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WILLIAM H. BROWN
THOMAS POON
www.wiley.com/college/brown
C H A P T E R E I G H T
Alcohols, Ethers,
and Thiols
Copyright 2014 John Wiley & Sons, Inc. All rights reserved.
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Alcohols - Structure
The functional group of an alcohol
is an -OH (hydroxyl) groupbonded to an sp3hybridized
carbon.
Bond angles about the hydroxyl
oxygen atom are approximately109.5.
Oxygenis also sp3hybridized.
Two sp3hybrid orbitals form sigma
bonds to carbon and hydrogen.
The remaining two sp3hybridorbitals each contain an unshared
pair of electrons.
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Alcohols - Nomenclature
IUPAC names
The parent chain is the longest chain that
contains the -OH group.
Number the parent chain in the direction that
gives the -OH group the lower number. Change the suffix -eto -ol.
Common names
Name the alkyl group bonded to oxygen followedby the word alcohol.
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Alcohols - Nomenclature
Compounds containing
two -OH groups are named as diols,
three -OH groups are named as triols.
-OH groups on adjacent carbons are called
glycols.
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Physical Properties
Figure 8.2 Polarity of the C-O-H bond in methanol.
(a) Partial positive charges on carbon and hydrogenand a partial negative charge on oxygen. (b) Anelectron density map showing the partial negative
charge in red and the partial positive charge in blue.
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Hydrogen Bonding
Alcohols associate in the liquid state by hydrogen
bonding.
Hydrogen bonding: The attractive force between a
partial positive charge on hydrogen and a partial
negative charge on a nearby oxygen, nitrogen, or
fluorine atom.
The strength of hydrogen bonding in alcohols is
approximately 2 to 5 kcal/mol.
Hydrogen bonds are considerably weaker thancovalent bonds(for example, 110 kcal/mol for an OH
bond).
Nonetheless, hydrogen bonding can have a significant
effect on physical properties.
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Boiling Points
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Acidity of Alcohols
Most alcohols are about the same or slightly
weaker acids than water.
Aqueous solutions of alcohols have the same pH
as that of pure water.
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Acidity of Alcohols
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Basicity of Alcohols
In the presence of strong acids, the oxygen
atom of an alcohol behaves as a weak base. Proton transfer from the strong acid forms an
oxonium ion.
Thus, alcohols can function as both weak acidsand weak bases.
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Reaction with Active Metals
Alcohols react with Li, Na, K, and other active
metals to liberate hydrogen gas and form metalalkoxides.
Na is oxidized to Na+and H+is reduced to H2.
Alkoxides are somewhat stronger bases that OH.
Alkoxides can be used as nucleophiles innucleophilic substitution reactions.
They can also be used as bases in !-eliminationreactions.
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Conversion of ROH to RX
Water-soluble3 alcohols react very rapidly
with HCl, HBr, and HI.
Low-molecular-weight 1 and 2 alcohols areunreactive under these conditions.
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Conversion of ROH to RX
Water-insoluble3 alcohols react by bubbling gaseous
HCl through a solution of the alcohol dissolved in diethylether or THF.
1 and 2 alcohols require concentrated HBr and HI toform alkyl bromides and iodides.
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Reaction of a 3 ROH with HX
3 Alcohols react with HX by an SN1
mechanism Step 1: Add a proton. Rapid and reversible proton
from the acid to the OH group.
This proton-transfer converts the leaving group fromOH, a poor leaving group, to H2O, a better leavinggroup.
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Reaction of a 3 ROH with HX
Step 2: Break a bond to form a stable molecule or ion.
Loss of H2O gives a 3 carbocation.
Step 3: Reaction of an electrophile and a nucleophile to
form a new covalent bond completes the reaction.
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Reaction of a 1 ROH with HX
1 alcohols react with HX by an SN2mechanism.
Step 1: Add a proton.Proton transfer to OH converts
OH, a poor leaving group, to H2O a better leaving group.
Step 2: Reaction of a nucleophile and an electrophile
to form a new covalent bondand break a bond.
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Reaction of ROH with HX
Reactions are governed by a combination of
electronic and steric effects
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Reaction with SOCl2
Thionyl chloride, SOCl2, is the most widely
used reagent for conversion of primary and
secondary alcohols alcohols to alkyl
chlorides.
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Dehydration of Alcohols
An alcohol can be converted to an alkene by
elimination of H and OH from adjacent
carbons
1 alcohols must be heated at high
temperaturein the presence of an acidcatalyst, such as H2SO4or H3PO4.
2 alcohols undergo dehydration at somewhat
lower temperatures.
3 alcohols often require temperatures at or only
slightly above room temperature.
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Dehydration of Alcohols
examples:
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Dehydration of Alcohols
When isomeric alkenes are obtained, the
more stable alkene (the one with the greater
number of substituents on the double bond)
generally predominates (Zaitsevs rule).
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Dehydration of a 2 Alcohol
A three-step mechanism
Step 1: Add a proton. Proton transfer from H3O+
to the OH group converts OH, a poor leaving
group, into H2O, a better leaving group.
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Dehydration of a 2 Alcohol
Step 2: Break a bond to form a stable molecule or ion.
Loss of H2O gives a carbocation intermediate.
Step 3: Take a proton away.Proton transfer from an
adjacent carbon to H2O gives the alkene and regenerates
the acid catalyst.
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Dehydration of a 1 Alcohol
A two-step mechanism
Step 1: Add a proton. Proton transfer from the acid gives
an oxonium ion.
Step 2: Take a proton away and loss of H2Ogives the
alkene and regenerates the acid catalyst.
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Hydration-Dehydration
Acid-catalyzed hydration of an alkene and
dehydration of an alcohol are competing
processes.
Large amounts of water favor alcoholformation.
Scarcity of water or experimental conditions
where water is removed favor alkeneformation.
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Oxidation of Alcohols
Oxidation of a 1 alcohol gives an aldehyde or a
carboxylic acid, depending on the oxidizing agentand experimental conditions. Oxidation of a 2alcohol gives a ketone.
The most common oxidizing agent is chromic acid.
Chromic acid oxidation of 1-octanol gives octanoic acid.
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Oxidation of Alcohols
To oxidize a 1 alcohol to an aldehyde, use PCC.
PCC oxidation of geraniol gives geranial.
Tertiary alcohols are not oxidized by either of these
reagents; they are resistant to oxidation.
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Ethers - Structure
The functional group of an ether is an oxygen
atom bonded to two carbon atoms.
Oxygen is sp3hybridized with bond angles of
approximately 109.5.
In dimethyl ether, the COC bond angle is110.3.
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Ethers - Nomenclature
IUPAC
The longest carbon chain is the parent alkane.
Name the -OR group as an alkoxy substituent.
Common names:
Name the groups bonded to oxygen followed by the wordether.
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Ethers - Nomenclature
Although cyclic ethers have IUPAC names,
their common names are more widely used.
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Ethers - Physical Properties
Figure 8.5 Ethers are polarmolecules.
Each C-O bond is polar covalent.
However, only weak attractive forces exist
between ether molecules in the pure liquid.
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Ethers - Physical Properties
Table 8.3 Boiling Points and Solubilities in Water of
Some Alcohols and Ethers of ComparableMolecular Weight
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Ethers - Physical Properties
Figure 8.6 Ethers are hydrogen bond acceptors
only. They are not hydrogen bond donors.
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Ethers - Physical Properties
The effect of hydrogen bonding is illustratedby comparing the boiling points of ethanol
and dimethyl ether.
Copyright 2014 John Wiley & Sons, Inc. All rights reserved.
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Reactions of Ethers
Ethers resemble hydrocarbons in their
resistance to chemical reaction.
They do not react with strong oxidizing agents
such as chromic acid, H2CrO4.
They are not affected by most acids and bases atmoderate temperatures.
Because of their good solvent properties and
general inertness to chemical reaction, ethersare excellent solvents in which to carry out
organic reactions.
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Epoxides
Epoxide:A cyclic ether in which oxygen is one
atom of a three-membered ring.
Ethylene oxide is synthesized from ethylene and O2.
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Epoxides
Other epoxides can be synthesized from an
alkene by oxidation with a peroxycarboxylicacid, RCO3H.
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Reactions of Epoxides
Ethers are generally unreactive to aqueous
acid.
Epoxides, however, react readily because of the
angle strain in the three-membered ring.
Reaction of an epoxide with aqueous acid gives aglycol.
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Other Epoxide Ring Openings
The mechanism of acid-catalyzed hydrolysis of an
epoxide involves three steps. Step 1: Add a proton
Step 2. Reaction of an electrophile and a nucleophileto form a new covalent bond.
Step 3. Take away a proton.
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Other Epoxide Ring Openings
The value of epoxides lies in the number of
nucleophiles that will bring about ring opening,and the combinations of functional groups thatcan be synthesized from them.
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Thiols - Structure
Figure 8.7 The functional group of a thiol is
an -SH (sulfhydryl) group bonded to an sp3hybridized carbon.
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Thiols - Nomenclature
IUPAC names
The parent chain is the longest chain containing the-SH group.Add -thiol to the name of the parent chain.
Common names
Name the alkyl group bonded to sulfur followed by the
word mercaptan.
Alternatively, indicate the -SH by the prefix mercapto.
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Thiols - Physical Properties
Low-molecular-weight thiols have a STENCH
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Thiols - Physical Properties
The difference in electronegativity between S and H
is 2.5 2.1 = 0.4 Because of their low polarity, thiols
show little association by hydrogen bonding.
have lower boiling points and are less soluble in waterthan alcoholsof comparable MW.
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Acidity of Thiols
Thiols are stronger acids than alcohols.
Thiols react with strong bases to form salts.
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Oxidation of Thiols
Thiols are oxidized by a variety of oxidizing
agents, including O2, to disulfides.
Disulfides, in turn, are easily reduced to thiols
by several reagents.
This easy interconversion between thiols and
disulfides is very important in protein chemistry.
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