Introduction to Examples of Alkanes
Alkanes are hydrocarbons with the general formula CnH2n+2. Alkanes can be divided into two main groups of compounds: Alliphatic and Alicyclic compounds. The alliphatics are open chain carbon compounds while alicyclic compounds are closed ring chain compounds. In a molecule of alkane there are present ony C-C sigma bonds. The open chain alkanes do show the presence of substituents of other alkyl groups on the carbons of the main chain, for example CH3CHCH3CH2CH3 is called 2-methyl-2-butane.
Thus open chain alliphatic alkanes are of two types 1) straight chains and 2)branched chains.
The straight chain alliphatic compounds form a series of compounds in which each member differs from the preceeding one by a -CH2 group. Such a series is called a homologous series. All of these alkanes show similar chemical properties though many of the physical properties may be different.
Nomenclature is Example of Alkanes
The alkanes are named according to the IUPAC system. The rules followed in naming these compounds are:
1) The longest straight chain of maximum carbon atoms is selected.
2) Numbering of the carbon atoms are done from one end in such a way that the substituent on the molecule gets the lowest number.
3) In case of more than one alkyl side chain the longest chain is numbered so that the simplest alkyl group gets the lowest number .
4) The substituents are named in alphabetical order along with the number of carbon atom to which it is bonded.
5) If two or more identical substituents are present the names of the identical substituents are not repeated but prefixes as di, tri, tetra, are used while writing the name of the compound as 2,2 dimethyl propane - CHC(CH3)2CH3 .
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Occurence of Examples of Alkanes
Alkanes occur naturally in the petroleum and natural gas.
Methane- a lower alkane with the molecular formula CH4 is produced in marshy places due to the bacterial decomposition of vegetable matter. It is called marsh gas. When it mixes with P2H4 it starts burning with a blue flame. P2H4 is produced as a result of putrefaction of animal bodies and P2H4 spontaneously burns in air liberating heat and in this heat methane burns.
Preparation of Examples of Alkanes
Though each alkane can be prepared by a unique method most suited economically in general alkanes, in general, alkanes are prepared as follows:
Let R represents an alkyl group.
1) Hyddrogenation of unsaturated hydrocarbons like alkenes and alkynes: When alkenes and alkynes are reduced with hydrogen in presence of finely powdered platinum or palladim catalyst, or nickel at 200 to 300°C, alkanes with the same number of carbon atoms are obtained. This reaction is called Sabatier Senderen's reaction.
RCH=CHR + H2 → RCH2=CH2R in presence of powdered nickel and heat.
alkene alkane
2) Reduction of Alkyl Halides: The reducing agents that can be used are zinc and acetic acid, zinc and sodium hydroxide, zinc couple and ethanol, alluminium amalgam in ethanol, lithium alluminium hydride(LiAlH4), sodium boro hydride(NaBrH4), hydrogen/Pd catalyst, hydrogen/Pt catalyst.
R-X → R-H in presence of zinc/ HCl
here X is a halogen atom.
3) By reduction of aldehydes and ketones: This reaction is called Clemensen Reduction. Reducing agent is zinc amalgam/ concentrated HCl.
O
║
R-C-H → R-CH3 in presence of Zn-Hg and conc HCl
alkane
R-CO-R’ → R-CH2-R in presence of Zn-Hg and conc HCl
alkane
4) By reducing alchohols, aldehydes or carboxylic acids or iodides using red phosphorous and HI.
ROH + 2HI → RH + I2 + H2O in presence of red phosphorous and 150°C
RI + HI → RH + I2 in presence of red phosphorous and 150°C
RCOOH + 6HI → RCH3 + 3I2 + 2H2O in presence of red phosphorous and 150°C
RCOR + 4HI → RCH3 in presence of red phosphorous and 150°C
RCHO + 4HI → RCH2R in presence of red phosphorous and 150°C
Red phosphorous reacts with iodine to produce HI again and this can be reused.
5) By hydrolysis of Grignard's Reagent: When Grignard's reagent is treated with cold water or dilute acid the nalkane is produced. example-
RMgX + H2O → R-H +Mg(OH)X
cold
6) When a carbonyl compond is reacted with hydrazine we get hydrazone which on heating to 170 to 180°C with NaOC2H5 at 180°C gives an alkane. Thus
i) R-CO-R + NH2NH2 → R2C=N-NH2
ii) R2C=N-NH2 → N2 +R2CH2 in presence of NaOC2H5 and at 180°C
This is also called Wolf Kishner Reduction.
7) Lower alkanes can be prepared from sodium salts of carboxylic acids by heating the anhydrous salt in presence of lime and caustic soda. As a result there is loss of one molecule of carbondioxide and the alkane is produced. Thus,
RCOONa + NaOH → RH + Na2CO3 in presence of CaO and heat.
8) By Wurtz Reaction: When an alkyl halide is treated with sodium in dry ether an alkane is obtained. Metallic sodium acts as a reducing agent while ether acts as a solvent.
While primary and secondary alkanes can be prepared by this method, tertiary alkanes cannot be prepared. THis method is not suitable for preparation of methane and nor for alkanes with odd number of carbon atoms.
CH3I + Na + C2H5I → CH3-CH3 + C2H5-CH3 + C2H5-C2H5
So isolation of the alkanes become difficult from this mixture and that is why this is not a good method.
9) By Kolbe's Synthesis: When potassium salt of a carboxylic acid is subjected to electrolysis between platinum electrodes then at the cathode potassium hydroxide (KOH) and hydrogen gas are formed and at the anode carbondioxide and alkane are formed. When the mixture obtained at anode is passed through caustic potash solution then carbon dioxide is absorbed and alkane is obtained.
10) Corey- House Alkane Synthesis: When an alkyl halide RX is treated with lithium in ether and cuprous halide is added to it, a complex lithium dialkyl cuprate ( R2CuLi) is produced. If this complex is treated with another alkyl halide R'X then an alkane (R-R') is obtained.
i) RX + Li → R-Li + X-
ether
ii) 2RLi + CuI → R2LiCu + LiI
ether
iii) R2LiCu + R'X → R-R' + RCu +LiX
ether
11) When inorganic carbides are treated with hot water we get the formation of methane
BeC2 + 4H2O → CH4 + 2 Be(OH)2
When trialkyl borane is treated with AgNO3 and NaOH at low temperature a higher alkane is obtained as
(C2H5)3Br → C2H5-C2H5-C2H5
Properties - Examples of Alkanes
The physical properties of alkanes: Most alkanes of low molecular weight are gases while the next higher molecular weight alkanes are liquids and alkanes with more than C18 are colourles solids. Alkanes are neutral in nature, not affected byacids and alkalis at room temperature. Alkanes are bad conductors of heat and electricity. Boiling point and melting point of liquid alkanes and solid alkanes respecively increase with increase in molecular weight. Alkanes are insoluble in water but soluble in organic solvents. Liquid alkanes are lighter than water but as molecular weight increase the density of the alkanes also increase.
Chemical properties of Alkanes:-
Alkanes are saturated hydrocarbons so they are linked by single covalent bonds and these bonds are hard to break thus they are stable and so lesser chemical reactivity is there. The chemical properties of Alkanes are:
1) Combustion reactions:-
a) Combustion in excess of air:- On burning alkanes in excess of air carbon-dioxide and water vapor is formed with evolution of heat. Amount of heat evolved depends on the number of Carbon atoms. More the number of carbons in the hydrocarbon more are the heat evolved.
General equation:
CnH2n+2 + (3n+1/2) O2 -------------> nCO2 + (n+1) H2O + Heat.
Example:- CH4 + 2O2 --------------> CO2 + 2H2O + Heat.
2C2H6 + 7O2 -----------> 4CO2 + 6H2O + Heat.
b) Combustion in limited supply of air:- When combustion takes place in limited supply of oxygen then carbon monoxide is produced instead of carbon dioxide. Carbon monoxide is a poisonous gas. Heat produced is also less.
Example:- 2CH4 + 3O2 -----------> 2CO + 4H2O + Heat.
2C2H6 + 5O2 ------------> 4CO + 6H2O + Heat.
2) Controlled Oxidation:-
a) Hydrocarbon ---------> Alcohol.
When 1volume of methane is mixed with 9volumes of oxygen is passed through Copper tube at 200°C, methyl alcohol is formed.
2CH4 + O2 –--------Cu(tube) 200°C --------> 2CH3OH.
b) Hydrocarbon ---------> Aldehyde.
When mixture of methane and oxygen is passed over heated molybdenum oxide (catalyst) formaldehyde is formed when the temperature range is 300°C- 500°C.
CH4 + O2 ----------MoO (350°-500°C) --------> HCHO + H2O.
Chemical properties of Alkanes (Some more reactions) :-
1) Cracking/ Pyrolysis: -
When alkanes are heated to high temperatures in absence of air, mixture of saturated and unsaturated hydrocarbons is formed. This can also be done at a lower temperature by use of catalyst such as silica or alumina. This type is called catalytic cracking.
Example:- C2H6 ------(SiO2)--------> C2H4 + H2.
2) Substitution Reactions: -
Alkanes undergo substitution reactions on reaction with Halogens . In presence of UV light and excess halogen reaction is chain reaction and proceeds till all the hydrogen of hydrocarbons are replaced.
Example:-CH4 + Cl2---------> CH3Cl + HCl
CH3Cl + Cl2 -------> CH2Cl2 + HCl
CH2Cl2 + Cl2 ------> CHCl3 + HCl
CHCl3 + Cl2 --------> CCl4 + HCl.
Uses of Alkanes
1) The lower members of the homologous series of alkanes are used as fuels.
2) Methane gas is now finding wide use in biogas fuels.
3) Propane is used as a refrigerant in the petroleum industry.
4) Many other alkanes are used in the chemical industry.
Alkanes are hydrocarbons with the general formula CnH2n+2. Alkanes can be divided into two main groups of compounds: Alliphatic and Alicyclic compounds. The alliphatics are open chain carbon compounds while alicyclic compounds are closed ring chain compounds. In a molecule of alkane there are present ony C-C sigma bonds. The open chain alkanes do show the presence of substituents of other alkyl groups on the carbons of the main chain, for example CH3CHCH3CH2CH3 is called 2-methyl-2-butane.
Thus open chain alliphatic alkanes are of two types 1) straight chains and 2)branched chains.
The straight chain alliphatic compounds form a series of compounds in which each member differs from the preceeding one by a -CH2 group. Such a series is called a homologous series. All of these alkanes show similar chemical properties though many of the physical properties may be different.
Nomenclature is Example of Alkanes
The alkanes are named according to the IUPAC system. The rules followed in naming these compounds are:
1) The longest straight chain of maximum carbon atoms is selected.
2) Numbering of the carbon atoms are done from one end in such a way that the substituent on the molecule gets the lowest number.
3) In case of more than one alkyl side chain the longest chain is numbered so that the simplest alkyl group gets the lowest number .
4) The substituents are named in alphabetical order along with the number of carbon atom to which it is bonded.
5) If two or more identical substituents are present the names of the identical substituents are not repeated but prefixes as di, tri, tetra, are used while writing the name of the compound as 2,2 dimethyl propane - CHC(CH3)2CH3 .
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Occurence of Examples of Alkanes
Alkanes occur naturally in the petroleum and natural gas.
Methane- a lower alkane with the molecular formula CH4 is produced in marshy places due to the bacterial decomposition of vegetable matter. It is called marsh gas. When it mixes with P2H4 it starts burning with a blue flame. P2H4 is produced as a result of putrefaction of animal bodies and P2H4 spontaneously burns in air liberating heat and in this heat methane burns.
Preparation of Examples of Alkanes
Though each alkane can be prepared by a unique method most suited economically in general alkanes, in general, alkanes are prepared as follows:
Let R represents an alkyl group.
1) Hyddrogenation of unsaturated hydrocarbons like alkenes and alkynes: When alkenes and alkynes are reduced with hydrogen in presence of finely powdered platinum or palladim catalyst, or nickel at 200 to 300°C, alkanes with the same number of carbon atoms are obtained. This reaction is called Sabatier Senderen's reaction.
RCH=CHR + H2 → RCH2=CH2R in presence of powdered nickel and heat.
alkene alkane
2) Reduction of Alkyl Halides: The reducing agents that can be used are zinc and acetic acid, zinc and sodium hydroxide, zinc couple and ethanol, alluminium amalgam in ethanol, lithium alluminium hydride(LiAlH4), sodium boro hydride(NaBrH4), hydrogen/Pd catalyst, hydrogen/Pt catalyst.
R-X → R-H in presence of zinc/ HCl
here X is a halogen atom.
3) By reduction of aldehydes and ketones: This reaction is called Clemensen Reduction. Reducing agent is zinc amalgam/ concentrated HCl.
O
║
R-C-H → R-CH3 in presence of Zn-Hg and conc HCl
alkane
R-CO-R’ → R-CH2-R in presence of Zn-Hg and conc HCl
alkane
4) By reducing alchohols, aldehydes or carboxylic acids or iodides using red phosphorous and HI.
ROH + 2HI → RH + I2 + H2O in presence of red phosphorous and 150°C
RI + HI → RH + I2 in presence of red phosphorous and 150°C
RCOOH + 6HI → RCH3 + 3I2 + 2H2O in presence of red phosphorous and 150°C
RCOR + 4HI → RCH3 in presence of red phosphorous and 150°C
RCHO + 4HI → RCH2R in presence of red phosphorous and 150°C
Red phosphorous reacts with iodine to produce HI again and this can be reused.
5) By hydrolysis of Grignard's Reagent: When Grignard's reagent is treated with cold water or dilute acid the nalkane is produced. example-
RMgX + H2O → R-H +Mg(OH)X
cold
6) When a carbonyl compond is reacted with hydrazine we get hydrazone which on heating to 170 to 180°C with NaOC2H5 at 180°C gives an alkane. Thus
i) R-CO-R + NH2NH2 → R2C=N-NH2
ii) R2C=N-NH2 → N2 +R2CH2 in presence of NaOC2H5 and at 180°C
This is also called Wolf Kishner Reduction.
7) Lower alkanes can be prepared from sodium salts of carboxylic acids by heating the anhydrous salt in presence of lime and caustic soda. As a result there is loss of one molecule of carbondioxide and the alkane is produced. Thus,
RCOONa + NaOH → RH + Na2CO3 in presence of CaO and heat.
8) By Wurtz Reaction: When an alkyl halide is treated with sodium in dry ether an alkane is obtained. Metallic sodium acts as a reducing agent while ether acts as a solvent.
While primary and secondary alkanes can be prepared by this method, tertiary alkanes cannot be prepared. THis method is not suitable for preparation of methane and nor for alkanes with odd number of carbon atoms.
CH3I + Na + C2H5I → CH3-CH3 + C2H5-CH3 + C2H5-C2H5
So isolation of the alkanes become difficult from this mixture and that is why this is not a good method.
9) By Kolbe's Synthesis: When potassium salt of a carboxylic acid is subjected to electrolysis between platinum electrodes then at the cathode potassium hydroxide (KOH) and hydrogen gas are formed and at the anode carbondioxide and alkane are formed. When the mixture obtained at anode is passed through caustic potash solution then carbon dioxide is absorbed and alkane is obtained.
10) Corey- House Alkane Synthesis: When an alkyl halide RX is treated with lithium in ether and cuprous halide is added to it, a complex lithium dialkyl cuprate ( R2CuLi) is produced. If this complex is treated with another alkyl halide R'X then an alkane (R-R') is obtained.
i) RX + Li → R-Li + X-
ether
ii) 2RLi + CuI → R2LiCu + LiI
ether
iii) R2LiCu + R'X → R-R' + RCu +LiX
ether
11) When inorganic carbides are treated with hot water we get the formation of methane
BeC2 + 4H2O → CH4 + 2 Be(OH)2
When trialkyl borane is treated with AgNO3 and NaOH at low temperature a higher alkane is obtained as
(C2H5)3Br → C2H5-C2H5-C2H5
Properties - Examples of Alkanes
The physical properties of alkanes: Most alkanes of low molecular weight are gases while the next higher molecular weight alkanes are liquids and alkanes with more than C18 are colourles solids. Alkanes are neutral in nature, not affected byacids and alkalis at room temperature. Alkanes are bad conductors of heat and electricity. Boiling point and melting point of liquid alkanes and solid alkanes respecively increase with increase in molecular weight. Alkanes are insoluble in water but soluble in organic solvents. Liquid alkanes are lighter than water but as molecular weight increase the density of the alkanes also increase.
Chemical properties of Alkanes:-
Alkanes are saturated hydrocarbons so they are linked by single covalent bonds and these bonds are hard to break thus they are stable and so lesser chemical reactivity is there. The chemical properties of Alkanes are:
1) Combustion reactions:-
a) Combustion in excess of air:- On burning alkanes in excess of air carbon-dioxide and water vapor is formed with evolution of heat. Amount of heat evolved depends on the number of Carbon atoms. More the number of carbons in the hydrocarbon more are the heat evolved.
General equation:
CnH2n+2 + (3n+1/2) O2 -------------> nCO2 + (n+1) H2O + Heat.
Example:- CH4 + 2O2 --------------> CO2 + 2H2O + Heat.
2C2H6 + 7O2 -----------> 4CO2 + 6H2O + Heat.
b) Combustion in limited supply of air:- When combustion takes place in limited supply of oxygen then carbon monoxide is produced instead of carbon dioxide. Carbon monoxide is a poisonous gas. Heat produced is also less.
Example:- 2CH4 + 3O2 -----------> 2CO + 4H2O + Heat.
2C2H6 + 5O2 ------------> 4CO + 6H2O + Heat.
2) Controlled Oxidation:-
a) Hydrocarbon ---------> Alcohol.
When 1volume of methane is mixed with 9volumes of oxygen is passed through Copper tube at 200°C, methyl alcohol is formed.
2CH4 + O2 –--------Cu(tube) 200°C --------> 2CH3OH.
b) Hydrocarbon ---------> Aldehyde.
When mixture of methane and oxygen is passed over heated molybdenum oxide (catalyst) formaldehyde is formed when the temperature range is 300°C- 500°C.
CH4 + O2 ----------MoO (350°-500°C) --------> HCHO + H2O.
Chemical properties of Alkanes (Some more reactions) :-
1) Cracking/ Pyrolysis: -
When alkanes are heated to high temperatures in absence of air, mixture of saturated and unsaturated hydrocarbons is formed. This can also be done at a lower temperature by use of catalyst such as silica or alumina. This type is called catalytic cracking.
Example:- C2H6 ------(SiO2)--------> C2H4 + H2.
2) Substitution Reactions: -
Alkanes undergo substitution reactions on reaction with Halogens . In presence of UV light and excess halogen reaction is chain reaction and proceeds till all the hydrogen of hydrocarbons are replaced.
Example:-CH4 + Cl2---------> CH3Cl + HCl
CH3Cl + Cl2 -------> CH2Cl2 + HCl
CH2Cl2 + Cl2 ------> CHCl3 + HCl
CHCl3 + Cl2 --------> CCl4 + HCl.
Uses of Alkanes
1) The lower members of the homologous series of alkanes are used as fuels.
2) Methane gas is now finding wide use in biogas fuels.
3) Propane is used as a refrigerant in the petroleum industry.
4) Many other alkanes are used in the chemical industry.