Alkanes (CnH2n+2)
- Alkanes are the simplest members of the hydrocarbon family.
- The general formula of alkane family is CnH2n+2
- All bonds between carbon-carbon atoms are single.
Physical Properties of Alkanes:
- They are colorless and odorless.
- They have low densities, and low melting and boiling points.
- But all these quantities increase by increasing number of carbon atoms.
- At ordinary conditions, first four members of the unbranched alkanes are gases, C5-C17 unbranched alkanes are liquid and the unbranched alkanes with 18 and more carbon atoms are solids.
- Molecules of alkanes are non-polar. So they are insoluble in water. (Water molecule is polar and polar substances cannot dissolve non-polar ones.)
Alkane |
Alkane Name |
Alkyl |
Alkyl name |
CH4 |
Methane |
CH3- |
Methyl |
C2H6 |
Ethane |
C2H5- |
Ethyl |
C3H8 |
Propane |
C3H7- |
Propyl |
Name |
# carbons |
Structure |
Methane |
1 |
CH4 |
Ethane |
2 |
CH3-CH3 |
Propane |
3 |
CH3-CH2-CH3 |
Butane |
4 |
CH3-(CH2)2-CH3 |
Pentane |
5 |
CH3-(CH2)3-CH3 |
Hexane |
6 |
CH3-(CH2)4-CH3 |
Heptane |
7 |
CH3-(CH2)5-CH3 |
Octane |
8 |
CH3-(CH2)6-CH3 |
Nonane |
9 |
CH3-(CH2)7-CH3 |
Decane |
10 |
CH3-(CH2)8-CH3 |
Nomenclature of Alkanes:
Knowing names of alkyl groups and some very basic rules you can easily name branched alkanes. Here are these rules:
- Indicate the longest continuous carbon chain. This chain determines the parent name of the alkane.
All above possess 5 carbon atoms in their longest continuous carbon chains. So all are of them are pentanes.
- Number the carbon atoms of the longest continuous chain, beginning with the end of the chain nearer the substituent.
- Designate the location of the substituent group by the use of numbers obtained by application of second rule. Numbers are separated from words by a dash (-).
2-Methylbutane 3-Ethylhexane 3-Methylhexane
- When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain. To separate the numbers use a comma.
3-Ethyl-2-methylheptane 3-Bromo-4-ethyloctane
- When two substituents are present on the same carbon atom, use that number twice.
3-Ethyl-3methylpentane 2-chloro-2-methylbutane
- When two or more substituents are identical, indicate this by the use of prefixes di-, tri-, tetra-, and so on.
2,3-Dimethylbutane 3,4,5-Trimethylhexane 2-Bromo-2,3,4-trimethylhexane
Isomerism
The existence of different compounds that have the same molecular formula is called isomerism.
Molecules that have the same kinds and numbers of atoms but different arrangements are called isomers.
Compounds having the same molecular formula but differing in structural formula are called structural isomers. Isomers have different physical and chemical properties.
n-butane and 2-methylpropane are the isomers of each other.
Note: n represent normal
isomers of C4H9Cl
1-Chlorobutane 2-Chlorobutane 1-Chloro-2-methylpropane 2-Chloro-2-methylpropane
Chemical Properties:
Alkanes are relatively inert, that is, they have little affinity for most chemical reagents. They are insoluble in water and do not react with aqueous solutions of (most) acids or alkalis. Alkanes do not react with oxidizing agents or reducing agents, even with very strong oxidizing agent potassium permanganate KMnO4. However they do react with some reagents such as oxygen and halogens.
- Combustion reaction: Alkanes give highly exothermic reactions with oxygen. That’s why many alkanes are used as fuels. The products of these reactions are water and carbon dioxide.
C3H8+5O2 --> 3CO2+4H2O + Δ
- Substitution Reactions: Hydrogen atom(s) of the alkanes may displace with another atom or a group of atoms. Such reactions are called substitution reactions. Substitution reactions are characteristic for alkanes. Halogens (Cl2, Br2), and nitric acid give substitution reactions with alkanes. The reactions with chlorine and bromine are often called chlorination and bromination reactions.
+HO-NO2--> 
+H2O
Alkanes give substitution reactions with halogens only in the presence of sunlight or heat
light or heat
CH4+Cl2 --> CH3Cl + HCl
Chloromethane
If there is enough chlorine, further chlorination is possible.
Cl2 Cl2 Cl2
CH3Cl --> CH2Cl2
--> CHCl3
--> CCl4
Chloromethane Dichloromethane trichloromethane tetrachloromethane
So the overall reaction can be written as follows:
CH4+ 4Cl2 --> CCl4+ 4HCl
- Dehydrogenation reaction: Dehydrogenation reactions are the reactions in which water molecule(s) are removed from the alkane. Such reactions take place only in the presence of proper catalysts (Pt, Ni, Al2O3 or Cr2O3) and at high temperatures.
CH3-CH3 --cat,t--> CH2=CH2+H2
- Cracking: Cracking is the name of the process of breaking down chemical compounds by heat. Cracking of alkanes give a smaller alkane and an alkene. For example the molecule C15H32 might be broken down into C8H18 and C7H14.
C15H32 --t--> C8H18 +C7H14
- Decomposition reactions: Alkanes decompose when they are heated. Methane, for example, at 1000oC decomposes to carbon and hydrogen. But when the temperature is over 1500oC the products are hydrogen gas and acetylene, C2H2.
CH4 --1000oC--> C+2H2
2CH4
--1500oC--> 
+3H2
Preparation of Alkanes:
- Hydrogenation of unsaturated hydrocarbons: Alkenes and alkynes may add hydrogen molecules to give saturated hydrocarbons in the presence of nickel or platinum catalyst.
cat., t
CH3-CH=CH2 +H2
-->CH3-CH2-CH3
- Wurtz synthesis: When alkyl halides (R-X) are heated with sodium, alkanes with higher carbon content are produced.
R-R + 2NaX (or)
R-X+ 2Na+X-R` R-R` + 2NaX (or)
R`-R` + 2NaX
For example, if we use chloromethane and chloroethane in Wurtz synthesis the possible products will be ethane, propane and butane.
--> C2H6+2NaCl (or)
CH3Cl+ 2Na+C2H5Cl
--> C3H8+2NaCl (or)
--> C4H10+2NaCl
- Additional methods for preparation of methane:
- Synthesis from its simple substances:
cat., t
C+2H2 --> CH4
- Synthesis in lab from aluminum carbide:
Al4C3+12H2O --> 4Al(OH)3+ 3CH4
Uses of Alkanes
- As consumers we have little direct use of ethane but many of us use propane as a commercial fuel, LPG.
- Several of the higher molecular weight molecules beginning with pentane are liquids and are major components of gasoline.
- Larger alkanes are oily or greasy liquids or waxy solids at room temperature. Candle wax is an example for these.
- Alkanes as a class are compounds of very low toxicity. Because of their very low toxicity the higher alkanes are useful as medical lubricants.
CYCLOALKANES (CnH2n)
Carbon atoms in alkanes can be attached to one another in a ring formation and such compounds are called cycloalkanes. The general formula of cycloalkanes is CnH2n.
Cycloalkanes are named by adding the prefix cyclo - to the names of the alkanes. For example, the cycloalkane with three carbon atoms is called cyclopropane.