Chapter 10 Alkenes Addition Reactions PDF

Chapter 10 Alkenes Addition reactions 1 Addition reactions are basically methods to add substituents to multiple bonds Discussion will deal with mostly double bonds; primarily alkenes Other type of double bonds will come up in discussions throughout the course, but basics...

Chapter 10 Alkenes Addition reactions 1 Addition reactions are basically methods to add substituents to multiple bonds Discussion will deal with mostly double bonds; primarily alkenes Other type of double bonds will come up in discussions throughout the course, but basics are essentially the same Most slides up to slide 16 are concepts that have been discussed previously 2 Alkenes are also called olefins. (next few slides are a review) Naming: …ene.. refers to a chain with a double bond cyclo….. ene …. Refers to a cyclic with a double bond 3  Cis and Trans Isomers of Alkenes Alkenes having one alkyl group bonded to each carbon atom can be differentiated using the prefixes – cis and trans. cis has the two alkyl groups on the same side of the double bond. trans has the two alkyl groups on opposite sides of the double bond. 4  Cyclic Alkenes Cycloalkenes having fewer than eight carbon atoms have a cis geometry. 5 Alkenes summary 6  Prefixes E and Z Step : Assign priorities to the two substituents on each end of the C=C bond by using the priority rules for R,S nomenclature. Divide the double bond in half, and assign the numbers 1 and 2 to indicate the relative priority of the two groups on each end. 7  Prefixes E and Z Step : Assign E or Z based on the location of the two higher-priority groups (1). The E isomer has the two higher-priority groups on the same opposite sides. The Z isomer has the two higher-priority groups on the same side. 8  Common Names of Alkenes and Alkene Substituents Some alkene or alkenyl substituents have common names. The simplest alkene, CH2=CH2, named in the IUPAC system as ethene, is often called ethylene. 9 Useful Products Formed From Ethylene 10 Naturally Occurring Alkenes 11  Fatty Acids..alkenes in natural products Increasing the number of double bonds in the fatty acid side chains decreases the melting points 12  Preparation of Alkenes Alkenes can be prepared from alkyl halides, tosylates, and alcohols via elimination reactions. These are used to make the alcohol into a good leaving group HOH 13  Regioselectivity and Stereoselectivity of Alkene Formation The most stable alkene (Zaitsev product) is usually formed as the major product. 14  **Addition Reactions** Basic: the π bond is broken and two new σ bonds are formed alkenes act as nucleophiles and react with electrophiles. So (generally) alkenes do not react with nucleophiles or bases, reagents that are themselves electron rich. 15  Syn and Anti Addition to Alkenes Syn addition takes place when both X and Y are added from the same side. Anti addition takes place when X and Y are added from opposite sides. 16  Addition Reactions of Cyclohexene (as an example..any alkene can be generalized this way) In each reaction, the pi bond is broken and two new sigma bonds are formed. Figure 10.7 17  Hydrohalogenation—Electrophilic Addition of HX Basic reaction: HX adds to the double bond; first with the electrophilic portion of the molecule 18 Examples 19  Heat of Formation for Electrophilic Addition Addition reactions are exothermic because the two σ bonds formed in the product are stronger than the σ and π bonds broken in the reactants. H degrees for the addition of HBr to ethylene is –14 kcal/mol. Figure 10.8 20  Mechanism of Electrophilic Addition The mechanism of electrophilic addition consists of two successive Lewis acid- base reactions. Step – the alkene is the Lewis base that donates an electron pair to H-Br, the Lewis acid. Step – Br− is the Lewis base that donates an electron pair to the carbocation, the Lewis acid. 21  Energy Diagram for Electrophilic Addition Each step has its own energy barrier with a transition state energy maximum. Since step has a higher energy transition state, it is rate-determining. H degrees for step is positive because more bonds are broken than formed, whereas H degrees for step is negative because only bond making occurs. Figure 10.9 22  Carbocation Rearrangements (Chapter 9 & pg 415) (generally) less stable carbocation will be converted into a more stable carbocation by a shift of a hydrogen or an alkyl group, called a rearrangement. How do we know that a rearrangement occurred? the double bond may be in an unexpected location. The carbon skeletons of the reactant and product are different. 23  Carbocation Rearrangements 1,2-shift moves with two bonding electrons, also moves the net positive (+) charge. Movement of a H atom is called a 1,2-hydride shift. Movement of an alkyl group is called a 1,2-alkyl shift. 24  Markovnikov’s Rule With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers, but only one is actually formed: H atom adds to the less substituted carbon atom; H atom adds to the carbon that has the greater number of H atoms to begin with. 25  Carbocation Stability and Markovnikov’s Rule Markovnikov’s rule: formation of a carbocation in the rate-determining step of the mechanism. In an unsymmetrical alkene, the H atom is added to the less substituted carbon to form the more stable, more substituted carbocation. 26 27  **Hammond Postulate and Electrophilic Addition According to the Hammond postulate, the blue path is faster because formation of the carbocation is an endothermic process. The transition state to form the more stable 2o carbocation is lower in energy, so The 2o carbocation is formed faster. The Ea for formation of the more stable 2o carbocation is lower than the Ea for formation of the 1o carbocation. Figure 10.10 28  Stereochemistry of Electrophilic Addition Trigonal planar atoms react with reagents from two directions with equal probability. Achiral starting materials yield achiral products. Sometimes new stereogenic centers are formed from hydrohalogenation. 29  Stereochemistry of Carbocation Formation mechanism of hydrohalogenation……..two enantiomers are formed. Initial addition of H+ occurs from either side of the planar double bond. Both modes of addition generate the same achiral carbocation. 30  Stereochemistry of Nucleophilic Attack Nucleophilic attack of Cl− on the trigonal planar carbocation also occurs from two different directions, forming two products, A and B, having a new stereogenic center. A and B are enantiomers. Since attack from either direction occurs with equal probability, a racemic mixture of A and B is formed. 31  Hydrohalogenation—Summary Table 10.3 Summary: Electrophilic Addition of HX to Alkenes Observation Mechanism The mechanism involves two steps. The rate-determining step forms a carbocation. Rearrangements can occur. Regioselectivity Markovnikov’s rule is followed in unsymmetrical alkenes, H bonds to the less substituted C to form the more stable carbocation. Stereochemistry Syn and anti addition occur. 32  Hydration—Electrophilic Addition of Water Hydration is the addition of water to an alkene to form an alcohol. 33  Mechanism of Hydration: note the acid “protonates” the water 34 35  Electrophilic Addition of Alcohols Alcohols add to alkenes, forming ethers by the same mechanism. RO-H + C=C RO-C-C-H as “IF” the H and the RO from the alcohol was “directly” added to the double bond (remember the actual mechanism) Mike up to here 36 Hydrohalogenation and addition of alcohols are similar 37  Halogenation—Electrophilic Addition of Halogen Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide. 38  Halogenation Details Halogens are polarizable. The double bond induces a dipole in the halogen molecule, one halogen atom becomes electron deficient and the other electron rich (Xδ+—Xδ−) so addition can occur Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration. No rearrangements occur. Only anti addition of X2 is observed. These facts suggest that carbocations are not intermediates in this reaction 39 Halogenation Mechanism 40  Stability of Cation Intermediates Carbocations are unstable because they have only six electrons that surround carbon. Bridged halonium ions are unstable because of ring strain. 41  Stereochemistry of Halonium Formation Determined from cyclic alkenes Chlorination of cyclopentene affords both enantiomers of trans- 1,2-dichlorocyclopentane, with no cis products. the electrophile Cl+ can add from ether side of the double bond 42  Stereochemistry of Halonium Ring Opening nucleophilic attack of Cl− must occur from the backside. the two Cl atoms in the product are oriented trans to each other. Backside attack occurs with equal probability at either carbon of the three-membered ring to yield a racemic mixture. 43  So what happens to linear alkenes example: 2-butene cis-2-Butene yields two enantiomers trans-2-butene yields a single achiral meso compound. Figure 10.11 44 halogenation 45  Halohydrin Formation Treatment of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the groups of X and OH to the double bond. As “IF” X2 and HOH formed “X-OH” the mechanism then is similar to the halonium mechanism (X adds first as electrophile followed by OH- 46  Mechanism of Halohydrin Formation Even though X− is formed in step of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X− is the nucleophile. 47  Anti Stereochemistry in Halohydrin Formation Because the bridged halonium ion is opened by backside attack of H2O, addition of X and OH occurs in an anti fashion and trans products are formed. example 48 unsymmetrical alkenes preferred product has the electrophile X+ bonded to the less substituted carbon, and the nucleophile H2O bonded to the more substituted carbon. 49  Regiochemistry of Halohydrin Formation nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate the partial positive charge in the transition state. 50  Summary of Halohydrin Formation Table 10.4 Summary: Conversion of Alkenes to Halohydrins Observation Mechanism The mechanism involves three steps The rate –determining step forms a bridged halonium ion. No rearrangements can occur. Regioselectivity The electrophile bonds to the less substituted carbon. Stereochemistry Anti addition occurs. 51  Borane and Hydroboration hydroboration–oxidation: addition H and BH2 to the π bond 52 Hydroboration Mechanism Four centered 53  Regiochemistry of Hydroboration With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom. larger boron atom bonds to the less sterically hindered, more accessible carbon atom. Note: this is anti-markonikov addition (no carbocation intermediate 54  Electronic factors of Hydroboration Figure 1.14 boron bears a δ− charge in the transition state and carbon bears a δ+ charge. Since alkyl groups stabilize a positive charge, the more stable transition state has the partial positive charge on the more substituted carbon 55  Oxidation Following Hydroboration workup to the alcohol alkylboranes are oxidized, without isolation with basic hydrogen peroxide (H2O2, −OH), Alkyl borane is not isolated Produce the alcohol with retention of configuration syn addition in an “anti-Markovnikov” fashion of H and OH 56  Summary of Hydroboration— Oxidation Table 10.5 Summary: Hydroboration-Oxidation of Alkenes Observation Mechanism The addition of H and occurs in one step. No rearrangements can occur. Regioselectivity The OH group bonds to the less substituted carbon atom. Stereochemistry Syn addition occurs. OH replaces with retention of configuration. 57 58  Use of Alkenes in Synthesis Suppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol. To solve this problem we must: 59  Retrosynthetic Analysis Working backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis. 60 61 Chapter 10 HW 46,b,e; 49 c,d; 54 b; 55; 63c 62