Hydrocarbons Experiment Essay

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EXERCISE 6 HYDROCARBONS I. INTRODUCTION Hydrocarbons are compounds containing only hydrogen and carbon atoms. They are classified based on the presence of certain functional groups in their structures: alkanes (no functional group), alkenes (C = C), alkynes (C ≡ C), and aromatic hydrocarbons (pi system of electrons). Alkanes and their cyclic analogues have only two types of bonds in their molecules: C – C sigma bonds and C – H sigma bonds. Both types of bonds may also be present in other hydrocarbons. All carbon atoms in alkanes are sp3 hybridized. Also since both C – C and C – H bonds are non – polar, ionic reactions are unusual for alkanes under ordinary conditions. Instead, alkanes typically undergo reactions involving free radicals. An example of a reaction of alkanes is halogenation, which is a free radical substitution reaction involving C – H bonds. The reaction of a halogen with an alkane is initiated by heat or UV light and is given by: UV light or R–H + X–X High temp. R–X + H–X When the halogen used is bromine (Br2), the reaction is accompanied by loss of the red – orange color of molecular bromine and the evolution of a gas, hydrogen bromide. These are signs indicating that a free radical substitution reaction has occurred. Alkenes and their cyclic analogues contain at least one C = C in their molecule. The double bond contains a pi bond which, arises from the p orbital of each carbon atom (Cp – Cp) and a sigma bond which, is due to Csp2 – Csp2 orbital overlap. Since the double bond is a site of high electron density (i.e. it is electron rich), alkenes react with electrophillic and oxidizing reagents. Alkenes react with halogens in a reaction known as electrophillic addition. This reaction takes place at the double bond and does not require the presence of heat or light. Free radical addition also occurs but at a slower rate. The general

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