Chapter 10 Structure and Synthesis of Alcohols

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Chapter 10Structure and Synthesis of Alcohols
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Jo Blackburn Richland College, Dallas, TX Dallas
County Community College District ã 2003,
Prentice Hall
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Structure of Alcohols

• Hydroxyl (OH) functional group
• Oxygen is sp3 hybridized.

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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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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
2-methylpropanol
Butan-2-ol
2-methylpropan-2-ol
3-bromo-3-methylcyclohexanol
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Unsaturated Alcohols

• Hydroxyl group takes precedence. Assign that
  carbon the lowest number.
• Use alkene or alkyne name.

pent-4-ene-2-ol
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Naming Priority

• Acids
• Esters
• Aldehydes
• Ketones
• Alcohols
• Amines

• 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

4-hydroxybutanoic acid
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Common Names

• Alcohol can be named as alkyl alcohol.
• Useful only for small alkyl groups.
• Examples

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.

hexan-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-,2diol
propylene glycol
ethylene glycol
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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.

4-methylphenol
para-cresol
3-chlorophenol
meta-chlorophenol
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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
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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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Methanol

• 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

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Ethanol

• 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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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
O
H







H
O
H
p
K



1
5
.
7
a
p
K



1
0
a
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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

• 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.

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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
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Grignard and Ester (1)

• Grignard attacks the carbonyl.
• Alkoxide ion leaves! ? !

Ketone intermediate
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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 -CC-H. Grignard reagent is destroyed,
  becomes an alkane.
• No other electrophilic multiple bonds, like CN,
  CN, SO, or NO.

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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 CC bonds.

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Thiols (Mercaptans)

• 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