Chemistry 1103 Charlie Bond MCS Rm 4.16/4.27
[email protected] What is Organic Chemistry? Organic Reactions I II Alkanes (Ch 21) Conformational Analysis (Ch 21) Stereochemistry I II III (Ch 22) Alkyl Halides I II (Ch 24) Alcohols and Ether I II (Ch 24)
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 -e to -ol
• Common names – name the alkyl group bonded to oxygen followed by the word alcohol 2
Alcohols - Nomenclature OH OH Ethanol (Ethyl alcohol)
OH 1Propanol (Propyl alcohol)
2Propanol (Isopropyl alcohol)
OH OH 1Butanol (Butyl alcohol)
OH 2Butanol (secButyl alcohol) OH
2Methyl1propanol (Isobutyl alcohol) OH
Cyclohexanol 2Methyl2propanol (tertButyl alcohol) (Cyclohexyl alcohol)
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Alcohols - Nomenclature • Compounds containing – two -OH groups are named as diols, – three -OH groups are named as triols, etc.
CH2 CH2 OH OH 1,2Ethanediol (Ethylene glycol)
CH3 CHCH2 HO OH 1,2Propanediol (Propylene glycol)
CH2CHCH2 HO HO OH 1,2,3Propanetriol (Glycerol, Glycerin) 4
Polyols and sugars Larger alcohol-containing molecules are of massive importance in biochemistry as they are used by cells as fuel, starting materials for making essential molecules (e.g. ribose in DNA), and to store information.
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Ignose and Godnose Albert Szent-Gyorgyi isolated ascorbic acid and wanted to call the compound Ignose in his publication (as it resembles the sugars glucose and fructose). The journal editor refused and so he tried to call it Godnose instead.
Eventually they settled on hexuronic acid, until Charles King discovered that it was the same compound as vitamin C isolated from lemons.
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Physical Properties • Alcohols are polar compounds – both the C-O and O-H bonds are polar covalent H H H
δ+ δC O
δ+ H
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Hydrogen Bonding – Association of ethanol molecules in the liquid state (only two of the three possible hydrogen bonds to the upper oxygen are shown here).
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Boiling Points
– alcohols have higher boiling points and are more soluble in water than hydrocarbons Molecular Boiling Weight Point Solubility (g/mol) (°C) in Water
Structural Formula
Name
CH3OH CH3CH3
methanol ethane
32 30
65 89
infinite insoluble
CH3CH2 OH CH3CH2 CH3
ethanol propane
46 44
78 42
infinite insoluble
CH3CH2 CH2OH CH3CH2 CH2CH3
1propanol butane
60 58
97 0
infinite insoluble
CH3CH2 CH2CH2CH2OH 1pentanol
88
2.3 g/100 g
HOCH2CH2CH2CH2 OH 1,4butanediol CH3CH2 CH2CH2CH2CH3 hexane
90 86
138 230 69
infinite insoluble
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Acidity of Alcohols • Most alcohols are about the same or slightly weaker acids than water CH3 O H +
O H H
+ CH3 O + H O H H
[CH3 O- ][H3 O+] K a = = 3.2 x 1016 [CH3 OH] pK a = 15.5
– aqueous solutions of alcohols have the same pH as that of pure water 10
Acidity of Alcohols – pKa values for several low-molecular-weight alcohols Compound
Structural Formula
hydrogen chloride acetic acid
HCl CH3 COOH
7 Stronger acid 4.8
CH3 OH
15.5
H2 O
15.7
CH3 CH2 OH
15.9
methanol water ethanol 2propanol 2methyl2propanol
pKa
(CH3 )2 CHOH 17 (CH3 )3 COH
18
Weaker acid
*Also given for comparison are pKa values for water, acetic acid, and hydrogen chloride.
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Acidity of Phenols • Phenols are significantly more acidic than alcohols OH + H2 O Phenol CH3 CH2 OH + H2 O Ethanol
-
O + H3 O+
pKa = 9.95
Phenoxide ion +
CH3 CH2 O- + H3 O Ethoxide ion
pKa = 15.9
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Acidity of Phenols – the greater acidity of phenols compared with alcohols is the result of the greater stability of the phenoxide ion relative to an alkoxide ion O:
O:
:
: O:
:
:
:
:
:O:
O:
:
: :
These three Kekulé structures are equivalent
These three contributing structures delocalize the negative charge onto the carbons of the ring 13
Rule of Thumb for OH pKa • • • • •
Compound Alkylsulfonates Carboxylic acids Phenols Alcohols
pKa 0 5 10 15
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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 H2SO4 + + + H O H CH3 CH2 -O H + O H CH3 CH2 -O-H •• H H H Ethanol Hydronium ion Ethyloxonium ion (pKa 1.7) (pKa 2.4)
– thus, alcohols can function as both weak acids and weak bases 15
Reaction with Active Metals • Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides – Na is oxidized to Na+ and H+ is reduced to H2 2CH3 CH2 OH + 2Na
2CH3 CH2 O-Na+ + H2 Sodium ethoxide
– alkoxides are somewhat stronger bases that OH– alkoxides can be used as nucleophiles in nucleophilic substitution reactions – they can also be used as bases in β -elimination reactions 16
Conversion of ROH to RX • Conversion of an alcohol to an alkyl halide involves substitution of halogen for -OH at a saturated carbon – the most common reagents for this purpose are the halogen acids, HX, and thionyl chloride, SOCl2
Is -OH a good leaving group? 17
Conversion of ROH to RX – water-soluble 3° alcohols react very rapidly with HCl, HBr, and HI CH3 25°C CH3COH + HCl CH3 2Methyl2 propanol
CH3 CH3 CCl + H2 O
CH3 2Chloro2 methylpropane
– low-molecular-weight 1° and 2° alcohols are unreactive under these conditions 18
Conversion of ROH to RX – water-insoluble 3° alcohols react by bubbling gaseous HCl through a solution of the alcohol dissolved in diethyl ether or THF OH CH3 1Methyl cyclohexanol
+
HCl
0°C ether
Cl
+ H2O
CH3 1Chloro1methyl cyclohexane
– 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides OH + HBr Br + H2 O 1Butanol
1Bromobutane (Butyl bromide) 19
Reaction of a 3° ROH with HX • 3° Alcohols react with HX by an SN1 mechanism – Step 1: a rapid, reversible acid-base reaction transfers a proton to the OH group CH3
CH3 H + + CH3-C O CH H 3
••
O H
• •
•• ••+ CH3-C O H + H O H •• H CH3 2Methyl2propanol (tertButyl alcohol)
rapid and reversible
H
An oxonium ion
– this proton-transfer converts the leaving group from OH-, a poor leaving group, to H2O, a better leaving group 20
Reaction of a 3° ROH with HX – Step 2: loss of H2O from the oxonium ion gives a 3° carbocation intermediate CH3 H + CH3-C O CH H
slow, rate determining SN 1
3
An oxonium ion
CH3 +
CH3 -C
+
CH3 A 3° carbocation intermediate
O H H
– Step 3: reaction with halide ion completes the reaction CH3 CH3-C+ + CH3
Cl
fast
CH3 CH3-C Cl
CH3 2Chloro2methylpropane (tertButyl chloride)
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Reaction of a 1° ROH with HX • 1° alcohols react by an SN2 mechanism – Step 1: proton transfer to OH converts the leaving group from OH-, a poor leaving group, to H2O a better leaving group rapid and reversible + OH + H O H
O
H
+ O H
H An oxonium ion
H
H
of H2O by Br– Step 2: nucleophilic displacement slow, rate Br
-
+
O H
H determining SN2
Br
+ O
H H
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Reaction of ROH with HX • Same as for alkyl halides: Reactions are governed by a combination of electronic and steric effects never react by SN2
governed by steric factors Increasing rate of displacement of H2 O
SN2
3° alcohol 2° alcohol 1° alcohol SN 1
Increasing rate of carbocation formation
never react by SN1
governed by electronic factors 23
From MasteringChem • What is wrong with this mechanism
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Dehydration of Alcohols • An alcohol can be converted to an alkene by elimination of H and OH from adjacent carbons (a β -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 only at or slightly above room temperature 25
Dehydration of Alcohols CH3 CH2 OH OH
H2 SO4 o
180 C
CH2 =CH2 + H 2 O
H2 SO4
+ H2 O
140oC Cyclohexanol
CH3 CH3 COH CH3
Cyclohexene
H2 SO4 50oC
CH3 CH3 C=CH2 + 2Methylpropene (Isobutylene)
H2 O
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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 (Zaitsev’s rule) rule OH 85% H3PO4 CH3CH2CHCH3 CH3CH=CHCH3 + CH3CH2 CH=CH2 heat 2Butanol 2Butene 1Butene (80%) (20%) 27
Dehydration of a 2° Alcohol • A three-step mechanism – Step 1: proton transfer from H3O+ to the -OH group converts OH-, a poor leaving group, into H2O, a better leaving group rapid and HO + reversible CH3 CHCH2 CH3 + H O H H
H
+H O
CH3 CHCH2 CH3 + O H An oxonium ion H
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Dehydration of a 2° Alcohol – Step 2: loss of H2O gives a carbocation intermediate + H
O
H
CH3CHCH2 CH3
slow, rate determining
+ CH3CHCH2CH3 + H2O A 2° carbocation intermediate
– Step 3: proton transfer from an adjacent carbon to H2O gives the alkene and regenerates the acid catalyst H
+ rapid CH3 -CH-CH-CH3 + O H H E1
+ CH3 -CH=CH-CH3 + H O H H
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Dehydration of a 1° Alcohol • A two-step mechanism – Step 1: proton transfer gives an oxonium ion CH3 CH2 -O-H + H O H H
rapid and reversible
H CH3 CH2 O
+ O H H
H
– Step 2: proton transfer to solvent and loss of H2O gives the alkene and regenerates the acid catalyst H
H slow, rate H O + H C CH2 O determining E2 H H H
H H + HO H + C C + O H H H H H
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Hydration-Dehydration • Acid-catalyzed hydration of an alkene and dehydration of an alcohol are competing processes C C
+ H2O
acid catalyst
An alkene
C
C
H OH An alcohol
– large amounts of water favor alcohol formation – scarcity of water or experimental conditions where water is removed favor alkene formation – Le Chatelier’s Principle 31
