Download Alcohols

Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 10
Structure and Synthesis
of Alcohols
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2006, Prentice Hall
Structure of Alcohols
• Hydroxyl (-OH) functional group
• Oxygen is sp3 hybridized.
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Chapter 10
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Classification
• 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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Chapter 10
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Classify these:
HOH
CH3
R
C
OH
R
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Chapter 10
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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, starting from the end
closest to the -OH group.
• Number and name all substituents. =>
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Name these:
HOH
CH3
R
C
OH
R
2-methyl-1-propanol
2-methylpropan-1-ol
2-methyl-2-propanol
2-methylpropan-2-ol
2-butanol
butan-2-ol
3-bromo-3-methylcyclohexanol
3-bromo-3-methylcyclohexan-1-ol
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Unsaturated Alcohols
• Hydroxyl group takes precedence. Assign
that carbon the lowest number.
• Use alkene or alkyne name.
4-penten-2-ol
pent-4-ene-2-ol
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Naming Priority
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Acids
Esters
Aldehydes
Ketones
Alcohols
Amines
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•
•
•
Alkenes
Alkynes
Alkanes
Ethers
Halides
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Hydroxy Substituent
• When -OH is part of a higher priority class of
compound, it is named as hydroxy.
• Example:
also known as GHB
4-hydroxybutanoic acid
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Common Names
• Alcohol can be named as alkyl alcohol.
• Useful only for small alkyl groups.
• Examples:
HOH
CH3
R
C
OH
R
isobutyl alcohol
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.
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
propane-1,2- diol
ethylene glycol
propylene glycol
Chapter 10
=> 12
Naming Phenols
• -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.
3-chlorophenol
meta-chlorophenol
4-methylphenol
para-cresol
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Physical Properties
• Unusually 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
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Solubility in Water
Solubility decreases as the size
of the alkyl group increases.
Chapter 10
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16
Methanol
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•
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•
“Wood alcohol”
Industrial production from synthesis gas
Common industrial solvent
Fuel at Indianapolis 500
Fire can be extinguished with water
High octane rating
Low emissions
But, lower energy content
Invisible flame
Chapter 10
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17
Ethanol
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•
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•
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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 ethanol, 100 mL methanol
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2-Propanol
• “Rubbing alcohol”
• Catalytic hydration of propene
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Acidity of Alcohols
• pKa range: 15.5-18.0 (water: 15.7)
• Acidity decreases as alkyl group
increases.
• Halogens increase the acidity.
• Phenol is 100 million times more acidic
than cyclohexanol!
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Table of Ka Values
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Formation of Alkoxide
Ions
React methanol and ethanol with sodium
metal (redox reaction).
React less acidic alcohols with more
reactive potassium.
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Formation of
Phenoxide Ion
Phenol reacts with hydroxide ions to form
phenoxide ions - no redox is necessary.
O
O H
+
OH
+
HOH
pKa = 15.7
pKa = 10.0
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Synthesis (Review)
• Nucleophilic substitution of OH- on alkyl
halide
• Hydration of alkenes
water in acid solution (not very effective)
oxymercuration - demercuration
hydroboration - oxidation
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Glycols (Review)
• Syn hydroxylation of alkenes
osmium tetroxide, hydrogen peroxide
cold, dilute, basic potassium
permanganate
• Anti hydroxylation of alkenes
peroxyacids, hydrolysis
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Organometallic
Reagents
• Carbon is bonded to a metal (Mg or Li).
• Carbon is nucleophilic (partially
negative).
• It will attack a partially positive carbon.
C - X
C = O
• A new carbon-carbon bond forms.
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Grignard Reagents
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•
•
•
Formula R-Mg-X (reacts like R:- +MgX)
Stabilized by anhydrous ether
Iodides most reactive
May be formed from any halide
primary
secondary
tertiary
vinyl
aryl
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Some Grignard
Reagents
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Organolithium Reagents
• Formula R-Li (reacts like R:- +Li)
• Can be produced from alkyl, vinyl, or
aryl halides, just like Grignard reagents.
• Ether not necessary, wide variety of
solvents can be used.
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Reaction with Carbonyl
• R:- attacks the partially positive carbon in the
carbonyl.
• The intermediate is an alkoxide ion.
• Addition of water or dilute acid protonates the
alkoxide to produce an alcohol.
Chapter 10
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30
Synthesis of 1° Alcohols
Grignard + formaldehyde yields a primary
alcohol with one additional carbon.
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Synthesis of 2º Alcohols
Grignard + aldehyde yields a secondary
alcohol.
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Synthesis of 3º Alcohols
Grignard + ketone yields a tertiary alcohol.
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How would you
synthesize…
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Grignard Reactions with
Acid Chlorides and Esters
• Use two moles of Grignard reagent.
• The product is a tertiary alcohol with
two identical alkyl groups.
• Reaction with one mole of Grignard
reagent produces a ketone
intermediate, which reacts with the
second mole of Grignard reagent.
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Grignard + Acid
Chloride (1)
• Grignard attacks the carbonyl.
• Chloride ion leaves.
Ketone intermediate
Chapter 10
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36
Grignard and Ester (1)
• Grignard attacks the carbonyl.
• Alkoxide ion leaves! ? !
Ketone intermediate
Chapter 10
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37
Second step of reaction
• Second mole of Grignard reacts with the
ketone intermediate to form an alkoxide ion.
• Alkoxide ion is protonated with dilute acid.
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How would you
synthesize...
Using an acid chloride or ester.
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Grignard Reagent +
Ethylene Oxide
• Epoxides are unusually reactive ethers.
• Product is a 1º alcohol with 2 additional
carbons.
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Limitations of Grignard
• No water or other acidic protons like
O-H, N-H, S-H, or -C—C-H. Grignard
reagent is destroyed, becomes an
alkane.
• No other electrophilic multiple bonds,
like C=N, CN, S=O, or N=O.
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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
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Sodium Borohydride
• Hydride ion, H-, 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.
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Lithium Aluminum Hydride
• Stronger reducing agent than sodium
borohydride, but dangerous to work with.
• Converts esters and acids to 1º alcohols.
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Comparison of
Reducing Agents
• LiAlH4 is stronger.
• LiAlH4 reduces more
stable compounds
which are resistant
to reduction.
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Catalytic Hydrogenation
• Add H2 with Raney nickel catalyst.
• Also reduces any C=C bonds.
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Thiols (Mercaptans)
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Sulfur analogues of alcohols, -SH.
Named by adding -thiol to alkane name.
The -SH group is called mercapto.
Complex with heavy metals: Hg, As, Au.
More acidic than alcohols, react with
NaOH to form thiolate ion.
• Stinks!
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Thiol Synthesis
Use a large excess of sodium
hydrosulfide with unhindered alkyl
halide to prevent dialkylation to R-S-R.
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Thiol Oxidation
• Easily oxidized to disulfides, an
important feature of protein structure.
Vigorous oxidation with KMnO4,
HNO3, or NaOCl produces sulfonic acids.
•
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End of Chapter 10
Chapter 10
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