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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
201
Alkanes
(no branch), isopentane (one branch) and neopentane
This is because in branched chain molecules, the
molecules tend to be spherical and so available surface
area for intermolecular attraction is decreased and hence
the boiling point gets lowered.
5. Melting points: The melting point of a substance not
only depends upon the size and shape of its molecules but also on how perfectly they are packed in
the crystal lattice (Fig. 4.4). The melting points of alkanes do not show a regular gradation with the
increase in molecular size. For instance, let us consider the melting points of propane to n-octane.
Alkane Propane
n-Butane n-Pentane n-Hexane n-Heptane n-Octane
3H8
) 4H10) 5H12) 6H14) H16) 8H18)
85.9 135 143.3 183 216
It may be noted that the increase in melting point is much more in moving from an alkane having
odd number of carbon atoms to the higher alkane than that of even number of carbon atoms to the
chain of carbon atoms, the terminal methyl groups lie on the opposite side of the zigzag chain.
On the other hand, in case of n-alkanes having odd number of carbon atoms, the terminal methyl
groups lie on the same side of the chain as shown below:
H3C
CH
2
CH
2
CH
2
CH
2
CH
3
H3C
CH
2
CH
2
CH
2
CH
3
m-Pentane
(Odd number of C atoms)
n-Hexane
(Even number of C atoms)
It appears that an alkane having an odd
number of carbon atoms has less symmetrical
in so closely together in crystal structure as
compared to an alkane with an even number
of carbon atoms having more symmetrical
structure and hence its molecules show more
close packing in the crystal lattice.
Learning Plus
Pentane, hexane, heptane, and octane are
classed as dangerous for the environment and
harmful. The straight-chain isomer of hexane is
a neurotoxin.
100
50
479 11 13 15 17 19 2321
Melting points (K)
Number of carbon
atoms per molecule
5
140
180
220
260
300
340
380
420
Figure 4.4 Increase in melting points of n-alkanes with
increase in the number of carbon atoms per molecule.
Learning Plus
Neurotoxin
A neurotoxin is a toxin that acts specifically on
nerve cells, usually by interacting with membrane
proteins such as ion channels.
next odd carbon atoms alkane, the increase in size is partially
neutralized by a decrease in symmetry. That is the reason for
slight increase in melting point in moving from even-number
alkane to the higher alkane.
But when we move from odd number alkane to the next even
carbon atom alkane, there is an increase in the size as well as an increase in symmetry. That explains the
4.6 CHEMICAL PROPERTIES OF ALKANES
parum–little.
afns–
–1
mole–1422O or
reducing agents like lithium aluminium hydride, sodium borohydride, etc.
2.
bonds are only slightly polarized. Moreover, alkane molecules are non-polar in nature (dipole
3.
Lewis acids.
4. The bond angles in alkanes are 109°28', due to sp
3
less reactive towards common reagents.
However, under strong conditions, alkanes undergo a few reactions the most important being substitution
(replacement of hydrogen atoms of alkanes by corresponding number of other atoms or groups) and thermal
decomposition (cracking). These important reactions of alkanes are discussed below:
1. Halogenation: The process of replacement of hydrogen atom of all alkanes by a halogen atom is
known as halogenation of alkanes. The decreasing order of reactivity of halogens towards alkanes is:
F2 > Cl2 > Br2 > I
2
a. Fluorination: Flourine reacts with alkanes with a violent explosion and it involves the cleavage of
F
2
CH
4
Methane
C + HF + (CH
3
F + CH2F2 + CHF3 + CF4 + C3F8)
Learning Plus
Inhalation of butane can cause problems like
euphoria, drowsiness, narcosis, asphyxia, cardiac
arrhythmia, etc.
202
203
Alkanes
b. Chlorination
i. Chlorination of lower members of alkanes (methane):
Methane reacts with chlorine in the presence of
diffused sunlight or ultraviolet light or when healed
replaced by chlorine atoms, for example,
Diffused sunlight or UV
or heated 600 K
CH
4
+ Cl
2
Methane
CH
3
Cl + HCl
Methylchloride
(Chloromethane)
CH3Cl + Cl
2
CH2Cl2 + HCl
Methylene chloride
(Dichloromethane)
CH2Cl2 + Cl
2
CHCl3 + HCl
Choloform
(Trichloromethane)
CHCl3 + Cl
2
CCl4 + HCl
Carbon tetrachloride
(Tetrachloromethane)
In the presence of direct sunlight or ultraviolet radiation, the reaction is very explosive
Direct sunlight
CH4 + 2Cl
2
C + 4HCl
ii. Chlorination of higher members of alkanes: Halogenation of higher alkanes (propane, butane,
etc.) yields a mixture of all possible isomeric products:
Cl
2
UV of diffused sulight
or heated 600
K
C
2H6
C2H5Cl + HCl
Ethyl chloride
Cl
2
UV or
diffused sunlight
Isopropyl chloride
(2-Chloro-propane)
55%
Propane
n-Propyl chloride
(1-Chloro-propane)
45%
CH
3
CH
3
CH
2
+
CH
3
CH2ClCH
2
Cl
CH
3
CH
3
CH
Remember
Halogen-rich alkanes like chloroform can be
carcinogenic.
Methyl iodide
CH
4
+ l
2
CH3l + HI
204
Thus, the case of substitution of a hydrogen atom by a halogen atom is in the order:
Tertiary > Secondary > Primary
c. Bromination: It is similar to chlorination, but not so vigorous.
Br
2
–HBre
Br
, Light
Light
2
–HBr
tetrabromide
CH
Methylene
bromide
CBr
4
Carbon
2 Br2
CH
4
Methane
Br
, Light
2
–HBr
Br
Light
2
–HBr
CHBr
Bromo-
form
Br
CH
3
Methyl
bromide
3
d. Iodination: The reaction with iodine is extremely slow and reversible.
Iodination is, therefore, carried out in the presence of an oxidizing agent such as iodic acid or
nitric acid or mercuric oxide, which converts hydrogen iodide formed into iodine.
5HI + HIO
3
3I2 + 3H2O
2. Nitration: The process which involves the replacement of hydrogen atom of alkanes by nitro group.
2) is known as nitration of alkanes.
nitration.
a. Liquid phase nitration:
3
C
6H13
Hexane
– H
+ HO – NO
2
Fuming nitric acid
413 K, high
pressure (40 h)
C
6H13NO2
Nitrohexane
+ H2O
b. Vapour phase nitration:
3
CH3 – H + HO – NO
Methane
C2H5 – H + HO – NO
Ethane
750 K
2
CH
3NO2
+ H2O
Nitromethane
725 K
2
C
2H5NO2
+ H2O
Nitromethane
Alkanes
C3H
8
Propane
Heat
CH
4
Methane
C
3H8
Propane
C
2H4
Ethane
H
2
Hydrogen
+
+
3. Sulphonation: The process which involves the replacement of hydrogen atom of alkanes with sulphonic
acid group ( 3H) is known as sulphonation of alkanes.
Only higher alkanes (hexane onwards) undergo sulphonation. In this process, alkane is heated with
fuming sulphuric acid or oleum or pyrosulphuric acid, H22O, which is a mixture of conc. H24 3
gas. For example,
SO
3
C6H13 – H
Hexane
+ HO – SO3 H
Sulphuric
acid
C
6H13SO3
Hexanesulphonic
H + H2O
acid
4. Thermal decomposition or pyrolysis or cracking:
fragments. For example,
205
H bonds. The reaction follows a
free radical mechanism.
The nature of products formed on cracking of alkanes depends upon the following:
1. The structure of alkane
2. The pressure under which cracking is carried out
3. The nature of catalyst
The cracking process is of prime importance for the manufacture of gasoline from high boiling fractions
of petroleum. It is also used for the production of oil gas used in the laboratories from kerosene or petrol.
Changes occurring during cracking: During cracking following important changes take place:
point than initial alkane.
Applications of cracking
1.
Preparation of oil gas: Oil gas is obtained by cracking of kerosene oil by dropping it over red hot
iron retorts. The kerosene oil is converted into gaseous hydrocarbons (methane, ethane, propane and
butane), hydrogen along with a large amount of volatile tarry matter. The cracked products are bubbled
through water in the hydraulic syphon where the vapours are condensed and removed. The gas thus
obtained is commonly known as oil gas.
2. Preparation of petrol gas: Petrol gas is obtained by cracking
of petrol by passing it through electrically heated coils or
retorts.
3. To provide extra gasoline: The cheap heavy oils, which
are much less in demand, are cracked to produce extra
gasoline. About 50% of our gasoline is now obtained by
this method.
4. As a source of alkanes: The low-molecular-weight alkanes obtained in the cracking process are very
good starting material for a large number of products.
Types of cracking: There are two main types of cracking.
1. Thermal cracking: This is carried out by heating the lubricating oil or diesel within the temperature
a. Liquid phase cracking
usually 10–5 atm.
b. Vapour phase cracking
3–4 atm pressure.
2. Catalytic cracking
(2 atm) in the presence of catalyst, which is a mixture of silica, alumina and manganese dioxide.
• Steam cracking: It is a process in which the hydrocarbon is diluted with steam, heated for a fraction of a
commercial hydrocarbons like ethane, propylene, butadiene, cyclopentadiene, etc.
• Hydrocracking: It is a process in which the hydrocarbon is mixed with hydrogen under pressure and
low hydrocarbons. The preparation of other organic compounds in petrochemical industry.
5. Reforming or aromatization: It is a process similar to cracking in which straight chain molecules in
the presence of a catalyst are effectively broken up and then reassembled as branched chain molecules
and aromatic compounds. Reforming is widely used to increase the octane rating of gasoline.
(10–20 atm) in the presence of a catalyst like chromium oxide supported on aluminium oxide. But
aluminium-based catalysts are now extensively used. The straight-chain hydrocarbons may be converted
to branched chain hydrocarbons by isomerization or alkylation, alkanes to cycloalkanes by cyclization and
cycloalkanes to aromatic hydrocarbons by dehydrogenation or aromatization. Thus, n-hexane changes into
benzene, n-heptane changes into toluene.
Learning Plus
Butane when blended with propane and other
hydrocarbons, it is referred to commercially as
liquid petroleum gas (LPG).
206
207
Alkanes
CH
3
CH
2
CH
3
CH
2
H2C
H
2
C
CH
2
CH
2
C
C
H
2
C
H
2
C
H
2
H
2
CH
CH
C
C
HC
HC
H
H
Cyclization
−3H
2
Dehydrogenation
−3H
2
Cyclohexane Benzenen-Hexane
Aromatization of a gasoline increases its octane number because unsaturated hydrocarbons are
better anti-knock compounds.
Reforming carried out in the presence of hydrogen is known as hydroforming.
6. Oxidation
a. Combustion
CnH
2n + 2
+ O2 (Excess) → nCO2 + (n + 1) H2O Heat
For example,
CH4 + 2O
2
CO2 + 2H2O;
DH = –890 KJ
2C
2H6
+ 7O
2
4CO2 + 6H2O;
DH = –3110 KJ
hydrocarbons obtained from petroleum are used as fuels
for the production of heat and power by combustion.
b. Incomplete combustion
supply of air or oxygen results in the formation of soot
or carbon black. This is used in the manufacture of
rubbers.
the presence of suitable metallic catalysts, lower alkanes are oxidized to alcohols and aldehydes
while higher homologues yield long-chain fatty acids. For example, when a mixture of methane and
methane is oxidized to methanol
.
475/100 atm
Cu tube
2CH
4
+ O
2
2CH3OH
Molybdenum
Oxide
CH
4
+ O
2
HCHO + H2O
Formaldehyde
Learning Plus
Burning of methane produces less amount of
carbon dioxide for each unit of heat released as
compared to any other hydrocarbon fuels such as
petrol, diesel, etc.
Alk. KMnO
4
Tert-butyl alcohol
(2-Methyl propane 2-ol)
Isobutane
(2-Methyl propane)
CH
3
CH
3
H + [O]C
CH
3
CH
3
CH
3
OHC
CH
3
Methyl free radical
°
H
H
H + Cl
C
H
H
HC
H
+ HCl
°
Methyl chloride
H
H
+ Cl
ClC°
H
+ Cl
°
H
HClC
H
208
ii. Chemical oxidation:
22O,
4
etc., but an alkane containing tertiary hydrogen atom gets oxidized by such reagents to corresponding
alcohol. For example,
4.7 MECHANISM OF HALOGENATION
Halogenation of alkanes is an example of chain reaction, a reaction that involves a series of steps, each of
which generates a reactive species that brings about the next step. The mechanism involves the following
three steps (chlorination of methane).
1.
Chain initiation step:
or heat, a halogen molecule is symmetrically dissociated into two halogen atoms (or free radicals),
which initiate the reaction.
Cl – Cl Cl° + Cl°
Heat or
UV light
Free radicals
radicals have no charge but because of the unpaired electron, these are highly reactive.
2.
Chain propagation steps: The chlorine radical (or atom) so formed removes a hydrogen atom from
methane to produce methyl free radical as shown below in step (a). The methyl radical in turn reacts
with another chlorine molecule to produce methyl chloride and regenerates another chlorine free radical
as shown below in step (b).
a.
b.
An atom or group of atoms possessing an odd (unpaired) electron is called a free radical. Free
Alkanes
°
Free radical
°
H
HCl + ClC
H
H
Cl + HCl
C
H
Methylene chloride
(Dichloromethane)
°
H
Cl + Cl
C
H
Cl
°
Cl
H
Cl + Cl
C
H
Free radical
°
Cl
H
Cl + Cl
C
H
°
Cl
Cl + HCl
C
H
Chloroform
(Trichloromethane)
°
Cl
Cl + ClC
H
Cl
°
Cl
Cl Cl + ClC
H
Free radical
°
°
Cl
Cl
Cl + Cl
C
H
Cl
Cl
Cl + HCl
C
Carbon tetrachloride
(Tetrachloromethane)
°
Cl
Cl Cl + ClC
Cl
°
Cl
Cl Cl + ClC
Cl
209
c.
d.
e.
f.
g.
h.
410 K
Pb
+
°
4C2H
5
Ethyl free radical
Pb
C
2H5
C2H
5
C2H
5
H5C
2
210
3. Chain terminating steps: At the end of the reaction, the radicals may combine with each other to
stop or terminate the chain reaction, for example,
C°l + C°H
C°l + C°l CI
,
2
3
CH3CI,
Methylchloride
C°H3 + C°H
General mechanism of halogenation of alkanes:
a. Chain initiation step
X – X X° + X
Heat or
sunlight
Halogen free radical
b. Chain propagating steps
R – H + X
°
R° + HX,
Alkyl free radical
R° + X – X R – X + X
and so on.
c. Chain terminating steps
°
X° + X
X – X, R° + R
Halogen
°
R – R,
Alkane
4.7.1 Evidences in Support of Free Radical Mechanism
°
CH3 – CH
Ethane
Alkyl halide
3
°
R – R
Alkane halide
3
°
R° + X
1. The reaction does not take place in dark at ordinary temperature but takes place in presence of light
molecules into halogen free radicals.
2. If the reaction actually follows free radical mechanism, the addition of substances that are the
a. Effect of tetraethyl lead (TEL):
2 to produce chlorine free radical, which propagates the
chain reaction: