alcohols, ethers and thiols bettelheim, brown, campbell and farrell chapter 14
TRANSCRIPT
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Alcohols, Ethers and Thiols
Bettelheim, Brown, Campbell and Farrell
Chapter 14
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Review of Carbon Classification
Type of C Attached to
1o (primary) C one other carbon
2o (secondary) C two other carbons
3o (tertiary) C three other carbons
4o (quaternary) C four other carbons
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Alcohols• Alcohol:Alcohol: Has an -OHOH (hydroxyl) group(hydroxyl) group bonded
to a tetrahedral carbon– methanol, CH3OH, is the simplest alcohol
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Classification of Alcohols
• Depends on the C which has the –OH group attached
CH2CH3 OH
ethanol
CH2CH3 OH
ethanol CH3CHCH3
OH
2-propanol
CH3CHCH3
OH
2-propanol
2-methyl-2-propanol
CH3CCH3
OH
CH32-methyl-2-propanol
CH3CCH3
OH
CH3
1o Alcohol—attached to one C
2o Alcohol—
attached to
two Cs
3o Alcohol--
attached to three Cs
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Alcohol Nomenclature1.Find longest carbon chain that contains the -OH group
(parent chain)
2.Number chain from end that gives the -OH the lower number
3.Change the ending -ee to -olol
4.Use a number to show the location of the -OH group
For cyclic alcohols, the carbon with the -OH group is C-1
5.Name and number substituents and list them in alphabetical order
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Nomenclature
Ethanol(Ethyl alcohol)
1-Propanol(Propyl alcohol)
2-Propanol(Isopropyl alcohol)
1-Butanol(Butyl alcohol)
OH
OH
OHOH
2-Butanol(sec-Butyl alcohol)
2-Methyl-1-propanol(Isobutyl alcohol)
2-Methyl-2-propanol(tert-Butyl alcohol)
OH
Cyclohexanol(Cyclohexyl alcohol)
OHOH
OH
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Nomenclature• Write the IUPAC name for each alcohol
(a) (b)OH OH
(c) (d) OHOH
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Nomenclature
(a) (b)OH OH
(c) (d) OHOH
1
22
2 2
4
trans-2-Methylcyclohexanol4-Methyl-2-pentanol
2-Heptanol
7
5
3
2,2-Dimethyl-1-propanol
1
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Nomenclature– Alcohols containing more than one hydroxyl
groups are named as a dioldiol, a trioltriol, and so on– IUPAC names for diols, triols, and so on retain
the final "-e" in the name of the parent alkane– Compounds containing two hydroxyl groups
on adjacent carbons are commonly called glycolsglycols
CH3CHCH2HO OH
CH2CH2OH OH
CH2CHCH2
OH OHOH1,2-Ethanediol
(Ethylene glycol)1,2-Propanediol
(Propylene glycol)1,2,3-Propanetriol
(Glycerol, Glycerin)
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Name the following
CH2CH2CH2
OHOHHO
OH
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Name the following
CH2CH2CH2
OHOHHO
OH
1,3-propanediol trans-1,3-cyclohexanediol
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Physical Properties• Alcohols are polar molecules
– the C-O and O-H bonds are both polar covalent
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Physical Properties
Alcohol molecules can form hydrogen bonds with each other
Alcohols can also form hydrogen bonds with water
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Physical Properties of Alcohols– Boiling point increases as MW increases– Solubility in water decreases as MW increases
CH3CH2CH2OH
CH3CH2CH2CH3
CH3OHCH3CH3
CH3CH2OH
CH3CH2CH3
CH3CH2CH2CH2OH
CH3CH2CH2CH2CH3
Structural Formula NameMolecularWeight
bp(°C)
Solubilityin Water
methanol 32 65 infiniteethane 30 -89 insoluble
ethanol 46 78 infinite
propane 44 -42 insoluble
1-propanol 60 97 infinite
butane 58 0 insoluble
8 g/100 g117741-butanol
pentane 72 36 insoluble
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Chemical Properties of Alcohols
• Combustion Reactions (they burn in O2)
• Acid-Base Chemistry
• Dehydration
• Oxidation
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Acidity of Alcohols
• Alcohols have about the same pKa values as water– Aqueous solutions of alcohols have the same
pH as that of pure water
– Phenols have lower pKa values than normal alcohols do (are more acidic)
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Acidity of Alcohols
• Alcohols and phenols both contain an OH group– Phenols are weak acids and react with strong bases
such as NaOH to form water-soluble salts– pKa = 9.89 for phenol
– Alcohols are much weaker acids and do not react in this manner
OH NaOHH2O
O-Na+ H2O+
Phenol Sodium phenoxide(a water-soluble salt)
+
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Why are phenols more acidic than alcohols?
Aromatic ring and O have delocalization of electrons. Less pull on H, so it is more acidic.
O H
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Dehydration
• Dehydration:Dehydration: Removal of water from adjacent carbons to form an alkene
– most often occurs by heating an alcohol with either 85% H3PO4 or concentrated H2SO4
CH3CH2OHH2SO4 CH2=CH2 H2O+
Ethanol Ethylene180°C
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• Dehydration essentially the reverse of hydration (addition of water to double bond)
• Conditions required depend on kind of alcohol
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Acid-Catalyzed DehydrationCH3CH2OH
H2SO4 CH2=CH2 H2O+Ethanol Ethylene
180°C
OHH2SO4 H2O140°C
Cyclohexanol Cyclohexene
+
CH3CCH3
OH
CH3H2SO4 CH3C=CH2
CH3
H2O50°C
2-Methyl-2-propanol (tert -Butyl alcohol)
2-Methylpropene (Isobutylene)
+
1o ca 180oC needed
Hard
2o lower temperatures
Moderate
3o slightly above room temperature
Easy
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Dehydration– Zaitsev’s Rule: When two alkene products are
possible, the alkene having the greater number of alkyl groups (i.e., C atoms) on the double bond generally predominates (forms major product)
– Major product favored.
CH3CH2CHCH3
OH H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2
1-Butene (20%)
2-Butene (80%)
2-Butanol
+-H2O
CH3CHCHCH3OH
CH3 H2SO4CH3C=CHCH3
CH3
CH3CHCH=CH2
CH3
3-Methyl-1-butene2-Methyl-2-butene (major product)
3-Methyl-2-butanol
+-H2O
(minor product)
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CH3CHCHCH3OH
CH3 H2SO4CH3C=CHCH3
CH3
CH3CHCH=CH2
CH3
3-Methyl-1-butene2-Methyl-2-butene (major product)
3-Methyl-2-butanol
+-H2O
CH3CH2CHCH3
OH H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2
1-Butene (20%)
2-Butene (80%)
2-Butanol
+-H2O
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Dehydration-Hydration Related• Acid-catalyzed hydration of alkenes and acid-
catalyzed dehydration of alcohols are competing reactions
• Equilibrium between alkene and alcohol exists
– Large amounts of water favor alcohol formation, while removal of water from the equilibrium mixture favors alkene formation (Le Chatelier's principle)
An alkene An alcohol
C CH OH
+ H2Odehydration
hydrationC C
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Dehydration vs Oxidation
• Dehydration: Remove –OH and –H– OH from one C: H from adjacent C
• Oxidation: Remove two –H atoms – One H from OH group: Other H from C that
has the –OH attached
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Oxidation of Alcohols• Oxidation of a 1° alcohol gives an aldehyde or
a carboxylic acid, depending on the experimental conditions– Oxidation of a 1° alcohol to a carboxylic acid is
carried out using an oxidizing agent such as potassium dichromate, K2Cr2O7, in aqueous sulfuric acid
CH3(CH2)6CH2OHK2Cr2O7
CH3(CH2)6CHO K2Cr2O7
H2SO4 H2SO4CH3(CH2)6COH
O
Octanal Octanoic acid1-Octanol
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Oxidation of Alcohols
– Can sometimes stop the oxidation at the aldehyde stage by distilling the mixture (Aldehydes usually have lower boiling points than either the 1° alcohol or the carboxylic acid)
CH3(CH2)6CH2OHK2Cr2O7
CH3(CH2)6CHO K2Cr2O7
H2SO4 H2SO4CH3(CH2)6COH
O
Octanal Octanoic acid1-Octanol
Carboxylic acidAldehydeAlcohol
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Oxidation of Alcohol– Oxidation of a 2° alcohol gives a ketone
– Tertiary alcohols are resistant to oxidation
CH3
OH
K2Cr2O7
H2SO4
1-Methyl-cyclopentanol
(no oxidation)
OH
K2Cr2O7
H2SO4 O
2-Isopropyl-5-methyl-cyclohexanone(Menthone)
2-Isopropyl-5-methyl-cyclohexanol(Menthol)
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Common Alcohols
• Ethanol– Ethyl alcohol, “grain alcohol”– Produced by fermentation– Natural fermentation of sugars and starches
in grains, corn, molasses, fruits, vegetables– Found in beer, wine, and other “spirits”
C6H12O6 → 2 CH3CH2OH + 2 CO2
glucose ethanol carbon dioxide
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Common Alcohols
• Methanol – Methyl alcohol or “wood alcohol”– Distilled from wood pulp originally– Very toxic, even in small amounts– Reacts to make formaldehyde (a fixative)
CH3OH → CH2=O
methanol formaldehyde
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Common Alcohols
• Isopropyl alcohol– 2-propanol “rubbing alcohol”– Evaporates quickly—used to reduce fever– Solvent in many cosmetics, perfumes, etc.
CH3CHCH3
OH
2-propanol
CH3CHCH3
OH
2-propanol
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Common Alcohols
• 1,2-ethanediol– Ethylene glycol – Very soluble in water– Used as antifreeze
• Glycerol– 1,2,3-propanetriol – Very soluble in water– Sweet taste– Part of fats and oils
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Preparation of Important Alcohols
Many alcohols (or their derivatives) can be formed by reacting smaller molecules.
CO
CO2H2 CH3OH
O2 CH2O
CH3COOHCoalor
methane Carbonmonoxide
Methanol
Acetic acid
Formaldehydeoxidation
catalyst
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Preparation of Important Alcohols
H2O, H2SO4
CH3CH=CH2
CH3CHCH3
OH
HOCH2CHCH2OHOHPropene
2-Propanol
Glycerin, glycerol
several steps
CH2=CH2
O2
H2O, H2SO4
OH2C CH2
CH3CH2OH
H2O, H2SO4
H2SO4
HOCH2CH2OH
CH3CH2OCH2CH3
Ethylene
Ethanol
Ethylene glycol
180°C Diethyl ether
catalystEthylene
oxide
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Ethers
• The functional group of an etherether is an oxygen atom bonded to two carbon atoms – Simplest ether is dimethyl ether– Most common ether is diethyl ether
• Often called just “ether”
CH3-O-CH3 CH3CH2-O-CH2CH3
Dimethyl ether Diethyl ether
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Ether Nomenclature• Generally use common names for small (low-
molecular-weight) ethers– common names are derived by listing the alkyl
groups bonded to oxygen in alphabetical order and adding the word "ether”
– alternatively, name one of the groups on oxygen as an alkoxy group
OCH3CH3CH2OCH2CH3
Cyclohexyl methyl ether(Methoxycyclohexane)
Diethyl ether
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Ether Nomenclature• Cyclic ether:Cyclic ether: an ether in which one of the atoms
in a ring is oxygen– Cyclic ethers also known by their common names– ethylene oxide is an important building block for the
organic chemical industry; it is also used as a fumigant in foodstuffs and textiles, and in hospitals to sterilize surgical instruments
– tetrahydrofuran is a useful laboratory and industrial solvent
Ethylene oxide Tetrahydrofuran (THF)
OO
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Physical Properties of Ethers• Ethers are polar compounds
– O has a partial negative charge – Each C bonded to it has a partial positive charge
• Only weak forces of attraction exist between ether molecules in the pure liquid
• Boiling points of ethers are close to those of hydrocarbons of similar molecular weight
• Have lower boiling points than alcohols of the same molecular formula
CH3CH2OH CH3OCH3
bp -24°C
Ethanol
bp 78°C
Dimethyl ether
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Reactions of Ethers• Ethers not very reactive (like hydrocarbons)
– Do not react with oxidizing agents such as potassium dichromate
– Do not react with reducing agents such as H2 in the presence of a transition metal catalyst
– Not affected by most acids or bases at moderate temperatures
• Good solvents for carrying out organic reactions, because of their general inertness and good solvent properties– Diethyl ether and THF (tetrahydrofuran)
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Thiols
• Thiol:Thiol: a compound containing an -SHSH (sulfhydryl)(sulfhydryl) groupgroup– Also called mercaptans– Low-molecular-weight thiols STINK– Responsible for smells such as those from rotten
eggs and sewage– the scent of skunks is due primarily to these two
thiols
CH3CH=CHCH2SH
CH3
CH3CHCH2CH2SH3-Methyl-1-butanethiol2-Butene-1-thiol
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Thiols - Nomenclature• IUPAC names are derived in the same manner
as are the names of alcohols– Retain the final -e-e of the parent alkane and add the
suffix -thiol-thiol – Common names for simple thiols are derived by
naming the alkyl group bonded to -SH and adding the word "mercaptanmercaptan"
CH3CH2SH
CH3
CH3CHCH2SHEthanethiol
(Ethyl mercaptan)2-Methyl-1-propanethiol
(Isobutyl mercaptan)
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Thiol Physical Properties• S-H bonds are nonpolar covalent
– Small difference in electronegativity between sulfur and hydrogen (2.5 - 2.1 = 0.4)
– No hydrogen bonding– Lower boiling points than comparable alcohols– Less soluble in water and other polar solvents than
comparable alcohols
11778
65
1-butanolethanol
methanol
9835
6
1-butanethiolethanethiol
methanethiol
bp (°C)Alcoholbp (°C)Thiol
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Reactions of Thiols
• Thiols are weak acids (pKa~10), and are comparable in strength to phenols– thiols react with strong bases such as NaOH
to form water-soluble thiolate salts
CH3CH2SH NaOHH2O
CH3CH2S-Na+ H2O+Ethanethiol
(pKa 10)
+Sodium
ethanethiolate
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Reactions of Thiols• The most important thiol reaction in biological
systems is their oxidation to disulfides – Functional group is a disulfidedisulfide (-S-S-)(-S-S-) bondbond
• Disulfide bonds important in protein structure
– Thiols are readily oxidized to disulfides by O2
– -SH group very susceptible to oxidation and must be protected from contact with air during storage
– disulfides, in turn, are easily reduced to thiols by several reducing agents.
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Reactions of Thiols
Oxidation of –SH groups to form S-S bondsReduction of S-S bonds to form –SH groups
2HOCH2CH2SH HOCH2CH2S-SCH2CH2OHA disulfide
oxidationreductionA thiol