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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
Alkanes
1s
C
H
C
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
H
H
H
H
H
1s
1s
1s
1s
1s
C
H
109°28′
109°28′
H
H
C
H
H
H
Bond length
= 1.54
Å
Bond length
= 1.10
Å
H
C
C
H
H
H
H
H
C
H
H
CH3CH2CH2CH
3
H3CHC
CH
3
CH
3
n-Butane
(Butane)
Isobutane
(Methylpropane)
b. The remaining three sp 3 hybrid orbitals of each carbon atom overlap with the 1s orbitals of hydrogen
H bonds, as shown in Fig. 4.2.
In the case of higher alkanes, the carbon–carbon chains possess a zigzag structure so that the repulsion
between four orbitals on each carbon is minimum, for example, structure of propane is shown in Fig. 4.2 (b).
(a)
191
4.3 STRUCTURAL ISOMERISM
Alkanes show chain isomerism,
represents two or more compounds having different structural formulae due to the different arrangement of
carbon chains.’
433323
not show chain isomerism because only one arrangement of carbon atoms is possible.
Butane and higher alkanes show chain isomerism.
Chain isomerism in butane
(b)
Figure 4.2 (a) Orbital structure of ethane. (b) Structure of propane.
4H10). It has two chain isomers.
Isopropane
(2-Methyl butane)
HC
CH
3
CH
3
H3C
Neopentane
(2,2-Dimethylpropane)
CH
3
CH
3
CH
3
CH3C
CH
3
CH2CH2CH2CH
3
n-Pentane
(2-Methylpentane)
CH
2
CH CH
3
CH
3
CH
3
CH
2
3-Methylpentane
CHCH
2
CH
3
CH
3
CH
3
CH
2
3
CH
3
CH3CH
3
Neohexane
(2, 2-Dimethylbutane)
CH
3
CH
3
CH
3
CH
3
C CH
2
192
Chain isomerism in pentane 5H12). It has three chain isomers.
Chain isomerism in hexane 6H14
1.
2.
3.
4.
5.
CH2 CH2 CH2 CH2 CH3
CH
3
n-Hexane (Hexane)
Isohexane-(2-Methylpentane)
3-Methylpentane
2,3-Dimethylbutane
Neohexane-(2,2-Dimethylbutane)
4.4 GENERAL METHODS OF PREPARATION OF ALKANES
Besides the natural sources, alkanes can also be prepared by the following methods:
1. From unsaturated hydrocarbons: Hydrocarbons containing double or triple bonds can react with
a. From alkenes
2
Ni, 523–573 K
Cn H
2n + 2
Cn H2n + H
Alkene Alkane
For example,
Alkanes
193
+ H
2
Ni, 523–573 K
Ni, 523–573 K
2
CH3 CH
CH3 CH3 CH
3
3
CH2 CH
Ethene Ethane
CH3 CH CH
Propene Propane
2
+ H
2
b. From alkynes
Cn H
Alkyne Alkane
2n – 2
+ H
Ni, 523–573 K
2
Cn H
2n + 2
For example,
CH
CH
Acetylene (Ethyne) Ethane
CH
C
CH + 2H
3
Methylacetylene
(Propyne)
+ 2H
2
2
Ni, 523–573 K
CH3 CH
CH3 CH2 CH
Propane
3
3
Platinum, palladium and Raney nickel are more active catalysts and can bring about hydrogenation even
as Sabatier–Senderens reaction. Al
The yield of alkane is very high. Methane, however, cannot be prepared by this method as the starting
alkenes or alkynes must contain at least two carbon atoms.
2. From alkyl halides: Alkyl halides have the general formula RX where R is an alkyl group and X is a
a. Reduction of alkyl halides: Different reducing agents can be used for reducing alkyl halides to obtain
alkanes.
3
and alcohol reduces alkyl halides to alkanes:
R – X +
2 [H]
Nascent
hydrogen
RH
Alkane
+ HX
Butane
2-Bromobutane
Br
CH
3
CH3 + 2[H]
CH
2
CH
CH
3
CH3 + HBr
CH
2
CH
2
Zn–Cu/C2H5OH
194
For example,
Zn – Cu/C2 H5 OH
C2 H5 I + 2 [H] C2H6 + HI
Ethyl iodide Ethane
(ii) Catalytic reduction: Alkyl halides can also be reduced by hydrogen in the presence of palladium as a
catalyst.
Pd
C2 H5 Br + H
Ethyl bromide Ethane
2
C2 H6 + HBr
(iii) Hydriodic acid, HI, is a powerful reducing agent that reduces alkyl halides (particularly iodides) to
C2 H5 I + HI
Ethyl iodide
Red P
D
C2 H6 + I
Ethane
2
Iodine produced may react with alkanes and so to avoid that, red phosphorus is added.
2P + 3I
2PI
2
3
(iv) Alkyl halides can also be reduced by complex metal hydrides like LiAlH44. LiAlH4 reduces
4 reduces only secondary and tertiary alkyl halides.
4RX + LiAlH
4
4RH + LiAlX
4
The reduction takes place in four independent steps, each step involving a nucleophilic displacement
by hydride ion, H–.
b. Wurtz synthesis:
a symmetrical higher alkane (R–R) is formed (ascending of series). The method is used for stepping
up the homologous series, i.e. conversion of lower member to higher member:
R – X
Alkyl halide
+ 2Na + X – R R – R + 2NaX
Dry
ether
Alkane
For example,
Isopropylbromide
(2-Bromopropane)
2, 3-Dimethylbutane
CH
CH
3
CH
3
CH
3
CH
3
HC + 2NaI
Dry
ether
CH
3
CH
CH
3
Br + 2Na + Br
HC
CH
3
CH
3
Alkanes
195
CH
– I + 2Na
3
Methyl iodide
C
– I + 2Na
2H5
Ethyl iodide
+ I – CH
+ I – C2 H
3
5
Dry
ether
Dry
ether
CH3 – CH3 + 2NaI
Ethane
C2 H5 – C2 H5 + 2NaI
Butane
Limitations:
1. 9), i.e. alkanes containing odd number of carbon atoms cannot be prepared by
concentration of methyl bromide and ethyl bromide, ethane and butane are obtained as by-products.
C
Br + 2Na
2 H5
Ethyl
bromide
+ BrCH
Methyl
bromide
3
Dry
ether
– C H3 + 2NaBr
C
2 H5
Propane
CH
Br + 2Na + BrCH
3
Methyl
bromide
C
Br + 2Na + BrC2 H
2 H5
Ethyl
bromide
2. Methane cannot be prepared by this method.
3. This reaction fails with tertiary alkyl halides.
of a new carbon–carbon bond.
Mechanism:
1.
Ionic mechanism: It operates when the reaction is carried out in solution. It has been suggested that an atom
of sodium reacts with an alkyl halide to form an alkyl sodium compound having a carbanion character.
Two possible mechanisms, depending upon the conditions of the reaction, have been proposed.
3
ether
Dry
Dry
5
ether
– C H3 + 2NaBr
CH
3
Ethane
– C2 H5 + 2NaBr
C
2 H5
Butane
X + 2Na
R Na + NaX
R
:
⊕
SN
2
R + X
R
:
R+RX
:
Ethane
CH
3
CH
2
CH2CH2I
+ H
CH
3
CH
3
CH2 + I+ CH
2
CH3CH
2
+ H
CH
2
CH
2
CH3CH3CH
2
+
CH
2
Ethane Ethene
°
°
196
The alkyl sodium thus formed provides the nucleophilic alkyl carbonion,
which displaces the halide ion
2 mechanism, resulting in the formation of an alkane:
This mechanism also explains the formation of by-products. For example, ethyl iodide on treatment
with sodium metal in the presence of ether gives n-butane as the major product and ethene and ethane
as the minor products:
2. Free radical mechanism:
disproportionate to form the observed products.
R – X + Na R° + NaX
R° + °R R – R
32
Coupling reaction
C2H5° + C2H5° C2H5 – C2H
5
Disproportionation reaction
c. Corey–House reaction:
alkanes containing both even and odd number of carbon atoms. Higher alkanes including branched chain
between an alkyl halide and a lithium dialkyl copper.
RX + 2Li R – Li + LiX
2R – Li + CuI R2CuLi + Lil
Lithium dialkyl copper
Dry ether
Alkane
R2 CuLi + R´ X R – R´ + RCu + LiX
(R and R' may be same or different)
Alkanes
Neo-pentane
(2, 2-Dimethylpropane)
2-Chloro 2-methyl propane
(Tertiary butyl chloride)
+ Zn
CH
3
Cl
CH
3
CH
3
CH
3
CH
3
C
Zn
CH
3
CH
3
CH3C
CH
3
CH
3
Cl
+
CH3CH2Mgl + CH3OH
CH3CH3 + Mg
OCH
3
I
Ethyl magnesium
iodide
Methanol Ethane
For good yields, the alkyl halide (RX) is generally a primary alkyl halide but alkyl group (R) in the
organometallic compound can be either primary, secondary or tertiary. For example,
197
CH3CH2 – Br CH3CH2 – Li (CH3CH2)2Cu Li
Li CuLi
Lithium diethyl copper
(CH3CH2)2 – CuLi + CH3CH2CH2 – Br
n
-Propyl bromide
Dry ether
CH
– CH2CH2CH3 + CH3CH2Cu + LiBr
3CH2
Pentane
d. Frankland’s method:
formed:
RX + Zn + XR´ R – R´ + ZnX
Alkane
C2H5I + Zn + IC2H
5
C4H10 + ZnI
2
2
only.
3. By using Grignard’s reagent:
This compound reacts with active hydrogen obtained from water, alcohol, ammonia and primary
amine to form alkanes. For example,
CH3CH2Br + Mg
Ethyl bromide
CH3CH2MgBr + H2O
Ethyl magnesium
bromide
Ether
CH3CH2MgBr
(Ethyl magnesium, bromide)
(Grignard’s reagent)
CH3CH3 + Mg(OH)Br
Ethane
HO:+ O:
CR
:
O:
:
O:
:
:O:
:
:
: :
C
OH
:O:
:
:O:
:
:O:
:
:O:
O:
R
R:
+OHC
Carboxylate ion
Proton
transfer
H
HC+
Alkane
Carbonate ion
198
4. From carboxylic acids: The general formula of carboxylic acid is R
carboxylic group and R is an alkyl group. There are two methods by which alkanes are obtained from
carboxylic acids.
a. By heating sodium salt of the acid with soda lime (decarboxylation):
is formed. The process of elimination of carbon dioxide from a carboxylic acid is known as
fusion:
For example,
Decarboxylation is a method of preparing an alkane with one carbon atom less than the starting
carboxylic acid (descending of series). For the lower homologues, the yield of alkanes is good but
for higher homologues, it is poor.
Mechanism: A probable mechanism of this reaction can be proposed as follows:
CaO
Heat
CaO
Heat
Alkane
Methane
2
CO
3
3
RCOONa
Sodium salt of
carboxylic acid
CH
COONa
3
Sodium acetate
(Sodium ethanoate)
+ NaOH R – H + Na
+ NaOH CH4 + Na2 CO
b. By Kolbe’s electrolytic method:
carboxylic acid is electrolysed, higher alkane is obtained as shown below:
Alkanes
2RCOONa 2RCOO
−
2Na
+
+
2Na
(at cathode)
+2e
−
−2e
−
R
R + 2CO
2
(at anode)
2CH3CO
O
⊕
Na 2CH3CO
O
2Na
+
⊕
Ionization
2H2O
⊕
2OH + 2H
Ionization
−2e
H3C° + °CH
3
H3CCH
3
::
O:
2CH3C
O
:
O:
°
2CH
3
C
O
+
°
O
2CH
3
C
O
:
O:
°
2CH
3
C
O
2H + 2e
H
2
199
2RCOONa + 2H2 O R – R + 2NaOH + H2 + 2CO
Electrolysis
2
For example,
2CH3 COONa + 2H2 O
Sodium acetate
Electrolysis
CH3 – CH3 + 2NaOH + H2 + 2CO
Ethane
2
or,
This reaction is believed to occur through the following steps:
At anode:
3 is lower than that of 3 ions are preferably
discharged at anode, OH
At cathode:
+ + ion, therefore H+ ions are preferably
discharged at anode, OH
–
remain the solution.
–
remains in the solution.
600
500
400
300
200
100
50
12 16 20840
Boiling points (K)
Number of carbon
atoms per molecule
200
NOTEWORTHY POINTS
Methane is not toxic; however, it is extremely flammable and may form explosive mixtures with air.1.
Methane is also an asphyxiate gas. It may displace oxygen in an enclosed space and can lead to 2.
suffocation.
Methane may be transported as a refrigerated liquid (liquefied natural gas or LNG).3.
It is unaffected by many common chemical reagents but reacts violently with chlorine or fluorine in the 4.
presence of light and is therefore important as a starting material for the synthesis of solvents, e.g.
methylene chloride, chloroform and carbon tetrachloride, and of some of the Freon refrigerants.
Formaldehyde is formed by the oxidation of methane and commercial solutions of formaldehyde in 5.
water, commonly called formalin, were formerly used as disinfectants and for preservation of biological
specimens.
Methane is used in the manufacturing of methanol which is used as good solvent for analysis of various 6.
drugs.
4.5 PHYSICAL PROPERTIES OF ALKANES
1. State:
2. Density: The densities of alkanes increase with increase in their molecular masses but become
3. Solubility:
4. Boiling points: The boiling points of n-alkanes increase gradually with increase in molecular masses
5
18 onwards are waxy solids at ordinary temperature.
constant at about 0.8 g/cm3. In general, it can be stated that all alkanes are lighter than water.
dissolves like,’ these are insoluble in polar solvents like water but are soluble in non-polar (organic)
solvents such as benzene, ether and carbon tetrachloride. Their solubility increases with an increase
in their molecular masses.
2 group (Fig. 4.3).
The increase in boiling point can be explained
in terms of intermolecular forces of attraction.
Alkanes are non-polar compounds possessing
forces) of attraction between their molecules.
These forces act along the surface of the
molecules and their magnitude increases
with an increase in surface area. As the
molecular size of alkanes increases, the
surface area increases, and thus the boiling
point accordingly increases.
It may be noted that branched chain
isomers, in general, have lower boiling points
than the corresponding straight chain isomers.
3 For example, the boiling points of n-pentane
Figure 4.3 Increase in the boiling point of
n-alkanes with the increase in the number of
carbon atoms per molecule.