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Structure: • The alkene functional group consists of two sp2 hybridized C atoms bonded to each other via a σ and a π bond. • The π bond is produced by the side-to- side overlap of the π-orbitals not utilized in the hybrids (see left). • The substituents are attached to the C=C unit via σ bonds. • The 2 C of the C=C and the 4 atoms attached directly to the C=C are all in the same plane. Reactivity: • A π bond is a region of high electron density (red) so alkenes are typically nucleophiles. • Alkenes react with + + electrophiles (e.g. H , X ) • Alkenes typically undergo addition reactions in which the π bond is converted to two new stronger σ bonds. • Overall reaction : Electrophilic addition Electrophilic addition reactions are an important class of reactions that allow the interconversion of C=C and C≡C into a range of important functional groups. Conceptually, addition is the reverse of elimination What does the term "electrophilic addition" imply ? A electrophile, E+, is an electron poor species that will react with an electron rich species (the C=C) An addition implies that two systems combine to a single entity. Depending on the relative timing of these events, slightly different mechanisms are possible: + • Reaction of the C=C with E to give a carbocation (or another cationic intermediate) that then reacts with a Nu- • Simultaneous formation of the two s bonds The following pointers may aid your understanding of these reactions: • Recognise the electrophile present in the reagent combination • The electrophile adds first to the alkene, dictating the regioselectivity. • If the reaction proceeds via a planar carbocation, the reaction is not stereoselective • If the two new s bonds form at the same time from the same species, then syn addition is observed • If the two new s bonds form at different times from different species, then anti addition is observed Carbocations (review) Stability: The general stability order of simple alkyl carbocations is: (most stable) 3o > 2o > 1o > methyl (least stable) This is because alkyl groups are weakly electron donating due to hyperconjugation and inductive effects. Resonance effects can further stabilize carbocations when present. Structure: Alkyl carbocations are sp2 hybridized, planar systems at the cationic C center. The p-orbital that is not utilised in the hybrids is empty and is often shown bearing the positive charge since it represents the orbital available to accept electrons. Reactivity: As they have an incomplete octet, carbocation s are excellent electrophile s and react readily with nucleophile s (substitutio n). Alternativel y, loss of H+ can generate a p bond (elimination ). The electrostati c potential diagrams clearly show the cationic center in blue, this is where the nucleophile will attack. Rearrangements: Carbocations are prone to rearrangement via 1,2-hyride or 1,2-alkyl shifts provided it generates a more stable carbocation. For example: Notice that the "predicted" product is only formed in 3% yield, and that products with a different skeleton dominate. The reaction proceeds via protonation to give the better leaving group which departs to give the 2o carbocation shown. A methyl group rapidly migrates taking its bonding electrons along, giving a new 2o carbocation to 3o carbocation skeleton and a more stable 3o carbocation which can then lose H+ to give the more stable alkene as the major product. This is an example of a 1,2-alkyl shift. The numbers indicate that the alkyl group moves to an adjacent position. Similar migrations of H atoms, 1,2-hydride shifts are also known. Reactions involving carbocations: 1. Substitutions via the SN1 2. Eliminations via the E1 + 3. Additions to alkenes and alkynes (HX, H3O ) Reaction Type: Electrophilic Addition Summary • Alkenes can be reduced to alkanes with H2 in the presence of metal catalysts such as Pt, Pd, Ni or Rh. • The two new C-H s bonds are formed simultaneously from H atoms absorbed into the metal surface. • The reaction is stereospecific giving only the syn addition product. • This reaction forms the basis of experimental "heats of hydrogenation" which can be used to establish the stability of isomeric alkenes. Related reactions • Hydrogenation of alkynes CATALYTIC HYDROGENATION Step 1: Hydrogen gets absorbed onto the metal surface. Step 2: Alkene approaches the H atoms absorbed on the metal surface. Step 3: C=C reacts with the H atoms on the surface forming the two new C-H s bonds. Reaction type: Electrophilic Addition Summary • When treated with HX alkenes form alkyl halides. • Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order). • Regioselectivity predicted by Markovnikov's rule : "For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present" • Reaction proceeds via protonation to give the more stable carbocation intermediate. • Not stereoselective since reaction proceeds via planar carbocation. • For HBr, care must be taken to avoid the formation of radicals when an alternate mechanism occurs. MECHANISM FOR REACTION OF ALKENES WITH HBr Step 1: An acid/base reaction. Protonation of the alkene to generate the more stable carbocation. The p electrons act as a Lewis base. Step 2: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide. Markovnikov's Rule • Markovnikov's rule (1870s), based on experimental observation, states that : "when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen to the carbon having the fewer number of hydrogen substituents" • Modern mechanistic knowledge indicates reaction occurs via protonation to give the more stable carbocation. Propene can protonate to give two different carbocations, one 2o and the other o o 1 . Formation of the more stable 2 carbocation is preferred. • The carbocation then reacts with the nucleophile to give the alkyl bromide Reaction of Alkenes with HBr (radical) Reaction type: Radical Addition Summary • When treated with HBr, alkenes form alkyl bromides. • However, under these conditions, the regioselectivity is anti Markovnikov . • Peroxides facilitate the formation of a bromine radical, RO + HBr → ROH + Br. • Reaction proceeds via the more stable radical intermediate. • Contrast with non-radical alternative: Regular Radical Conditions HBr (dark, N2 atmosphere) HBr (peroxides, uv light) + . Electrophile H Br Intermediate carbocation radical Regioselectivity Markovnikov Anti-Markovnikov • The change in selectivity can be rationalized by the reversal of the order in which the -H and -Br add MECHANISM FOR RADICAL REACTION OF ALKENES WITH HBr Step 1: An electrophilic bromine radical adds to the alkene to generate the 2o radical. Step 2: Radical abstracts a H atom from another molecule of HBr, creating the alkyl bromide and another bromine radical. Reaction type: Electrophilic Addition Summary • When treated with aq. acid, most commonly H2SO4, alkenes form alcohols. • Regioselectivity predicted by Markovnikov's rule • Reaction proceeds via protonation to give the more stable carbocation intermediate. • Not stereoselective since reactions proceeds via planar carbocation. MECHANISM FOR REACTION OF ALKENES + WITH H3O Step 1: An acid / base reaction. Protonation of the alkene to generate the more stable carbocation. The p electrons act as a Lewis base. Step 2: Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion. Step 3: An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst. Reaction type: Electrophilic Addition Summary. • Overall transformation : C=C to H-C-C-OH • Reagents (two steps) 1. BH3 or B2H6 then 2) NaOH/ H2O2 • Regioselectivity : Anti-Markovnikov, since the B is the electrophile. • Stereoselectivity : Syn since the C-B and C-H bonds form simultaneously from the BH3. • The alcohol is formed over a series of steps involving the B center (see below), with retention of configuration at the C. • Compliments simple hydration with opposite regiochemistry and stereospecificity. Reactivity of Borane The image shows the electrostatic potential for borane, BH3. The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. • The low electron density (blue) is on B atom, after all, it has an incomplete octet. • The high electron density (red) is on the H atoms. • Therefore : d- H-B d+ , the B is the electrophile. • Borane reacts with alkenes via a concerted mechanism • Simultaneous making of C-B and C-H bonds as C=C and B-H break. • Electrophilic B atom adds at the least substituted end of the alkene. • Although a carbocation is not involved, the reaction occurs as if the electrophilic B atom were making the more stable carbocation. MECHANISM FOR REACTION OF ALKENES WITH BH3 Step 1: A concerted reaction. The p electrons act as the nucleophile with the electrophilic B and the H is transferred to the C with syn stereochemistry. Step 2: First step repeats twice more so that all of the B-H bonds react with C=C Step 3: Peroxide ion reacts as the nucleophile with the electrophilic B atom. Step 4: Migration of C-B bond to form a C-O bond and displace hydroxide. Stereochemistry of C center is retained. Step 5: Attack of hydroxide as a nucleophile with the electrophilic B displacing the alkoxide. Step 6: An acid / base reaction to form the alcohol. Reaction type: Electrophilic Addition Summary. • Overall transformation : C=C to X-C-C-X • Reagent : normally the halogen (e.g. Br2) in an inert solvent like methylene chloride, CH2Cl2. • Regioselectivity : not relevant since both new bonds are the same, C-X. • Reaction proceeds via cyclic halonium ion. • Stereoselectivity : anti since the two C-X bonds form in separate steps - one from Br2 the bromide ion Br .
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