Alkene

structure and reactivity

1. Alkenes Structure, Nomenclature, and an introduction to Reactivity Thermodyn a mics and Kinetics ANEES BABER MSc Chemistry

2. ALKENES molecular formula = C n H 2 n +2 – 2 hydrogens for every π bond or ring degree of unsaturation = the total numbers of π bonds and ring The general molecular formula for an acyclic alkene is also C n H 2n because the double bond means the alkene has two fewer hydrogens than an alkane with the same number of carbons. Thus, the general molecular formula for a cyclic alkene must be C n H 2n-2 . The general molecular formula for a noncyclic alkane is C n H 2n+2 and the general formula for Cyclic alkene C n H 2n

3. Compounds with Molecular Formula = C 8 H 14 C n H 2 n +2 = C 8 H 18 Therefore, C 8 H 14 has two degrees of unsaturation . Thus, the general molecular formula for a hydrocarbon is C n H 2n+2 minus two hydrogens for every π bond or ring in the molecule The total number of π bonds and rings is called the compound’s degree of unsaturation . Thus, C 8 H 14 , which has four fewer hydrogens than an acyclic alkane with eight carbons (C n H 2n+2 = C 8 H 18 ), has two degrees of unsaturation. So we know that the sum of the compound’s π bonds and rings is two.

4. Saturated and Unsaturated Hydrocarbons Saturated hydrocarbons have no double bonds. Unsaturated hydrocarbons have one or more double bonds .

5. Nomenclature of Alkenes Replace “ ane ” of alkane with “ ene . ” 1- The functional group gets the lowest possible number . Number the longest continuous chain containing the functional group in the direction that gives the functional group suffix the lowest possible number.

6. Nomenclature of Dienes two double bonds = diene 2- For a compound with two double bonds, the “ne” ending of the corresponding alkane is replaced with “ diene .” 3- The name of a substituent is stated before the name of the longest continuous chain that contains the functional group, together with a number to designate the carbon to which the substituent is attached. if a compound’s name contains both a functional group suffix and a substituent, the functional group suffix gets the lowest possible number.

7. Number in the direction so that the functional group gets the lowest number. Nomenclature of Alkenes

8. Substituents are stated in alphabetical order . Nomenclature of Alkenes 4- If a chain has more than one substituent, the substituents are stated in alphabetical order. Then the appropriate number is assigned to each substituent.

9. Nomenclature of Alkenes 5- If counting in either direction results in the same number for the alkene functional group suffix, the correct name is the one containing the lowest substituent number.

10. Nomenclature of Cyclic Alkenes 6- A number is not needed to denote the position of the double bond in a cyclic alkene because the ring is always numbered so that the double bond is between carbons 1 and 2. To assign numbers to any substituents, count around the ring in the direction (clockwise or counterclockwise) that puts the lowest number into the name. Notice that 1,6-dichlorocyclohexene is not called 2,3-dichlorocyclohexene because the former has the lowest substituent number (1), even though it does not have the lowest sum of substituent numbers ( 1 + 6 = 7 versus 2 + 3 = 5 ).

11. Nomenclature of Alkenes 7- If counting in either direction leads to the same number for the alkene functional group suffix and the same lowest number or numbers for one or more of the substituents , then ignore those substituents and choose the direction that gives the lowest number to one of the remaining substituents .

12. Chapter 4 12 IUPAC Nomenclature of Dienes Find the longest chain containing both double bonds 1 2 3 4 5 3-butyl-1,4-pentadiene

13. Chapter 4 13 IUPAC Nomenclature of Dienes Use corresponding alkane name but replace the “ ne” ending with “ diene” “pentane” changed to “pentadiene” 3-butyl-1,4-pentadiene

14. Chapter 4 14 IUPAC Nomenclature of Dienes Number in the direction that gives the lowest number to a double bond 1,5-heptadiene not 2,6-heptadiene

15. Chapter 4 15 IUPAC Nomenclature of Dienes List substituents in alphabetical order 5-ethyl-2-methyl-2,4-heptadiene

16. Chapter 4 16 IUPAC Nomenclature of Dienes Place numbers indicating the double bond positions either in front of the parent compound or in the middle of the name immediately before the diene suffix 5-ethyl-2-methyl-2,4-heptadiene or 5-ethyl-2-methyl-hepta-2,4-diene

17. Vinylic and Allylic Carbons vinylic carbon : the sp 2 carbon of an alkene allylic carbon : a carbon adjacent to a vinylic carbon

18. Nomenclature Notice how these Groups and some others can be used as substituents names in Systematic nomenclature;

19. Double Bonds Have Restricted Rotation Rotation about a double bond breaks the π bond . Each double-bonded carbon of an alkene has three sp 2 orbitals. Each of these orbitals overlaps an orbital of another atom to form a σ bond, one of which is one of the bonds in the double bond. Thus, the σ bond of the double bond is formed by the overlap of an sp2 orbital of one carbon with an sp2 orbital of the other carbon, and the other bond of the double bond is a π bond formed from side-to-side overlap of the remaining π orbital on each of the sp2 carbons. For the two p orbitals to be parallel, all six atoms of the double-bond system must be in the same plane .

20. Stereoisomers are Named Using a cis or trans Prefix

21. Six Atoms of an Alkene are in the Same Plane © 2014 Pearson Education, Inc.

22. Alkenes Have Cis – Trans Isomers Cis: Hydrogen on same side of the ring Trans: The hydrogens are on opposite sides of the ring. They have different configurations; they can be separated.

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24. The E,Z System of Nomenclature Z = Zusammen (together) E = Entgegen (opposite) The E,Z system of nomenclature was devised for alkenes that do not have a hydrogen attached to each of the sp2 carbons. To name an isomer by the E,Z system, we first determine the relative priorities of the two groups bonded to one of the sp2 carbons and then the relative priorities of the two groups bonded to the other sp2 carbon

25. Relative Priorities The relative priorities of the two groups depends on the atomic numbers of the atoms attached to the sp 2 carbon .

26. The E and Z Isomers If the two atoms attached to an sp 2 carbon are the same (there is a tie), then consider the atomic numbers of the atoms that are attached to the “tied” atoms. The C of the CH 2 Cl group is bonded to Cl, H, H, and the C of the CH 2 CH 2 Cl group is bonded to C,H,H. Cl has a greater atomic number than C, so the CH 2 Cl group has the higher priority. Both atoms attached to carbon on the right are Cs (in a CH2OH group and a CH(CH 3 ) 2 group), so there is a tie on this side as well. The C of the CH 2O H group is bonded to O , H , H, and the C of the CH(CH 3) 2 group is bonded to C, C , H. Of these six atoms, O has the greatest atomic number, so CH2 OH has the higher priority.

27. E and Z If an atom is doubly bonded to another atom, the priority system treats it as if it were singly bonded to two of those atoms. If an atom is triply bonded to another atom, the priority system treats it as if it were singly bonded to three of those atoms. The groups attached to the CH2 groups to break the tie. One of these groups is CH 2 OH, and the other is C CH; the C of the CH 2 OH group is bonded to H, H , O; the triple-bonded C is considered to be bonded to C,C,C. Of the six atoms, O has the greatest atomic number, so CH 2 OH has the higher priority.   On the right, the first carbon of the CH2CH3 group is bonded to C,H,H ; the first carbon of the CH=CH2 group is bonded to an H and doubly bonded to a C, so it is considered to be bonded to H, C , C . One C cancels in each of the two groups, leaving H and H in the CH2 CH3 group and H and C in the CH=CH2 group. C has a greater atomic number than H, so CH=CH2 has the higher priority.

28. E and Z If two isotopes (atoms with the same atomic number, but different mass numbers) are being compared, the mass number is used to determine the relative priorities. The Cs that are attached to the sp2 carbon on the right are both bonded to C,C,H, so we must go to the next set of atoms to break the tie. The second carbon of the CH(CH3)2 group is bonded to H,H,H, whereas the second carbon of the CH=CH2 group is bonded to H, H, C .

29. Reactions of Alkenes . Alkenes undergo Electrophilic or Nucleophilic addition reaction All compounds with a carbon–carbon double bond react in the same way, whether the compound is a small molecule like ethene or a large molecule like cholesterol.

30. Electrophiles An electrophile has : a positive charge, a partial positive charge, or an incomplete octet. ELECTROPHILE : Electron loving (likes minus: e- and anions)

31. Nucleo p hiles A nucleophile has a negative charge or a lone pair of electrons NUCLEOPHILE : Nuclei loving (likes protons: H+) Nucleophiles : electron-rich atoms or molecules that react with electrophiles . An alkene is an electron rich molecule, a nucleophile? Electrophiles are Lewis acids and nucleophiles are Lewis bases.

32. Nucleo p hiles The π bond is localized above and below the C– C σ bond. The π electrons are relatively far away from the nuclei and are therefore loosely bound. An electrophile will attract those electrons, and can pull them away to form a new bond. This leaves one carbon with only 3 bonds and a + ve charge ( carbocation ). The double bond acts as a nucleophile (attacks the electrophile).

33. 33 Electrophilic Addition of HBr to Alkene A two step reaction. Mechanistic path of a reaction: How reactants form products . How can a mechanism be illustrated? i.e. bond making & bond breaking The step-by-step description of the process by which reactants (in this case, alkene + HBr ) are changed into products (an alkyl halide) is called the mechanism of the reaction .

34. 34 Reaction Mechanisms We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact. Each arrow represents the simultaneous movement of two electrons (an electron pair) from an electron-rich center (at the tail of the arrow) toward an electron-deficient center (at the point of the arrow). In this way, the arrows show which bonds are formed and which bonds are broken .

35. A Nucleophile Reacts with an Electrophile is a two-step reaction .

36. Reaction Mechanism First step of the reaction An arrow is drawn to show that the two electrons of the π bond of the alkene are attracted to the partially positively charged hydrogen of HBr . The hydrogen is not immediately free to accept this pair of electrons because it is already bonded to a bromine. However, as the π electrons of the alkene move toward the hydrogen, the H–Br bond breaks, with bromine keeping the bonding electrons. Notice that the π electrons are pulled away from one sp2 carbon, but remain attached to the other. Thus, the two electrons that originally formed the π bond now form a new σ bond between carbon and the hydrogen from HBr . The product is positively charged, because the sp2 carbon that did not form the new bond with hydrogen has lost a share in an electron pair

37. The curved arrow shows where the electrons start from and where they end up . Curved Arrows Second step of the reaction In the second step of the reaction, a lone pair on the negatively charged bromide ion forms a bond with the positively charged carbon of the carbocation. Notice that in both steps of the reaction, a nucleophile reacts with an electrophile.

38. 38 Reaction Mechanis ms Curved arrows are drawn only from the electron-rich site to the electron deficient site The overall reaction involves the addition of 1 mole of HBr to 1 mole of the alkene. The reaction, therefore, is called an addition reaction . Because the first step of the reaction is the addition of an electrophile ( H+) to the alkene, the reaction is more precisely called an electrophilic addition reaction .

39. How to Draw Curved Arrows An arrow is used to show both the bond that forms and the bond that breaks. Draw the arrows so that they point in the direction of the electron flow; the arrows should never go against the flow. This means that an arrow will point away from a negatively charged atom or toward a positively charged atom.

40. How to Draw Curved Arrows Curved arrows are meant to indicate the movement of electrons. Never use a curved arrow to indicate the movement of an atom. For example, do not use an arrow as a lasso to remove a proton, as shown in the equation on the right: The head of a curved arrow always points at an atom or at a bond. Never draw the head of the arrow pointing out into space.

41. How to Draw Curved Arrows A curved arrow starts at an electron source; it does not start at an atom. In the following example, the arrow starts at the electron-rich π bond, not at a carbon atom:

42. 42 Relative Stabilities of Alkenes The more alkyl substituents attached to a double bond the more stable the double bond.

43. Thermodynamics and Kinetics To understand the energy changes that take place in a reaction such as the addition of HBr to an alkene, we need to understand some of the basic concepts of thermodynamics , which describes a reaction at equilibrium, and kinetics , which explains the rates of chemical reactions;

44. A Reaction Coordinate Diagram The more stable the species, the lower its energy.

45. The Equilibrium Constant Thermodynamics is the field of chemistry that describes the properties of a system at equilibrium . If the reactants are more stable than the products, there will be a higher concentration of reactants than products at equilibrium, and K eq will be less than 1. The relative concentrations of reactants and products at equilibrium can be expressed by an equilibrium constant, Keq The relative concentrations of products and reactants at equilibrium depend on their relative stabilities: the more stable the compound, the greater its concentration at equilibrium. Thus, if the products are more stable (have a lower free energy) than the reactants , there will be a higher concentration of products than reactants at equilibrium, and Keq will be greater than 1.

46. Exergonic and Endergonic Reactions Gibbs free-energy change , or Δ G . The symbol indicates standard conditions, which means that all species are at a concentration of 1 M, a temperature of 25 °C, and a pressure of 1 atm. The difference between the free energy of the products and the free energy of the reactants under standard conditions is called the Gibbs free-energy change. Δ G

47. 47 Thermodynamics When G ° is negative the reaction is exergonic G ° will be negative if the products have a lower free energy (are more stable) than the reactants. In other words, the reaction will release more energy than it consumes; such a reaction is called an exergonic reaction .

48. 48 Thermodynamics When G° is positive the reaction is endergonic I f the products have a higher free energy (are less stable) than the reactants, G° will be positive, and the reaction will consume more energy than it releases; such a reaction is called an endergonic reaction

49. Thermodynamics So , We have seen that whether reactants or products are favored at equilibrium can be indicated by the equilibrium constant ( Keq ) or by the change in free energy ( G° ). These two quantities are related by the equation; A small change in G° corresponds to a large change in Keq and, therefore, a large change in the amount of product obtained at equilibrium

50. ? The G° for conversion of “axial” fluorocyclohexane to “equatorial” fluorocyclohexane at 25C is -0.25kcal/ mol . Calculate the percentage of fluorocyclohexane molecules that have the fluoro substituent in an equatorial position at equilibrium.

51. Gibbs Free- Energy Change (∆ G ° ) The enthalpy term ( H ) is the heat given off or the heat consumed during the course of a reaction. Heat is given off when bonds are formed, and heat is consumed when bonds are broken. If the bonds that are formed in a reaction are stronger than the bonds that are broken, more energy will be released in the bond-forming process than will be consumed in the bond-breaking process, and ∆H will be negative. A reaction with a negative ∆H is called an exothermic reaction . If the bonds that are formed are weaker than those that are broken, ∆H will be positive. A reaction with a positive ∆H is called an endothermic reaction

52. Calculating ∆ H °

53. Entropy Entropy ∆S is a measure of the freedom of motion in a system. Restricting the freedom of motion of a molecule decreases its entropy. For example, in a reaction in which two molecules come together to form a single molecule, the entropy of the product will be less than the entropy of the reactants because two separate molecules can move in ways that are not possible when they are bound together in a single molecule. In such a reaction, ∆S will be negative. In a reaction in which a single molecule is cleaved into two separate molecules, the products will have greater freedom of motion than the reactant, and ∆S will be positive.

54. Entropy A reaction with a negative G° has a favorable equilibrium constant ( Keq > 1 ); that is, the reaction is favored as written from left to right because the products are more stable than the reactants. If you examine the expression for the Gibbs standard free-energy change, you will find that negative values of ∆H and positive values of ∆S contribute to make G° negative. In other words, the formation of products with stronger bonds and greater freedom of motion causes G ° to be negative. Notice that the entropy term is temperature dependent and, therefore, becomes more important as the temperature increases. As a result, a reaction with a positive ∆ S may be endergonic at low temperatures, but exergonic at high temperatures.

55. 57 Kinetics Kinetics is the field of chemistry that studies the rates of chemical reactions and the factors that affect those rates. G° of a reaction will not tell us how fast it will occur or if it will occur at all  G° describes only the difference between the stability of the reactants and the stability of the products. We need to know the rate of reaction The rate of a reaction is dependant on the height of the energy barrier for the reaction, G ‡ , called the free energy of activation G ‡ = (free energy of the transition state) -- (free energy of the reactants)

56. Reaction Coordinate Diagrams for Fast and Slow Exergonic and Endergonic Reactions As G ‡ decreases, the rate of the reaction increases. Thus, anything that makes the reactants less stable or makes the transition state more stable will make the reaction go faster.

57. Free Energy of Activation (∆ G ‡ ) The free energy of activation is the energy barrier of the reaction. Some exergonic reactions have small free energies of activation and therefore can take place at room temperature. In contrast, some exergonic reactions have free energies of activation that are so large that the reaction cannot take place unless energy is supplied in addition to that provided by the existing thermal conditions .

58. Thermodynamic Vs Kinetic Stability

59. Rate of reaction 1- The number of collisions that take place between the reacting molecules in a given period of time. The rate of the reaction increases as the number of collisions increases. 2- The fraction of collisions that occur with sufficient energy to get the reacting molecules over the energy barrier. If the free energy of activation is small, then more collisions will lead to reaction than if the free energy of activation is large. 3- The fraction of collisions that occur with the proper orientation. 2-Butene and HBr will react only if the molecules collide with the hydrogen of HBr approaching the π bond of 2-butene. If a collision occurs with the hydrogen approaching a methyl group of 2-butene, no reaction will take place, regardless of the energy of the collision.

60. Rate of a Reaction Increasing the concentration increases the rate. Increasing the temperature increases the rate. The rate of a reaction can also be increased by a catalyst

61. The Rate of a Reaction versus The Rate Constant for a Reaction The rate constant tells us how easy it is to reach the transition state (how easy it is to get over the energy barrier). Low-energy barriers are associated with large rate constants , whereas high-energy barriers are associated with small rate constants The rate of a reaction is a measure of the amount of product that is formed per unit of time. The rate is the product of the rate constant and the reactant concentration(s), so reaction rates depend on concentration, whereas rate constants are independent of concentration .

62. Kinetics The value of A pertains to the frequency and orientation of collisions that occurs with the proper orientation for the reaction e –E a /RT is the fraction of collisions with the minimum energy ( E a ) needed for reaction. ( R is the gas constant, T is the temperature in kelvins, and E a is the experimental energy of activation, which is an approximate value of the activation energy; Therefore, when we compare two reactions to see which one occurs more readily, we must compare their rate constants and not their concentration-dependent rates of reaction. Although rate constants are independent of concentration, they are dependent on temperature. The Arrhenius equation relates the rate constant of a reaction to the experimental energy of activation and to the temperature at which the reaction is carried out:

63. An Electrophilic Addition Reaction Transition states have partially formed bonds . We have seen that the addition of HBr to 2-butene is a two-step process. In each step, the reactants pass through a transition state as they are converted into products. The structure of the transition state for each of the steps is shown here in brackets.

64. Reaction Coordinate Diagram A reaction coordinate diagram can be drawn for each step of a reaction. In the first step of the addition reaction, the alkene is converted into a carbocation that is higher in energy (less stable) than the reactants. The first step, therefore, is endergonic ( G° > 0). In the second step, the carbocation reacts with a nucleophile to form a product that is lower in energy (more stable) than the carbocation reactant. This step, therefore, is exergonic (  G° is < 0).

65. Reaction Coordinate Diagram for the Addition of HBr to 2-Butene The rate-limiting step of the reaction is the step that has its transition state at the highest point of the reaction coordinate diagram.

66. 68 Rate-Determining Step Formation of the carbocation intermediate is the slower of the two steps It is the rate-determining step Carbocation intermediates are consumed by bromide ions as fast as they are formed

67. 69 Transition States and Intermediates It is important to distinguish between a transition state and a reaction intermediate A transition state is a local maximum in the reaction coordinate diagram has partially formed and partially broken bonds

68. 70 Transition States and Intermediates An intermediate is at a local minimum energy in the reaction coordinate diagram may be isolated in some cases

69. A Catalyst A catalyst provides a pathway for a reaction with a lower energy barrier. A catalyst does not change the energy of the starting point (the reactants) or the energy of the end point (the products).

70. Enzymes Most biological reactions require a catalyst. Most biological catalysts are proteins called enzymes . The reactant of a biological reaction is called a substrate .

71. The Active Site of an Enzyme An enzyme binds its substrate at its active site .

72. Enzyme Side Chains that Bind the Substrate Some enzyme side chains bind the substrate . Some enzyme side chains are acids , bases , an d nucleophiles that catalyze the reaction .

73. Enzyme Side Chains that Catalyze the Reaction Some enzyme side chains are acids , bases , and nucleophiles that catalyze the reaction.

74. Thank you

75. Calculating Kinetic Parameters

76. Bases in Nucleic acids

77. Isomers for Compounds with Two Double Bonds