Download Chapter 7 Notes - Alcohols, Ethers, Thiols Functional groups alcohol

Chapter 7 Notes - Alcohols, Ethers, Thiols
Functional groups
alcohol: C-O-H
ether: C-O-C
thiol: C-S-H
Alcohol Classification
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based on the carbon the OH group is attached to:
1° , 2° , 3°
methyl alcohol
CH3OH
ethyl alcohol
CH3CH2OH (1°)
isopropyl alcohol
(CH3)2CHOH (2°)
t-butyl alcohol
(CH3)3COH (3°)
Alcohol Nomenclature
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OH group takes priority (even over -ene or -yne)
- it must be in the parent chain
- the direction of numbering gives it the lowest possible number
-ol suffix with number designation
name other substituents and multiple bonds as usual
Alcohol Examples
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common names for alcohols:
alkyl alcohol
cyclohexyl alcohol or cyclohexanol
trans-4-methylcyclohexanol
Alcohol Example
(R)-3-methyl-5-hexen-3-ol
Ether Nomenclature
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alkyl alkyl ether (common)
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benzyl methyl ether
alkoxy substituent (IUPAC)
(S)-2-ethoxypentane
Sulfur Functional Groups
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thiols: C-S-H group
(analogous to alcohols)
sulfides: C-S-C group
(analogous to ethers)
disulfides: C-S-S-C group
(analogous to peroxides)
similar to oxygen analogs except:
better nucleophiles
easier to oxidize
Hydrogen Bonding
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sp3 O with two covalent bonds and two lone pairs
O lone pairs attract polar H bonds
covalent O-H bond strength ~ 100 kcal/mole
O...H (H-bond) strength ~ 5 kcal/mole
Effects of H-Bonding
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alcohols have higher boiling points than alkanes (nonpolar)
or alkyl halides (polar, but no H-bonds)
ethers are polar but have no H-bonds
(pentane and diethyl ether both boil at about 35°)
H-bonds hold together the strands of DNA ("velcro" effect)
Acid-Base Reactions
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remember analogy with water
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reactions as bases:
H2O + H+ <==> H3O+
ROH + H+ <==> ROH2+ (an oxonium ion)
reactions as acids:
H2O + B- <==> B-H + OHROH + B- <==> B-H + RO- (an alkoxide ion)
Acidity of Alcohols
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alcohols about as acidic as water
MeOH more acidic, EtOH less acidic
3° alcohols much weaker acids
pKa values: 3° > 2° > 1° > MeOH
18 , 17, 16, 15.5 (compare H2O: pKa = 15.7)
tBuOH + NaOH ---> unfavorable
Alkoxide Anions
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deprotonation of alcohols gives alkoxide anions
CH3OH + NaNH2 ---> NH3 + CH3O- Na+ (sodium methoxide)
most commonly made by direct reaction with active metals
CH3OH + Na ---> 1/2 H2 + CH3O- Na+
(CH3)3COH + K ---> 1/2 H2 + (CH3)3CO-K+
Oxygen Functional Groups
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alcohols are just the first of the many possible oxygen functional groups
oxidation leads to increasing number of bonds to oxygen
alkane --> alcohol --> carbonyl --> carboxyl --> CO2
reduction leads to decreasing number of bonds to oxygen
CO2 --> carboxyl --> carbonyl --> alcohol --> alkane
Synthesis of Alcohols
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hydration of alkenes
follows Markovnikov's Rule
1-hexene --(H+, H2O)--> 2-hexanol
reduction of carbonyl and carboxyl compounds
reducing agents:
sodium borohydride (NaBH4)
lithium aluminum hydride (LiAlH4)
Alcohol Redox Reactions
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reductions to prepare alcohols:
aldehydes or ketones plus NaBH4
carboxylic acids or esters plus LiAlH4
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oxidations of alcohols:
1° alcohol to aldehyde with PCC
2° alcohol to ketone with CrO3
1° alcohol to carboxylic acid with CrO3
Alcohol Redox Examples
Reactions of Alcohols
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acid/base reactions
oxidation reactions
elimination (dehydration)
substitution (C-O bond cleavage)
Substitution Reactions
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substitution by halogens using HX
OH is a poor leaving group
but initial protonation creates a good leaving group (H2O)
halide substitution (SN1 mechanism)
tBuOH + HBr --> tBuOH2+ --> tBu+ --> tBuBr
favored by relatively stable carbocation (3°)
halide substitution (SN2 mechanism)
MeOH + HBr --> MeOH2+ + Br- --> MeBr + H2O
concerted displacement of H2O by Br- (unstable carbocation)
Dehydration (Elimination) Reactions
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same start as for substitution
initial protonation creates a good leaving group (H2O)
carbocation may lose H+ to form an alkene (E1 mechanism)
tBuOH + H2SO4 --> tBuOH2+ --> tBu+ --> (CH3)2C=CH2 + H+
favored by relatively stable carbocation (3°), absence of nucleophile, high temperature
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less-substituted alcohols (unstable carbocations) show
concerted loss of H2O and H+ (E2 mechanism)
The Zaitsev Rule
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predicts regiochemistry of alkene formation
the major product in an elimination reaction is the more substituted alkene
(generally more stable)
dehydration of an alcohol forms the more stable (more substituted) alkene
Ether Reactions
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ethers are generally unreactive
(make good solvents)
react with strong acid
(protonated form can undergo SN1 or SN2 substitution)
Epoxides
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cyclic 3-membered ring ethers
named as 1,2-epoxyalkane
prepared from alkenes with peroxyacids
unlike other ethers, these react easily to undergo ring opening
Epoxide Reactions
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acid-catalyzed hydration
base-catalyzed hydration
product is trans diol
(backside attack as in the reaction of bromonium ions)