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Chapter 4

Chapter 4. Reactions of Alkenes and Alkynes. The most important reaction of alkenes is the addition to the C=C double-bond of various reagents X-Y to yield saturated products A second characteristic reaction of alkenes is the formation of chain-growth polymers . Reactions of alkenes.

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Chapter 4

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  1. Chapter 4 Reactions of Alkenes and Alkynes • The most important reaction of alkenes is the addition to the C=C double-bond of various reagents X-Y to yield saturated products • A second characteristic reaction of alkenes is the formation of chain-growth polymers

  2. Reactions of alkenes Electrophilic addition reactions • Addition of HX (Hydrohalogenation) • Addition of H2O • Addition of X2 • Addition of H2 • Hydroxylation with KMnO4 • Oxidative cleavage of alkenes with acidic KMnO4 • Polymerization of alkenes

  3. Addition of HX to Alkenes: Hydrohalogenation The of halogen acids, HX, to alkenes is a general reaction that allows chemists to prepare a variety of halo-substituted alkane products

  4. A regiospecific reaction: The reactions are regiospecific (regioselective) when only one of two possible directions of addition occurs ClHHCl | | | | CH3— C — CH2 CH3— C — CH2 | | CH3 CH3

  5. Orientation of Alkene Addition Reactions:Markovnilov’s Rule • 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 • Electrophile; H+

  6. Carbocation Structure and Stability • The electronic structure of a carbocation • Bond angles about the positively charged carbon are 120° • Carbon uses sp2 hybrid orbitals to form sigma bonds to the three attached groups • The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons

  7. More highly substituted carbocation are more stable • Alkyl groups tend to donate electrons to the positively charged carbon atom • The more alkyl groups there are, the more electron donation there is and the more stable the carbocation

  8. Addition of H2O to Alkenes: Hydration • Addition of water is called hydration • Acid-catalyzed hydration of an alkene is regioselective - H adds to the less substituted carbon of the double bond • Require high temperature and strongly acidic condition

  9. Other methods

  10. Addition of X2 to Alkenes: Halogenation • Carried out with either the pure reagents or in an inert solvent such as CCl4 or CH2Cl2

  11. A test for a double bond Br2 (red) → no color

  12. Anti stereochemistry • Stereoselective reaction: a reaction in which a single starting material has the capacity to form two or morestereoisomeric products but forms one of them in greater ammounts

  13. Addition of H2 to Alkenes: Hydrogenation • Most alkenes react with H2 in the presence of a transition metal catalyst to give alkanes • commonly used catalysts are Pt, Pd, Ru, and Ni • The process is called catalytic reduction or catalytic hydrogenation • Oxidation: the loss of electrons • Reduction: the gain of electrons

  14. Syn stereochemistry

  15. Oxidation of Alkenes: Epoxidation, Hydroxylation and Cleavage • The addition of oxygen • Alkenes are oxidized to give epoxides on treatment with a peroxyacid, RCOOOH

  16. Epoxides undergo an acid-catalyzed ring-opening reaction with water (a hydrolysis) to give the corresponding dialcohol, or diol, also called a glycol • Hyrdoxylation, the addition of an -OH group

  17. The hydroxylation of the alkene can also be carried out byreaction with potassium permanganate, KMnO4, in basic solution • The reaction occurs with syn stereochemistry and yields a 1,2-dialcohol, or cis-diol, product (also called glycol)

  18. When oxidation of the alkene is carried out with KMnO4 in acidic solution, cleavage of the double bond occurs and carbonyl-containing products are obtained • The double bond carbons • contain two substutuents: the products are ketone • contain one substutuent: the products are carboxylic acid • contain two hydrogens: the products are CO2

  19. Addition of Radical to Alkenes:Polymers • A polymer is a large molecule built up by repetitive bonding together of many smaller molecules (called monomer) • Cellulose (sugar) • Proteins (amino acid) • Nucleic acid (nucleotide) • Synthetic polymers

  20. Many simple alkenes undergo rapid polymerization when treated with a small amount of a radical as catalyst • High pressure (1000-3000 atm) • High temperature (100-250℃) (several thousand monomers)

  21. Radical polymerization of an alkene involves three kinds of steps: • Initiation • Propagation • Termination In the mechanism, a curved half-arrow, or “fishhook,” is used to show the movement of a single electron

  22. Step 1 Initiation: Reaction begins when a few radicals are generated by the catalyst • Benzoyloxy peroxide is used as initiator, the O-O bond is broken on heating to yield benzoyloxy radicals • The benzoyloxy radicals then adds to the C=C bond of ethylene to generate a carbon radical

  23. Step 2 Propagation: • Polymerization occurs when the carbon radical formed in step 1 adds to another ethylene molecule • Repetition of this step for hundreds or thousands of times builds the polymer chain

  24. Step 3 Termination: Polymerization eventually stops when a reaction that consumes the radical occurs Combination of two growing chains is one possible chain-terminating reaction 2 R-CH2CH2· → R-CH2CH2CH2CH2-R

  25. Conjugated Dienes A compound has altering single and double bonds – so-called conjugated compound -- • If the double bonds are well separated in a molecule, they react independently, but they are close together, they may interact with one another Buta-1,3-diene is a conjugated diene, whereas penta-1,4-diene is a non-conjugated diene with isolated double bonds

  26. There is an electronic interaction between the two double bonds of a conjugated diene because of p orbital overlap across the central single bond • This interaction of p orbitals across a single bond gives conjugated dienes some unusual properties

  27. HX adds to a conjugated diene, mixtures of products are oftenobtained • 3-Bromobut-1-ene is the typical Markovnikov product of 1,2-addition, but 1-bromobut-2-ene appears unusual (1,4-addition)

  28. Allylic carbocation Next to the double bond More stable than nonallylic

  29. Stability of Allylic Carbocations: Resonance • All three carbon atoms are sp2-hybridized, and each has a p orbital • The p orbital on the central carbon can overlap equally well with p orbitals on either of the two neighboring carbons • The two electrons are free to move about over the entire three-orbital array

  30. The two individual structures of an allylic carbocation are called resonance forms • The only difference between the resonance forms is the position of the bonding electrons • The atoms remain in exactly the same place in both resonance forms – connections and 3-D shapes • An allylic carbocation has a single, unchanging structure called a resonance hybrid that is blend of the two individual forms • The greater the number of possible resonance forms, the greater the stability –resonance leads to stability

  31. Drawing and Interpreting Resonance Forms The lengths of the two C-O bonds are identical The acetate ion is simply a resonance hybrid of the two resonance forms, with both oxygens sharing the p electrons and the negative charge equally

  32. Individual resonance forms are imaginary. • The real structure is a resonance hybrid of the different resonance forms • Resonance forms differ only in the placement of their p or non-bonding electrons

  33. Different resonance forms of a substance don’t have to be equivalent

  34. Resonance forms must be valid Lewis structures and obey normal rules of valency • Resonance leads to stability • The greater the number of resonance forms, the more stable of the substance

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