Fundamentals of ChemistryDOE-HDBK-1015/1-93CHEMICAL BONDINGRev. 0CH-01Page 29Figure 11 AlkaneFigure 12 AlkeneThe general formula for alkanes is C H. The alkanes aren 2n+2colorless, practically odorless, insoluble in water, and readilysoluble in nonpolar solvents such as benzene or ether. Alkanes are low in reactivity. The reactions they do undergoare called halogenation, thermal decomposition (cracking),and combustion. These are summarized below.Halogenation occurs when a hydrogen atom is replacedwith a halogen atom. This is referred to as asubstitution reaction. There is no limit to how manyhydrogen atoms can be replaced in one molecule. Thermal decomposition or cracking is the process of breaking large molecules intosmaller ones. Using heat as a catalyst, propane can be broken into methane andethylene:Combustion occurs when an alkane is burned, the products being carbon dioxide gas,water, and heat. These reactions are highly exothermic and as such the hydrocarbonsare frequently used for fuel.AlkenesAlkenesare hydrocarbons containing two fewer hydrogenatoms than the corresponding alkane. The general formula foralkenes is C H . These molecules will have a double bond asn 2nillustrated in Figure 12.Because there are fewer hydrogen atoms than the maximumpossible, alkenes are unsaturated hydrocarbons. The chiefsource for alkenes is the cracking of alkanes.
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