ALKANES

 

Introduction.

 

Alkanes are saturated hydrocarbons that is to say compounds of carbon and hydrogen which only have single, covalent, bonds holding the atoms together. The first ten members of this class of compounds have the following structures :

 

                  Name                                      Expanded structure                                Condensed structure

 

Methane

 

 

Ethane

 

 

Propane

 

 

 

Butane

 

 

 

 

Pentane

 

 

 

 

Hexane

 

 

 

 

Heptane

 

 

 

 

Octane

 

 

 

 

 

Nonane

 

 

 

 

 

Decane

 

 

                                                                                                                                         Table 1 Alkanes

 

 

They can all be represented by the general formula CnH2n+2   where n=1,2,3,4,etc

 

 

The above compounds have a very similar general make-up and this results in the members of the alkane group, or series, having similar properties. One member of the series differs from the next by a CH2 group only. Hence the mass and the molecular size increase steadily as one goes from the simplest member methane to the heavier ones. In fact, as the mass increases, the melting and boiling points increase with it. (Van der Waals intermolecular interactions, which are the forces holding the molecules together, increase with the surface area as the molecules grow in size.) 

 

Series of related compounds like the alkanes above are said to form

HOMOLOGOUS SERIES.

 

(Members of a homologous series must

(i)                                                      contain the same elements,

(ii)                                                    have the same functional groups and thus similar chemical properties,

(iii)                                                   have a difference of a –CH2- between successive members which leads to

(iv)                                                  members of the series having a gradual change in physical properties and,

(v)                                                    finally, they must be represented by a generalized formula )

Eg:

 

Alkane

Formula

M.pt.

B.pt.

Radical name

Radical

 

Methane

 

 

- 182.5

 

- 161.7

 

Methyl

 

 

Ethane

 

 

- 183.3

 

- 88.6

 

Ethyl

 

 

Propane

 

 

- 187.7

 

- 42.1

 

Propyl

 

 

Butane

 

 

- 138.3

 

- 0.5

 

Butyl

 

 

Pentane

 

 

- 129.8

 

36.1

 

Pentyl

 

 

Hexane

 

 

- 95.3

 

68.7

 

Hexyl

 

 

Heptane

 

 

-90.6

 

98.4

 

Heptyl

 

 

Octane

 

 

- 56.8

 

125.7

 

Octyl

 

 

Nonane

 

 

- 53.5

 

150.8

 

Nonyl

 

 

Decane

 

 

- 29.7

 

174.0

 

Decyl

 

                                                                                                                                                Table 2 Alkanes

 

Nomenclature

 

The names for the first ten straight chain alkanes have been given as the naming for organic compounds always depends on the name of the longest uninterrupted carbon chain. This forms the base name, i.e. the name of the parent molecule chosen, and is shown in green and underlined in Table 2 Alk above. The ending then tells us about the functionality present - and the number, when present, tells us where that functionality is or where it starts.

For alkanes, with no functionality, the ending of –ane is used.

 

Rules for naming compounds are :

(i)                  Identify the longest uninterrupted, i.e. unbroken, chain. This will form the base name as though the compound is derived from this parent molecule.

(ii)                Identify the groups or side chains which have not been included in the basic (parent) molecule and the number of each group present

(iii)               Number the molecule starting with the end which gives the side groups or substituents the lowest numbers possible i.e start numbering from the end nearest to a substituent. (If there are very similar numbers for a given structure, the numbering chosen would be the one with the smallest number at the first point of difference e.g. 2,2,5-trimethylhexane and not 2,5,5-trimethylhexane. [The first point of difference here would be a choice between the second number : a 2 is smaller than a 5])

(iv)              The substituents are listed alphabetically but similar substituents are grouped together. Two methyl groups will give dimethyl (di when there are two, tri for three, tetra, penta, hepta, hexa, octa, nona, deca, etc for four, five, six,  etc). However, for naming purposes, the m in dimethyl is considered and not the d of the multiple di. The substituents are preceded by their position numbers on the base molecule, one number for each substituent with each number separated from the other by a comma. A dash / hyphen separates the numbers from the words.

(v)                For saturated ring structures i.e. cycloalkanes, the base molecule is the cycloalkane itself. All other groups are considered as substituents. The ring is numbered clockwise or anticlockwise starting from the group or radical or substituent, which comes first alphabetically.

 

 

 

E.g

 

 The molecule on the right is a decane derivative.

A quick look at the same molecule below, where the parent molecule has been identified and numbered, clearly shows this. The substituents or groups not included in the parent structure, (alkyl groups here), have also been isolated by placing them in boxes and are also coloured blue for easier identification.

 The missing substituents are two ethyl groups on carbons number 5 and 7 and a methyl group on carbon number 3

 

Therefore, the systematic name (i.e. name according to accepted rules) would be :

 

5,7-diethyl-3-methyldecane and not

4,6-diethyl-8-methyldecane as the alternative numbering would give.

 

 

 

Note on representations of a 3-dimensional structure on a 2-dimensional surface.

 

It is worth remembering, particularly when drawing and naming structures, that the molecules we are trying to represent are in fact 3-dimensional entities. Molecules that appear to be different when drawn on paper may in fact turn out to be only one compound. Taking chloroethane as the simplest of examples, only one chloroethane molecule can exist despite showing the chlorine atom to appear on different sides of the molecule when representing its molecule on paper. Even turning the molecule round can give the impression that the chlorine is now on a different carbon.

 

 

 

 

General methods of preparation of alkanes

 

A.                 From Alkenes.  Hydrogenation (“reduction”) of alkenes.

 

The addition of hydrogen to alkenes is known as a “hydrogenation” or “reduction”. In this reaction a molecule of hydrogen is added to the alkene molecule at the site of unsaturation i.e. where the double bond is. This is achieved under mild conditions when a catalyst is used to bring about this change.

 

 

Suitable catalysts for this reaction are : (a)  Raney Nickel,     (b) Pd on C    or (c) Pt on C

The conditions to be used can be moderate temperatures and pressures although catalytic hydrogenations often occur at room temperature and pressure. (Any increase in pressure is bound to bring about the expected reaction in better yields – Le Chatelier).

 

 

This is by far the most important way of generating alkanes and is in many ways the easiest reaction to carry out.

 

 

 

 

B.           From Carboxylic Acids and their salts - by decarboxylation reactions i.e. removal of CO2

 

(i)                                                      Using Sodalime.            

 

When a carboxylic acid, or its salt (sodium, potassium or calcium salts are commonly used) is heated strongly with sodalime (which is essentially sodium hydroxide mixed with calcium oxide to give a non-deliquescent solid), the carboxylic acid looses CO2 and gives the alkane :

 

 

 

 

or by way of a generalized representation of this reaction :

 

where R represents any alkyl (or even aryl) group, and NaOH here specifically refers to sodalime. (Some authors prefer to use Ca(OH)2 as the reagent in sodalime then giving CaCO3 as the inorganic product)

This reaction gives the required alkane albeit in low yields. The problem being that it is physically difficult to get an infinitely intimate mixture of solids for reaction to proceed efficiently when the heating commences.

 

 

(ii)                                                    By Electrolysis. (The Kolbe Process)

 

The electrolysis of aqueous solutions  of carboxylic acid salts similarly gives a decarboxylation of the anion  (at the anode) and two alkyl residues (i.e. groups) combine together to give an alkane.

 

or in general :

 

 

 

The limitation with this technique is that it only produces alkanes that have an even number of carbons. If alkanes with an odd number of carbon atoms are needed, mixtures of carboxylate salts will have to be used. Such procedures would however also make it possible for other, perhaps undesirable, products to be formed. The nature of the alkanes is very much dependent on the chance collisions of the various alkyl radicals present close to the anode where the decarboxylation processes take place.

 

 

C.                 From Haloalkanes.

 

(i)                                                      By reduction of haloalkanes - using an “active” (atomic?) form of hydrogen.

Haloalkanes can easily be made to give the alkanes if these are exposed to an environment where hydrogen atoms are generated. Such conditions are found for example when sodium/mercury or aluminium/mercury amalgams are in contact with ethanol. Similarly, a zinc/copper couple with ethanol gives the same result, as would nickel/aluminium + NaOH or zinc/mercury + HCl.

 

 

General reaction :

 

                                  Sources of H :    Na/Hg + C2H5OH   ;   Al/Hg + C2H5OH;   Cu/Zn + C2H5OH;

                                                            Ni/Al + NaOH;    Zn/Hg + HCl.

 

(ii)                                                    Halogen removal via Grignard reaction.

In this reaction, the haloalkane is first made to give the Grignard reagent as an intermediate and this is then hydrolysed (with water!) to give the alkane. This is not the best use of Grignard chemistry but it does give the alkane as a product if one so wishes.

 

 

or

 

 

 

(iii)                                                   Dehalogenation using metallic sodium. The Wurtz Reaction

 

In this procedure, where the haloalkane is heated with metallic sodium in an inert solvent such as ether, the haloalkane loses its halogen and two radicals then come together to give an alkane with a longer carbon chain. (In fact, the chain doubles in length).

 

 

for which the generalized reaction would be :

 

 

In this reaction, same as for one of the decarboxylation reactions, only even numbered alkanes can be made. If the alkanes with an odd number of carbons have to be produced, mixed haloalkanes must be used. However, this will again then give rise to a number of products where the different alkyl groups come together to give the final alkane. Hence, a mixture of RX and R’X yields RR, RR’ and R’R’ as the alkanes.

 

 

 

D.                 From Alcohols. 

 

Aggressive reduction of alcohols by very strong reducing agents, will remove the alcohol functionality (i.e. the OH group) replacing it with an H atom. A Reducing agent that is good enough for this transformation is red P + conc. HI(aq) heated under pressure.

 

 

Reaction represented in a general way would be :

 

 

 

E.                  From Carbonyl compounds i.e. aldehydes and ketones – By strong reducing agents.

 

Using the same conditions for reduction of alcohols also gives alkanes from the carbonyl compounds such as aldehydes (RCHO) and ketones (RCOR).

 

i.e.

 

 

 

 

 

Special synthetic methods for methane.

 

a.                                           From Aluminium carbide.

 

 The reaction of aluminium carbide with water gives methane and aluminium hydroxide as the only products.

 

 

 

b.                                          Fischer –Tropsch Process (Ind)

 

Industrial synthesis of methane can be achieved using CO and hydrogen (both easily obtained from water and carbon) using Nickel as catalyst and the reaction can be carried out at a modest 300oC.

 

           

 

 

 

You can take me back, now.