Structures of Alcohols, Phenols, Thiols and Ethers

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Structures of Alcohols, Phenols, Thiols and Ethers

• Alcohols, phenols, thiols and ethers consist of a
  hydrocarbon singly bonded to an oxygen or a
  sulfur
• Alcohols have an -OH group attached to an alkane,
  phenols have an -OH group attached to a benzene,
  thiols have an -SH group attached to an alkane
  and ethers have an O bonded to two Cs

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Naming Alcohols

• Parent name ends in -ol
• Find longest chain containing the C to which the
  OH group is attached
• Number Cs starting at end nearest OH group
• Locate and number substituents and give full name
• - use a number to indicate position of OH group
• - cyclic alcohols have cyclo- before the parent
  name numbering begins at the OH group, going in
  direction that gives substituents lowest possible
  numbers
• - use a prefix (di-, tri-) to indicate multiple
  OH groups in a compound

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Nomenclature
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Classification of Alcohols

• Alcohols can be classified as methyl, primary,
  secondary or tertiary
• Classification is based on the number of alkyl
  groups attached to the carbon to which the OH
  group is attached
• If OH is attached to a 1? C, its a 1? alcohol,
  etc.

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Naming Phenols

• Phenol is the common name for an OH group
  attached to a benzene, and is accepted by IUPAC
• Compounds with additional substituents are named
  as substituted phenols
• Ortho, meta and para are used when there is only
  one other substituent
• If there are two or more additional substituents,
  each must be numbered, beginning at the OH and
  going in direction that gives substituents lowest
  numbers (or alphabetical if same in both
  directions)

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Many phenols have pleasant odors, and some are
bioactive - Euganol (from cloves) is a topical
anesthetic - Thymol (from thyme) is an antiseptic
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Naming Thiols

• Parent name ends in -thiol
• Find longest chain containing the C to which the
  SH group is attached
• Number Cs starting at end nearest SH group
• Parent name is alkane name of carbon portion of
  longest chain, followed by thiol
• Locate and number substituents and give full name
• - use a number to indicate position of SH group
• - cyclic thiols have cyclo- before the parent
  name numbering begins at the SH group, going in
  direction that gives substituents lowest possible
  numbers
• - use a prefix (di-, tri-) to indicate multiple
  SH groups in a compound

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Thiols - Nomenclature

• Common names for simple thiols are derived by
  naming the alkyl group bonded to -SH and adding
  the word "mercaptan"

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Naming Ethers

• Simple ethers are named by their common names
• For common names name each alkyl group attached
  to the oxygen followed by ether
• For complex ethers IUPAC names are used
• For IUPAC names
• 1. Name as an alkane, with larger alkyl group
  being the parent chain
• 2. The smaller alkyl group and the O are named
  together as an alkoxy group (replace -yl with
  -oxy)
• 3. Number chain starting at end nearest alkoxy
  group
• 4. Use a number to give location of alkoxy group

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Naming Cyclic Ethers

• Cyclic ethers are generally named by their common
  names (we will not study the IUPAC names)
• A cyclic ether containing two carbons is called
  ethylene oxide (generally known as epoxides)
• A cyclic ether containing 4 carbons (with 2
  double bonds) is called a furan
• A cyclic ether containing 5 carbons (with 2
  double bonds) is called a pyran
• A cyclic ether containing 4 carbons and 2 oxygens
  is called a dioxane

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Naming Examples
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Physical Properties of Alcohols, Phenols, Thiols
and Ethers

• All of these types of compounds have a bent
  geometry around the O or the S, and are polar
  compounds
• Alcohols and phenols contain a very polarized O-H
  bond, and they can H-bond with themselves and
  with other alcohols or water
• - Small alcohols (4 or less Cs) are soluble in
  water
• - Phenol is soluble in water (even with 6 Cs)
  because it partially ionizes in water (its a
  weak acid)
• - Alcohols and phenols have relatively high
  boiling points
• Thiols are much less polar than alcohols because
  the electronegativity of S is the same as that of
  C (2.5), much less than that of O (3.5), so C-S
  and S-H bonds are not polar
• - thiols do not H-bond and have relatively low
  boiling points
• Ethers do not H-bond with themselves, so have
  boiling points similar to hydrocarbons
• -ethers are only slightly soluble in water and
  are highly flammable

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Physical Properties

• bp increases as MW increases
• solubility in water decreases as MW increases

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Acidity and Basicity of Alcohols and Phenols

• Alcohols and phenols, like water, can act as
  either weak acids or weak bases (although phenol
  is more acidic)
• Phenols are more acidic because the anion that
  forms upon loss of the proton is stabilized by
  resonance

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Combustion Reactions of Alcohols and Ethers

• Both alcohols and ethers can burn with oxygen to
  produce water, carbon dioxide and heat (just like
  hydrocarbons)
• However, ethers are much more flammable than
  alcohols and care should be taken when working
  with ethers in the laboratory (just a spark from
  static electricity can set off ether fumes)
• Examples
• CH3CH2OH 3O2 ? 2CO2 3H2O
  Heat
• CH3-O-CH3 3O2 ? 2CO2 3H2O
  Heat

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Dehydration of Alcohols to Form Alkenes

• An alcohol can lose a water molecule to form an
  alkene using an acid catalyst such as H2SO4 and
  heat (an elimination reaction)
• This is the reverse of the addition of H2O to an
  alkene
• Dehydration is favored by using heat (endothermic
  reaction) and a solvent other than water (lower
  concentration of H2O)
• When more than one alkene can be formed,
  Zaitsevs rule states that the more substituted
  alkene will be the major product
• Order of reactivity 3? gt 2? gt (1? gt
  methyl)
• - In fact this reaction only works with 3? and
  2? alcohols

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Mechanism of Acid-Catalyzed Dehydration of an
Alcohol

• First, the acid catalyst protonates the alcohol
• Next, H2O is eliminated to form a carbocation
• Finally, a proton is removed to form an alkene
  H3O