Chemistry

Overview of Hydrocarbons Reactivity



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Hydrocarbons are a group of organic compounds which contain carbon and hydrogen in its molecular structure.  Carbon is a unique atom due to its ability to form polymers from its compounds.  Only silicon mimics carbon in this trend of polymer formation.  Due to the comparable electronegativities of carbon and hydrogen the C-H bond is usually not reactive unless there is an unusual bonding of the carbon atom to other atoms which are much more electronegative.  An example is an electron withdrawing groups such as a nitro group. 


Carbon compounds especially hydrocarbons are usually stable compounds to organic reactions that can be kept at room temperature for a long period of time without decomposition.  Petroleum is an example of a mixture of hydrocarbons which are very stable and are present beneath the earth surface for a long period of time already. 


Although hydrocarbons are unreactive in most cases they are combustible due to the presence of C-H bonds in them.  In the combustion process of hydrocarbons they are oxidized with oxygen in air to form CO2 and H2O with the concomittant release of heat.  For simplicity of this discussion I will take the simplest hydrocarbons that are present. 


These are ethane and ethylene in addition to acetylene.  The difference between these compounds is in the degree of unsaturation or put it more clearly they differ by the amount of double bonds between the two carbons in each compound and compound.   The trend in reactivity in these compounds is that as we add more double bonds we increase the reactivity of the compound. 


Therefore ethylene is more reactive than ethane by several folds.  It has a widespread ability for organic reactions such as addition of lewis acid across the double bond.  Acetylene is even more reactive than the other two compounds.  Besides its ability to add acids to its double bonds it can function as an acid releasing a proton from the carbon that is attached to the triple bond. 


Due to the presence of the double bonds in ethylene and acetylene they are compounds which can be described as hungry for hydrogens.  Their structure with the double bonds is very reactive due to the presence of the pi bond which is higher energetically than the usual sigma bond between the two carbons in ethane. 


Therefore, the process of adding hydrogen to acetylene and ethylene in an exothermic process which releases energy due to the replacement of the pi bond by the energetically lower sigma bond in ethane.  The manner of hybridization in these molecules differ based on the number of double bonds in the compound. 


The hybridization in carbon that is attached to one pi bond is of the sp2 type.  Namely there are participation of three orbitals in carbon to form the bonds to one carbon and two hydrogens while the other p orbital is reserved for the pi bond.  In sp2 hybridization there is 33.3% participation of the s orbital on carbon. 


In ethane where there is no double bonds in this compound the hybridization is of the sp3 type.  Namely, there are three p orbitals on carbon and one s orbital that are mixed with a certain proportion to form equal four orbitals that are energetically the same.  The percent of s character in sp3 hybridization is 25% which is less than that of ethylene.  This can explain why ethylene has more acidic protons than ethane.  We also note that the percent of s character is increased from ethane to ethylene. 


In acetylene where there are two pi bonds in addition to one sigma bond between the two carbons the type of hybridization is that of sp type.  Namely there is 50% participation of the s orbital on carbon and 50% participation in the p orbital.  The amount of s character of 50% is higher than that of ethylene and acetylene.  Therefore, the acidity of the C-H bond in acetylene is significantly higher than that in ethylene and ethane.  This is so due to the retention of the sigma orbital electron pair by the energetically lower s orbital of the sp bond. 


In the organic lab it is usually feasible to deprotonate the C-H bond in acetylene using a strong base such as butyllithium or BuLi.  These unsaturated compounds: ethylene and acetylene will add a hydrogen molecule across their double bond to the energetically more stable ethane molecule with concomittant release of energy. 

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