ExA1. Unsaturated Hydrocarbons. Olefins. Experiment: Next Week. Structure Addition Reactions Mechanisms

Transcription

1 ExA1 Unsaturated Hydrocarbons Olefins Structure Addition Reactions Mechanisms Experiment: part A - Setup part B - Reaction part C - Isolation part D - Analysis Next Week 1

2 Alkenes & Alkynes Hydrocarbons are molecules composed of only hydrogen and carbon. Carbons form four bonds. Alkanes are hydrocarbons that have only carbon-carbon single bonds. Alkenes are hydrocarbons that have one or more carboncarbon double bonds. Carbons that form double bonds are said to have a sp 2 shape. The bonds connected to a sp 2 carbon have a 120 angle between them. Alkynes are hydrocarbons that have one or more carboncarbon triple bonds. Carbons that form triple bonds are said to have a sp shape. The bonds connected to a sp carbon form a single line, they have a 180 angle around the carbon. Alkenes and alkynes have unique properties. 2

3 Alkenes in Biochemistry Ethene, the simplest alkene is a plant hormone, it causes fruit to ripen. Plastic bags from the supermarket are made of polyethylene (many ethenes stuck together). The longer you leave fruit in those bags, the faster that hormone will cause them to ripen. Alkene structures are important in many biological structures, like the pheromone of silkworm moths. 3

4 Alkynes in Industry Burning alkynes produces less heat than alkanes. But they also produce less gases. The gases produced in burning alkynes have more heat per mole of product Those gases have a higher temperature. Alkynes burn hotter. If you want to put heat into your house, burn an alkane. If you want to create gases to push pistons in your car, burn an alkane. But if you want concentrated heat to cut metal, burn an alkyne. Like ethyne (also called acetylene). 4

5 Saturated & Unsaturated We describe hydrocarbons are either saturated or unsaturated. Saturated hydrocarbons are hydrocarbons with only single carbon-carbon bonds. This means they have the most hydrogens their carbon skeletons can possibly have. Alkanes are saturated hydrocarbons. Unsaturated hydrocarbons have double and triple carbon-carbon bonds. Unsaturated hydrocarbons are not saturated with hydrogen, their carbon skeletons could hold more hydrogen. Alkenes & Alkynes are unsaturated hydrocarbons. Hydrocarbons can be monounsaturated (one unsaturation) or polyunsaturated (more than one unsaturation). Each unsaturation in a compound reduces the number of hydrogen atoms in the molecular formula by 2. CnH(2n) CnH(2n+2) CnH(2n-2) C 2H 2 C 2H 6 5 C 2H 4 Sunflower oil is a mixture of monounsaturated and polyunsaturated hydrocarbon oils. The world's largest sunflower oil producers are Ukraine, Russia and Argentina.

6 Cis-Trans Isomers The concept of stereo isomers can quickly take us beyond the scope of this class. We will confine our discussions of geometric isomers to cis trans relationships within the backbone of the hydrocarbon. There is more than one way to indicate the stereochemistry of a double bond. We will use only the cis-trans naming convention. Cis isomers are isomers where the two hydrogens on a double bond are on the same side. Prefix the full name of the substances with cis. cis-2-pentene or cis-pent-2-ene Trans isomers are isomers where the two hydrogens on a double bond are on opposite sides. Prefix the full name of the substances with trans. trans-2-pentene or trans-pent-2-ene 6

7 ExA1 Unsaturated Hydrocarbons Olefins Structure Addition Reactions Mechanisms Experiment: part A - Setup part B - Reaction part C - Isolation part D - Analysis Next Week 7

8 Types of Reactions Organic chemists describe and group chemical reactions into different classes to organize their palette or collection of tools. Some reaction types you re already familiar with are: Combustion Reactions Double Displacement Reactions Reduction-Oxidation Reactions Neutralization Reactions The pi-bonds in unsaturated hydrocarbons are weak and easy to break. Many chemical reactions take advantage of this to add new atoms to carbon chains at those points. + Br2 Addition Reactions are a type of reaction that describe any reaction which adds new atoms to the carbon chain. We ll introduce you to a handful of very common addition reactions. 8

9 Bromination Bromination is the addition of bromine to a carbon skeleton. These reactions occur easily when bromine is mixed with an alkene. Bromination is an addition reaction. Bromination can be used to test hydrocarbons for unsaturations. Bromine does not react with an alkane because the alkane contains only single C-C bonds which cannot add the bromine. Alkanes merely dilute the red-brown bromine color to an orange or yellow color in the absence of a strong catalyst. Due to their C=C double bonds which can be broken, alkenes react readily with bromine to produce saturated dibromoalkanes. When an alkene is reacted with bromine, the red-brown color of the bromine is immediately lost due to the reaction of the bromine. Br2 + Alkane Br2 + Alkene + Br2 red liquid colorless oil + Br2 N/R + Br2? 9

10 Hydrogenation Hydrogenation is the addition of hydrogen to a carbon skeleton. The reaction between hydrogen and alkenes is less favorable than with bromine. But occurs with a catalyst like Pd, Ni or Pt. Hydrogenation is an addition reaction. + H2 N/R needs a catalyst + H2 Pd, Ni or Pt Ni + H2 N/R already saturated with hydrogen + H2 Ni harder, but possible with Ni 10

11 Hydration Hydration is the addition of the atoms of water to a carbon skeleton. Acid is used as a catalyst to drive this reaction. Hydration is an addition reaction. + H2O N/R needs a catalyst + H2O H + molecules with an OH group of atoms attached to them are alcohols we ll be learning more about alcohols and other functional groups in chapter 12 H + H2O X the OH group adds selectively to the more substituted side of the alkene bond (when there is a difference)

12 Oxidation with KMnO4 The per oxy-ion of manganese in water, permanganate ion, adds two OH groups to alkene carbons. Acid is used as a catalyst to drive this reaction. This is an addition reaction. OH 1- + MnO4 1- H2O + MnO2 2- dark purple clear Permanganate can be used to test hydrocarbons for unsaturations. The permanganate ion is dark purple in solution. If it s consumed by an alkene, it produces a colorless ion. The disappearance of permanganate color indicates the presence of an unsaturated hydrocarbon.

13 Reactions of Alkenes Predict the products or indicate the reactants required for the following reactions. + Br2 + H2 Pd, Ni or Pt + H2O H + KMnO4 13

14 ExA1 Unsaturated Hydrocarbons Olefins Structure Addition Reactions Mechanisms Experiment: part A - Setup part B - Reaction part C - Isolation part D - Analysis Next Week 14

15 15 Hydrogenation

16 Hydrogenation (plus enantiomer) 16

17 17 Hydration

18 18 Diol Addition

19 Halogenation How does the bromine add to the double bond? Do both bromine atoms attach at the same time? Do they attach in separate steps? Or is there more to the story? + Br2 Let s do an experiment to answer these questions. 19

20 ExA1 Unsaturated Hydrocarbons Olefins Structure Addition Reactions Mechanisms Experiment: part A - Setup part B - Reaction part C - Isolation part D - Analysis Next Week 20

21 Bromination? trans-cinnamic acid 1 21

22 Experiment Setup Pre-weigh 5.0 ml R.B. flask Add: 0.20 g trans-cinnamic acid 1.4 ml CH2Cl ml 10% Br2 Stir Bar Attach air condenser Lower into sand bath Heat C (gentle boil) Reflux for 25 minutes Maintain If reflux is not visible reduce heat cool if needed Don t Loose all Solvent If color fades, Add Br2 solution drop wise to return color After 25 minutes, cool to room temperature 22

23 Experiment Isolation Place cool reaction flask in ice bath solid should appear If no product appears after 5 min, add 1 ml of dh2o, cool another 5 min Collect solid by Hirsch funnel vacuum filtration Wash crystals with 1 ml ice cold CH2Cl2, three times Purification Place crystals in 10 ml Erlenmeyer flask Add 0.8 ml hot ethanol Use hot place to make dissolve If needed, add 0.1 ml increments of EtOH until dissolved 23

24 Analysis & Report Determine yield and melting point for the product recovered. In your report: Conclude what product was formed. Conclude which isomer was formed 2S 3S and enantiomer or 2R 3S and enantiomer Propose a mechanism based on the isomer you identified (it s OK to research mechanisms, show intermediate). Predict the product of KMnO4 and trans-cinnamic acid. Explain the differences between these two reactions. 24

25 ExA1 Unsaturated Hydrocarbons Olefins Structure Addition Reactions Mechanisms Experiment: part A - Setup part B - Reaction part C - Isolation part D - Analysis Next Week 25

26 Next Meeting For next Meeting: Bring to class: Notebook You will not be turning in notebooks, but this permanent record of your preparations, observations and notes will be essential to success in this class. Textbook, calculator, pencils (yes, you can use pen) Safety Glasses (you cannot participate without them) Read through and take notes on: Experiment 17 (p296) Hydrogenation and Olefin addition reactions lecture textbook. Produce and bring to class: Your pre-lab for exp 17 (p296) Scheme and measured quantities required. Substances used & their properties. Procedure summary. 26

27 Questions?