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chem lec.docx - ADDITION REACTIONS Addition at the...

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ADDITION REACTIONS Addition at the carbon-carbon double bond is the mostcommon reaction of alkenes. Hydrogen, halogens, hydrogen halides, sulfuricacid, and water are some of the reagents that can be added to unsaturatedhydrocarbons.HALOGENATION: Addition of a Halogen, X2 (Cl2;Br2)HYDRATION: Addition of Water, H2O – this reaction requires a trace of strongacid (H+) as acatalyst. The product is an Alcohol.HYDROGENATION: Addition of Hydrogen, H2HYDROHALOGENATION: Addition of Hydrogen Halide, HX (HCl, HBr, or HI)SULFONATION: Addition of Sulfuric Acid, H2SO4CATALYZED OXIDATION. This is another reaction of alkenes at the doublebond. For example, when shaken with a cold, dilute solution of PotassiumPermanganate, KMnO4, an alkene is converted to a glycol (glycols aredihydroxy alcohols).CRACKING ALKANES: The commercial method of producing alkenes is bycracking petroleum.• Cracking is a process in which complex organic molecules are broken upinto simpler molecules.• This process involves the action of heat and sometimes involves a catalyst.• Cracking that involves heat alone is known as thermal cracking, while thecracking process that involves heat and requires a catalyst is called catalyticcracking.DEHYDRATION OF ALCOHOLS: Ethene can be prepared in the laboratory bydehydrating (removing water) ethyl alcohol by using hot concentratedsulfuric acid as the dehydrating agent.DEHYDROGENATION (an Elimination Reaction). Hydrogen is lost from anorganic compound during dehydrogenation.CRACKING - breaking up large molecules Alkane to form smaller ones (Alkane+ Alkene).ISOMERIZATION - rearrangement of molecular structures.HALOGENATION (a substitution reaction). In this type of reaction, a “halogen”is substituted for a hydrogen atom.• If the specific halogen used in the reaction is “chlorine”, the reaction iscalled “chlorination”.Acetylene is also prepared by the cracking of Methane in an electric arc.Alkynes like Acetylene can be prepared inexpensively from the reaction ofCalcium Carbide and Water.The boiling points of alkenes are slightly lower than the alkanes since theyhave a slightlysmaller molar mass.The smaller alkenes (2-5 carbons) are gases at room temperature.As the chain lengthens (6 -17 carbons), the alkenes are liquid, and above 17-carbons, theyare solid.Alkenes are non-polar, like the other hydrocarbons, and so are insoluble inwater but
The melting and boiling points of hydrocarbons increase with increasingmolecular weights.1. Ethene– the simplest alkene is produced by ripening fruits, it is a plantgrowth substance. It isproduced during fruit ripening, during leaf fall and flower senescence, as wellas under conditions ofstress, including wounding, heat, cold, or water stress, and disease.2. Isoprene– is the building block molecule of the polyenes, alkenes withseveral double bonds found innature. The molecules that are produced are called Isoprenoids, or Terpenes.

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Term
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Acetylene

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