Hydrocarbons 1 Flashcards
methods of preparation of alkanes: catalytic hydrogenation
Dihydrogen gas adds to alkenes and alkynes
in the presence of finely divided catalysts like
platinum, palladium or nickel to form alkanes.
This process is called hydrogenation. These
metals adsorb dihydrogen gas on their surfaces
and activate the hydrogen – hydrogen bond.
Platinum and palladium catalyse the reaction
at room temperature but relatively higher
temperature and pressure are required with
nickel catalysts.
methods of preparation of alkanes: aklyl halide
i) Alkyl halides (except fluorides) on
reduction with zinc and dilute hydrochloric
acid give alkanes.
ii) Alkyl halides on treatment with sodium
metal in dry ether (free from moisture)
solution give higher alkanes. This reaction
is known as Wurtz reaction and is used
for the preparation of higher alkanes
containing even number of carbon
atoms.
methods of preparation of alkanes:From carboxylic acids
Sodium salts of carboxylic acids on heating
with soda lime (mixture of sodium
hydroxide and calcium oxide) give alkanes
containing one carbon atom less than the
carboxylic acid. This process of elimination
of carbon dioxide from a carboxylic acid is
known as decarboxylation.
methods of preparation of alkanes: Kolbe’s electrolytic method
An aqueous
solution of sodium or potassium salt of a
carboxylic acid on electrolysis gives alkane containing even number of carbon atoms
at the anode.
physical properties of alkanes: physical state and nature
Alkanes are almost non-polar molecules
because of the covalent nature of C-C and C-H
bonds and due to very little difference of
electronegativity between carbon and
hydrogen atoms. They possess weak van der
Waals forces. Due to the weak forces, the first
four members, C1 to C4
are gases, C5
to C17 are
liquids and those containing 18 carbon atoms
or more are solids at 298 K. They are colourless
and odourless.
explain the variation of boiling point with molecular mass
there is a steady increase in boiling point with
increase in molecular mass. This is due to the
fact that the intermolecular van der Waals
forces increase with increase of the molecular
size or the surface area of the molecule.
As the number of branches increases the boilin gpoint decreases. this is because as branching increases, the molecular surface area decreases leading to lesser area of contact between the molecules. Hence there are very little vander waals forces of attraction, which can be broken easily.
what are substitution
One or more hydrogen atoms of alkanes can
be replaced by halogens, nitro group and
sulphonic acid group. Halogenation takes
place either at higher temperature
(573-773 K) or in the presence of diffused
sunlight or ultraviolet light. Lower alkanes do
not undergo nitration and sulphonation
reactions. These reactions in which hydrogen
atoms of alkanes are substituted are known
as substitution reactions.
rate of reaction of halogens
rate of substitution of free radicals
It is found that the rate of reaction of alkanes
with halogens is F2 > Cl2
> Br2
> I2
. Rate of
replacement of hydrogens of alkanes is :
3° > 2° > 1°.
Fluorination is too violent to be
controlled. Iodination is very slow and a
reversible reaction. It can be carried out in the
presence of oxidizing agents like HIO3
or HNO3
.
CH I CH I HI 4 2 3 + + (13.15)
HIO 5HI 3I 3H O 3 + → +2 2 (13.16)
Halogenation is supposed to proceed via
free radical chain mechanism involving three
steps namely initiation, propagation and
termination
what is complete and incomplete combustion
Alkanes on heating in the presence of air or
dioxygen are completely oxidized to carbon
dioxide and water with the evolution of large
amount of heat.
During incomplete combustion of
alkanes with insufficient amount of air or
dioxygen, carbon black is formed which is
used in the manufacture of ink, printer ink,
black pigments and as filters.
what is controlled oxidation
Alkanes on heating with a regulated supply of
dioxygen or air at high pressure and in the
presence of suitable catalysts give a variety of
oxidation products.
explain isomerisation reactions
n-Alkanes on heating in the presence of
anhydrous aluminium chloride and hydrogen
chloride gas isomerise to branched chain
alkanes. Major products are given below. Some
minor products are also possible which you
can think over. Minor products are generally
not reported in organic reactions.
what is aromatisation or reforming
n-Alkanes having six or more carbon atoms
on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of
vanadium, molybdenum or chromium
supported over alumina get dehydrogenated
and cyclised to benzene and its homologues.
This reaction is known as aromatization or
reforming.
what is steam reaction
Methane reacts with steam at 1273 K in the
presence of nickel catalyst to form carbon
monoxide and dihydrogen. This method is
used for industrial preparation of dihydrogen
gas.
mixture of carbon monoxide and steam is called water gas
what is pyrolysis or cracking
Higher alkanes on heating to higher
temperature decompose into lower alkanes,
alkenes etc. Such a decomposition reaction
into smaller fragments by the application of
heat is called pyrolysis or cracking.
Pyrolysis of alkanes is believed to be a
free radical reaction. Preparation of oil gas or
petrol gas from kerosene oil or petrol involves
the principle of pyrolysis. For example,
dodecane, a constituent of kerosene oil on
heating to 973K in the presence of platinum,
palladium or nickel gives a mixture of heptane
and pentene.
what are conformers/ rotamers/ conformations
spatial
arrangements of atoms which can be
converted into one another by rotation around
a C-C single bond are called conformations
or conformers or rotamers. Alkanes can thus
have infinite number of conformations by
rotation around C-C single bonds.
