10/29/ Stability of Alkenes. Stability of Alkenes. Stability of Alkenes

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1 7.5 Stability of cis and trans isomers Interconversion does not occur spontaneously Cis isomers are less stable than trans isomers because of the steric strain between the two larger substituents on the same side of the double-bond Determining relative stabilities of alkene isomers Establish a cis-trans equilibrium through treatment with strong acid Trans isomer is more stable than cis isomer by 2.8 kj/mol (0.66 kcal/mol) at room temperature which corresponds to a 76:24 ratio More general method to find relative stabilities of alkene isomers is by hydrogenation reaction Hydrogenate alkene to give corresponding alkane using H 2 gas in the presence of a catalyst such as palladium or platinum 1

2 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 than the trans isomer 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 than the trans isomer Compare heats of hydrogenation ( H hydrog ) for cis and trans isomers Trans isomer is more stable than cis isomer by 4kJ/mol 2

3 Alkenes become more stable with increasing substitution Stability order due to two factors Hyperconjugation Stabilizing interaction between the C=C bond and adjacent C-H σ bonds on substituents The more substituents there are the greater the stabilization of the alkene 3

4 Bond strength A bond between an sp 2 carbon and an sp 3 carbon is somewhat stronger than a bond between two sp 3 carbons 7.6 Electrophilic Addition Reactions of Alkenes Electrophilic addition reactions of alkenes Carbon-carbon double bonds behave as nucleophiles (Lewis bases) in polar reactions by donating a pair of electrons to an electrophile (Lewis acid) Successful not only with H 2 O but also with HBr, HCl, and HI (addition reactions of halogen acids is not common in living organisms) Mechanism of the acid-catalyzed electrophilic addition of H 2 O to 2-methylpropene to give the alcohol 2-methylpropan-2-ol 4

5 Mechanism of the acid-catalyzed electrophilic addition of H 2 O to 2-methylpropene to give the alcohol 2-methylpropan-2-ol Mechanism of the acid-catalyzed electrophilic addition of H 2 O to 2-methylpropene to give the alcohol 2-methylpropan-2-ol This is a good time to mention that organic reaction equations are sometimes 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 C to emphasize that both reactants are equally important for the purposes of the discussion. The solvent and notes about other reaction conditions, such as temperature, are written either above or below the reaction arrow: 5

6 Alternatively, we might write the same reaction in a format to emphasize that 2-methylpropene is the reactant whose chemistry is of greater interest. The second reactant, HCl, is placed above the reaction arrow together with notes about solvent and reaction conditions: Alkynes also undergo electrophilic addition reactions Reactivity is substantially less than that of alkenes 7.7 Orientation of Electrophilic Addition: Markovnikov s Rule Regiospecific reactions - reactions where only one of two possible orientations occurs Markovnikov s Rule (Vladimir Markovnivov,1869) In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents 6

7 Orientation of Electrophilic Addition: Markovnikov s Rule Orientation of Electrophilic Addition: Markovnikov s Rule When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results Orientation of Electrophilic Addition: Markovnikov s Rule (Restated) Markovnikov s rule (restated) In the addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one 7

8 Orientation of Electrophilic Addition: Markovnikov s Rule (Restated) Worked Example 7.2 Predicting the Product of an Electrophilic Addition Reaction What product would you expect from reaction of HCl with 1-ethylcyclopentene? Worked Example 7.3 Synthesizing a Specific Compound What alkene would you start with to prepare the following alkyl halide? (There may be more than one possibility.) 8

9 7.8 Carbocation Structure and Stability Why Markovnikov s Rule works Structure of carbocations Carbocations are planar The trivalent carbon is sp 2 hybridized and has a vacant p orbital perpendicular to the plane of the carbon Carbon has three attached groups Carbocation Structure and Stability Stability Thermodynamic measurements show that the stability of carbocations increases with increasing substitution. Carbocation Structure and Stability More highly substituted carbocations are more stable than less highly substituted ones Inductive effect The more alkyl groups that are attached to the positively charged carbon, the more electron density shifts toward the charge and the more inductive stabilization of the cation occurs 9

10 Carbocation Structure and Stability Hyperconjugation the stabilizing interaction between a p orbital and properly oriented C-H σ bonds on neighboring carbons The more alkyl groups that are on the carbocation the more stable the carbocation 10