Alcohols
Alcohols are hydrocarbon compounds
which have hydroxyl group bonded to sat- urated carbon atom. (-OH) is defined
as the functional group in these compounds. Their general formula is R-OH or
(CnH2n+ 2O).
Alcohols are classified according
to the hydroxyl group that is attached to carbon atom. If the carbon atom with
hydroxyl group is attached to another carbon atom, it is a primary (1°)
alcohol; if the carbon with the hydroxyl group is attached to two carbon atoms,
it’s a secondary (2°) alcohol; if the carbon with the hydroxyl group is
attached to three carbon atoms, it is called as a tertiary (3°) alcohol.
Nomenclature of Alcohols
Alcohols are named according to
IUPAC system as follows:
1. The longest chain which has the hydroxyl group is
chosen. The carbon with the hydroxyl group is given the lowest number.
2. Naming is done according to the number of carbons. The
–ol suffix
is added to the end of the corresponding alkane.
3. The number and place of the alkyl groups on the chain
is mentioned.
Synthesis of Alcohols
Alcohols can be synthesized
through many methods in industry and laboratories. Here we will explain one of
the laboratory methods.
Addition of Water to Alkenes
Addition of water to alkenes is
achieved via hot concentrated sulfuric acid used as a catalyst. Alkene reacts first
to produce alkyl sulfonic acid (Markovnikov’s rule). This compound reacts with
water and forms alcohol and sulfuric acid again.
Example 1
What are the probable structural
formulas for alcohols that have a molecular weight of 74 g/mol?
Solution:
Physical Properties of Alcohols
Alcohols have got low molecular
weights. They are poisonous and they have sharp odors. They have strong
intermolecular bonds. Therefore, their boiling points are much higher than of
alkynes. Besides, alcohols can form hydrogen bonds between their molecules and
with water molecules. Therefore, alcohols between (C1-C3) dissolve in water
very well.
Chemical Properties of Alcohols
Alcohols are compounds that tend
to react. They react with polarized or ionic compounds. Some important
reactions of alcohols are given below:
A) Alcohols react with sodium or
potassium metals and produce hydrogen gas.
B) They react with phosphorous trichloride (PCl3) or phosphorous pen- tachloride (PCl5) and form alkyl halides.
C) Reactions of Alcohols with
Acids
This reaction occurs fast with
tertiary alcohols especially when concentrated hydrochloric acid is used. But
with primary and secondary alcohols, it is quite slow. And the reaction rate
also depends on the type of acid.
HI > HBr >
HCl
In the reaction, solution of zinc
chloride in hydrochloric acid is used. This mix- ture is called as Lucas’
reagent. It is used to differentiate primary, secondary and tertiary alcohols.
Tertiary alcohols readily react with this cation and form alkyl halides which
does not dissolve in the solution. While secondary alcohols react with Lucas’
reagent after 5 minutes, primary alcohols do no react with it at room
temperature.
Example 2
Differentiate 1-propanol,
2-propanol and 2-methyl-2-propanol chemically.
Solution:
We can differentiate those
compounds using dry ZnCl2/HCl Lucas’ reagent. In this process, it is mixed with
2-propanol (secondary alcohol) and 5 minutes later, a layer of 2-propane
chloride forms. While it reacts with 2-methyl-2-pro- panol (tertiary alcohol)
readily, 1-propanol (primary alcohol) does not react with Lucas’ reagent at
room temperature.
This way we can differentiate
primary, secondary and tertiary alcohols using Lucas’ reagent.
1. For primary alcohols to react with Lucas’ reagent,
high temperature is neces- sary and they do not react at room temperature.
Therefore, no turbidity is visible in solution.
2. In secondary alcohols, turbidity appears 2-5 minutes
later.
3. Through reaction of tertiary alcohol with Lucas’
reagent, slightly water solu- ble alkyl halide forms and it causes turbidity in
solution.
D) Synthesis of water from Alcohol
Ethyl alcohol reacts with
concentrated sulfuric acid at 170 °C and ethylene forms.
Mechanism:
The mechanism of
the reaction above follows the following steps:
1. Addition of proton to alcohol
molecule:
2. The cleavage of water molecule from the compound above and formation of more stable carbonium ion:
3. The cleavage of proton from
carbonium ion and formation of double bond and cation:
According to Saytzeff’s rule, OH-
group leaves carbon atom and hydrogen atom leaves adjacent carbon atom which
has fewer hydrogen.
E) Oxidation
The structure of products which
form when alcohols are oxidized depends on the type of alcohol and reaction
conditions. Substances widely used in oxida- tion reactions are K2Cr2O7 /H2SO4
or KMnO4/ H2SO4 mixtures. This reaction is a method used to differentiate
alcohols.
Primary alcohols (1°) oxidize to
aldehydes first then to carboxylic acids.
Secondary alcohols (2°) oxidize to
ketones.
Tertiary alcohols (3°) do not
oxidize as the carbon atom which carries the OH- group does not have any
hydrogen atoms.
References
1. K. J. Denniston c J. J.Topping c and R. L.Caretc
“General Organic and Biochemistry”c Mc-Graw- Hillc New York
(2004).
2. K.W. Whittenc R.E. Davis and L. M. Peckc “General Chemistry” 7th ed.
Holt Rinehart and Winstonc New York (2010).
3. Clayden, J.; Greeves, N. and Warren, S. (2012) Organic
Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.
4. Streitwieser, Andrew; Heathcock, Clayton H.; Kosower,
Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4:
Medtech (Scientific International, reprint of revised 4th edition, Macmillan,
1998). ISBN 978-93-85998-89-8.
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