Alcohols and Phenols
Alcohols and Phenols
Chapter 17 Part 1
Alcohols and Phenols Alcohols contain an OH group connected to a
saturated C (sp3) They are important solvents and synthesis
intermediates Enols also contain an group connected to an
unsaturated C (sp3) Phenols contain an OH group connected to a carbon
in a benzene ring
OH an enol
Alcohols and Phenols Methanol, CH3OH, called methyl alcohol, is a
common solvent, a fuel additive, produced in large quantities Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beverage Phenol, C6H5OH (“phenyl alcohol”) has diverse uses - it gives its name to the general class of compounds
1
Methanol CO + 2 H2
400oC ZnO/Cr2O3
CH3OH
Converted to formaldehyde for the
manufacture of resins and plastics
Also used as a solvent, as an antifreeze, and
as a fuel
Colorless liquid, B.P. = 65°C, and is miscible
with water
Poisonous
Ethanol C6H12O6
yeast
H2C CH2 +
H2O
2 CH3CH2OH + 2 CO2 H2SO4
CH3CH2OH
Colorless liquid, B.P. =78°C, and is miscible
with water Used as a solvent or chemical intermediate.
Isopropyl alcohol OH CH3CHCH3 Colorless liquid, B.P. =78°C, and is miscible
with water Used as a solvent
2
17.1 Naming Alcohols General classifications of alcohols based on
substitution on C to which OH is attached
IUPAC Rules for Naming Alcohols Select the longest carbon chain containing the
hydroxyl group, and derive the parent name by replacing the -e ending of the corresponding alkane with -ol Number the chain from the end nearer the hydroxyl group Number substituents according to position on chain, listing the substituents in alphabetical order
3
Many Alcohols Have Common Names These are accepted by IUPAC
Give the IUPAC names for these compounds: OH OH
Br
H
OH H
HO OH
OH H3C
CH3
Draw the following structures: 2 -
Ethyl- 2 - buten- 1 - ol Cyclohexen - 1 - ol trans - 3 - Chlorocycloheptanol 1,4 - Pentanediol 3 -
4
Naming Phenols Use “phene” (the French name for benzene)
as the parent hydrocarbon name, not benzene Name substituents on aromatic ring by their position from OH
17.2 Properties of Alcohols and Phenols: Hydrogen Bonding The structure around O of the alcohol or phenol is
similar to that in water, sp3 hybridized Alcohols and phenols have much higher boiling
points than similar alkanes and alkyl halides
5
Alcohols Form Hydrogen Bonds A positively polarized OH hydrogen atom
from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another molecule This produces a force that holds the two molecules together These intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution
6
17.3 Properties of Alcohols and Phenols: Acidity and Basicity Weakly basic and weakly acidic Alcohols are weak Brønsted bases Protonated by strong acids to yield oxonium ions,
ROH2+
Alchols and Phenols are Weak Brønsted Acids Can transfer a proton to water to a very small
extent Produces H3O+ and an alkoxide ion, RO−, or
a phenoxide ion, ArO−
Brønsted Acidity Measurements The acidity constant, Ka, measure the extent to which a Brønsted acid transfers a proton to water
Ka =
[A − ][H3O + ] [HA]
and pKa = −log Ka
Relative acidities are more conveniently presented on
a logarithmic scale, pKa, which is directly proportional to the free energy of the equilibrium Differences in pKa correspond to differences in free energy Table 17.1 presents a range of acids and their pKa values
7
Relative Acidities of Alcohols Simple alcohols are about as acidic as water Alkyl groups make an alcohol a weaker acid The more easily the alkoxide ion is solvated
by water the more its formation is energetically favored Steric effects are important
8
Inductive Effects Electron-withdrawing groups make an alcohol a
stronger acid by stabilizing the conjugate base (alkoxide)
Generating Alkoxides from Alcohols Alcohols are weak acids – requires a strong
base to form an alkoxide such as NaH, sodium amide NaNH2, and Grignard reagents (RMgX)
Alkoxides are bases used as reagents in
organic chemistry
CH3OH
Methanol
+ NaH
CH3CH2OH + NaNH2 Ethanol
OH CH3CHCH3 + CH3Li Isopropyl alcohol
+ CH3MgBr
OH Cyclohexanol
CH3 CH3 C OH
+
2K
CH3 tert-Butyl alcohol
9
Phenol Acidity Phenols (pKa ~10) are much more acidic than
alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion
Phenols react with NaOH solutions (but alcohols do
not), forming soluble salts that are soluble in dilute aqueous A phenolic component can be separated from an
organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution
Substituted Phenols Can be more or less acidic than phenol itself An electron-withdrawing substituent makes a phenol
more acidic by delocalizing the negative charge Phenols with an electron-donating substituent are
less acidic because these substituents concentrate the charge
10
Nitro-Phenols Phenols with nitro groups at the ortho and para
positions are much stronger acids The pKa of 2,4,6-trinitrophenol is 0.6, a very strong
acid
Consider: p -
Nitrobenzyl alcohol is more acidic than benzyl alcohol. Explain.
17.4 Preparation of Alchols: an Overview Alcohols are derived from many types of compounds The alcohol hydroxyl can be converted to many other
functional groups
This makes alcohols useful in synthesis
11
Review: Preparation of Alcohols by Regiospecific Hydration of Alkenes Hydroboration/oxidation: syn, non-Markovnikov
hydration
Oxymercuration/reduction: Markovnikov hydration
Preparation of 1,2-Diols Review: Cis 1,2-diols from hydroxylation of an alkene
with OsO4 followed by reduction with NaHSO3 In Chapter 18: Trans-1,2-diols from acid-catalyzed
hydrolysis of epoxides
12
17.5 Alcohols from Reduction of Carbonyl Compounds Reduction of a carbonyl compound in general gives
an alcohol
Note that organic reduction reactions add the
equivalent of H2 to a molecule
Reduction of Aldehydes and Ketones Aldehydes gives primary alcohols Ketones gives secondary alcohols
Catalytic Hydrogenation: O RCH aldehyde
+
H2
Pt, Pd or Ni
RCH2OH primary alcohol
O Pt, Pd or Ni RCR' + H2 ketone
OH RCHR' secondary alcohol
13
Reduction Reagent: Sodium Borohydride NaBH4 is not sensitive to moisture and it does not
reduce other common functional groups
Lithium aluminum hydride (LiAlH4) is more powerful,
less specific, and very reactive with water
Both add the equivalent of “H-”
Mechanism of Reduction The reagent adds the equivalent of hydride to the
carbon of C=O and polarizes the group as well
14
O CH
1. NaBH4, ethanol
?
2. H3O
O2N
m-Nitrobenzaldehyde
O CH3(CH2)5CH Heptanal
1. LiAlH4, ether 2. H3O
O CH3CCH2C(CH3)3
?
1. NaBH4, ethanol
2. H3O 4,4-Dimethyl-2-pentanone
O (C6H5)2CHCCH3
1. LiAlH4, ether
2. H3O 1,1-Diphenyl-2-propanone
?
?
Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are reduced to give
primary alcohols LiAlH4 is used because NaBH4 is not effective
15
1. LiAlH4, ether
COOH
?
2. H3O
cyclopropanecarboxylic acid
Note: the aluminum hydride places two hydrogens on the carbonyl carbon and the acid (or water) is the source of the hydrogen on the hydroxyl group.
O COCH2CH3 1. LiAlH4, ether 2. H3O
?
ethyl benzoate
Note: The reduction of esters yields two alcohols
Which reagent would you use to accomplish each of the following reactions? O
O
?
CH3CCH2CH2COCH3
O
O
CH3CCH2CH2COCH3
OH
O
CH3CCH2CH2COCH3
?
OH CH3CCH2CH2CH2OH
16
What carbonyl compounds give the following alcohols on reduction with LiAlH4? CH2OH
OH CHCH3
OH H
(CH3)2CHCH2OH
Problem: Give the structure of an ester that will yield a
mixture containing equal amounts of - propanol and 2 1 - propanol on the reduction with lithium aluminum hydride.
17
Chapter 17 Part 1
Alcohols and Phenols Alcohols contain an OH group connected to a
saturated C (sp3) They are important solvents and synthesis
intermediates Enols also contain an group connected to an
unsaturated C (sp3) Phenols contain an OH group connected to a carbon
in a benzene ring
OH an enol
Alcohols and Phenols Methanol, CH3OH, called methyl alcohol, is a
common solvent, a fuel additive, produced in large quantities Ethanol, CH3CH2OH, called ethyl alcohol, is a solvent, fuel, beverage Phenol, C6H5OH (“phenyl alcohol”) has diverse uses - it gives its name to the general class of compounds
1
Methanol CO + 2 H2
400oC ZnO/Cr2O3
CH3OH
Converted to formaldehyde for the
manufacture of resins and plastics
Also used as a solvent, as an antifreeze, and
as a fuel
Colorless liquid, B.P. = 65°C, and is miscible
with water
Poisonous
Ethanol C6H12O6
yeast
H2C CH2 +
H2O
2 CH3CH2OH + 2 CO2 H2SO4
CH3CH2OH
Colorless liquid, B.P. =78°C, and is miscible
with water Used as a solvent or chemical intermediate.
Isopropyl alcohol OH CH3CHCH3 Colorless liquid, B.P. =78°C, and is miscible
with water Used as a solvent
2
17.1 Naming Alcohols General classifications of alcohols based on
substitution on C to which OH is attached
IUPAC Rules for Naming Alcohols Select the longest carbon chain containing the
hydroxyl group, and derive the parent name by replacing the -e ending of the corresponding alkane with -ol Number the chain from the end nearer the hydroxyl group Number substituents according to position on chain, listing the substituents in alphabetical order
3
Many Alcohols Have Common Names These are accepted by IUPAC
Give the IUPAC names for these compounds: OH OH
Br
H
OH H
HO OH
OH H3C
CH3
Draw the following structures: 2 -
Ethyl- 2 - buten- 1 - ol Cyclohexen - 1 - ol trans - 3 - Chlorocycloheptanol 1,4 - Pentanediol 3 -
4
Naming Phenols Use “phene” (the French name for benzene)
as the parent hydrocarbon name, not benzene Name substituents on aromatic ring by their position from OH
17.2 Properties of Alcohols and Phenols: Hydrogen Bonding The structure around O of the alcohol or phenol is
similar to that in water, sp3 hybridized Alcohols and phenols have much higher boiling
points than similar alkanes and alkyl halides
5
Alcohols Form Hydrogen Bonds A positively polarized OH hydrogen atom
from one molecule is attracted to a lone pair of electrons on a negatively polarized oxygen atom of another molecule This produces a force that holds the two molecules together These intermolecular attractions are present in solution but not in the gas phase, thus elevating the boiling point of the solution
6
17.3 Properties of Alcohols and Phenols: Acidity and Basicity Weakly basic and weakly acidic Alcohols are weak Brønsted bases Protonated by strong acids to yield oxonium ions,
ROH2+
Alchols and Phenols are Weak Brønsted Acids Can transfer a proton to water to a very small
extent Produces H3O+ and an alkoxide ion, RO−, or
a phenoxide ion, ArO−
Brønsted Acidity Measurements The acidity constant, Ka, measure the extent to which a Brønsted acid transfers a proton to water
Ka =
[A − ][H3O + ] [HA]
and pKa = −log Ka
Relative acidities are more conveniently presented on
a logarithmic scale, pKa, which is directly proportional to the free energy of the equilibrium Differences in pKa correspond to differences in free energy Table 17.1 presents a range of acids and their pKa values
7
Relative Acidities of Alcohols Simple alcohols are about as acidic as water Alkyl groups make an alcohol a weaker acid The more easily the alkoxide ion is solvated
by water the more its formation is energetically favored Steric effects are important
8
Inductive Effects Electron-withdrawing groups make an alcohol a
stronger acid by stabilizing the conjugate base (alkoxide)
Generating Alkoxides from Alcohols Alcohols are weak acids – requires a strong
base to form an alkoxide such as NaH, sodium amide NaNH2, and Grignard reagents (RMgX)
Alkoxides are bases used as reagents in
organic chemistry
CH3OH
Methanol
+ NaH
CH3CH2OH + NaNH2 Ethanol
OH CH3CHCH3 + CH3Li Isopropyl alcohol
+ CH3MgBr
OH Cyclohexanol
CH3 CH3 C OH
+
2K
CH3 tert-Butyl alcohol
9
Phenol Acidity Phenols (pKa ~10) are much more acidic than
alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion
Phenols react with NaOH solutions (but alcohols do
not), forming soluble salts that are soluble in dilute aqueous A phenolic component can be separated from an
organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution
Substituted Phenols Can be more or less acidic than phenol itself An electron-withdrawing substituent makes a phenol
more acidic by delocalizing the negative charge Phenols with an electron-donating substituent are
less acidic because these substituents concentrate the charge
10
Nitro-Phenols Phenols with nitro groups at the ortho and para
positions are much stronger acids The pKa of 2,4,6-trinitrophenol is 0.6, a very strong
acid
Consider: p -
Nitrobenzyl alcohol is more acidic than benzyl alcohol. Explain.
17.4 Preparation of Alchols: an Overview Alcohols are derived from many types of compounds The alcohol hydroxyl can be converted to many other
functional groups
This makes alcohols useful in synthesis
11
Review: Preparation of Alcohols by Regiospecific Hydration of Alkenes Hydroboration/oxidation: syn, non-Markovnikov
hydration
Oxymercuration/reduction: Markovnikov hydration
Preparation of 1,2-Diols Review: Cis 1,2-diols from hydroxylation of an alkene
with OsO4 followed by reduction with NaHSO3 In Chapter 18: Trans-1,2-diols from acid-catalyzed
hydrolysis of epoxides
12
17.5 Alcohols from Reduction of Carbonyl Compounds Reduction of a carbonyl compound in general gives
an alcohol
Note that organic reduction reactions add the
equivalent of H2 to a molecule
Reduction of Aldehydes and Ketones Aldehydes gives primary alcohols Ketones gives secondary alcohols
Catalytic Hydrogenation: O RCH aldehyde
+
H2
Pt, Pd or Ni
RCH2OH primary alcohol
O Pt, Pd or Ni RCR' + H2 ketone
OH RCHR' secondary alcohol
13
Reduction Reagent: Sodium Borohydride NaBH4 is not sensitive to moisture and it does not
reduce other common functional groups
Lithium aluminum hydride (LiAlH4) is more powerful,
less specific, and very reactive with water
Both add the equivalent of “H-”
Mechanism of Reduction The reagent adds the equivalent of hydride to the
carbon of C=O and polarizes the group as well
14
O CH
1. NaBH4, ethanol
?
2. H3O
O2N
m-Nitrobenzaldehyde
O CH3(CH2)5CH Heptanal
1. LiAlH4, ether 2. H3O
O CH3CCH2C(CH3)3
?
1. NaBH4, ethanol
2. H3O 4,4-Dimethyl-2-pentanone
O (C6H5)2CHCCH3
1. LiAlH4, ether
2. H3O 1,1-Diphenyl-2-propanone
?
?
Reduction of Carboxylic Acids and Esters Carboxylic acids and esters are reduced to give
primary alcohols LiAlH4 is used because NaBH4 is not effective
15
1. LiAlH4, ether
COOH
?
2. H3O
cyclopropanecarboxylic acid
Note: the aluminum hydride places two hydrogens on the carbonyl carbon and the acid (or water) is the source of the hydrogen on the hydroxyl group.
O COCH2CH3 1. LiAlH4, ether 2. H3O
?
ethyl benzoate
Note: The reduction of esters yields two alcohols
Which reagent would you use to accomplish each of the following reactions? O
O
?
CH3CCH2CH2COCH3
O
O
CH3CCH2CH2COCH3
OH
O
CH3CCH2CH2COCH3
?
OH CH3CCH2CH2CH2OH
16
What carbonyl compounds give the following alcohols on reduction with LiAlH4? CH2OH
OH CHCH3
OH H
(CH3)2CHCH2OH
Problem: Give the structure of an ester that will yield a
mixture containing equal amounts of - propanol and 2 1 - propanol on the reduction with lithium aluminum hydride.
17
