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Alkanes

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are good source of energy and are used as fossil fuels. Based on the nature of bonds between the carbon atom they are classified as alkanes, alkenes and alkynes. We will study some of the properties of alkanes and cyclo alkanes in the unit.

 

What are Alkanes?

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Alkanes are saturated hydrocarbons with general formula CnH2n+2. They have highest hydrogen-carbon ratio and hence burn with blue flame. They are least reactive as they cannot undergo addition reaction, a special type of reaction where atoms/groups are added across carbon-carbon bond.
The carbon atom in alkane is sp3 hybridized and hence all the carbon atoms will have tetrahedral shape.

Structure of Alkanes

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The outermost electronic configuration of carbon is 2s1,2p3 in the excited state. It undergoes sp3 hybridization to give 4sp3 hybridized orbitals. The four sp3 hybridized orbitals project in tetrahedral shape.Hence the structure of all alkanes can be understood as a combination of many tetrahedral shape. For example the structure of methane is in tetrahedral shape.

In the similar way the structure of propane can be viewed as a combination of three tetrahedral shapes end to end

Cycloalkanes

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Cycloalkanes are the types of alkanes with one or more rings in their chemical structure. Even in cyclo alkanes carbon- carbon bond exist but carbon atoms joined up in a ring.

The general formula for cycloalkanes are different from that of alkanes. The formula is CnH2n.

Some examples of cycloalkanes are:

Examples of Cycloalkanes

Ring strain


The structures mentioned above are not planar. All the cycloalkanes from cyclopentane upwards exist as "puckered rings". All the carbon atoms in cycloalkane is SP3 hybridised. So the cycloalkanes exists in chair form which is considered to be most stable form of cycloalkanes.

The chair form is shown below
Cyclohexane

Preparation of Cycloalkanes

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Cycloalkanes are prepared by any of the following methods.

1. By intermolecular Wurtz reaction of dihalides:


Organic compounds containing halogens at the terminals on reacting with sodium in the presence of ether undergo ring formation to give cycloalkanes. This is called as intramolecular wurtz reaction or Freunt reaction. For example 1,4 dichloro butane on reacting with sodium or zinc dust in the presence of ether gives cyclobutane.
Cl-CH2-CH2-CH2-Cl + 2Na →

2. By reduction of aromatic compounds:


Aromatic compounds like benzene, toluene on reduction with zinc dust gives cycloalkanes. Benzene on reduction with zinc dust gives cyclohexane.

3. By Clemmenson reduction:


Cyclic ketones on treating with zinc amalgam and concentrated hydrochloric acid gives cyclo alkanes. This is the effective method of preparing cycloalkanes in the pure form as cycloalkanes can easily be prepared from calcium salts of dicarboxylic acids. For example cyclopentanone on reduction with zinc amalgam and concentrated hydrochloric acid gives cyclopentane.

4. From alkenes:


Alkenes on reacting with methylene iodide gives cycloalkanes. For example propene on treating with methylene iodide gives 1-methyl cyclopropane.

CH3-CH=CH2 + CH2I2 1 Methyl Cyclopropane

Chemical Reactivity

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The relative reactivity of various hydrocarbons is given here for reference

alkanes< alkenes< alkynes

This order of reactivity is due to increase in the number of unsaturation across the carbon atoms. Hence the valence of carbon is not completed and they can add up atoms/groups.

Similarly cyclo alkanes > linear alkanes. This is due to the ring strain in the cyclo alkane which decreases the stability.

Properties of Alkanes

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  1. All the alkanes are colorless.
  2. The lower members (first four to five carbon atoms) are gases while the alkanes with up to 15 carbon atoms are liquids. The rest are solids.
  3. They have least carbon ratio comparing with alkene and alkyne and hence they can burn completely with blue flame to give maximum possible energy.
  4. Similarly the melting and boiling point increases as we increase the number of carbon atoms.
  5. But the branched alkanes will have less boiling point than linear alkanes owing to symmetry and packing efficiency.

Uses of Alkanes

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  1. Alkanes are used as fuels. They have high calorific value and produce less pollution as the carbon ratio is very small. Generally we use the term Octane number to denote the efficiency of a hydrocarbon like aviation fuel etc. Fuels with high octane number are ideal for high compression engines while with low octane number are ideal for low compression engines like diesel engines. The other term used to determine the quality of fuels is flash point. Flash point is the lower temperature at which a fuel can give sufficient vapor to form explosive mixture with air. LPG have very low flash point while diesel has high flash point.
  2. They are used in the manufacture of polymers.
  3. They are intermediates in the synthesis of many other important chemicals like ethanol

Properties of Cycloalkanes

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The properties of cycloalkanes are similar to properties of alkanes like combustion, substitution etc. The important distinguishing properties are discussed below.
  1. Addition reaction: Cycloalkanes having more ring strain are unstable and hence they undergo addition reaction with bromine like alkenes. For example cyclopropane on treating with bromine gives 1,3 dibromo propane. Other cycloalkanes undergo addition reaction at high temperatures. For example cyclobutane undergo addition reaction with bromine at 1200C to give 1,4 dibromo propane.
  2. Oxidation reaction: Cycloalkanes undergo ring opening with alkaline potassium permanganate to give dicarboxylic acids. This is oxidation reaction by cleavage of carbon chain. For example cyclobutane on treating alkaline KMnO4 gives sodium salt of malonic acid.

Ring Stabilization and Reactivity

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The reactivity of cycloalkanes depends on its stability. The preparation of cycloalkanes (ring formation) and reactivity (ring opening) are determined by the stability. But cycloalkanes are not equally stable like linear alkanes. This is due to two factors.

1. Angle strain:


In linear/branched alkanes all the bond angles are 109.5 as in tetrahedral shape. But in cycloalkanes, the angle is somewhat reduced. For example in cyclopropane the bond angle between carbon atoms is 60. Hence bonding electrons come closer which creates a strain called as angle strain.

Angle Strain

2. Steric hindrance:   


In ring compounds the hydrogen atoms are closer and in eclipsed position they will try to move away. This is called as steric hindrance. The total of angle strain and steric hindrance is called as ring strain.

Ring strain is the direct indicator of stability of cycloalkanes. It is calculated from the combustion data. It is found that cyclopropane, butane have more ring strain and hence they are highly unstable. But cyclohexane has nearly zero ring strain and it is highly stable.

This shows that all the bond angles in the cyclohexane are 109.5. But this is not possible in planar hexagonal structure. Hence it was proposed that cyclohexane is existing in a chair form where all bond angles are 109.5.

Cracking of Petroleum as a Source of Alkanes

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  • Cracking means breaking a complex long hydrocarbons into smaller useful hydrocarbons by applying heat, pressure over a catalyst.
  • All the petroleum products like diesel, petrol and LPG are hydrocarbons with a varying chain length.
  • Generally cracking means breaking a higher fraction like crude oil residues to lower fraction like LPG. There are many types of cracking employed like hydro cracking, steam cracking and thermal cracking.
  • In hydro cracking the cracking process is usually carried in the environment of hydrogen. Cracking is a free radical process.
  • Cracking is usually done in a catalyst. Zeolites are catalysts used in cracking process. They are shape selective catalysts which allow only specific size of products pass through the pores.
  • Hence when higher molecules are cracked in zeolites they are broken to smaller fragments.
More topics in Alkanes
Constitutional Isomers Alkyl Groups
Nmr Spectroscopy Cracking
Petroleum Refining Alkane Formula
Chloromethane Chloroethane
Combustion of Methane
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