7. Alkenes: Reactions and Synthesis

Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) alcohols (add H-OH) alkanes (add H-H) halohydrins (add HO-X) Dihalides or dihaloalkanes (add X-X) halides or haloalkanes(add H-X) diols (add HO-OH) cyclopropane (add :CH2)

7.1 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation) Uses heat and KOH elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC)

Saytzeff Rule or Zaitsev Rule: In eliminations (dehydration or dehydrohalgenation) when a mixture of product alkenes can form, the major product will be the more substituted alkene: discussed again in chapter 11.

Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process. The bond strengths in the diatomic halogens are relatively small. F2 is too reactive and reacts explosively. The addition I2 is thermodynamically unfavorable and does not occur Cl2 reacts as Cl+ Cl- Br2 is similar The solvent used is an inert solvent such as CH2Cl2 which dissolves the reactants but is not a reactant.

Addition of Br2 (Cl2)to Cyclopentene Nucleophile: alkene; Electrophile: One of the Br of Br2(Cl of Cl2) The other Br (Cl) leaves as Br- (Cl-) Nucleophile: a lone pair on the attached Br (Cl); electrophilic: carbocation An unstable cyclic bromonium ion(chloronium) is formed. Nucleophile: Br- (Cl-); electrophile: more substituted C of double bond Addition is exclusively anti: stereospecific +

Bromonium Ion Mechanism Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile

Halohydrin Formation When the addition of halogens to alkene is carried out in the presence of water as solvent rather than the inert CH2Cl2, the halohydrin forms. This is formally the addition of HO-X to an alkene (with +OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrin Nucleophile: alkene; Electrophile: One of the Br of Br2(Cl of Cl2) The other Br (Cl) leaves as Br- (Cl-) Nucleophile: a lone pair on the attached Br (Cl); electrophilic: carbocation An unstable cyclic bromonium ion(chloronium) is formed. Nucleophile: H2O; electrophile: more substituted C of double bond Addition is exclusively anti: stereospecific

Mechanism of Formation of a Bromohydrin Br2 forms bromonium ion, then water adds Orientation toward stable C+ species Aromatic rings do not react

An Alternative to Bromine Bromine is a difficult reagent to use for this reaction N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

Acid-catalysed Addition of Water to Alkenes: Markovnikov product Hydration of an alkene is the addition of H-OH to to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

Addition of Water to Alkenes by Oxymercuration-Reduction: Markovnikov product For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Nucleophile: pi electrons of double bond; electrophile: Mercury(II)cation of mercury(II)acetate A cyclic mercurinium ion is formed; No carbocation intermediate formed Nucleophile: water; electrophile: more substituted C of double bond; Markovnikov orientation NaBH4 converts the C-Hg bond to a C-H bond; Solvent: water and THF

Addition of Water to Alkenes: Hydroboration-Oxidation: non-Markovnikov regiochemistry; syn stereochemistry Herbert Brown invented hydroboration (HB) Borane (BH3) is electron deficient is a Lewis acid Nucleophile: pi electrons of double bond; electrophile: borane H adds to more substituted C of double bond; BH2 to less substituted C Borane adds to an alkene to give an organoborane; solvent is THF Oxidation of the organoborane by hydrogen peroxide in basic medium( H2O2 in presence of NaOH) yields the alcohol In the oxidation step, The C-B is replaced by C-OH such that both the added groups (H from the hydroboration step and the OH from the oxidation step) add to the same face: syn addition

BH3 Is a Lewis Acid. It has only 6 electrons BH3 Is a Lewis Acid. It has only 6 electrons. It forms a coordinate bond with a lone pair on Oxygen in THF. In hydroboration, it coordinates with the pi electrons of the double bond Although it attaches itself so that a positive charge can form at the more substituted C atom of the double bond, there is no carbocation intermediate in the reaction. In a concerted manner the more substituted C becomes more electron-deficient and the B as it gains the pi electrons becomes more ready to allow the transfer one of the H attached to it to the C. The attachment of the BH3 to the less substituted C is motivated both by the stability of the incipient carbocation and steric factor.

Hydroboration-Oxidation Forms an Alcohol from an Alkene Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientation OH is added to carbon with most H’s H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH3 are across C=C

Hydroboration: Orientation in Addition Step Addition in least crowded orientation, syn Addition also is via most stable carbocation

Hydroboration, Electronic Effects Give Non-Markovnikov More stable carbocation is also consistent with steric preferences

Hydroboration - Oxygen Insertion Step H2O2, OH- inserts OH in place of B Retains syn orientation

Summary for hydration of Alkenes: Acid-catalyzed hydration: Advantages: cheaper chemicals; disadvantages: carbocation intermediate in mechanism: carbocations can rearrange which can result in a mixture of products. This method is regiospecific (Markovnikov addition: H adds to less substituted C of double bond; OH adds to the more substituted C of the double bond). But the addition is not stereospecific as the mechanism involves the addition of the OH (from H2O) to a trigonal planar carbocation. Reagents/solvent: alkene, H3PO4/H2O Oxymercuration-demercuration: regiospecific (Markovnikov addition: H adds to less substituted C of double bond; OH adds to the more substituted C of the double bond). No carbocation intermediate; intermediate is cyclic Mercurium ion and hence no rearrangements. Alkene, Hg(OAc)2/THF, H2O; NaBH4/H2O Hydroboration-Oxidation: regiospecific (nonMarkovnikov addition: H adds to more substituted C of double bond; OH adds to the less substituted C of the double bond). No carbocation intermediate ion and hence no rearrangements. Stereospecific syn addition. Alkene, BH3/THF; H2O2, NaOH

7.6 Addition of Carbenes to Alkenes The carbene functional group is “half of an alkene” Carbenes are electrically neutral with six electrons in the outer shell They symmetrically across double bonds to form cyclopropanes

Formation of Dichlorocarbene Base such as t-butoxide removes proton from chloroform Stabilized carbanion remains Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene The addition of the carbene is stereospecific: syn addition

Simmons-Smith Reaction Equivalent of addition of CH2: Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition

Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis

7.7 Reduction of Alkenes: Hydrogenation Addition of H-H across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)

Hydrogen Addition- Selectivity Selective for C=C. No reaction with C=O, C=N Polyunsaturated liquid oils become solids If one side is blocked, hydrogen adds to other

Mechanism of Catalytic Hydrogenation Heterogeneous – reaction between phases Addition of H-H is syn

7.8 Oxidation of Alkenes: Hydroxylation and Cleavage Hydroxylation adds OH to each end of C=C Catalyzed by osmium tetroxide Stereochemistry of addition is syn Product is a 1,2-dialcohol or diol (also called a glycol)

Osmium Tetroxide Catalyzed Formation of Diols Hydroxylation - converts to syn-diol Osmium tetroxide, then sodium bisulfate Via cyclic osmate di-ester

Summary of Hydroxylation Reactions: Syn Addition to create the syn-diol: 2 methods: Osmium tetroxide catalysed reaction & Potassium permanganate oxidation under alkaline conditions , in the presence of OH-(look at handout for mechanism) 2. Anti addition to form the anti-diol: epoxidation; Reagents: peroxy acid (peroxy benzoic acid; acidic medium, H2O)

Alkene Cleavage: Ozone Ozone, O3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes

Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene

Structure Elucidation With Ozone Cleavage products reveal an alkene’s structure

Permangante Oxidation of Alkenes Oxidizing reagents other than ozone also cleave alkenes Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

Cleavage of 1,2-diols Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compinds Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

7.10 Addition of Radicals to Alkenes: Polymers A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization Alkenes react with radical catalysts to undergo radical polymerization Ethylene is polymerized to poyethylene, for example

Free Radical Polymerization of Alkenes Alkenes combine many times to give polymer Reactivity induced by formation of free radicals

Free Radical Polymerization: Initiation Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule A bond is broken homolytically

Polymerization: Propagation Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times

Polymerization: Termination Chain propagation ends when two radical chains combine Not controlled specifically but affected by reactivity and concentration

Other Polymers Other alkenes give other common polymers

Unsymmetrical Monomers If alkene is unsymmetrical, reaction is via more highly substituted radical

Chain Branching During Polymerization During radical propagation chain can develop forks leading to branching One mechanism of branching is short chain branching in which an internal hydrogen is abstracted

Cationic Polymerization Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations

Calculate degree of unsaturation; If this number is ≥ 4 one good guess is the presence of a benzene ring in the structure; One benzene ring has a degree of unsaturation = 4 (1 ring + 3 double bonds) See if the structure if you have guessed is consistent with the products specified for the given reactions.