Concepts & Vocabulary 7.1 Industrial Preparation and Use of Alkenes Breaking up of large hydrocarbon molecules into smaller, useful molecules is called cracking. 7.2 Calculating Degree of Unsaturation Saturated molecules contain only single bonds and no rings. Saturated hydrocarbons have the formula C n H 2n +2 , where n can be any integer. Degrees of unsaturation account for the total number of rings and pi bonds in a molecule. Each degree of unsaturation reduces the number of hydrogens in the molecule by 2. 7.3 Naming Alkenes When the two largest groups are on the same side of the double bond (top or bottom) they are called cis or Z . When the two largest groups are on opposite sides of the double bond (top or bottom) they are called trans or E . Endocyclic double bonds occur when there is a pi bond within a ring. 7.4 Cis-Trans Isomerism in Alkenes 7.5 Alkene Stereochemistry and the E, Z Designation E and Z are less limited than cis and trans in naming. E and Z configurations use the same priority rules as R and S (CIP rules). 7.6 Stability of Alkenes Relative stability of alkenes can be measured by using heats of hydrogenation upon reduction to the related alkane. More substituted alkenes are more stable than less substituted. Alkenes with the largest groups trans are more stable than cis. 7.7 Electrophilic Addition Reactions of Alkenes In electrophilic addition reactions, the pi bond of the alkene acts as the nucleophile. Electrophilic addition reactions occur faster with larger hydrogen halides as well as more substituted alkenes. 7.8 Orientation of Electrophilic Additions: Markovnikov's Rule The more substituted carbocation intermediate forms during electrophilic addition reactions, since more substituted carbocations are more stable. This is known as Markovnikov's rule. 7.9 Carbocation Structure and Stability Molecules or ions that can disperse (delocalize) charge are more stable than structures with charge localized on a single atom. Due to inductive stabilization, carbocation stability follows the order: tertiary > secondary > primary > methyl Electron donating groups stabilize carbocations. Electron withdrawing groups destabilize carbocations. Resonance effects can stabilize a carbocation (some examples include benzylic and allylic carbocations). Vinylic carbocations are unstable and are unlikely to form. 7.10 The Hammond Postulate The Hammond Postulate states that transition state structure most resembles the nearest stable species. Based on the Hammond Postulate, transition states for exothermic reaction steps resemble reactants, while endergonic step transition states resemble products. 7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements Carbocations will rearrange from less stable to more stable isomers through hydride shifts or alkyl shifts. Skills to Master Skill 7.1 Calculate degree of unsaturation for organic molecular formulae. Skill 7.2 Draw isomers from a molecular formula. Skill 7.3 Name alkenes following IUPAC rules, including configuration ( E , Z ). Skill 7.4 Draw structures from IUPAC name. Skill 7.5 Describe bonding in alkenes including bond length, strength, angle and restricted rotation. Skill 7.6 Explain stability of alkenes. Skill 7.7 Rank alkenes in order of stability. Skill 7.8 Draw mechanism for electrophilic addition of HX to alkenes, including regiochemistry. Skill 7.9 Explain stability of carbocations. Skill 7.10 Explain transition states related to the Hammond Postulate. Skill 7.11 Explain products formed by carbocation rearrangements. Memorization Tasks MT 7.1 Memorize formula for saturated hydrocarbons C n H 2n +2 . MT 7.2 Memorize basic IUPAC naming rules. MT 7.3 Memorize relative stability of alkenes. MT 7.4 Memorize relative stability of carbocations. Contributors Layne Morsch (University of Illinois Springfield)