Alkenes and Alkynes - URI Department of Chemistry

10/29/20107.5 Stability of AlkenesStability of cis and trans isomers• Interconversion does not occur spontaneously• Cis isomers are less stable than trans isomers because of thesteric strain between the two larger substituents on the same sideof the double-bondStability of AlkenesDetermining relative stabilities of alkene isomers• Establish a cis-trans equilibrium through treatment withstrong acid• Trans isomer is more stable than cis isomer by 2.8 kJ/mol(0.66 kcal/mol) at room temperature which corresponds toa 76:24 ratioStability of Alkenes• More general method to find relative stabilities of alkeneisomers is by hydrogenation reaction• Hydrogenate alkene to give corresponding alkane using H 2gas in the presence of a catalyst such as palladium orplatinum1

10/29/2010Stability of AlkenesEnergy Diagram for Hydrogenation of cis- and trans- But-2-ene• Cis isomer is higher energy than the trans isomer• More energy released in the hydrogenation of the cis isomer than thetrans isomerStability of Alkenes• Energy Diagram for Hydrogenation of cis- and trans- But-2-ene• Cis isomer is higher energy than the trans isomer• More energy released in the hydrogenation of the cis isomer thanthe trans isomerStability of AlkenesCompare heats of hydrogenation ( H° hydrog ) for cis andtrans isomers• Trans isomer is more stable than cis isomer by 4kJ/mol2

10/29/2010Stability of AlkenesAlkenes become more stable with increasing substitutionStability of AlkenesStability of AlkenesStability order due to two factors• Hyperconjugation• Stabilizing interaction between the C=C bond and adjacent C-Hσ bonds on substituents• The more substituents there are the greater the stabilization of thealkene3

10/29/2010Electrophilic Addition Reactions of AlkenesMechanism of the acid-catalyzed electrophilic addition of H 2 O to2-methylpropene to give the alcohol 2-methylpropan-2-olElectrophilic Addition Reactions of AlkenesMechanism of the acid-catalyzed electrophilic addition of H 2 O to2-methylpropene to give the alcohol 2-methylpropan-2-olElectrophilic Addition Reactions of AlkenesThis is a good time to mention that organic reaction equations aresometimes written in different ways to emphasize different points.In describing a laboratory process, for example, the reaction of 2-methylpropene with HCl might be written in the format A + B → Cto emphasize that both reactants are equally important for thepurposes of the discussion. The solvent and notes about otherreaction conditions, such as temperature, are written either aboveor below the reaction arrow:5

10/29/2010Electrophilic Addition Reactions of AlkenesAlternatively, we might write the same reaction in a format toemphasize that 2-methylpropene is the reactant whosechemistry is of greater interest. The second reactant, HCl, isplaced above the reaction arrow together with notes aboutsolvent and reaction conditions:Electrophilic Addition Reactions of AlkenesAlkynes also undergo electrophilic addition reactions• Reactivity is substantially less than that of alkenes7.7 Orientation of Electrophilic Addition:Markovnikov’s RuleRegiospecific reactions - reactions where only one oftwo possible orientations occursMarkovnikov’s Rule (Vladimir Markovnivov,1869)• In the addition of HX to an alkene, the H attaches to thecarbon with fewer alkyl substituents and the X attaches tothe carbon with more alkyl substituents6

10/29/2010Orientation of Electrophilic Addition:Markovnikov’s RuleOrientation of Electrophilic Addition:Markovnikov’s Rule• When both double-bond carbon atoms have the samedegree of substitution, a mixture of addition products resultsOrientation of Electrophilic Addition:Markovnikov’s Rule (Restated)Markovnikov’s rule (restated)• In the addition of HX to an alkene, the more highly substitutedcarbocation is formed as the intermediate rather than the less highlysubstituted one7

10/29/2010Orientation of Electrophilic Addition:Markovnikov’s Rule (Restated)Worked Example 7.2Predicting the Product of an Electrophilic AdditionReactionWhat product would you expect from reaction of HClwith 1-ethylcyclopentene?Worked Example 7.3Synthesizing a Specific CompoundWhat alkene would you start with to prepare thefollowing alkyl halide?(There may be more than one possibility.)8

10/29/20107.8 Carbocation Structure and StabilityWhy Markovnikov’s Rule worksStructure of carbocations• Carbocations are planar• The trivalent carbon is sp 2hybridized and has a vacant porbital perpendicular to theplane of the carbon• Carbon has three attachedgroupsCarbocation Structure and StabilityStability• Thermodynamic measurements show that the stability ofcarbocations increases with increasing substitution.Carbocation Structure and Stability• More highly substituted carbocations are more stablethan less highly substituted ones• Inductive effect• The more alkyl groups that are attached to the positivelycharged carbon, the more electron density shifts toward thecharge and the more inductive stabilization of the cationoccurs9

10/29/2010Carbocation Structure and Stability• Hyperconjugation• the stabilizing interaction between a p orbital and properlyoriented C-H σ bonds on neighboring carbons• The more alkyl groups that are on the carbocation the morestable the carbocation10