chapter 12 - Introduction Chapter Twelve Unsaturated...

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1 Chapter Twelve ALKENE Alkenes Introduction square6 Unsaturated Unsaturated hydrocarbons hydrocarbons that that contain contain one or more C-C double bonds, triple bonds, or benzenes. boxshadowdwn Alkenes: one or more double bonds. (C n H 2n ) boxshadowdwn Alkynes: one or more triple bonds. (C n H 2n-2 ) square6 Thermal cracking cracking: major process for the production of ethylene. boxshadowdwn A saturated hydrocarbon is converted to an unsaturated hydrocarbon. C H 2 CH 2 CH 3 CH 3 + H 2 thermal cracking Alkenes square6 Structure: boxshadowdwn The VSEPR model predicts bond angles of 120° about each carbon of a double bond. boxshadowdwn In ethylene, the actual angles are close to 120°. boxshadowdwn In substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion between groups bonded to the double bond. H C C H H H 121.7° H 3 C C C H H H 124.7° Ethylene Propene Structures square6 Alkenes: under normal conditions, no rotation allowed around the carbon-carbon double bond. square6 Cis-Trans isomerism trans -2-Butene mp -106°C, bp 1°C cis -2-Butene mp -139°C, bp 4°C H C C CH 3 H 3 C H H C C H H 3 C CH 3
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2 Nomenclature of Alkenes and Alkynes square6 IUPAC system square6 Prefix---indicate carbon number in the chain. square6 Suffix - ene ---indicate alkene. square6 Suffix– yne ---indicate alkyne. Step 1 Step 1. Find the longest carbon chain that includes double bond (or triple bond). Step 2 Step 2. Number the chain so that double bond has the lower set of number. Designate the position of the double bond by its first C number. Step 3 Step 3. Branched alkenes are named in a manner similar to alkanes. Examples CH 3 CH 2 CH 2 CH 2 CH=CH 2 CH 3 CH 2 CHCH 2 CH=CH 2 CH 3 CH 3 CH 2 CHC=CH 2 CH 2 CH 3 CH 2 CH 3 1-Hexene 4-Methyl-1-hexene 2,3-Diethyl-1-pentene 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 CH 3 CHC CH CH 3 CH 3 CH 2
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