Alkanes

Introduction

An alkane is a saturated hydrocarbon, in which there are no double bonds, and is made up entirely of Hydrogen and Carbon atoms. The formula for an alkane is CnH2n+2  although this might be different if a ring is present in the hydrocarbon chain.

The naming of straight chain hydrocarbons is as follows:

tang-01-organic-chemistry-and-alkanes-10-638.jpg

which have been discussed in the previous Chapter: Introduction to Organic Chemistry.

Apart from straight chain alkanes, one can also find ring structures, with the most common being:

cyclic

which would have a formula of CnH2n.

Sometimes the angle is so small that there would be a strain in the molecule due to repulsion between different sets of paired electrons, which would result in a highly reactive structure. This can be obtained either by having a ring angle which is too small or an angle which is too big since this would affect the bonding electrons which are not in the ring.

These rings tend to try and obtain an angle closer to that of a tetrahedron structure, ie. a 109.5o. The most  common  isomerism  is  the  puckering  of  the  cyclohexane,  in  which  there  are  two  different structures, the boat and the chair conformation, which can be seen below:

chair

Solubility

The solubility of each alkane chain is very minimal in aqueous solutions due to the lack of polar bonds. On the other hand, an alkane would be highly soluble in other alkanes or other organic compounds. This is due to the fact then when dissolution occurs there are breaking of intermolecular bonds between the solvent atoms and formation of new intermolecular bonds between the solute and the solvent. If the energy needed to break the bonds is bigger than the energy produced in the formation of new bonds, then the dissolution would not occur, while if the energy needed to break the bonds is lower than that released in the formation of bonds dissolution would occur. The reason while polar solutes dissolve in polar solvents while non polar solutes dissolve in non polar solvents is due to the fact that once a polar solvent or solute is present too much energy would be needed to break the intermolecular bonds, H- bonding when compared to the energy released after dissolution (Van der Waals’ bonding) due to the fact that the latter bonds are much weaker then H-bonding..

Boiling Points

As one can evaluate the boiling point with an increase in molecular size one would immediately see that there is an increase in boiling point with an increase in molecular size. This would be explained by the fact that due to a lack of polarity there would only be weak Van der Waals bonding between different molecules and due to the fact that these types of bonds are very weak and thus small molecules would be gaseous. As the size increases the boiling point would increase.  The change to a liquid is observed with a Carbon chain of more than 5.

The difference in boiling point between straight chains alkanes and branched alkanes is that a branched alkane would have a lower boiling point then a straight chained alkane, as can be seen in the following table:

This is due to the fact that branching lowers to the surface area of the molecule and thus would lower the possibility of van der Waals’ bonding, which would reduce the energy obtained from van der  Waals bonds.

Reactions

There are three different types of reaction:

     Burning

     React with a halogen

     Cracking

Combustion

Combustion is the word used to define the burning of an alkane in the air. If enough Oxygen is present this would react to form Carbon Dioxide and Water only, but as the number of Carbon atoms increases, there is a bigger chance of incomplete combustion. It must be noted that in incomplete combustion it is the Hydrogen that reacts first, and then the Carbon would react with the Oxygen that is left.

Some equation showing complete combustion are the following:

C3H8 + 5O2 à 3CO2 + 4H2O

C4H10 +6.5O2 à 4CO2+5H2O

Cracking

There are two different types of cracking, thermal cracking and catalytic cracking.

Thermal cracking

This is performed in a chamber at a   temperature of 450 – 750oC and a pressure of up to 70atm. This would break a Carbon to Carbon bond forming free radicals which can then react in different ways.

A simple reaction might be C15H32 à 2C2H4 + C3H6 + C8H18    in which two alkenes and an alkane are formed from a long chained alkane.

Catalytical cracking

This is performed at a temperature of around 500oC but a much-lowered pressure using a catalyst such as a zeolite.

Halogenation

Halogenation of an alkane is normally performed as a free radical substitution, in which three steps can be observed, initiation, propagation and termination. Initiation is a reaction which produces free radicals from a neutral molecule, propagation is a transfer of the radical charge while the termination is when two radicals join together to form a neutral molecule removing the radical charge. In the halogenation of alkanes this is seen as follows:

free radical reaction

Apart from this if a halogen is reacted with a cycloalkane there is a chance that the ring would open up to form a straight-chain alkane with a halogen at each end. This is more the case in strained cycloalkane such as cyclopropane. If UV only is used then the reaction is as follows:

cycloalkane

while  if  there  is  no  control  on  the  wavelengths  used  the  reaction  might  continue  by  free  radical substitution. Unfortunately, there is no control over when the reaction will stop, and once the reaction starts it is only over once all of the halogen has reacted.

 

Advertisements