Chapter 19: Alkenes and Alkynes

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1 Chapter 19: Alkenes and Alkynes The vast majority of chemical compounds that we know anything about and that we synthesize in the lab or the industrial plant are organic compounds. The simplest organic compounds are alkanes pure carbon and hydrogen, C n H 2n+2, - but these compounds are not particularly reactive. Their octets are satisfied leaving them without a handle for reactivity. From the point of view of building complex organic compounds, the alkenes (and alkynes) are much more useful. Chapter 19 1

2 For example, the pigment/anti-oxidant β-carotene is generated from the isoprene unit: aka 2-methyl-1,3-butadiene We can see that 8 isoprene units can be linked: giving: Chapter

3 Chapter

4 Understanding the reactivity of a substance, such as ethylene, is a question of putting together electronic and physical characteristics of the molecule. For ethylene, the double bond is stronger (611 kj/mol) and shorter (113 pm) than the corresponding single bond in ethane (376 kj/mol; 154 pm). It is also rigid : This rigidity is observed in the activation energy for rotation. In single bond, E a 12 kj/mol. For a double bond, E a 268 kj/mol. Chapter

5 The reason for the rigidity is that a double bond is made from two orbitals from each atom overlapping. That is, the carbon atoms are sp 2 hybridized which form the sigma bonding network in the compound. The remaining p- orbital then forms a pi orbital that provides an additional linkage between the carbon atoms: Chapter

6 Things are a little more complicated and to fully understanding bonding in organic compounds, we need to adopt a molecular orbital theory approach. At its heart, M.O. Theory takes the orbitals from all of the atoms in a compound and combines them to form orbitals of the whole molecule. The one important distinction between a valence bond approach (or a hybridization approach ) and M.O. Theory is that the latter combines two atomic orbitals to give two molecular orbitals one of which is bonding and one of which is anti-bonding. Chapter

7 For double bonds, the extra p-orbital results in a π- bonding and π-anti-bonding orbital. The highest occupied molecular orbital is called the HOMO and the lowest unoccupied molecular orbital is the LUMO. Chapter

8 Recall that the nomenclature for alkenes and alkynes must account for the presence of the double or triple bond. We do this by naming the parent hydrocarbon chain - the longest chain that incorporates the double or triple bond and then add the suffix -ene or -yne (we replace the -ane ). We then number the chain so as to give the double or triple bond the lowest possible number all things being equal. If there are more than one double or triple bond, then we use di-, tri-, tetra-, etcetera. Chapter

9 Chapter

10 The internal barrier to rotation for double bonds adds an additional constraint to naming the compound. We need to distinguish between the two (or more) possible isomers. Chapter

11 Recall from the last chapter that ΔG = -4 kj/mol for this reaction. That gives a value of K eq = 5 consistent with the distribution (the difference is due to rounding). But why? Chapter

12 The IUPAC-approved convention is the E,Z-system of nomenclature. E stands for entgegen which is German for opposite. The highest priority groups are on opposite sides of the bond. Z stands for zusammen which is German for together. The highest priority groups are on the same side of the bond. For example: Chapter

13 Recall also that organic functional groups such as double and triple bonds can be identified by their characteristic spectroscopic features. For Infrared Spectroscopy (IR Spectroscopy), we have the general pattern: There are subtleties to this. Chapter

14 Stretches just below 3000 cm -1 correspond to the symmetric and anti-symmetric C-H stretches of an alkane; stretches just above 3000 cm -1 result from the symmetric and anti-symmetric C-H stretches associated with a C=C. A C=C stretching vibration is often found in the cm -1 region. =C-H cm -1 C=C cm -1 RCH=CH and 990 cm -1 R 2 C=CH cm -1 Chapter

15 13 C NMR Spectrocopy also has characteristic regions. Typical chemical shifts for alkenes are in the 110 to 150 ppm range. Even more information can be gleaned from the 1 HNMR spectrum but it is also more complicated to interpret. However, it does have broad ranges: Chapter

16 Chapter 19 16

17 In dealing with inorganic reactions the reactions of salts and such there are a serious of descriptions for different types of reactions such as double displacement or acid-base. Organic reactions also have classifications based on the mechanism and/or the type of products produced. In all cases, organic reactions can generally be described by the rearrangement of electron pairs to give products which is the basis for our mechanisms. Chapter

18 Addition Reactions: These occur when two or more molecules come together, resulting in a single new product with no leftover atoms. Chapter

19 Elimination Reactions: In a sense, an elimination reaction is simply the reverse of an addition. That is, a single small molecule is released and the resulting residual organic fragment regenerates its double bond. Chapter

20 Substitution Reactions (S N 1 and S N 2): These require the presence of a leaving group which is a group that can be displaced to give an ionic or neutral species. They require that the incoming group has a similar charge to the leaving group and is also a good nucleophile a species that interacts with a positively charged carbon. Chapter

21 Rearrangement Reactions: These occur when a molecule undergoes an internal reorganization of its atoms with no net gain or loss. This might involve the interconversion of isomers (i.e. cis to trans) but could also involve the shifting of groups within a molecular structure or the simple cycling of electrons. Chapter

22 Visualizing the mechanism of an organic reaction is not actually possible in an experimental sense. At best, we can measure some of the intermediate species along the way. But putting it together leads to an understanding of the patterns and processes that drive chemistry. Chapter

23 Many organic reactions involve polar groups nucleophiles and electrophiles and essentially are acidbase reactions where electrons move in pairs to form and break bonds. When electrons move in pairs, bonds break heterolytically and form in a heterogenic manner. Chapter

24 In radical reactions, the bond breaking and making proceeds via a one electron process similar to electrochemistry where each partner ends up with one electron from the bonding pair. Bond breaking is homolytic (the bond undergoes homolysis ) and bond making is homogenic. The reaction products may be free radicals depending upon the rate constant for their quenching. Chapter

25 The fundamental characteristic of all polar reactions is that electron-rich sites (nucleophiles) react with electron-poor sites (electrophiles) in another molecule (or even within the same molecule) because the negatively charged region of one molecule is attracted to the positively charged region of another. Each of these could be considered a Lewis Acid. Chapter

26 Chapter

27 A typical reaction and one of the simplest is the addition of HX to an alkene. For example: HCl + H 2 C=CH 2 H 3 C-CH 2 Cl Chapter

28 The reaction proceeds through the π-bonding electrons because they are more accessible: The fact that the π-bond is both electron-rich and electron accessible is consistent with the mechanism. Also, the +ve charged carbon (with only 6e - ) is called a carbocation and is unstable, so very reactive. Chapter

29 From an energy point of view: Chapter

30 With ethylene, it doesn t make any difference which way around the HX adds as the product is the same. However, with an unsymmetrical alkene, we can get different products: Chapter

31 These reactions are regioselective and consistent with Markovnikov s rule which says those that has, gets. Actually, it says that the more stable carbocation will form on the carbon with the greatest number of alkyl groups attached. When it s a tie, both compounds form: Chapter

32 The stability of carbocations: Predict the products: CH 3 CH 2 CH=CH 2 + HCl CH 3 CH=C(CH 3 ) 2 + HI Chapter

33 Addition of H 2 O to Double Bonds: Hydration Water consists of H + + OH - which means that it should add to a double bond in the same way that HX does. The mechanism is similar to the reaction with HX but there is an added step as it is OH 2 that acts as the nucleophile. It also follows Markovnikov s Rule. Chapter

34 Chapter

35 But the subtle difference between HX and H 2 O addition is heat. The hydration requires harsh conditions for example, H 3 PO 4 and 250 C. As a consequence, chemists have continually searched for better ways to do a hydration. Oxymercuration-demercuration (Markovnikov addition) Hydroboration (anti-markovnikov addition) Chapter

36 Further, hydration is matched with dehydration the formation of an double bond by the removal of water. There are various catalysts that will do this the most common is sulphuric acid. Many of the reactions that alkenes do in one direction can be reversed such as halogenation and hydrogenation. Chapter

37 Addition of X 2 to Double Bonds: Halogenation Again, although a little less obvious, Cl 2 and Br 2 can be thought of as Cl + + Cl - and Br + + Br -. These are not real ions but the halogens behave as if they are. Chapter

38 Mechanistically: Chapter

39 Addition of H 2 to Double Bonds: Hydrogenation Hydrogenation involves the reduction of a double bond when exposed to an atmosphere of hydrogen in the presence of a catalyst. Book says heterogeneous process in that the catalyst is a solid and the reaction happens at the surface. Not always. Reduction in organic chemistry refers to an increase in electron density on carbon atoms by forming a C-H bond or breaking a C-O, C-X, or C-N bond. Chapter

40 Oxidation of Double Bonds: Hydroxylation and Cleavage Hydroxylation is the addition of OH to each end of a C=C bond. There are a number of reagents that can accomplish this but KMnO 4 (in basic conditions) is representative: Oxidation in organic chemistry refers to decreasing the electron density on C atoms by breaking C-H bonds or by forming a C-O, C-X, or C-N bond. Chapter

41 Under acidic conditions, KMnO 4 leads to cleavage of the double bond. - If there are no hydrogens on either end, then the products are ketones. - If there is a hydrogen, then one of the products is a carboxylic acid. - If the double bond is terminal (=CH 2 ), then CO 2 is produced. Chapter

42 Free Radical Reactions: Polymerization These are a unique class of organic reactions. Industrially, these are hugely important. Three steps: 1) initiation: a radical species is generated followed by Chapter

43 2) Propogation: free radical addition of monomers 3) Termination: coupling of two free radicals to give a C-C bond 2R-CH 2 CH 2. R-CH 2 CH 2 -CH 2 CH 2 -R Many different types of monomers: Chapter

44 Chapter

45 Alkynes are hydrocarbons with a triple bond but we can think of them as a two double bonds. Shorter and stronger, the electrons are more exposed: Recall that the spectroscopy of these compounds must take into account the triple bond: Chapter

46 Generally, the reactivity is that of two double bonds: Addition of HX: Addition of X 2 : Addition of H 2 O: Chapter

47 Addition of H 2 : Alkynes can be converted to alkenes by the addition of 1 equivalent of hydrogen or all the way to alkanes by the addition of 2 equivalents. Chapter

48 Summarizing: Chapter 19 48

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