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Topic 3

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Western University
Chemistry 2223B
Felix Lee

Chemistry 2213a  Fall 2012  Western University Topic 3. Reactions of Alkenes (and Alkynes) A. Introduction to Alkene Reactions  The main reactions of alkenes are addition reactions, where some AB species reacts to break the  bond and form two new  bonds. Addition reactions are usually exothermic and occur spontaneously at room temperature. A B C C A B C C  To fully understand the structure and stereochemistry of the products formed, we need to look at the sequence of steps that occur as the reaction proceeds, i.e. the reaction mechanism. Reactions of Alkenes (and Alkynes)  2 B. Mechanism of the Electrophilic Addition Reaction  Electrophilic addition is the mechanism by which the addition of many reagents across the double bond occurs.  In Chapter 4 of the text, we saw that  bonds have electron density on both sides of the  bond (the two “faces” of the alkene). The  bond is reactive and will attract electrophiles, which are electron-seeking reagents.  T+e add–tion of AB t–a  bond proceeds well when AB is ionized (behaves as A and B ), polarized (A –B ), or electron-deficient (missing a full octet).  If A ≠ B, the reaction can form two constitutional isomers: W Y + - A B B A C C A B W C C Y W C C Y X Z X Z X Z 1 2  The constitutional isomers that can be formed from reactions are called regioisomers. If one of the regioisomers is formed in greater amount than the other, the reaction is said to be regioselective. Electrophilic addition reactions are usually regioselective. Reactions of Alkenes (and Alkynes)  3 Two-step mechanism Step 1 (electrophilic step) W Y +  A aor/ A slow C C A B W C C Y W C C Y X Z X Z X Z + B  The first step involves the electrophilic attack of the  electron pair by a positively charged species.  This results in: o The breaking of the original A–B bond, and the bonding electrons are transferred to B; o The breaking of the C=C  bond; and o The formation of the C–A  bond.  Two bonds are broken ( + ), but only one is formed (), so this step is endothermic. It is the slow, rate-determining step of the entire electrophilic addition reaction. Reactions of Alkenes (and Alkynes)  4  The product of this step is a positively charged intermediate known as a carbocation. This is consumed in the second step. Step 2 (nucleophilic step) A A B W C Y W Y X Z fast X Z B and/or and/or A B A W C Y W Y X Z X Z  In this step, a nucleophile (nucleus- or positive-seeking species) attacks the carbocation. A new C–B  bond is formed, and this step is fast and exothermic. Reactions of Alkenes (and Alkynes)  5 Energy diagram  If AB adds to asymmetrical alkenes, one of the two possible regioisomers is usually formed in a greater amount. + - A B B A W Y   C C A B W C C Y W C C Y X Z X Z X Z 1 2  To predict the TS 1 TS 2 major product, it less-stable carbocation is necessary to more-stable carbocation know how the g course of the e A n W C Y Ea2 reaction depends E X Z on the energy of W Y Ea1 W CA Y WA BY the carbocation C C XZ H X Z intermediate. X Z B A carbocation W Y intermediate X Z Reaction coordinate Reactions of Alkenes (and Alkynes)  6  The reaction coordinate diagram shows two possible routes. The shape of the plot is typical for the addition of most AB species to alkenes.  The E far the first step (E ) a1 the highest point on the curve, so the first step is the slow, rate-determining step.  The E for the second step (E ) is much smaller than E , so the second step a a2 a1 is fast once the carbocation is formed.  If the carbocation intermediate is lower in energy (more stable), the E requirea to form it is also lower. This has several implications: o The more stable the carbocation, the faster the reaction. o Of the two possible carbocations, a larger amount of the more-stable carbocation is made. o Therefore, the major regioisomer will be the one with the nucleophile bonded to the carbon atom of the alkene that formed the more-stable carbocation. th  This is a better and more modern restatement of a 19 -century rule known as Markovinov’s Rule. Reactions of Alkenes (and Alkynes)  7 Stabilization of carbocations  Carbocations can be stabilized by atoms or groups that are able to donate electron density to the positively charged carbon atom. These groups are called electron-donating groups (EDG). The two main methods of electron donation to the carbocation are resonance and induction. Resonance  A carbocation is sp -hybridized, with three  bonds and an empty p orbital. Resonance stabilization neighbouring groups ( bonds or electron-pair donors) is possible. Y W W C C C C C C Y X H H Z Z X W W or C D C D X X Reactions of Alkenes (and Alkynes)  8 Induction  Alkyl groups allow their  electrons to shift slightly towards to carbocation, thus stabilizing by induction.  The more alkyl groups attached to the carbocation, the more stable it is. Reactions of Alkenes (and Alkynes)  9 CH OCH  Note: The ability of an alkyl group to donate 3 3 electron density by induction is much less thH3C C H 3 C the ability of an atom or group to donate by resonance, if resonance is possible. CH 3 CH 3 less stable more stable C. Addition of HX, X 2 and H 2 to Alkenes  All three of these reactions follow the general mechanism of electrophilic addition to alkenes. Addition of HX (HCl, HBr, HI) H3C CH 3 H C H H3C H 3 H 3 CH 3 H 3 CH 3 H3C CH 3 CH 3 X CH3 H X X Reactions of Alkenes (and Alkynes)  10  If two regioisomers are possible, the major regioisomer formed is derived from the most-stable carbocation. HCl Cl + Cl 97% 3%  In some alkenes, both carbocations are of similar stability, so, both regioisomers are formed in roughly equal amounts. Cl HCl Cl Reactions of Alkenes (and Alkynes)  11  The relative rates at which different alkenes react depend on carbocation stability. For example, the relative rates of reaction with HCl are 2 > 1 > 3. Note that carbocation 2 is more stable than carbocation 1 because oxygen is a better electron donor (by resonance) than an alkyl group (by induction). O 1 2 3 most stable carbocations are: same stability as O 1 2 3 major products after reaction with HCl: Cl Cl Cl O Cl 1 2 3 Reactions of Alkenes (and Alkynes)  12 Addition of H O 2 H OH H + C C H OH C C  Although the H–OH bond is polar, the reaction requires a small amount of acid (usually a drop of a strong inorganic acid, such as H SO ) to initiate the 2 4 electrophilic step of the addition. Thus, the first step is: H H C C C C + + +  H is present as3H O , but we can write H for simplicity. What is really going on is: Reactions of Alkenes (and Alkynes)  13 –  In the acid-catalyzed addition of water, there is no OH , so the second step must use H 2 as the nucleophile. 2 O can function as such because it has a lone pair of electrons. H H H OH H OH C C H OH C C C C  After H2O forms a new  bond, the O atom has a formal charge of +1 and is + therefore cal+ed the oxonium ion (compare this to hydroniu3, H O ). This ion releases an H to give the uncharged product.  Note that in the overall mechanism, H is consumed in the first step but + regenerated in the second step. There is no overall change in H concentration, i.e. it is a catalyst.  Compounds written over or under a reaction arrow are usually catalysts or solvents. H 2O 4 OH H CHC C(CH ) H O CH CH C(CH ) 3 3 2 2 3 2 3 2 Reactions of Alkenes (and Alkynes)  14  Like the addition of HCl, the relative rates of hydration reactions can be explained by examining the relative stabilities of the carbocations. Alkene Rel. Rate for Carbocation Stability water addition rank OCH 3 OCH 3 17000 1 CH 3 CH3 7500 2 H H 1 3 Br Br 0.22 4 Note: Carbocation #4 is less stable than #3 because of destabilization by the inductively withdrawing Br. Reactions of Alkenes (and Alkynes)  15 Addition of X 2Cl 2nd Br ) 2 X X = Cl or Br C C X X inert solvent anti addition X  The inert solvent is usually CC4 or CH2Cl2. Anti addition means that the two X atoms adds to the opposite faces of the double bond (recall planar nature of C=C bond). Reactions of Alkenes (and Alkynes)  16  Anti addition is achieved by the formation of a cyclic (“bridged”) halonium ion in the first step of the reaction. If X = Cl, then the cyclic ion is called a chloronium ion. If X = Br, then it is called a bromonium ion. + - X C C X X C C X + –  The normally non-polar2X is polarized into  and  when the halogen molecule approaches the electron-rich  bond.  The positively charged X forms bonds to each of the alkene C, forming a three- membered ring and a +1 formal charge.  In the second step of the reaction, the intermediate halonium ion blocks attack by X from one side of the C–C bond. The X forms a new bond only from the opposite side; this is called anti addition. X X C C X C C X Reactions of Alkenes (and Alkynes)  17  Anti addition is most noticeable for cyclic alkenes, where a trans isomer is formed. Because one stereoisomer is preferentially formed in halogenation reactions, the reaction is also stereoselective. Br Br 2 Br  Anti addition is less noticeable for acyclic alkenes because the resulting carbon-carbon single bond freely rotates. What happens after the addition reaction doesn’t change the fact that the addition occurs in an anti manner. Me Me Br Et Br Br Br2 Me = Et Et Me Me Et Et Br Et Me + other Me Me conformations side view Et Et Reactions of Alkenes (and Alkynes)  18  Th
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