Chemistry 2213a Fall 2012 Western University
Topic 5. Alcohols, Ethers, and Thiols
A. Structure and Nomenclature: Alcohols and Thiols
Alcohols have the general formula R–OH, while thiols, the oxygen counterpart
of alcohols, have the general formula R–SH.
The O atom has a valence of 2, and one of these is taken by the H. Thus, the
OH group substitutes an H atom in general formulas. So, the O atom is always
ignored in unsaturation calculations.
C H O
C 2 6 CH 3H O2 6 6
0 units unsat. 4 units unsat.
In nomenclature, the OH or SH group is given the lowest possible
number in the carbon chain. These groups take priority over
bonds. 3-buten-2-ol Alcohols, Thiols, and Ethers 2
In complex molecules, OH can be treated as a hydroxy substituent.
Like alkyl halides, alcohols are classified as 1°, 2°, or 3° depending on the
number of carbon atoms bonded to the carbon bearing the OH group.
B. Physical Properties of Alcohols and Thiols
The properties of alcohols stem from the high electronegativity of H
oxygen, resulting in polar bonds. The effect is less in thiols b+caus-
sulfur is less electronegative than oxygen. O
R Alcohols, Thiols, and Ethers 3
Due to hydrogen bonding, alcohols have very high BP for their molar mass.
C H 36°
C5H 11 108° CH 4 –136°
C5H 11 136° H2O 100°
C. Reactions of Alcohols and Thiols
Alcohols and thiols undergo a series of common reactions: acid-base
reactions, conversion to alkyl halides, dehydrations to alkenes, and oxidations.
1. Acid-base reactions
Like water, alcohols and thiols are amphiprotic. In the presence of strong
bases, they can function as acids. Likewise, in the presence of strong acids,
they can function as bases.
ROH B RO BH
ROH H ROH 2
base Alcohols, Thiols, and Ethers 4
Alcohols, like water, also react with Group 1 and 2 metals to form alkoxide
salts, with the liberati2n of H gas.
RO Na 1/2 2
2. Conversion to alkyl halides
1. Reaction with HX (X = Cl, Br, I)
In acidic solutions, alcohols can be protonated. This protonation step
generates a good leaving group.
R OH H X R OH 2 X
Afterwards, anNS 1 reaction can occur if a stable carbocation is formed.
OH H O
X X Alcohols, Thiols, and Ethers 5
An S N reaction occurs if a stable carbocation cannot form.
X RCH OH RCH X H O
2 2 2 2
2. Reaction with thionyl chloride (S2Cl )
Good yields of chloroalkanes are obtained from 1° and 2° alcohols when
SOCl is a chlorine source. The by-products made are gases and bubble out of
solution, rendering the reaction irreversible.
RCH O2 + SOCl R2H Cl + HCl2+ SO 2
This mechanism is S N-like but involves a lot of sulfur chemistry, so we will not
examine it in detail. 3° alcohols will not react wit2 SOCl .
3. Dehydration to alkenes
Water can be eliminated in the presence of heat () and conc2H S4 or 3 PO4.
H OH conc
C C 2 4 C C H O
Alcohols, Thiols, and Ethers 6
The acid is necessary to protonate the OH, generating the good leaving group
H2O. The elimination proceeds E1 if a stable carbocation is formed.
H OH H OH
2 E1 or E2
C C H C C C C H O
Elimination to form double bonds is especially favoured when stable double
bonds are formed. Recall Zaitsev’s Rule: alkyl groups stabilize C=C bonds.
OH H2SO 4
stable Alcohols, Thiols, and Ethers 7
4. Oxidation of 1° and 2° alcohols
Primary and secondary alcohols are easily oxidized to aldehydes and ketones,
OH [O] O
Because oxidation removes H atoms from both the OH and the CH, only 1°
and 2° alcohols can be oxidized (not 3°) to simple carbonyl compounds.
CH3 CH 3 Alcohols, Thiols, and Ethers 8
Oxidation of alcohols is a great way to prepare carbonyl compoun