Alkanes

ALKANES

Alkanes are a class of saturated open chain hydrocarbons containing only carbon-carbon single bonds. Methane  is the simplest and the first member of this family. It is a colorless and odorless gas that is found in coal mines, natural gas, and in the digestive systems of ruminants.

When one hydrogen atom of methane is replaced by a methyl group (-CH3), the resulting molecule is ethane. Ethane is a colorless, odorless gas that is commonly used as a fuel for heating and cooking.

By replacing additional hydrogen atoms with methyl groups, we can generate a series of alkanes with increasing numbers of carbon atoms. For example, replacing two hydrogen atoms in ethane with methyl groups yields propane (C3H8), and replacing three hydrogen atoms in propane with methyl groups yields butane (C4H10).

This process can be continued to produce alkanes with any desired number of carbon atoms. The general formula for alkanes is CnH2n+2, where n is the number of carbon atoms in the molecule.

 

 

 

Alkanes are a family of hydrocarbons with the general formula CnH2n+2, where n is the number of carbon atoms in the molecule. They are also referred to as paraffins due to their relatively low reactivity towards acids, bases, and other reagents.

The structure of methane (CH4), which is the simplest alkane, is tetrahedral in shape according to the VSEPR theory. The carbon atom is located at the center of the tetrahedron, while the four hydrogen atoms are situated at the tetrahedron's corners. The bond angles between the H-C-H atoms are all 109.5 degrees.

 

In alkanes, the tetrahedra are joined together with C-C and C-H bond lengths of 154 pm and 112 pm, respectively. The C-C and C-H σ bonds are formed by the head-on overlapping of sp3 hybrid orbitals of carbon and 1s orbitals of hydrogen atoms.

The homologous series of alkanes can be extended by adding one or more methylene (-CH2-) groups to the molecule. Each member of the series differs from the previous member by a -CH2- group. The physical and chemical properties of the alkanes vary as the chain length increases due to an increase in the van der Waals' forces between the molecules, as well as an increase in surface area and molecular weight.

 

 Nomenclature and Isomerism

 

  • The nomenclature and isomerism in alkanes can be understood through examples.
  • The first three alkanes (methane, ethane, and propane) have only one structure each.
  • Higher alkanes can have more than one structure due to the possibility of branching in the carbon chain.
  • For example, C4H10 can exist in two isomeric forms: n-butane and isobutane.
  • Similarly, C5H12 can exist in three isomeric forms: n-pentane, isopentane, and neopentane.
  • Isomers of the same molecular formula can have different physical and chemical properties due to their different structures.
  • Structural isomers differ in their carbon atom arrangement, while chain isomers differ in the arrangement of the carbon chain.
  • Isomerism is important because it affects the physical and chemical properties of organic compounds, such as boiling points and reactivity.

 

 

The structural isomers of butane are structures I and II

I


Butane (n- butane), (b.p. 273 K)

II



2-Methylpropane (isobutane) (b.p.261 K)

The chain isomers of pentane are structures III, IV, and V.

 

III



Pentane (n-pentane)

(b.p. 309 K)

 

IV



 2-Methylbutane (isopentane)

(b.p. 301 K)

 

 

V

2,2-Dimethylpropane (neopentane)

(b.p. 282.5 K)

Carbon atoms can be classified as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) based on the number of carbon atoms attached to them. For example, a carbon atom that is attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called a primary carbon atom. Terminal carbon atoms are always primary. A carbon atom attached to two carbon atoms is known as secondary, while a tertiary carbon is attached to three carbon atoms, and a neo or quaternary carbon is attached to four carbon atoms.

For higher alkanes, there can be an even larger number of isomers. For example, C6H14 has five isomers, C7H16 has nine, and as many as 75 isomers are possible for C10H22. Therefore, the number of possible isomers increases rapidly with the increase in the number of carbon atoms in the molecule.

Nomenclature

 

 

It is important to be able to write the correct IUPAC name for a given structure, as well as the correct structure from a given IUPAC name. To write the structure, first identify the parent alkane and write its longest chain of carbon atoms. Number the carbons in the chain and then attach the substituents to the correct carbon atoms. Finally, satisfy the valence of each carbon atom by adding the correct number of hydrogen atoms.

For example, to write the structure of 3-ethyl-2,2-dimethylpentane:

i) Draw the chain of five carbon atoms:

C - C - C - C - C

ii) Give a number to each carbon atom:

C1 - C2 - C3 - C4 - C5

iii) Attach an ethyl group to carbon 3 and two methyl groups to carbon 2:

iv) Satisfy the valence of each carbon atom by adding the correct number of hydrogen atoms:

In this way, we can arrive at the correct structure.

 

Preparation of alkanes

 

From unsaturated hydrocarbons to alkanes

 

Alkanes can be prepared by the hydrogenation of alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium, or nickel. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen-hydrogen bond. Platinum and palladium catalyze the reaction at room temperature, but relatively higher temperature and pressure are required with nickel catalysts. This process is called hydrogenation.

 

 

 

From alkyl halides to alkanes

Alkanes can be prepared by the reduction of alkyl halides with zinc metal in the presence of concentrated hydrochloric acid. This process is called the Wurtz reaction.

 

Wurtz reaction

Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon
atoms.

 

From carboxylic acid to alkanes

Decarboxylation

The process of decarboxylation involves the removal of a carboxyl group (-COOH) from a carboxylic acid, resulting in the formation of an alkane with one less carbon atom than the original acid. This reaction is typically carried out by heating the sodium salt of the carboxylic acid with soda lime, which is a mixture of sodium hydroxide and calcium oxide. The reaction can be represented as:

 

 

Kolbe's electrolytic method

An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis

gives alkane containing even number of carbon atoms at the anode.



The reaction is supposed to follow the following path :

ii) At anode:

Acetate ion                 Acetate free radical              Methyl free radical

iii) 



iv) At cathode :

 

Physical properties of alkanes

Alkanes are almost non-polar molecules due to the covalent nature of C-C and C-H bonds and the small difference in electronegativity between carbon and hydrogen atoms. As a result, they possess weak van der Waals forces, which increase with the size of the molecule. The first four members of alkanes, C1 to C4, are gases, C5 to C17 are liquids, and those containing 18 carbon atoms or more are solids at 298 K. They are colorless and odorless.

 

Solubility of alkanes in water:

Based on their non-polar nature, alkanes are generally insoluble in water, which is a polar solvent. This is because like dissolves like, and polar substances dissolve in polar solvents, while non-polar substances dissolve in non-polar solvents. Thus, petrol, which is a mixture of hydrocarbons (including alkanes), is used as a fuel for automobiles and also for dry cleaning of clothes to remove grease stains. The greasy substance, which is a mixture of higher alkanes, is non-polar and hydrophobic in nature.

 

Boiling point of alkanes:

Boiling points of alkanes increase with an increase in molecular mass due to the increase in intermolecular van der Waals forces with an increase in the surface area of the molecule. The boiling points of isomeric alkanes, such as pentane, 2-methylbutane, and 2,2-dimethylpropane, can vary due to their different shapes.

For example, pentane has a continuous chain of five carbon atoms, making it a linear molecule. It has the highest boiling point (309.1K) among the three isomeric pentanes. On the other hand, 2,2-dimethylpropane is a highly branched alkane that has a spherical shape. It has a lower boiling point (282.5K) than pentane. This is because of its smaller area of contact, which results in weaker intermolecular forces between spherical molecules that can be overcome at a relatively lower temperature. The molecule with the most branching, 2,2-dimethylpropane, has the lowest boiling point among the three isomers.

 

Chemical properties of alkanes

Substitution reactions of alkanes

In this reaction, one hydrogen atom of methane  is replaced by a chlorine atom  to form chloromethane  and hydrogen chloride (HCl) is also produced as a by-product.

This reaction is an example of a substitution reaction, where one or more hydrogen atoms of an alkane are replaced by another atom or group of atoms. Halogenation, nitration, and sulfonation are common types of substitution reactions of alkanes.

Halogenation of alkanes can occur in the presence of halogens (fluorine, chlorine, bromine, or iodine) and requires heat or light to initiate the reaction. The halogen atoms substitute the hydrogen atoms of alkanes to form haloalkanes.

Nitration and sulfonation reactions, on the other hand, involve the substitution of hydrogen atoms of alkanes with nitro or sulfonic acid  groups, respectively. These reactions require strong acids, such as concentrated nitric or sulfuric acid, and high temperatures.

Overall, substitution reactions are important in organic chemistry because they allow for the synthesis of a wide range of organic compounds with different functional groups, which have various applications in industry and everyday life.

 

Halogenation of alkanes

Halogenation of alkanes involves the substitution of one or more hydrogen atoms of an alkane with a halogen atom, such as chlorine or bromine. The halogenation of alkanes is a substitution reaction, which is a type of organic reaction in which an atom or a group of atoms is substituted by another atom or group of atoms.

   

 

The rate of reaction of alkanes with halogens decreases in the order: F2 > Cl2 > Br2 > I2. This is due to the decreasing bond dissociation energy of the halogens in this order, which affects the strength of the C-Halogen bond formed during the reaction. Fluorination is indeed a very violent reaction and is not easily controlled, which is why it is not commonly used. Iodination, on the other hand, is a very slow and reversible reaction. It can be carried out in the presence of oxidizing agents like HIO3 or HNO3 to increase the reaction rate and yield.

The rate of replacement of hydrogens of alkanes by halogens is influenced by the degree of substitution of the carbon atom to which the hydrogen is attached. Tertiary carbons (3°) are more easily substituted than secondary carbons (2°) which in turn are more easily substituted than primary carbons (1°). This is due to the greater stability of the free radical intermediate formed in the case of 3° carbons, which makes it easier for the halogen to replace the hydrogen atom.

 

Mechanism

(i) Initiation : The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The Cl–Cl bond is weaker than the C–C and C–H bond and hence, is easiest to break.

(ii) Propagation : Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.

The methyl radical thus obtained attacks the second molecule of chlorine to form CH3 – Cl with the liberation of another chlorine free radical by homolysis of chlorine molecule.

The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible and may occur. Two such steps given below explain how more highly haloginated products are formed.

(iii) Termination: The reaction stops after some time due to consumption of reactants and / or due to the following side reactions :

The possible chain terminating steps are :

(a)

(b)

(c)

Though in (c), CH3 – Cl, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.

 

Combustion of alkanes

The combustion of alkanes is an exothermic reaction where the carbon-carbon and carbon-hydrogen bonds are broken and new bonds are formed between carbon and oxygen atoms. The products of combustion are carbon dioxide and water. The heat released during the reaction is due to the formation of new, more stable bonds between the carbon and oxygen atoms, which releases energy.

The general combustion equation for any alkane can be represented as:

This equation shows that for complete combustion, each molecule of alkane requires (3n+1/2) molecules of oxygen. Incomplete combustion of alkanes can result in the formation of other products such as carbon monoxide and soot, which can be harmful to the environment and human health.

 

Controlled oxidation of alkanes

 

Controlled oxidation of alkanes can be achieved by using various catalysts and conditions. For example, when alkanes are heated with a regulated supply of air or dioxygen and in the presence of suitable catalysts like platinum or palladium, they can undergo selective oxidation to produce various products such as alcohols, aldehydes, ketones, and carboxylic acids.

 

 

 

Isomerisation of alkanes

Isomerization of alkanes involves the conversion of straight chain alkanes into branched-chain isomers by rearranging the carbon skeleton of the molecule. This reaction can occur under high temperature and pressure and in the presence of a catalyst, such as aluminum chloride, hydrogen chloride or platinum. The reaction is an equilibrium process, meaning that the product distribution depends on the temperature and pressure used.

 

 

Isomerization of alkanes is an important industrial process, as it can improve the octane rating of gasoline. Branched-chain alkanes have higher octane ratings than straight-chain alkanes, as they are less prone to knocking, a type of uncontrolled combustion in the engine. Therefore, isomerization of straight-chain alkanes to branched-chain isomers is often used to produce high-octane gasoline.

 

Aromatization of alkanes

n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues. This reaction is known as aromatization or reforming.

 

 

 

Reaction with steam of alkanes

 Methane reacts with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. This method is used for industrial preparation of dihydrogen gas

 

 

Pyrolysis of alkanes

Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.

 

Conformations of alkanes

  • Alkanes have carbon-carbon sigma (σ) bonds.
  • The electron distribution of the sigma molecular orbital is symmetrical around the internuclear axis of the C-C bond.
  • This symmetrical distribution allows for free rotation around the C-C single bond.
  • Different spatial arrangements of atoms in space can be obtained by the rotation of the C-C single bond.
  • These spatial arrangements are called conformations, conformers, or rotamers.
  • Alkanes can have an infinite number of conformations due to the free rotation around the C-C single bond.
  • However, the rotation around a C-C single bond is not entirely free, and a small energy barrier of 1-20 kJ mol–1 exists due to weak repulsive interaction between adjacent bonds.
  • This repulsive interaction is known as torsional strain.

 

Conformation of ethane-

  • Ethane contains a carbon-carbon single bond, with each carbon atom attached to three hydrogen atoms.
  • When one carbon atom is kept stationary and the other is rotated around the C-C axis, an infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom are generated.
  • These spatial arrangements are called conformational isomers or conformers.
  • There are infinite conformations of ethane, but two extreme cases are commonly considered: the eclipsed and the staggered conformations.
  • In the eclipsed conformation, the hydrogen atoms attached to two carbons are as close together as possible, resulting in high energy and instability.
  • In the staggered conformation, the hydrogens are as far apart as possible, resulting in low energy and stability.
  • Any other intermediate conformation is called a skew conformation.
  • It is important to note that in all conformations, the bond angles and bond lengths remain the same.
  • The eclipsed and the staggered conformations can be represented by Sawhorse and Newman projections.

 

Sawhorse projections of ethane

Sawhorse projection of the eclipsed conformation:

  • The eclipsed conformation is one of the extreme conformations of ethane where the hydrogen atoms attached to two carbons are as close together as possible.
  • In the sawhorse projection of the eclipsed conformation, the front carbon is viewed directly and the rear carbon is viewed at a slight angle (about 30°) from above.
  • The central C-C bond is drawn as a longer line with the upper end slightly tilted towards the right or left side.
  • The three hydrogen atoms attached to each carbon are shown as three short lines, each inclined at an angle of 120° to each other.
  • The hydrogen atoms on the rear carbon are in close proximity to the hydrogen atoms on the front carbon, resulting in high torsional strain.

Sawhorse projection of the staggered conformation:

  • The staggered conformation is the other extreme conformation of ethane where the hydrogen atoms are as far apart as possible.
  • In the sawhorse projection of the staggered conformation, the front carbon is viewed directly and the rear carbon is viewed at a slight angle (about 30°) from below.
  • The central C-C bond is drawn as a longer line with the upper end slightly tilted towards the right or left side.
  • The three hydrogen atoms attached to each carbon are shown as three short lines, each inclined at an angle of 120° to each other.
  • The hydrogen atoms on the rear carbon are as far away as possible from the hydrogen atoms on the front carbon, resulting in the lowest torsional strain.

 

 

Newman projections of ethane

  • Newman projection is a way of representing the conformations of molecules.
  • It is viewed along the C-C bond axis.
  • The carbon atom nearer to the viewer is represented by a point and the rear carbon atom by a circle.
  • Three hydrogen atoms attached to the front carbon are shown by three lines inclined at an angle of 120° to each other.
  • The three hydrogen atoms attached to the rear carbon are shown by shorter lines also inclined at an angle of 120° to each other.
  • The Newman projection helps to visualize the staggered and eclipsed conformations of ethane.
  • In the staggered conformation, the two methyl groups (CH3) are as far apart as possible and the hydrogen atoms on one carbon are as far as possible from the hydrogen atoms on the other carbon. This conformation has the lowest energy and is the most stable conformation of ethane.
  • In the eclipsed conformation, the two methyl groups are as close together as possible, resulting in a high-energy conformation. This conformation is less stable than the staggered conformation and has a higher energy due to the torsional strain between the adjacent CH3 groups.

 

 

Relative stability of conformations

  • The staggered form of ethane is the most stable conformation due to minimum repulsive forces and minimum energy.
  • The eclipsed form of ethane has increased electron cloud repulsions, resulting in higher energy and lesser stability.
  • Torsional strain is the repulsive interaction between electron clouds and affects the stability of a conformation.
  • The magnitude of torsional strain depends on the dihedral angle or torsional angle, which is the angle of rotation about the C-C bond.
  • The energy difference between the staggered and eclipsed forms of ethane is small, around 12.5 kJ/mol.
  • Despite this small energy difference, at ordinary temperatures, ethane molecules have enough thermal or kinetic energy to overcome this energy barrier through intermolecular collisions.
  • As a result, rotation about the carbon-carbon single bond in ethane is almost free for all practical purposes.
  • Staggered conformation is the preferred conformation, and it is not possible to separate and isolate different conformational isomers of ethane.