1. 1 Chapter 3 ALKENES Introduction 3.1 Isomersm in the alkenes 3.2 Prep artion 3.3 Physical properties 3.4 Chemical reactions 3.5 Conjugated dienes 3.6 Nomenclature

2. 2 INTRODUCTION  Alkenes or olefins are unsatutared hidrocarbons that contain functional groups, carbon-carbon double bond, C=C.  They form a homologous series of hydrocarbons with the general formula C n H 2n.  Alkenes occur abundantly in nature, and many have important biological roles.

3. 3  Ethylene is a plant hormone that induces ripening in fruit;   -Pinene is the major constituent of turpentines.   -Carotene, which contains 11 double bonds.  - Pinene  -Carotene  It is an orange pigment responsible for the orange color of carots;   -Carotene is avaluable dietary source of vitamin A;

4. 4  Lycopene is responsible for the red color of tomato.

5. 5 3.1 Nomenclature  Common names (trivial name) used for simple alkenes: ethylene, propylene, isobutylene, and so on.  In the IUPAC system, the continuous-chain alkenes are named after their alkane parent, but with the –ane ending changed to –ene.  For example, CH 3 -CH 3 is ethane and CH 2 =CH 2 is ethene  A hydrocarbon with two double bonds is called a diene, while one with three double bond is called a triene, and so on.

6. 6 Alkenyl  Groups containing C=C double bonds are called alkenyl.  In general, some alkenyl groups have trivial names. They are shown below.

7. 7 3.2 Isomersm in the alkenes  All the alkenes with more 4 carbon atoms show structural isomerism.  This means that there are two or more different structural formulae that you can draw for each molecular formula.  Some alkenes have geometric isomers

8. 8 3.3 Preparation  The most common industrial synthesis of alkenes is based on cracking of petroleum.  Large alkanes are broken apart at high temperatures in the presence of a zeolite catalyst to give  alkenes and  smaller alkanes.

9. 9  the mixture of products is then separated by fractional distillation.  This is mainly used for the manufacture of small alkenes (up to six carbons). Cracking reaction

10. 10 a.The Wittig reaction  The reaction involves reaction of an aldehyde or ketone with a Wittig reagent (or phosphorane) of the type Ph 3 P=CR 3 R 4 to produce an alkene and Ph 3 P=O. alkene

11. 11 Ex:

12. 12 b. Synthesis in the laboratory  The principal methods for alkene is the elimination of alkyl halides, alcohols and similar compounds. b1. Dehydrohalogenation of alkyl halides. This reaction involves loss (elimination) of the halogen atom and a hydrogen atom from a carbon adjacent to the one losing the halogen. The reagent required is a base, whose function is to abstract the hydrogen as a proton. Ease of dehydrohalogenation of ankyl halides is 3 0 > 2 0 > 1 0

13. 13 Saytzeff Rule For unsymmetrical products the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate. For the given above case the chief product is 2-butene (but-2-ene) (Saytzeff's rule)

14. 14 b2. Dehydration of alcohols Dehydration meaning elimination of a molecule of water is generally carried out neither of two ways: (a)- by heating the alcohol with sulfuric acid or phosphoric acid; or (b)- by passing the alcohol vapor over a catalyst, commonly alumina (Al 2 O 3 ), at high temperatures.  The ease of dehydration of alcohols is: 3 0 > 2 0 > 1 0.

15. 15 b3. Dehalogenation of vicinal dihalides Dehalogenation of vicinal dihalides is carried out by treatment with zinc:

16. 16 b4. Reduction of alkynes A carbon-carbon double bond can be generated form a carbon-carbon triple bond by addition: Lindlar catalyst : Pd/CaCO 3 ho ặ c Pd/BaSO 4

17. 17 3.4 Physical properties of the alkenes The physical state depends on molecular mass.  The simplest alkenes, ethene, propene and butene are gases.  Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.  The alkene has a boiling point which is little lower than the corresponding alkane.  The B.P. depends on the shape of the molecule and the number of electrons it contains (Van der Waals forces).  Solubility. Alkenes are virtually insoluble in water, but dissolve in organic solvents.

18. 18 3.5 Chemical reactions Alkenes are more reactive than alkanes due to the presence of a carbon-carbon pi-bond. The typical reactions of alkenes are addition reactions. These reactions occur because:  An addition reaction results in the conversion of one  bond and one σ bond into two σ bonds with releasing energy (reactions are usually exothermic).

19. 19  The electrons of the  bond are exposed.  Because the  bond results from overlapping p orbitals,  electrons lie above and below the plane of the double bond:

20. 20  using a general molecule X-Y  The rather exposed electrons in the pi bond are particularly open to attack by agents which carry some degree of positive charge.  These are called electrophiles.

21. 21 Electrophiles  positive reagents proton (H + ),  neutral reagents such as bromine (because it can be polarized so that one end is positive),  Lewis acids: BH 3, BF 3 and AlCl 3.  Metal ions that contain vacant orbitals: silver ion (Ag + ), mercuric ion (Hg 2+ ), platinium ion (Pt 2+ ).

22. 22 Mechanism for the addition reaction  The addition reaction of alkenes involves electrophilic attack on the double bond. Ex:

23. 23  Step 1: Electrophile A + slowly attracts to  -bond to form carbocation  Step 2: carbocation rapidy links with B - to form additional product. carbocation

24. 24 3.5.1 Hydrohalogenation Addition of hydrohalic acids such as HCl, HBr or HI to alkenes yields the corresponding haloalkanes. CH 2 =CH 2 + HBr → CH 2 Br-CH 2 -H The reaction takes place in two steps:

25. 25 Regioselectivity of a reaction  The addition of the strong acids (HBr, HCl,...) to symmetrical alkenes (ethene, cyclohexene) leads to formation of only one product.  In the case of unsymmetrical alkenes (but-1-ene, 2-methylbut-2-ene and methylcyclohexene), the halogen is preferentially added to the carbon with fewer hydrogen substituents, forming only one constitutionally isomeric product.  reactions are regioselectivity.

26. 26 For 2-methylbut-2-ene: (CH 3 ) 2 C=CHCH 3 + H-Cl → (CH 3 ) 2 CCl–CHHCH 3 2-chloro-2-methylbutane (chief product) Not (CH 3 ) 2 CH–CHClCH 3 Similarly, butene (but-1-ene) reacts with HBr forming 2-bromobutane as the predominant product.

27. 27 Markovnikov Rule  When an acid HX adds to an unsymmetrically substituted double bond, the hydrogen of the acid bonds to that carbon of the double bond that has the greater number of hydrogen atoms.  Expression in another way: in the addition of HX to alkenes, the more highly substituted carbocation intermediate (the more stable) is formed rather than the less highly substituted one.

28. 28  The stability of carbocations increases with increasing alkyl groups substitution because 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. CH 3 (+) < CH 3 CH 2 (+) < (CH 3 ) 2 CH (+) ≈ CH 2 =CH-CH 2 (+) < C 6 H 5 CH 2 (+) ≈ (CH 3 ) 3 C (+)

29. 29 3.5.2 Hydration  A process in which water adds to alkenes to yield alcohols, ROH, is called hydration.  Hydration of an alkene with aqueous acid takes place by a mechanism similar to that of HX addition. Note: (A + )  The addition of water to an unsymmetrical alkene follows Markovnikov’s rule, giving more highly substituted alcohol as product.

30. 30 Ex: propene reacts with water Step 1: H + attracts to  -bond H 2 SO 4 is catalyst. Step 2: carbocation links with HO - The sub-product is CH 2 -CH 2 -CH 2 -OH Carbocation Main product

31. 31  The reactions of 2-methylpropene with mercuric acetate (oxymercuration, in the top) and boron hydrides (hydroboration, in the bottom) are illustrated as below:

32. 32 3.5.3 Halogenation  Addition of elementary bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes, respectively: CH 2 =CH 2 + Br 2 → BrCH 2 -CH 2 Br  The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes.

33. 33  Practice Problem Predict the favored product in examples as shown below:

34. 34  Practice Problem Write products for the following reactions:

35. 35 3.5.4 Hydrogenation  Addition of H 2 to the C=C bond of alkenes produces the corresponding alkanes.  carried out under pressure in the presence of a metallic catalyst, such as Pt, Raney nickel (alloy of Ni and Al) or Pd is often employed.  The catalytic hydrogenation of ethylene to yield ethane: CH 2 = CH 2 + H 2 → CH 3 -CH 3  The mechanism is syn-addition.

36. 36 3.5.5 Oxidation Alkenes are oxidized with a large number of oxidizing agents.  In the presence of oxygen, alkenes burn with a bright flame to produce carbon dioxide and water.  Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides

37. 37 The Ozonolysis reaction  The first step: the addition one ozone molecular with double bond leads to form ozonide,  The second step:then hydrolysis the ozonide in the present of Zn metal, yielding carbonyl products:

38. 38 The Ozonolysis reaction  Carbonyl products: aldehydes and ketones + Aldehydes in the present of an oxidative agent (H 2 O 2 ) are oxidized to form carboxylic acids. Ex: R 1, R 2, R 3 : hydrocarbon radicals; R 4 : H

39. 39 The Ozonolysis reaction  so, we can write the ozonolysis reaction as the diagram: 1.The first step 2.The second step If we know the structure of products, we can determine the structure of alkenes  is used to determine the position of a C=C bond in an unknown alkene.

40. 40 The Ozonolysis reaction  Some examples: Ex 1: Ex 2:

41. 41 The Ozonolysis reaction  Some examples: Ex 3: alkene + 1.O 3 ; 2. H 2 O/Zn to form axetanal (CH 3 CHO) and metanal (HCHO). Determine the structure of the alkene.  R 1 :CH 3 ; R 2 : H  R 3 = R 4 = H CH 3 -CH=O H-CH=O Alkene is: CH 3 CH=CH 2

42. 42 Hydroxylation  Hydroxylation is the addition of an –OH group to the alkene carbons.  can be carried out by reaction of the alkene with potassium permaganate, KMnO 4 in basic solution.  The reaction occurs with syn-stereochemistry and yields a 1,2-dialcohol, or diol, product (also called a glycol).

43. 43 Oxidative cleavage  When oxidation of the alkene is carried out with KMnO 4 in acidic solution, cleavage of the double bond occurs and carbonyl-containing products are obtained. Ex:

44. 44  If the double bond is tetrasubstituted, the two carbonyl- containing products are ketones;  If a hydrogen is present on the double bond, one of the carbonyl-containing products is a carboxylic acid;  If two hydrogen are present on one carbon, CO 2 is formed.

45. 45 3.5.6 Polymerization  Polymers play a very important role in our lives.  A polymer is a large molecule built up by repeating bonding together of many smaller molecules, called monomers.  Biological polymers in Nature: Cellulose is a polymer of sugar units; Proteins are polymers of amino acids; Nucleic acids are polymers of nucleotide units.

46. 46 3.6 Alkadienes The polyenes, having two double bonds, are called "dienes". The relative arrangement of the double bonds dictates the characteristic reactions of the systems. Double bonds can be arranged in three possible ways as below: (from most to least stable)

47. 47 Conjugated dienes  There is an electronic interaction between the two double bonds of a conjugated diene because the adjacent “p" orbitals can all overlap with each other across the central single bond.  Adjacent π systems makes the conjugated dienes the most stable type of diene.

48. 48 Isolated dienes  The double bond units occur separately.  The π systems are isolated from each other by sp 3 hybridised centers.  The isolated dienes have reactivity that is characteristic of simple alkenes

49. 49 Cumulated dienes The double bond units share a common sp hybridised C atom. The cumulated dienes have reactivity more like simple alkynes.  Allene, CH 2 =C=CH 2

50. 50 3.6.1 Preparation  By elimination reactions of unsaturated alcohols and alkyl halides.  The formation of the conjugated diene is usually favored over the isolated diene.

51. 51  Practice Problems What would the products of the following sequences be ?  But-1-ene reacted with N-bromosuccinimide (NBS) then treated with KOH in ethanol/ heat.  Cyclohexene reacted with Br 2 / CH 2 Cl 2 then with KOH in ethanol / heat.

52. 52 3.6.2 Reactions of Dienes  Dienes undergo electrophilic addition reactions in a similar fashion to alkenes.  Dienes can undergo addition reactions in which one or both of the π bonds are converted to new stronger σ bonds.

53. 53  Typical reactions of dienes:  Addition of hydrogen halides  Addition of halogens  Diels-Alder reaction

54. 54 Addition of Hydrogen Halides  Conjugated dienes undergo addition reactions in a similar manner to simple alkenes, but two modes of addition are possible.  These differ based on the relative positions of H and X in the products:

55. 55 Explaining distribution of the 1,2 and 1,4 addition- products.

56. 56  At low temperature, the reaction is under kinetic control and the major product is form 1,2- addition.  At room temperature, the reaction is under thermodynamic control and the major product is from 1,4- addition.

57. 57 Addition of Halogens to Dienes Halogens add to dienes via direct and conjugate addition pathways: The major 1,4-addition products are usually the more stable with the E configuration of C=C.

58. 58 Diels-Alder Reaction The Diels-Alder reaction (Nobel Prize in 1950) is addition of a conjugated diene to an alkene (the dienophile) to produce a cyclic product. The reaction of 1,3-butadiene with ethene to form forms cyclohexene, via a cyclic transition state, so it can also be described as a "cycloaddition".

59. 59 THE END OF ALKENES