Alcohols contain an OH group attached to a saturated sp3 carbon atom'

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Alcohols contain an OH group attached to a
saturated (sp3) carbon atom. Phenols contain an
OH group attached to one of the carbons of a
benzene ring (sp2). Ethers contain an O- atom
bonded to two carbon atoms which can be either
aliphatic or aromatic.
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Constitutional isomerism in alcohols can arise
from Different carbon skeletons Different
placement of the OH group on a carbon skeleton
No isomers are possible for the simplest
alcohols methanol, CH3OH, and ethanol, CH3CH2OH.
But two isomers are possible for C3H8O
For alcohols having the formula C4H10O, two
different carbon skeletons are possible
The OH group can be placed in two unique
positions on each carbon skeleton
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Alcohols are classified as primary (1),
secondary (2), or tertiary (3).
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Simple alcohols are usually referred to by their
common names which consist of the alkyl group
name followed by a space and the word alcohol.
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Alcohols are characterized by strong hydrogen
bonding and have Much higher boiling and melting
points than those of hydrocarbons Solubility in
water
Although not shown in the above illustration,
each alcohol molecule is hydrogen bonded to
several neighboring alcohol molecules. The high
melting and boiling temperatures of alcohols
compared to the corresponding hydrocarbons is a
direct result of the strengths of hydrogen bonds
compared to London forces.
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The difference in boiling points between
hydrocarbons and the corresponding alcohols
decreases as the chain length of the hydrocarbon
increases. London forces are directly related to
molecular size and the proportion of the total
molecular force due to hydrogen bonding becomes
less at higher molecular weights.
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Diols and triols have especially high boiling
points compared to alkanes of similar size due to
more extensive hydrogen bonding. HOCH2CH2OH BP
198C CH3CH2CH2OH BP 97C Alcohols containing 3
or fewer carbons are completely miscible in water
Alcohols containing 4 carbons are moderately
soluble, 5 carbons slightly soluble, and more
than 5 carbons are negligibly soluble. An
alcohol containing 5 or more carbons is soluble
only if it contains two or more alcoholic groups.
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Alcohols are usually considered to be neutral
compounds. Water solutions of alcohols have a pH
of 7, the same as for water itself. Alcohols are
very weakly amphoteric they are both very weak
acids and very weak bases.
An alcohol can accept a proton (act as a base)
from strong acids such as sulfuric acid (H2SO4)
The equilibrium lies far to the left only about
0.1 of the alcohol molecules become protonated,
however, this small concentration is important in
the dehydration reactions of alcohols. The
acidity of alcohols is too weak to be observed in
reactions with strong bases. However they slowly
react with active metals such as sodium to yield
hydrogen gas
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Alcohols can be dehydrated when heated with
catalytic amounts of a strong acid. The reaction
is called an intramolecular dehydration. An
Hatom is lost from one carbon and an OH group
from an adjacent carbon
All alcohols can be dehydrated if they contain an
H-atom on a carbon adjacent to the carbon holding
the OH group. Methanol, for instance, cannot
be dehydrated. It contains only one carbon.
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The dehydration of alcohols is the reverse of the
hydration of alkenes. LeChateliers principle
allows the equilibrium position to be adjusted in
favor of excess alcohol or excess
alkene When hydration is desired, a large
excess of water is used. When dehydration is
desired, the reaction is run at a temperature
above the boiling point of the alkene formed,
which distills out of the reaction mixture.
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Dehydration of secondary and tertiary alcohols
may follow a more complicated course than that
for primary alcohols. If the alcohol is
asymmetric, two different products can be formed
In this case, 2-butene is the major product (the
carbon with fewest H). So, the more stable the
alkene, the higher its yield in a dehydration
reaction and the more substituted the alkene, the
more stable it is
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In the dehydration of 2-butanol previously shown,
the predominate 2-butene product formed is a
mixture of cis and trans isomers. The trans
isomer predominates because it is more stable
than the cis isomer. Dehydration reactions occur
during physiological processes such as glycolysis
and the citric acid cycle.
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The weakest bonds in an alcohol are the O-H bond
and the adjacent C-H bond. These bonds only are
oxidized during selective oxidations. Typical
selective oxidizing agents are MnO4- and
Cr2O7-. Primary, secondary, and tertiary alcohols
respond differently to selective
oxidation Primary alcohols are oxidized in two
stages Stage 1 Simultaneous loss of hydrogens
(dehydrogenation) from the OH group and the
adjacent C-H carbon producing a carbonyl group
and a resulting compound called an
aldehyde. Stage 2 Oxidation of the H attached
to the carbonyl group of the aldehyde to OH,
producing a carboxylic acid.
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The reaction does not stop at the aldehyde when
permanganate or dichromate are used as the
oxidizing agent. Aldehydes can be produced using
milder oxidizing agents such as pyridinium
chlorochromate (PCC). Secondary alcohols
Oxidation cannot proceed beyond the carbonyl
stage. These alcohols are oxidized to ketones.
Tertiary alcohols Oxidation of the OH cannot
occur because no hydrogen atom is attached to the
same carbon as the OH group.
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Permanganate and dichromate can be used for
simple chemical diagnostic tests for primary and
secondary alcohols. Permanganate oxidation MnO4-
(purple solution) is converted into MnO2 (brown
precipitate) Dichromate oxidation Cr2O72-
(orange solution) is converted into Cr3 (green
solution) Uses and limitations Primary and
secondary alcohols ARE distinguished from
tertiary alcohols which are unaffected by
permanganate and dichromate. Primary and
secondary alcohols ARE distinguished from
alkanes, cycloalkanes, aromatics, and esters,
which do not undergo permanganate or dichromate
oxidations. Positive permanganate or dichromate
tests DO NOT distinguish primary and secondary
alcohols from alkenes, alkynes, and phenols,
which are also oxidized by these reagents.
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Phenols contain an OH group bonded to one of the
sp2 carbon atoms of a benzene ring. The phenol
family includes the parent compound, phenol, as
well as a wide variety of other compounds having
additional substituents attached to the phenol
ring.
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The O-H bond in phenols is more polar than that
in alcohols because the benzene ring has an
electron withdrawing effect which further
polarizes the O-H bond.
The greater polarity of this bond results in
stronger hydrogen bonding and correspondingly
higher boiling points, melting points, and water
solubility, compared to alcohols.
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Phenols are weak acids and are much more acidic
than alcohols.
The pH value of a 0.1 M aqueous solution of
phenol is about 5.
The higher acidity of phenols compared to
alcohols is due to the electron-withdrawing
effect of the phenyl ring compared to the R group
of an alcohol.
The negative charge on the oxygen atom is
dispersed around the benzene ring rather than
leaving it entirely on the oxygen, as in the
alcohol system. This effect is called charge
dispersal.
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Phenols do not undergo dehydration, because that
would form a triple bond within the benzene ring
destroying its aromatic nature. The oxidation of
phenols is the basis of many antioxidants, both
in physiological systems and in commercial
products.
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Ethers contain an oxygen atom bonded to two
different carbon atoms. The carbon atoms can be
either aliphatic or aromatic carbons. The common
nomenclature system uses the names of the groups
attached to oxygen, in alphabetical order,
followed by the word ether. For ethers containing
other than simple groups, the IUPAC system is
used. In this case the ether is named as a member
of some other family, with the more complex group
determining the base name. The simpler group and
the ether oxygen to which it is attached are
treated as an alkoxy (RO) or aryloxy(ArO)
group. CH3CH2-O-CH2CH3 Diethyl Ether (common) or
Ethoxyethane (IUPAC)
Common names are used for cyclic ethers
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The boiling point of dimethyl ether is higher
than that of the corresponding alkane, propane,
because the C-O-C bond is bent and the two
carbon-oxygen bonds are polar. This effect falls
off rapidly with increasing alkane chain length.
Diethyl ether has the same boiling point as its
corresponding alkane, pentane. Ethers as a class
have much lower boiling points than alcohols due
to the lack of hydrogen bonding between molecules.
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When a primary alcohol is dehydrated, it produces
an ether as well as an alkene
Intramolecular dehydration to form an alkene
competes with intermolecular dehydration to form
an ether Alkene is the major product at 180
C Ether is the major product at 140 C Secondary
and tertiary alcohols do not form ethers when
heated with an acid catalyst. The attached
substituents prevent two molecules from
approaching each other closely enough from
intermolecular dehydration to occur. This effect
is called steric hindrance.
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• Thiols, or mercaptans, are the sulfur analogues
  of alcohols.
• The SH group is called the mercapto, or
  sulfhydryl, group.
• The IUPAC nomenclature system adds the ending
  thiol to the name of the alkane, but without
  dropping the final e.
• Thiols have distinct odors and flavors, often
  disagreeable (skunk). The aromas and flavors of
  garlic and onions are due largely to disulfides.
• Thiols have considerably lower boiling points
  than those of alcohols, even though thiols have
  higher molecular masses, because thiols do not
  possess the strong hydrogen bonding of alcohols.
• Thiols are weak acids but much stronger than
  alcohols, because the S-H bond is weaker than the
  O-H bond, a consequence of the larger size of
  sulfur compared with oxygen. However, thiols are
  weaker acids than phenols.

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Thiols are easily oxidized to disulfides by
many oxidizing agents Disulfides are named by
naming the R groups attached to the sulfur atoms
followed by the word disulfide.
Disulfides are easily reduced back to thiols by
many reducing agents In the preceding
equations, the abbreviations (O) and (H)
indicate general conditions for selective
oxidation and reduction, respectively, without
specification of the exact oxidizing or reducing
agent.
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The reactions to form disulfides and insoluble
heavy metal salts are of physiological
importance. Many proteins contain thiol groups
and disulfides critical to maintaining their
three dimensional shape and their proper
functioning. Lead and mercury salts react with
thiol groups, altering the physiological
functioning of the proteins involved. A common
source of lead is from flakes of paint
manufactured prior to 1980. The use of lead
tetraethyl as a gasoline additive also introduces
lead into the environment, however, its use has
been phased out. Mercury from insecticides and
other sources enters the food chain through fish
from contaminated rivers, lakes, and streams.
Mercury has a physiological effect similar to
that of lead.