Download 102 Lecture Ch14b

Dehydration of Alcohols to form Ethers
• Simple, symmetrical ethers can be formed from the
intermolecular acid-catalyzed dehydration of 1° (or
methyl) alcohols (a “substitution reaction”)
• 2° and 3° alcohols can’t be used because they eliminate
(intramolecular dehydration) to form alkenes
OH
H3O+
+
OH
+
O
heat
H2O
•Unsymmetrical ethers can’t be made this way because a
mixture of products results:
+
OH
+
CH3-OH
H3O
heat
+
O
+
O
O
Mechanism of Formation of Ethers from Alcohols
• First, an alcohol is protonated by H3O+
• Next, H2O is displaced by another alcohol (substitution)
• Finally, a proton is removed by H2O to form the product
H
OH
H
O
+
+
O
H
H
O
H
H
H
H
OH
O
O
+
+
O
H
H
H
H
H
O
H
+
H
H
O
O
+
O
H
H
Oxidation of Alcohols
• Recall that oxidation is a loss of electrons and reduction is a
gain of electrons
• However, red-ox does not always involve ions
• Oxidation can also be defined as a gain in bonds to oxygen
• This is because O is more electronegative than all other
elements (besides F), so it removes electron density from any
element with which it forms a covalent bond
• Primary alcohols can be oxidized to aldehydes and carboxylic
acids and secondary alcohols can be oxidized to ketones
[O]
O
[O]
O
OH
H
OH
[O]
O
OH
Oxidation of Primary and Secondary Alcohols
• Primary alcohols are initially oxidized to aldehydes, but
aldehydes are easily oxidized to carboxylic acids
• In order to stop the reaction at the aldehyde a very mild
reducing agent must be used
• The most common reagent is PCC (pyridinium chlorochromate)
• Other oxidizing agents (like CrO3/H3O+) will oxidize primary
alcohols all the way to carboxylic acids
• Secondary alcohols can be oxidized with either reagent
PCC
O
OH
OH
H
CrO3
OH
H3O+
PCC
or
CrO3/ H3O +
O
OH
O
Oxidation of Thiols
• Oxidation can also be defined as a loss of bonds to hydrogen
• This is because H is less electronegative than all other
nonmetals (besides P which is the same), so adds electron
density to any element with which it forms a covalent bond
• Thiols can be oxidized to disulfides using I2 (or Br2)
• In proteins, disulfide bonds between sulfur-containing amino
acids (cysteines) are a major factor in preserving their shape
SH
+
I2
SH
S
S
+
2HI