Hydrocarbons-3 Flashcards
how to prepare alkyne from calcium carbide
On industrial
scale, ethyne is prepared by treating calcium
carbide with water. Calcium carbide is
prepared by heating quick lime with coke.
Quick lime can be obtained by heating
limestone
how to prepare alkynes from vicinal dihalide
Vicinal
dihalides on treatment with alcoholic
potassium hydroxide undergo
dehydrohalogenation. One molecule of
hydrogen halide is eliminated to form
alkenyl halide which on treatment with
sodamide gives alkyne.
phy properties of alkynes
First three
members are gases, the next eight are liquids
and the higher ones are solids. All alkynes are
colourless. Ethyene has characteristic odour.
Other members are odourless. Alkynes are
weakly polar in nature. They are lighter than
water and immiscible with water but soluble
in organic solvents like ethers, carbon
tetrachloride and benzene. Their melting point,
boiling point and density increase with
increase in molar mass.
why is the hydrogen bonded to triply bonded carbon highly acidic in nature?
(or)
why are alkynes weakly polar
hydrogen atoms in ethyne are attached to the sp
hybridised carbon atoms whereas they are
attached to sp
2
hybridised carbon atoms in
ethene and sp
3
hybridised carbons in ethane.
Due to the maximum percentage of s character
(50%), the sp hybridised orbitals of carbon
atoms in ethyne molecules have highest
electronegativity; hence, these attract the
shared electron pair of the C-H bond of ethyne
to a greater extent than that of the sp
2
hybridised orbitals of carbon in ethene and the
sp
3
hybridised orbital of carbon in ethane.
Thus in ethyne, hydrogen atoms can be
liberated as protons more easily as compared
to ethene and ethane. Hence, hydrogen atoms
of ethyne attached to triply bonded carbon
atom are acidic in nature.
what reactions is used for the distinction of alkyne from alkene and alkane?
sodium and sodamide are strong bases. they react with alkyne containing acidic hydrogen to give ethynides/ acetylide and hydrogen. this reaction is used for These reactions are not shown by alkenes
and alkanes, hence used for distinction
between alkynes, alkenes and alkanes
explain in general the addition reactions shown by alkynes
Alkynes contain a
triple bond, so they add up, two molecules of
dihydrogen, halogen, hydrogen halides etc.
The addition product formed depends upon
stability of vinylic cation. Addition in
unsymmetrical alkynes takes place according
to Markovnikov rule. Majority of the reactions
of alkynes are the examples of electrophilic
addition reactions.
addition of halogens
Reddish orange colour of the solution of
bromine in carbon tetrachloride is decolourised.
This is used as a test for unsaturation.
addition of hydrogen halides
Two molecules of hydrogen halides (HCl, HBr,
HI) add to alkynes to form gem dihalides (in
which two halogens are attached to the same
carbon atom)
addition of water
Like alkanes and alkenes, alkynes are also
immiscible and do not react with water.
However, one molecule of water adds to alkynes
on warming with mercuric sulphate and dilute
sulphuric acid at 333 K to form carbonyl
compounds.
explain linear polymerisation of alkynes
Under suitable
conditions, linear polymerisation of ethyne
takes place to produce polyacetylene or
polyethyne which is a high molecular weight
polyene containing repeating units of
(CH = CH – CH = CH ) and can be represented
as —( CH = CH – CH = CH)— Under special n
conditions, this polymer conducts electricity Thin film of polyacetylene can be used as
electrodes in batteries. These films are good
conductors, lighter and cheaper than the metal
conductors.
explain cyclic polymerisation
Ethyne on passing
through red hot iron tube at 873K undergoes
cyclic polymerization. Three molecules
polymerise to form benzene, which is the
starting molecule for the preparation of
derivatives of benzene, dyes, drugs and large
number of other organic compounds. This is
the best route for entering from aliphatic to
aromatic compounds
