Transcript Chapter-1 ALCOHOLS21

Chapter-1
ALCOHOLS
Contents
Introduction
Nomenclature
Preparation
Reactions
Introduction
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Introduction, classification, nomenclature and
isomerism of alcohols :
The hydroxy derivatives of aliphatic hydrocarbons (compounds having
their carbon atoms in chains and not in the form of rings) are called
alcohols. When one, two or more hydrogen atoms of a hydrocarbon are
replaced by a corresponding number of hydroxyl groups (-OH), alcohols
can be obtained.
Classification of Alcohols
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They can be classified as:
Alcohols with one hydroxyl group - Monohydric alcohol
Alcohols with two hydroxyl groups - Dihydric alcohol
Alcohols with three hydroxyl groups - Trihydric alcohols
Alcohols with four or more hydroxyl groups - Polyhydric alcohols
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Secondary Alcohol
Here the carbon atom bearing the hydroxyl group is attached
to two other carbon atoms.
Tertiary Alcohol
Here the carbon atom bearing the hydroxyl group is attached
to three other carbon atoms
Nomenclature of alcohols
In the IUPAC system, the names of saturated alcohols are
derived from corresponding alkenes by replacing 'e' of alkenes
by 'ol'
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Some examples are shown below
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The numbering is done such that the carbon atom attached to the,-
OH group gets the lowest number
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For naming polyhydric alcohols, the name of the alkane is retained
and the ending -e is not dropped. Thus dihydric alcohols are named
as alkane diols and trihydric alcohols are named as alkene triols
Isomerism Of Alcohols
Alcohols exhibit following types of isomerism:
1. Chain isomerism
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Alcohols with four or more carbon atoms exhibit this type of
isomerism in which the carbon skeleton is different.
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2. Position isomerism
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Alcohols with three or more carbon atoms can exhibit position
isomerism.
3.
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Functional isomerism
Alcohols with two or more carbon atoms can exhibit functional
isomerism with ethers.
4. Optical isomerism
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Alcohols containing chiral centrescen exhibit enantiomerismor
optical isomerism.
Preparation
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A common source for producing alcohols is from
carbonyl compounds. The choice of carbonyl type
(ketone, aldehyde, ester, etc) and the type of
reaction (Grignard addition or Reduction), will
determine the product(s) you will get.
There are primarily two types of reactions used
to create alcohols from carbonyls: Grignard
Addition reactions and Reduction reactions.
Grignard Addition Reactions :
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Grignard reagents are created by reacting
magnesium metal with an alkyl halide . The
magnesium atom then gets between the alkyl
group and the halogen atom with the general
reaction:
R-X + Mg → R-Mg-X
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Mechanism of Grignard reagent reacting with a
carbonyl:
The general mechanism of a Grignard reagent reacting
with a carbonyl (except esters) involves the creation of
a 6-membered ring transition state .
Synthesis from an Aldehyde /
Ketone/Ester
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When a formaldehyde is the target of the Grignard's
attack, the result is a primary alcohol.
When an aldehyde is the target of the Grignard's
attack, the result is a secondary alcohol.
When a ketone is the target of the Grignard's attack,
the result is a tertiary alcohol .
Synthesis of alcohol from an epoxide
and Grignard reagent
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The reaction of Grignard reagents with epoxides
is regioselective. The Grignard reagent attacks at
the least substituted side of the carbon-oxygen
bonds, if there is one.
Organolithium Alternative
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Organolithium reagents are slightly more
reactive, but produce the same general results as
Grignard reagents
Reduction
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From an Aldehyde
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From Ketone
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From an Ester
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From carboxylic acid
Acidity
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In an O-H bond, the O steals the H's electron
due to its electronegativity, and O can carry a
negative charge (R-O-).
. This makes the -OH group (and alcohols)
Bronsted acids. Alcohols are weak acids, even
weaker than water.
On the other hand, alcohols are also weakly
basic, As a Bronsted base, the oxygen atom in
the -OH group can accept a proton (hydrogen
ion.) This results in a positively-charged
species known as an oxonium ion.
Reactions
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Conversion of alcohols to haloalkanes :
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This process can occur through SN2 (backside attack)
or SN1 (carbocation intermediate) mechanisms
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SN2 conversion of an alcohol to a haloalkane:
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R-O-H + H+ + X- → R-O+-H2 + X- → R-X
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+ H2O
SN1 conversion of an alcohol to a haloalkane:
R-O-H + H+ + X- → R-O+-H2 + X- → R+ + H2O + X→ R-X + H2O
Oxidation
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With regards to alcohol, oxidizing reagents can
be strong or weak. Weak reagants are able to
oxidize a primary alcohol group into a aldehyde
group and a secondary alcohol into a ketone
Strong reagents will further oxidize the
aldehyde into a carboxylic acid (COOH). Tertiary
alcohols cannot be oxidized
An example of a strong oxidizing reagent is
chromic acid (H2CrO4). An example of a weak
oxidizing reagent is pyridinium chlorochromate
(PCC) (C5H6NCrO3Cl)
Glycol
Ethylene glycol
 IUPAC name:Ethan-1,2-diol
 Other names:1,2Ethanediol
Ethylene Alcohol,
Hypodicarbonous acid ,
Monoethylene glycol
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Ethylene glycol (IUPAC name: ethane-1,2-diol) is
an organic compound widely used as
anautomotive antifreeze and a precursor to polymers. In its
pure form, it is an odorless, colorless, syrupy, sweet-tasting
liquid. Ethylene glycol is toxic, and ingestion can result in
death.
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Ethylene glycol was first prepared in 1859 by
the French chemist Charles-Adolphe Wurtzfrom
ethylene glycol diacetate
via saponification with potassium hydroxide and,
in 1860, from the hydration of ethylene oxide.
Current method
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Ethylene glycol is produced
from ethylene (ethene), via the
intermediate ethylene oxide. Ethylene oxide
reacts with water to produce ethylene glycol
according to the chemical equationC2H4O + H2O → HOCH2CH2OH
This reaction can be catalyzed by
either acids or bases
Reactions
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Ethylene glycol is used as a protecting
group for carbonyl groups in organic synthesis.
Treating a ketone or aldehyde with ethylene
glycol in the presence of an acid catalyst (e.g., ptoluenesulfonic acid; BF3•Et2O) gives the
corresponding a 1,3-dioxolane, which is resistant
to bases and other nucleophiles. The 1,3dioxolane protecting group can thereafter be
removed by further acid hydrolysis.
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Uses:
The major use of ethylene glycol is as a
medium for convective heat transfer in,
for example, automobiles and liquid cooled
computers. Ethylene glycol is also
commonly used in chilled water air
conditioning systems
In the plastics industry, ethylene glycol is
important precursor to polyester fibers
and resins.
Niche Applications
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Minor uses of ethylene glycol include the manufacture
of capacitors, as a chemical intermediate in the
manufacture of 1,4-dioxane
. Ethylene glycol is also used in the manufacture of
some vaccines.
Ethylene glycol is commonly used as apreservative for
biological specimens, especially in secondary schools
during dissection as a safer alternative to formaldehyde