Class 11 hydrocarbon neb notes nepal.pdf

Notes

1. Hydrocarbon
Source of aliphatic hydrocarbon
(Hydrocarbons from petroleum)
Petroleum (Rock oil: Greek: petra= Rock; Oleum= oil) is a black viscous oily
liquid found under the surface of the earth. It is usually found under the dome
shaped rock along with natural gas .The natural gas consists of about 80%
methane with ethane, propane and butane in decreasing order.
Refining : The process of separating petroleum from useful fraction
having different boiling ranges with the simultaneous removal of
undesirable impurities is called refining.

3. Liquefied petroleum gas (LPG) is a mixture of propane, n-butane and
iso-butane compressed in cylinders under high pressure which is used
as domestic fuel for cooking and vehicle fuel.

5. Aromatic hydrocarbons from coal
Coal is complex mixture of organic compounds with varying amount of
free carbon. Depending upon the stage of formation, free carbon
content is different in coal as shown in table

6. When coal is heated to about 1000-1400
0
C in absence of air followed by
condensation, three different fractions are obtained
(i)solid residue obtained is called coke, which is used as heating fuel especially
in steel industries.
(ii) Uncondensed gases, which are mainly lower alkanes and alkenes are called
coal gas and are used as fuel.
(iii) Black viscous oily liquid is called coal tar which is a mixture of aromatic
hydrocarbons and their derivatives. It is subjected to fractional distillation to get
different useful fractions.

8. Crackingorpyrolysis
Pyrolysisisthedecompositionofasubstancebytheactionofheat.(Greek:pyr=fire,
lysis=breakdown).Pyrolysisofalkanesinpetroleumindustryiscalledthermal
crackingorsimplycracking.
Theprocessofbreakingofalkanesofhigherboilingpointsintothoseoflower
boilingpointsonheatingstronglyintheabsenceofairtoformamixtureofthelower
hydrocarbonscontainingbothalkanesandalkenesiscalledcrackingorpyrolysis.
Crackingcanbecarriedoutatlowertemperatureofabout450-500
0
Cinpresenceof
catalystlikeSiO
2-Al
2O
3,andtheprocessiscalledcatalyticcracking.
Whencrackingiscarriedoutonlyinthepresenceofheat,itiscalledthermal
cracking.

9. Reforming/Aromatization
The process of conversion of straight chain alkane containing 6 or more
carbon atoms into aromatic compound by heated to about 400°C in the
presence of catalyst such as Ptor Pdor Ni etcis called reforming or
aromatization.

10. Quality of fuel
Octane number
A fuel that produces minimum knocking is considered to be a good quality fuel. The
quality of a fuel(or anti-knocking property) is usually expressed in terms of octane
number.
Octane number is a scale that is used to determine the quality of fuel in the internal
combustion of an engine based on knocking property.
The straight-chain aliphatic hydrocarbons possess a higher knocking tendency than
branched-chain hydrocarbons.
Two pure hydrocarbons have been arbitrarily selected as standards for expressing
octane numbers.
n-heptane , a straight chain hydrocarbon which knocks very badly when used as a
fuel has been assigned an octane number of zero as a minimum.
On the other hand, iso-octane(2,2,4-trimethyl pentane), a branched chain
hydrocarbons has excellent anti-knocking property and hence it has been assigned
an octane number of 100 as maximum.

11. The octane number of other fuels is measured as the percent of isooctane in the
mixture of isooctane and n-heptane which causes the same knocking as the
fuels.
If a gasoline has octane number 80, it means that the anti knocking property of
that gasoline is equivalent to a sample which is a mixture of 80% isooctane and
20% n-heptane. The Greater the octane number, the better the fuel.
The octane number may be defined as the % of isooctane by volume in a mixture
of isooctane and n-heptane which has the same anti-knocking property as the
fuel under investigation.

12. Cetanenumber
The quality of diesel fuel used in diesel engine is generally expressed in
terms of cetanenumber.
Cetanenumber of the sample of diesel is the percentage of cetaneby
volume in a mixture of cetane(n-hexadecane) and α-methyl
naphthalene which has the same ignition properties as the diesel under
consideration.
Cetaneignites rapidly and has been assigned the cetanenumber 100
while α-methyl naphthalene due to its extremely poor ignites
properties has been assigned a cetanenumber of zero.

13. Anti knocking agent (gasoline additive )
The chemicals which is added to gasoline to increase the anti knocking
properties or octane no is known as gasoline additive or anti knocking agent.
The anti knocking agent normally used is TEL (tetraethyl lead ) . It is added to
the extent of 0.1% in the gasoline . Such a gasoline is leaded gasoline .
Working of TEL
During the burning TEL decompose to give ethyl free radicals
which combines with straight chain hydrocarbon of the fuel
and convert them into branch form . Due to such action
octane no increased and knocking effect decreased .

14. Pb(C
2H
5)
4 C
2H
5+ Pb
. .
.
C
2H
5 + Straight chain hydrocarbon Branch chain hydro carbon
In this process lead starts to deposited in the cylinder . Which decreases the
efficiency of the engine So it is removed by adding 1,2 di bromoethane .
In the process it first decompose to ethene and bromine . The bromine thus
obtained combines with lead to form lead bromide which is volatile in
nature and easily escape through the engine during the process .
BrCH
2CH
2Br CH
2=CH
2+ Br
2
Br
2+ Pb PbBr
2

15. Qualitative analysis of organic compounds
Qualitative analysis of organic compound is simply the procedure for
the detection of elements present in the molecule. Carbon is always
present in an organic compound. Hydrogen is almost always present
while oxygen is generally present. Besides these, nitrogen , sulphurand
halogens may be commonly present. Phosphorous and a few metals
are rarely found in organic compounds.
Elements except carbon , hydrogen and oxygen are known as foreign
elements.

16. Detection of nitrogen, sulphurand halogens(Lassaigne'stest)
This method was developed by French chemist J. L. Lassaignein 1843 for the detection of
nitrogen, sulphurand halogens in organic compounds. In this method, little organic
compound is fused with dry sodium metal in a fusion tube at high temperature and the
product is extracted in distilled water. The resulting aqueous solution is called sodium
extract or Lassaigne'sextract.
Purpose for preparing sodium extract
Organic compounds are covalent compound but common laboratory reagents are ionic
compounds. Therefore, organic compounds do not interact with common laboratory
reagents easily.

17. When organic compound is fused with sodium metal , the covalent bond between carbon
and foreign element breaks and water soluble ionic compound of sodium are formed . The
ionic sodium salt contains foreign elements of the organic compound in the form of acid
radical, which can be easily detected by simple laboratory test using common ionic
laboratory reagents. By using lassaigne`sfiltrate we can detect nitrogen , sulphurand
halogens.

18. Q Why sodium extract is usually alkaline in nature?
a.Detection of nitrogen
If nitrogen is present in the organic compound, sodium cyanide is formed into
the sodium extract solution. About 1 mL sodium extract is taken in a test tube
and made alkaline by adding dilute NaOH, if not alkaline. Little freshly prepared
ferrous sulphatesolution is added into it, boiled then cooled and few drops of
ferric chloride solution is added. Finally, few drops of conc. HClor H
2SO
4is
added. Formation of prussianblue or green colourindicates the presence of
nitrogen in organic compound.

19. Azocompounds (organic compounds having azogroup i.e. -N = N-) do not give
this test because they lose N
2at moderate temperature.

20. b. Detection of nitrogen and sulphurtogether
If both nitrogen and sulphurelements are present in organic compound,
NaCNS(sodium thiocyanide) may be formed in the sodium extract solution,
which may give blood red solution on adding FeCl
3solution.

22. d. Detection of halogen
If halogen is present in the organic compound, sodium halide is formed into the sodium
extract solution.
The sodium halide is then detected by silver nitrate test.
A little sodium extract is taken in a test tube, few drops of nitric acid is added, boiled then,
cooled and finally few drops of aqueous silver nitrate is added.
i. If chlorine is present in the organic compound, white precipitate is obtained which is
soluble in NH
4OH and reappears on adding HNO
3.

23. ii. If bromine is present in the organic compound, pale yellow
precipitate is obtained which is sparingly soluble in NH
4OH.
iii. If iodine is present in the organic compound, yellow precipitate is
obtained which is insoluble in NH
4OH.