is not isolated

32. Oxidation: 2° ROH
2° alcohols are oxidized to ketones by chromic acid

33. Oxidation of Glycols
Glycols are cleaved by oxidation with periodic acid, HIO4

34. Thiols: Structure
The functional group of a thiol is an SH (sulfhydryl) group bonded to an sp3 hybridized carbon
The bond angle about sulfur in methanethiol is 100.3°, which indicates that there is considerably more p character to the bonding orbitals of divalent sulfur than there is to oxygen

35. Nomenclature IUPAC names: Common names:
the parent is the longest chain that contains the -SH group
change the suffix -e to -thiol
when -SH is a substituent, it is named as a sulfanyl group
Common names:
name the alkyl group bonded to sulfur followed by the word mercaptan

36. Thiols: Physical Properties
Because of the low polarity of the S-H bond, thiols show little association by hydrogen bonding
they have lower boiling points and are less soluble in water than alcohols of comparable MW
the boiling points of ethanethiol and its constitutional isomer dimethyl sulfide are almost identical
Thiol
bp (° C)
Alcohol
bp (° C) M
ethanethiol 6 M
ethanol 65 E
thanethiol 35 E
thanol 78
1-Butanethiol 98
1-Butanol
117

37. Thiols: Physical Properties
Low-molecular-weight thiols = STENCH
the scent of skunks is due primarily to these two thiols
a blend of low-molecular weight thiols is added to natural gas as an odorant; the two most common of these are

38. Thiols: acidity Thiols are stronger acids than alcohols
when dissolved an aqueous NaOH, they are converted completely to alkylsulfide salts

39. Thiols: oxidation
The sulfur atom of a thiol can be oxidized to several higher oxidation states
the most common reaction of thiols in biological systems in interconversion between thiols and disulfides, -S-S-