Transcript Alcohols

Alcohols
Structure of Water and Methanol
• Oxygen is sp3 hybridized and tetrahedral.
• The H—O—H angle in water is 104.5°.
• The C—O—H angle in methyl alcohol is 108.9°.
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Classification of Alcohols
• Primary: carbon with —OH is bonded to one
other carbon.
• Secondary: carbon with —OH is bonded to
two other carbons.
• Tertiary: carbon with —OH is bonded to
three other carbons.
• Aromatic (phenol): —OH is bonded to a
benzene ring.
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Examples of Classifications
OH
C H3
C H3
CH
C H 2OH
*
Primary alcohol
C H3
CH
*
C H 2C H 3
Secondary alcohol
C H3
C H3
*
C
OH
C H3
Tertiary alcohol
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IUPAC Nomenclature
• Find the longest carbon chain containing the
carbon with the —OH group.
• Drop the -e from the alkane name, add -ol.
• Number the chain giving the —OH group the
lowest number possible.
• Number and name all substituents and write
them in alphabetical order.
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Examples of Nomenclature
OH
C H3
C H3
3
CH
2
1
C H 2OH
2-methyl-1-propanol
2-methylpropan-1-ol
C H3
2 1
C H3 C
OH
C H3
1
CH
2
C H 2C H 3
3 4
2-butanol
butan-2-ol
2-methyl-2-propanol
2-methylpropan-2-ol
C H3
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Alkenols (Enols)
• Hydroxyl group takes precedence. Assign the carbon
with the —OH the lowest number.
• End the name in –ol, but also specify that there is a
double bond by using the ending –ene before -ol
OH
C H2
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C HC H 2C HC H 3
4 3
2
1
4-penten-2-ol
pent-4-ene-2-ol
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Naming Priority
Highest ranking
Lowest ranking
1. Acids
2. Esters
3. Aldehydes
4. Ketones
5. Alcohols
6. Amines
7. Alkenes
8. Alkynes
9. Alkanes
10. Ethers
11. Halides
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Hydroxy Substituent
• When —OH is part of a higher priority class of
compound, it is named as hydroxy.
carboxylic acid
OH
C H 2C H 2C H 2C OOH
4
3
2
1
4-hydroxybutanoic acid
also known as g-hydroxybutyric acid (GHB)
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Common Names
• Alcohol can be named as alkyl alcohol.
• Useful only for small alkyl groups.
OH
C H3
C H3
CH
C H 2OH
isobutyl alcohol
C H3
CH
C H 2C H 3
sec-butyl alcohol
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Naming Diols
• Two numbers are needed to locate the two
—OH groups.
• Use -diol as suffix instead of -ol.
1
2
3
4
5
6
hexane-1,6- diol
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Glycols
• 1, 2-diols (vicinal diols) are called glycols.
• Common names for glycols use the name of the alkene
from which they were made.
ethane-1,2- diol
ethylene glycol
propane-1,2- diol
propylene glycol
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Phenol Nomenclature
• —OH group is assumed to be on carbon 1.
• For common names of disubstituted phenols, use
ortho- for 1,2; meta- for 1,3; and para- for 1,4.
• Methyl phenols are cresols.
OH
OH
H3C
Cl
3-chlorophenol
(meta-chlorophenol)
4-methylphenol
(para-cresol)
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Solved Problem 1
Give the systematic (IUPAC) name for the following alcohol.
Solution
The longest chain contains six carbon atoms, but it does not contain the carbon bonded to the hydroxyl
group. The longest chain containing the carbon bonded to the —OH group is the one outlined by the
green box, containing five carbon atoms. This chain is numbered from right to left in order to give the
hydroxyl-bearing carbon atom the lowest possible number.
The correct name for this compound is 3-(iodomethyl)-2-isopropylpentan-1-ol.
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Physical Properties
• Alcohols have high boiling points due to
hydrogen bonding between molecules.
• Small alcohols are miscible in water, but
solubility decreases as the size of the alkyl
group increases.
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Boiling Points of alcohols
• Alcohols have higher boiling points than ethers and
alkanes because alcohols can form hydrogen bonds.
• The stronger interaction between alcohol molecules will
require more energy to break them resulting in a higher
boiling point.
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Solubility in Water
Small alcohols are miscible in water, but solubility
decreases as the size of the alkyl group increases.
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Methanol
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“Wood alcohol”
Industrial production from synthesis gas
Common industrial solvent
Toxic Dose: 100 mL methanol
Used as fuel at Indianapolis 500
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–
–
–
–
Fire can be extinguished with water
High octane rating
Low emissions
Lower energy content
Invisible flame
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Ethanol
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Fermentation of sugar and starches in grains
12–15% alcohol, then yeast cells die
Distillation produces “hard” liquors
Azeotrope: 95% ethanol, constant boiling
Denatured alcohol used as solvent
Gasahol: 10% ethanol in gasoline
Toxic dose: 200 mL
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Formation of Alkoxide Ions
• Ethanol reacts with sodium metal to form sodium
ethoxide (NaOCH2CH3), a strong base commonly used for
elimination reactions.
• More hindered alcohols like 2-propanol or tert-butanol
react faster with potassium than with sodium.
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Formation of Phenoxide Ion
The aromatic alcohol phenol is more acidic than aliphatic
alcohols due to the ability of aromatic rings to delocalize
the negative charge of the oxygen within the carbons of
the ring.
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Synthesis of Alcohols (Review)
• Alcohols can be synthesized by nucleophilic
substitution of alkyl halide.
• Hydration of alkenes also produce alcohols:
– Water in acid solution (suffers from
rearragements)
– Oxymercuration–demercuration
– Hydroboration–oxidation
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Synthesis of Alcohols (Review)
• Alcohols can be synthesized by nucleophilic
substitution of alkyl halide.
• Hydration of alkenes also produce alcohols:
Chapter 10
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Reduction of Carbonyl
• Reduction of aldehyde yields 1º alcohol.
• Reduction of ketone yields 2º alcohol.
• Reagents:
– Sodium borohydride, NaBH4
– Lithium aluminum hydride, LiAlH4
– Raney nickel
Chapter 10
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Sodium Borohydride
• NaBH4 is a source of hydrides (H-)
• Hydride attacks the carbonyl carbon,
forming an alkoxide ion.
• Then the alkoxide ion is protonated by
dilute acid.
• Only reacts with carbonyl of aldehyde or
ketone, not with carbonyls of esters or
carboxylic acids.
Chapter 10
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Lithium Aluminum Hydride
• LiAlH4 is source of hydrides (H-)
• Stronger reducing agent than sodium
borohydride, but dangerous to work with.
• Reduces ketones and aldehydes into the
corresponding alcohol.
• Converts esters and carboxylic acids to 1º
alcohols.
Chapter 10
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Reducing Agents
• NaBH4 can reduce
aldehydes and ketones
but not esters and
carboxylic acids.
• LiAlH4 is a stronger
reducing agent and will
reduce all carbonyls.
Chapter 10
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Catalytic Hydrogenation
• Raney nickel is a hydrogen rich nickel powder that is more
reactive than Pd or Pt catalysts.
• This reaction is not commonly used because it will also
reduce double and triple bonds that may be present in the
molecule.
• Hydride reagents are more selective so they are used
more frequently for carbonyl reductions.
Chapter 10
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Alcohol Reactions
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Dehydration to alkene
Oxidation to aldehyde, ketone
Substitution to form alkyl halide
Reduction to alkane
Esterification
Williamson synthesis of ether
Chapter 11
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Summary Table
Chapter 11
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Oxidation States
• Easy for inorganic salts (reduced, organic oxidized)
– CrO42- reduced to Cr2O3
– KMnO4 reduced to MnO2
• Oxidation: loss of H2, gain of O, O2, or X2
• Reduction: gain of H2 or H-, loss of O, O2, or X2
• Neither: gain or loss of H+, H2O, HX
Chapter 11
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1º, 2º, 3º Carbons
Chapter 11
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Oxidation of 2° Alcohols
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2° alcohol becomes a ketone
Reagent is Na2Cr2O7/H2SO4 = H2CrO4
Active reagent probably H2CrO4
Color change: orange to greenish-blue
OH
CH3CHCH2CH3
Na2Cr2O7 / H2SO4
O
CH3CCH2CH3
=>
Chapter 11
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Oxidation of 1° Alcohols
• 1° alcohol to aldehyde to carboxylic acid
• Difficult to stop at aldehyde
• Use pyridinium chlorochromate (PCC) to
limit the oxidation.
• PCC can also be used to oxidize 2° alcohols
to ketones.
OH
N H CrO3Cl
CH3CH2CH2CH2
O
CH3CH2CH2CH
Chapter 11
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3° Alcohols Don’t Oxidize
• Cannot lose 2 H’s
• Basis for chromic acid test
Chapter 11
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Reduction of Alcohols
• Dehydrate with conc. H2SO4, then add H2
OH
CH3CHCH3
alcohol
H2SO4
CH2
CHCH3
alkene
Chapter 11
H2
Pt
CH3CH2CH3
alkane
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Reaction with HBr
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-OH of alcohol is protonated
-OH2+ is good leaving group
3° and 2° alcohols react with Br- via SN1
1° alcohols react via SN2
R O H
H3O
+
H
R O H
Chapter 11
-
Br
R Br
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Reaction with HCl
• Chloride is a weaker nucleophile than
bromide.
• Add ZnCl2, which bonds strongly with
-OH, to promote the reaction.
• The chloride product is insoluble.
• Lucas test: ZnCl2 in conc. HCl
– 1° alcohols react slowly or not at all.
– 2 alcohols react in 1-5 minutes.
– 3 alcohols react in less than 1 minute.
Chapter 11
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Dehydration Reactions
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Conc. H2SO4 (or H3PO4) produces alkene
Carbocation intermediate
Zaitsev product
Bimolecular dehydration produces ether
Low temp, 140°C and below, favors ether
High temp, 180°C and above, favors alkene
Chapter 11
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Esterification
• Fischer: alcohol + carboxylic acid
• Nitrate esters
• Phosphate esters
Chapter 11
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Fischer Esterification
• Acid + Alcohol yields Ester + Water
• Sulfuric acid is a catalyst.
• Each step is reversible.
O
CH3
C OH
CH3
+ H O CH2CH2CHCH3
+
H
O
CH3
CH3C OCH2CH2CHCH3
+ HOH
Chapter 11
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End Alcohols