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Chapter 3 Alkenes


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Chapter 3 Alkenes

  2. 2. ALKENES <ul><li>Also called olefins </li></ul><ul><li>Contain at least one carbon-carbon double bond (C=C) </li></ul><ul><li>General formula, C n H 2n (n=2,3,…) </li></ul><ul><li>Classified as unsaturated hydrocarbons (compound with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. </li></ul><ul><li>sp 2 -hybridized </li></ul><ul><li>For example: </li></ul><ul><li>C 2 H 4 - ethylene </li></ul>
  3. 3. Naming Alkenes
  4. 4. IUPAC RULES <ul><li>RULE 1. Select the longest continuous carbon chain that contains a double bond. </li></ul>This chain contains 6 carbon atoms
  5. 5. <ul><li>RULE 2. Name this compound as you would an alkane, but change –ane to –ene for an alkene. </li></ul>This is the longest continuous chain. Select it as the parent compound. Name the parent compound octene. This chain contains 8 carbon atoms
  6. 6. RULE 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon to indicate the position of the double bond. Place this number in front of the alkene name.
  7. 7. IUPAC RULES This end of the chain is closest to the double bond. Begin numbering here.
  8. 8. IUPAC RULES The name of the parent compound is 1-octene. 8 7 4 3 2 1 6 5
  9. 9. RULE 4. Branched chains and other groups are treated as in naming alkanes. Name the substituent group, and designate its position on the parent chain with a number.
  10. 10. IUPAC RULES This is an ethyl group. 8 7 4 3 2 1 6 5 The ethyl group is attached to carbon 4. 4 4-ethyl-1-octene
  11. 11. <ul><li>A compound with more than one double bond. </li></ul><ul><li>Two double bond: diene </li></ul><ul><li>Three double bond: triene </li></ul><ul><li>Four double bond: tetraene </li></ul><ul><li>* Numbers are used to specify the locations of the double bonds. </li></ul>
  12. 12. ALKENES AS SUBSTITUENTS <ul><li>Alkenes names as substituents are called alkenyl groups . </li></ul><ul><li>Can be named systematically as ethenyl, propenyl, etc. or by common names such as vinyl, ally, methylene and phenyl groups. </li></ul>
  13. 13. CYCLOALKENES <ul><li>Contains C=C in the ring </li></ul><ul><li>Nomenclature of cycloalkenes: </li></ul><ul><li>Similar to that alkenes </li></ul><ul><li>Carbons atoms in the double bond are designated C1 and C2 </li></ul>
  14. 14. NOMENCLATURE OF cis-trans ISOMERS <ul><li>cis – two particular atoms (or groups of atoms) are adjacent to each other </li></ul><ul><li>trans – the two atoms (or groups of atoms) are across from each other </li></ul>
  15. 15. PHYSICAL PROPERTIES OF ALKENES <ul><li>Boiling points and densities: </li></ul><ul><li>- Most physical properties of alkenes are similar to those alkanes. </li></ul><ul><li>- Example: the boiling points of 1-butene, cis -2-butene, trans -2-butene and n -butane are close to 0 o C. </li></ul><ul><li>- Densities of alkenes: around 0.6 or 0.7 g/cm 3 . </li></ul><ul><li>- Boiling points of alkenes increase smoothly with molecular weight. </li></ul><ul><li>- Increased branching leads to greater volatility and lower boiling points. </li></ul>
  16. 16. <ul><li>Polarity: </li></ul><ul><li>- relatively nonpolar. </li></ul><ul><li>- insoluble in water but soluble in non-polar solvents such as hexane, gasoline, halogenated solvents and ethers. </li></ul><ul><li>- slightly more polar than alkanes because: </li></ul><ul><li>i) electrons in the pi bond is more polarizable (contributing to instantaneous dipole moments). </li></ul><ul><li>ii) the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment). </li></ul>
  17. 17. <ul><li>propene, μ = 0.35 D </li></ul><ul><li>Alkyl groups are electron donating toward double bond, helping to stabilize it. This donating slightly polarizes the vinylic bond, with small partial positive charge on the alkyl group and a small negative charge on the double bond carbon atom. </li></ul><ul><li>For example, propene has a small dipole moment of 0.35 D. </li></ul>Vector sum = propene, μ = 0.33 D cis -2-butene, bp 4 o C Vector sum = 0 propene, μ = 0 trans -2-butene, bp 1 o C Vinylic bonds
  18. 18. <ul><li>In a cis -disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. </li></ul><ul><li>In a trans -disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans -disubstituted, the dipole moment is zero. </li></ul>Vector sum = propene, μ = 0.33 D cis -2-butene, bp 4 o C Vector sum = 0 propene, μ = 0 trans -2-butene, bp 1 o C
  19. 19. <ul><li>Cis - and trans- 2-butene have similar van der Waals attractions, but only cis isomer has dipole-dipole attractions. </li></ul><ul><li>Because of its increased intermolecular attractions, cis- 2-butene must be heated to a slightly higher temperature (4 o C versus 1 o C) before it begins to boil. </li></ul>Vector sum = propene, μ = 0.33 D cis -2-butene, bp 4 o C Vector sum = 0 propene, μ = 0 trans -2-butene, bp 1 o C
  20. 20. PREPARATION OF ALKENES <ul><li>Alkenes can be prepared in the following ways: </li></ul><ul><li>i) Dehydration of alcohols </li></ul>conc. H 2 SO 4 R-CH 2 -CH 2 -OH R-CH=CH 2 + H 2 O ii) Dehydrohalogenation of haloalkanes NaOH/ethanol R-CH 2 -CH 2 -X reflux R-CH=CH 2 + HX NaOH can be replaced by KOH
  21. 21. <ul><li>Saytzeff rule: </li></ul><ul><li>- A reaction that produces an alkene would favour the formation of an alkene that has the greatest number of substituents attached to the C=C group. </li></ul>Dehydration of alcohols Dehydrohalogenation of haloalkanes
  22. 22. REACTIVITY OF ALKENES <ul><li>More reactive than alkanes because: </li></ul><ul><li>A carbon-carbon double bond consists of a σ and a π bond. It is easy to break the π bond while the σ bond remains intact. </li></ul><ul><li>The π electrons in the double bond act as a source of electrons (Lewis base). Alkenes are reactive towards electrophiles which are attracted to the negative charge of the π electrons. </li></ul><ul><li>π bond will broken, each carbon atom becomes an active site which can form a new covalent bond with another atom. One π bond is converted into 2 σ bonds. </li></ul>
  23. 23. REACTIONS OF ALKENES <ul><li>Catalytic hydrogenation: </li></ul><ul><li>- hydrogenation: addition of hydrogen to a double bond and triple bond to yield saturated product. </li></ul><ul><li>- alkenes will combine with hydrogen in the present to catalyst to form alkanes. </li></ul><ul><li>Plantinum (Pt) and palladium (Pd) – Catalysts </li></ul><ul><li>Pt and Pd: temperature 25-90 o C </li></ul><ul><li>Nickel can also used as a catalyst, but a higher temperature of 140 o C – 200 o C is needed. </li></ul>
  24. 25. <ul><li>Addition of halogens: </li></ul><ul><li>i) In inert solvent: </li></ul><ul><li>- alkenes react with halogens at room temperature and in dark. </li></ul><ul><li>- the halogens is usually dissolved in an inert solvent such as dichloromethane (CH 2 Cl 2 ) and tetrachloromethane (CCl 4 ). </li></ul><ul><li>- Iodine will not react with alkenes because it is less reactive than chlorine and bromine. </li></ul><ul><li>- Fluorine is very reactive. The reaction will produced explosion. </li></ul>
  25. 26. EXAMPLES:
  26. 27. <ul><li>Addition of halogens: </li></ul><ul><li>ii) In water / aqueous medium: </li></ul><ul><li>- chlorine dissolves in water to form HCl and chloric (l) acid </li></ul><ul><li>(HOCl). </li></ul><ul><li>Cl 2 (aq) + H 2 O(l) HCl(aq) + HOCl (aq) </li></ul><ul><li>- same as bromine </li></ul><ul><li>Br 2 (aq) + H 2 O(l) HBr(aq) + HOBr(aq) </li></ul><ul><li>* Reaction of alkenes with halogens in water (eg. chlorine water and bromine water) produced halohydrins (an alcohol with a halogen on the adjacent carbon atom). </li></ul>
  27. 28. <ul><li>EXAMPLES: </li></ul>
  28. 29. <ul><li>Addition of hydrogen halides: </li></ul><ul><li>- Addition reaction with electrophilic reagents. </li></ul><ul><li>- Alkenes react with hydrogen halides (in gaseous state or in aqueous solution) to form addition products. </li></ul><ul><li>- The hydrogen and halogen atoms add across the double bond to form haloalkanes (alkyl halides). </li></ul><ul><li>- General equation: </li></ul><ul><li>Reactivity of hydrogen halides : HF < HCl < HBr < HI </li></ul>
  29. 30. * Reaction with HCl needs a catalyst such as AlCl 3
  30. 31. MARKOVNIKOV’S RULE <ul><li>There are 2 possible products when hydrogen halides react with an unsymmetrical alkene . </li></ul><ul><li>It is because hydrogen halide molecule can add to the C=C bond in two different ways. </li></ul>
  31. 32. <ul><li>Markovnikov’s rules: </li></ul><ul><li>- the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms. </li></ul>
  32. 33. Step 2: Rapid reaction with a negative ion. The negative ion (Y - ) acts as nucleophile and attacks the positively charged carbon atom to give product of the addition reaction. Mechanism of electrophilic addition reactions: - C=C : electron rich part of the alkene molecule - Electrophiles: electron-seeking Step 1: Formation of carbocation. Attack of the pi bond on the electrophile to form carbocation. δ + δ -
  34. 36. <ul><li>Addition reaction with concentrated sulfuric acid: hydration of alkenes </li></ul><ul><li>- the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15 o C). </li></ul><ul><li>- involves the addition of H atom and HSO 4 group across the carbon-carbon double bond. </li></ul><ul><li>- follows Markovnikov’s rule. </li></ul>
  35. 38. <ul><li>Addition reaction with acidified water (H 3 O + ): hydration of alkenes </li></ul><ul><li>Hydration : The addition of H atoms and –OH groups from water molecules to a multiple bond. </li></ul><ul><li>Reverse of the dehydration reaction. </li></ul><ul><li>Direct hydration of ethene: </li></ul><ul><li>- passing a mixture of ethene and steam over phosphoric (v) acid (H 3 PO 4 ) absorbed on silica pellets at 300 o C and a pressure of 60 atmospheres. </li></ul><ul><li>- H 3 PO 4 is a catalyst. </li></ul>H +
  36. 39. <ul><li>Markovnikov’s rule is apply to the addition of a water molecule across the double bond of an unsymmetrical alkene. </li></ul><ul><li>For examples: </li></ul>H + = catalyst H +
  37. 40. H + = catalyst
  38. 41. <ul><li>When HBr is added to an alkene in the absence of peroxides it obey Markovnikov’s rule. </li></ul><ul><li>When HBr (not HCl or HI) reacts with unsymmetrical alkene in the presence of peroxides (compounds containing the O-O group) or oxygen, HBr adds in the opposite direction to that predicted by Markovnikov’s rule . </li></ul><ul><li>The product between propene and HBr under these conditions is 1-bromopropane and not 2-bromopropane. </li></ul>ANTI-MARKOVNIKOV’S RULE: FREE RADICAL ADDITION OF HYDROGEN BROMIDE
  39. 42. <ul><li>Anti-Markovnikov’s addition: </li></ul><ul><li>- peroxide-catalysed addition of HBr occurs through a free radical addition rather than a polar electrophilic addition. </li></ul><ul><li>- also observed for the reaction between HBr and many different alkenes. </li></ul><ul><li>- not observed with HF, HCl or HI. </li></ul>
  40. 43. <ul><li>Combustion of alkenes: </li></ul><ul><li>The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. </li></ul><ul><li>For example, ethene burns as follows : </li></ul><ul><li>C 2 H 4 + 3O 2 -> 2CO 2 + 2H 2 O </li></ul>
  41. 44. <ul><li>Oxidation : reactions that form carbon-oxygen bonds. </li></ul><ul><li>Oxidation reaction of alkenes: </li></ul><ul><li>i) epoxidation </li></ul><ul><li>ii)hydroxylation </li></ul><ul><li>iii)Ozonolysis </li></ul>OXIDATION
  42. 45. EPOXIDATION OF ALKENES <ul><li>Epoxide / oxirane: a three-membered cyclic ether. </li></ul><ul><li>Examples of epoxidizing reagent: </li></ul>
  43. 46. Examples:
  44. 47. <ul><li>Hydroxylation: </li></ul><ul><li>- Converting an alkene to a glycol requires adding a hydroxyl group to each end of the double bond. </li></ul><ul><li>Hydroxylation reagents: </li></ul><ul><li>i) Osmium tetroxide (OsO 4 ) </li></ul><ul><li>ii)Potassium permanganate (KMnO 4 ) </li></ul>HYDROXYLATION OF ALKENES glycol
  45. 48. * Also known as Baeyer’s test
  46. 49. <ul><li>Ozonolysis: </li></ul><ul><li>- The reaction of alkenes with ozone (O 3 ) to form an ozonide, followed by hydrolysis of the ozonide to produce aldehydes and /or ketone. </li></ul><ul><li>- Widely used to determine the position of the carbon-carbon double bond. </li></ul><ul><li>- Ozonolysis is milder and both ketone and aldehydes can be recovered without further oxidation. </li></ul>OZONOLYSIS OF ALKENES
  47. 50. EXAMPLES:
  49. 52. <ul><li>Polymer : A large molecule composed of many smaller repeating units (the monomers) bonded together. </li></ul><ul><li>Alkenes serves as monomers for some of the most common polymers such as polyethylene (polyethene), polypropylene, polystyrene, poly(vinyl chloride) and etc. </li></ul><ul><li>Undergo addition polymerization /chain-growth polymer: </li></ul><ul><li>- a polymer that results from the rapid addition of one molecule at a time to a growing polymer chain, usually with a reactive intermediate (cation, radical or anion) at the growing end of the chain. </li></ul>POLYMERIZATION OF ALKENES
  50. 53. repeating unit
  51. 54. <ul><li>Reactions of alkenes with KMnO 4 </li></ul><ul><li>- KMnO 4 is a strong oxidising agent. </li></ul><ul><li>- alkenes undergo oxidation reactions with KMnO 4 solution under two conditions: </li></ul><ul><li>a) Mild oxidation conditions using cold, dilute, alkaline KMnO 4 (Baeyer’s test). </li></ul><ul><li>b) Vigorous oxidation conditions using hot, acidified KMnO 4. </li></ul>UNSATURATION TESTS FOR ALKENES
  52. 55. <ul><li>Reaction of alkenes with cold, dilute, alkaline KMnO 4 (Baeyer’s test) </li></ul><ul><li>- the purple colour of KMnO 4 solution disappears and a cloudy brown colour appears caused by the precipitation of manganese (IV) oxide, MnO 2 . </li></ul><ul><li>- test for carbon-carbon double or triple bonds. </li></ul><ul><li>- a diol is formed (containing two hydroxyl groups on adjacent carbon atoms). </li></ul>
  53. 57. <ul><li>b) Bromine </li></ul><ul><li>- A solution of bromine in inert solvent (CH 2 CI 2 or CCI 4 ) and dilute bromine water are yellow in colour. </li></ul><ul><li>- The solution is decolorised when added to alkenes or organic compounds containing C=C bonds. </li></ul>
  54. 59. <ul><li>a) Ozonolysis of alkenes: </li></ul><ul><li>- For example, ozonolysis of an alkene produces methanal and propanone. </li></ul>DETERMINATION OF THE POSITION OF THE DOUBLE BOND
  55. 60. <ul><li>b) Reaction of alkenes with hot, acidified KMnO 4 </li></ul><ul><li>- by using hot, acidified KMnO 4 , the diol obtained is oxidised further. </li></ul><ul><li>- cleavage of carbon-carbon bonds occurs and the final products are ketones, carboxylic acids or CO 2 . </li></ul>
  56. 61. <ul><li>Example: </li></ul><ul><li>An alkene with the molecular formula C 6 H 12 is oxidised with hot KMnO 4 solution. The carboxylic acids, butanoic acid (CH 3 CH 2 CH 2 COOH) and ethanoic acid (CH 3 COOH), are produced. Identify the structural formula of the alkene. </li></ul>
  57. 62. <ul><li>Ethylene and propylene are the largest-volume industrial organic chemicals. </li></ul><ul><li>Used to synthesis a wide variety of useful compounds. </li></ul>USES OF ALKENES
  58. 63. <ul><li>The most popular plastic. </li></ul><ul><li>Uses: </li></ul><ul><li>i) Grocery bags </li></ul><ul><li>ii)Shampoo bottles </li></ul><ul><li>iii)Children's toy </li></ul><ul><li>iv)Bullet proof vests </li></ul><ul><li>v)Film wrapping </li></ul><ul><li>vi)Kitchenware </li></ul>POLYETHENE (PE)
  59. 64. POLYVINYL CHLORIDE (PVC) <ul><li>USES OF PVC: </li></ul><ul><li>Clothing </li></ul><ul><li>- PVC fabric has a sheen to it and is waterproof. </li></ul><ul><li>- coats, shoes, jackets, aprons and bags. </li></ul><ul><li>As the insulation on electric wires. </li></ul><ul><li>Producing pipes for various municipal and industrial applications. For examples, for drinking water distribution and wastewater mains. </li></ul><ul><li>As a composite for the production of accessories or housings for portable electronics. </li></ul><ul><li>uPVC or Rigid PVC is used in the building industry as a low-maintenance material. </li></ul><ul><li>Ceiling tiles. </li></ul>
  60. 65. USES OF ETHANOL <ul><li>Motor fuel and fuel additive. </li></ul><ul><li>As a fuel to power Direct-ethanol fuel cells (DEFC) in order to produce electricity. </li></ul><ul><li>As fuel in bipropellant rocket vehicles. </li></ul><ul><li>In alcoholic beverages. </li></ul><ul><li>An important industrial ingredient and use as a base chemical for other organic compounds include ethyl halides, ethyl esters, diethyl ether, acetic acid, ethyl amines and to a lesser extent butadiene. </li></ul><ul><li>Antiseptic use. </li></ul><ul><li>An antidote. </li></ul><ul><li>Ethanol is easily miscible in water and is a good solvent. Ethanol is less polar than water and is used in perfumes, paints and tinctures. </li></ul><ul><li>Ethanol is also used in design and sketch art markers. </li></ul><ul><li>Ethanol is also found in certain kinds of deodorants. </li></ul>