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1 Chapter 10 Alkenes. 2 Introduction—Structure and Bonding.

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Presentation on theme: "1 Chapter 10 Alkenes. 2 Introduction—Structure and Bonding."— Presentation transcript:

1 1 Chapter 10 Alkenes

2 2 Introduction—Structure and Bonding

3 3 Alkenes Strength of the  Bonding Restricted rotation: Stereoisomerism: Stability:

4 4 Alkenes Cycloalkenes having fewer than eight carbon atoms have a cis geometry. trans-Cyclooctene is the smallest isolable trans cycloalkene less stable than cis-cyclooctene, making it one of the few alkenes having a higher energy trans isomer. Introduction—Structure and Bonding

5 5 Alkenes general structural formula C n H 2n : acyclic alkenes, Cycloalkanes Each  bond or ring removes two hydrogen atoms from a molecule, and this introduces one degree of unsaturation. The number of degrees of unsaturation for a given molecular formula can be calculated by comparing the actual number of H atoms in a compound to the maximum number of H atoms possible for the number of carbons present if the molecule were a straight chain alkane. This procedure gives the total number of rings and/or  bonds in a molecule. Calculating Degrees of Unsaturation : n-m for C n H 2(n-m) Quick assessment of molecular structure from molecular formula n-m = # of rings + # of  bonds

6 6 Alkenes Calculating Degrees of Unsaturation w/ hetero atoms Halogens (F, Cl, Br, I) - Add the number of halogens to the number of hydrogens in the formula. Oxygen – Ignore the number of oxygens in the fomula. Nitrogen – Subtract the number of nitrogens from the number of hydrogens in the formula. Examples. Deduce the number of degrees of unsaturation (d.u.) in the following molecular formulas and suggest one possible structure for each: (a)C 6 H 11 Cl; (b) C 5 H 8 O; (c) C 8 H 9 N.

7 7 Alkenes Nomenclature of Alkenes: alkenes are identified by the suffix –ene.

8 8 Alkenes Nomenclature of Alkenes Compounds with two double bonds : the suffix “–adiene”. three double bonds trienes, and so forth. Always choose the longest chain that contains both atoms of the double bond. In cycloalkenes, the double bond is located between C1 and C2, and the “1” is usually omitted in the name. The ring is numbered clockwise or counterclockwise to give the first substituent the lower number. Compounds that contain both a double bond and a hydroxy group are named as alkenols and the chain (or ring) is numbered to give the OH group the lower number.

9 9 Nomenclature of Alkenes : stereoisomers Alkenes Zusammen (together) Entgegen (opposite)

10 10 Alkenes Some alkene or alkenyl substituents have common names. The simplest alkene, CH 2 =CH 2, named in the IUPAC system as ethene, is often called ethylene. Nomenclature of Alkenes

11 11 Alkenes physical properties are similar to alkanes of comparable molecular weight. Alkenes have low melting points and boiling points. Melting and boiling points increase as the number of carbons increases because of increased surface area. Alkenes are soluble in organic solvents and insoluble in water. The C—C single bond between an alkyl group and one of the double bond carbons of an alkene is slightly polar because the sp 3 hybridized alkyl carbon donates electron density to the sp 2 hybridized alkenyl carbon. Physical Properties

12 12 Alkenes A consequence of this dipole is that cis and trans isomeric alkenes often have somewhat different physical properties. cis-2-Butene has a higher boiling point (4 ° C) than trans-2-butene (1 ° C). In the cis isomer, the two C sp 3 —C sp 2 bond dipoles reinforce each other, yielding a small net molecular dipole. In the trans isomer, the two bond dipoles cancel. Physical Properties

13 13 Alkenes Interesting Alkenes

14 14 Alkenes Triacyl glycerols are hydrolyzed to glycerol and three fatty acids of general structure RCOOH. Lipids As the number of double bonds in the fatty acid increases, the melting point decreases.

15 15 Alkenes Lipids

16 16 Alkenes Fats and oils are triglycerols with different physical properties. Fats have higher melting points—they are solids at room temperature. Usually from animal sources Oils have lower melting points—they are liquids at room temperature. Usually from vegitable sources The identity of the three fatty acids in the triacylglycerol determines whether it is a fat or an oil. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains. Lipids

17 17 Alkenes Increasing the number of double bonds in the fatty acid side chains decreases the melting point of the triacylglycerol. Fats are derived from fatty acids having few or no double bonds. Oils are derived from fatty acids having a larger number of double bonds. An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains. Lipids

18 18 Alkenes alkenes can be prepared from alkyl halides and alcohols via elimination reactions. Preparation of Alkenes

19 19 Alkenes elimination reactions are stereoselective and regioselective. Preparation of Alkenes

20 20 Reactions of Alkenes The characteristic reaction of alkenes is addition—the  bond is broken and two new  bonds are formed. Introduction to Addition Reactions Alkenes are electron rich. Because alkenes are electron rich, simple alkenes do not react with nucleophiles or bases, reagents that are themselves electron rich. Alkenes react with electrophiles.

21 21 Stereochemical outcome of addition to alkenes Introduction to Addition Reactions When the addition is selective, only one set of enantiomers forms.

22 22 Addition Reactions to Alkenes

23 23 Alkenes Hydrohalogenation—Electrophilic Addition of HX Addition reactions are exothermic

24 24 Alkenes Hydrohalogenation—Electrophilic Addition of HX What about the stereochemistry & Regiochemistry of the reaction ?

25 25 Alkenes The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. Mechanism of Hydrohalogenation

26 26 Alkenes Hydrohalogenation—Electrophilic Addition of HX

27 27 Alkenes Markovnikov’s Rule : Regioselectivity of addition Markovnikov’s rule (1869) in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with.

28 28 Alkenes The basis of Markovnikov’s rule : the formation of a carbocation in the rate-determining step of the mechanism. Markovnikov’s Rule

29 29 Alkenes According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process. Hydrohalogenation—Markovnikov’s Rule

30 30 The Hammond postulate relates reaction rate to stability. It provides a quantitative estimate of the energy of a transition state. The Hammond postulate : the transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy. in an endothermic step, TS resembles the products, in an exothermic step, TS resembles the reactants.

31 31 Alkenes According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process. Hydrohalogenation—Markovnikov’s Rule

32 32

33 33 Alkenes Recall that trigonal planar atoms react with reagents from two directions with equal probability. Achiral starting materials yield achiral products or racemic mixture. Hydrohalogenation—Reaction Stereochemistry A racemic mixture 

34 34 Hydrohalogenation—Reaction Stereochemistry

35 35

36 36 Alkenes Hydrohalogenation—Summary

37 37 Alkenes Hydration forms an alcohol. Hydration—Electrophilic Addition of Water - Reaction mechanism is very siminar to hydrohalogenation -

38 38

39 39 Alkenes Alcohols add to alkenes, forming ethers by the same mechanism in presence of acid. Hydration—Electrophilic Addition of Alcohols Note that there are three consequences to the formation of carbocation intermediates: 1.Markovnikov’s rule holds. 2.Addition of H and OH occurs in both syn and anti fashion. 3.Carbocation rearrangements can occur.

40 40 Alkenes Halogenation is the addition of X 2 (X = Cl or Br) to an alkene to form a vicinal dihalide. Halogenation—Addition of Halogen

41 41 Alkenes Halogens add to  bonds because halogens are polarizable. Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration. No rearrangements occur Only anti addition of X 2 is observed These facts suggest that carbocations are not intermediates. Halogenation—Addition of Halogen

42 42 Addition reaction of Br 2 to alkenes : the fact! How can we explain this outcome ?

43 43 Alkenes Mechanism of Halogenation—Addition of Halogen Carbocations are unstable because they have only six electrons around carbon. Halonium ions are unstable because of ring strain.

44 44

45 45 Non-classical carbocation By George Olah

46 46 Even though X¯ is formed in step [1] of the mechanism, its concentration is small compared to H 2 O (often the solvent), so H 2 O and not X¯ is the nucleophile. Halohydrin Formation : addition of X-OH

47 47 Alkenes Halohydrin Formation Bromohydrins are also formed with N-bromosuccinimide (NBS) in aqueous DMSO [(CH 3 ) 2 S=O]. In H 2 O, NBS decomposes to form Br 2, which then goes on to form a bromohydrin by the same reaction mechanism.

48 48 Alkenes Halohydrin Formation : Selectivity

49 49 Halohydrin Formation application

50 50 Alkenes Halohydrin Formation

51 51 Alkenes Hydroboration—Oxidation Hydroboration—oxidation is a two-step reaction sequence that converts an alkene into an alcohol.

52 52 Alkenes Hydroboration—Oxidation Hydroboration—oxidation results in the addition of H 2 O to an alkene.

53 53 Alkenes Hydroboration—Oxidation BH 3 is a reactive gas that exists mostly as a dimer, diborane (B 2 H 6 ). Borane is a strong Lewis acid that reacts readily with Lewis bases. The first step in hydroboration

54 54 Alkenes Hydroboration—Oxidation The proposed mechanism involves concerted addition of H and BH 2 from the same side of the planar double bond:

55 55 Alkenes Hydroboration—Oxidation BH 3 can react with three equivalents of alkene to form a trialkylborane.

56 56 Alkenes Hydroboration—Oxidation 9-borabicyclo[3.3.1]nonane (9-BBN)

57 57 Hydroboration—Selectivity With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom.

58 58 Alkenes Hydroboration—Oxidation

59 59 Alkenes Hydroboration—Oxidation Oxidation replaces the C—B bond with a C—O bond, forming a new OH group with retention of configuration. The overall result of this two-step sequence is syn addition of the elements of H and OH to a double bond in an “anti-Markovnikov” fashion.

60 60 Alkenes Hydroboration—Oxidation

61 61

62 62

63 63 Summary of Addition Reactions of Alkenes ReagentProductMechanismRegioselectivityStereochemistry HXAlkyl halide 2 step via rate-determining formation of R +. Rearrangements possible MarkovnikovSyn and anti addition H2OH2OAlcoholAs above X2X2 Vicinal (1,2) dihalide 2 step via rate-determining formation of bridged halonium ion No rearrangements -Anti addition Stereospecific X 2 /H 2 OHalohydrin (2- haloalcohol) 3 steps, but similar to halogenation Markovnikov: X + bonds to less substituted C. Anti addition BH 3 (or equivale nt), then H 2 O 2 /OH - Alcohol2 steps: one-step hydroboration, then oxidation. No rearrangements Anti- Markovnikov Syn addition in hydroboration step; retention of configuration in oxidation step

64 64 keeping track of all the reactions? Alkyl halides – Substitution and elimination (they have good leaving groups). Alcohols – As alkyl halides, but only if OH group has been converted to a good leaving group. Alkenes – Addition (  bond is easily broken) Firstly, check the basic reaction types for a functional group. This provides an overall organization of reactions. Then, learn the specific reagents for each reaction. This helps to classify the reagent according to its major properties. Is it basic or acidic? Is it electrophilic or nucleophilic? Is it an oxidizing agent or a reducing agent? Think mechanism ! – reasonable ones….

65 65 Alkenes Alkenes in Organic Synthesis: combination of reactions Suppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol. To solve this problem we must:

66 66 Alkenes Alkenes in Organic Synthesis Working backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis.

67 67 10.40, 10.46, 10.51, 10.52, 10.53, 10.57, 10.61, 10.63, 10.64 Homework

68 68 Preview of Chapter 11 Alkynes Extension of chapter 10 Preparation of alkynes dehydrohalogenation Reactions of alkynes Addition reaction Reaction of acetylide anions

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