Chapter 12: Alkenes and Alkynes

1. Chapter 12: Alkenes and Alkynes Alkene: A hydrocarbon that contains one or more carbon-carbon double bonds. • Ethylene is the simplest alkene. Alkyne: A hydrocarbon that contains one or more carbon-carbon triple bonds. • Acetylene is the simplest alkyne.

2. Chapter 12: Alkenes and Alkynes Cis-trans isomerism: Double Bond = No Free rotation!!! • Because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.

3. Chapter 12: Alkenes and Alkynes To name an alkene: • The parent name is that of the longest chain that contains the C=C. • Number the chain from the end that gives the lower numbers to the carbons of the C=C. • Locate the C=C by the number of its first carbon. • Use the ending -ene to show the presence of the C=C • Branched-chain alkenes are named in a manner similar to alkanes in which substituted groups are located and named.

4. Chapter 12: Alkenes and Alkynes To name an alkyne: Follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond. Common names are still used for some alkenes and alkynes, particularly those with low molecular weight.

5. Chapter 12: Alkenes and Alkynes To name a cycloalkene: • Number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first. • Note that it is not necessary to explicitly number the position of the double bond in a cycloalkene as in linear alkenes. • Number and list substituents in alphabetical order.

6. Chapter 12: Alkenes and Alkynes • Alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so forth. • Those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many). Alkenes and alkynes physical properties are very similar to the alkanes properties

7. Chapter 12: Alkenes and Alkynes Terpene: A compound whose carbon skeleton can be divided into five-carbon units identical with the carbon skeleton of isoprene. Terpenes illustrate an important principle of the molecular logic of living systems. • In building large molecules, small subunits are bonded together by a series of enzyme-catalyzed reactions and then chemically modified by additional enzyme-catalyzed reactions.

8. Chapter 12: Terpenes

9. Chapter 12: Alkenes and Alkynes The most common reaction is addition to the double bond.

10. Chapter 12: Reactions Combustion (but we already spoke about it in chp.11) Addition to the double bond. Polymerization

11. Chapter 12: Alkenes and Alkynes Most alkene addition reactions are exothermic. • the products are more stable (lower in energy) than the reactants. • Just because they are exothermic doesn’t mean that alkene addition reactions occur rapidly. • Reaction rate depends on the activation energy. • Many alkene addition reactions require a catalyst.

12. +HX Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane. • H adds to one carbon of the C=C and X to the other. • Reaction is regioselective. One direction of bond forming (or bond breaking) occurs in preference to all other directions. • Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

13. +HX Chemists account for the addition of HX to an alkene by a two-step reaction mechanism. • We use curved arrows to show the repositioning of electron pairs during a chemical reaction. • The tail of an arrow shows the origin of the electron pair (either on an atom or in the double bond). • The head of the arrow shows its new position. • Curved arrows show us which bonds break and which new ones form.

14. +HX Step 1 • Reaction of the carbon-carbon double bond with H+ gives a secondary (2°) carbocation intermediate, a species containing a carbon atom with only three bonds to it and bearing a positive charge. Step 2 • Reaction of the carbocation intermediate with chloride ion completes the addition.

15. +H20 Addition of water is called hydration. • Hydration is acid catalyzed, most commonly by H2SO4. • Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.

16. +H20 Step 1 Step 2 Step 3

17. +Cl2 / +Br2 Addition takes place readily at room temp. • Reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent. • Addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond. • Br2 has a deep red color; dibromoalkanes are colorless.

18. +H2 Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni.

19. +H2 ? < Wilhelm Normanninvented what he called fat hardening. Yoday known as “Hydrogenation” “The process of hydrogenation adds hydrogen atoms to cis-unsaturated fats, eliminating double bonds and making them into partially or completely saturated fats. However, partial hydrogenation converts a part of cis-isomers into trans-unsaturated fats instead of hydrogenating them completely “. Coronary heart disease http://en.wikipedia.org/wiki/Trans_fat

20. Polymerization From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization: • polymer: Greek: poly, many, and meros, part; any long-chain molecule synthesized by bonding together many single parts, called monomers. • monomer: Greek: mono, single and meros, part. polypropene (polypropylene)

21. Polymerization Low-density polyethylene (LDPE): • A highly branched polymer; polymer chains do not pack well and London dispersion forces between them are weak. • Softens and melts above 115°C. • Approximately 65% of all LDPE is used for the production of films for packaging and for trash bags. High-density polyethylene (HDPE): • Only minimal chain branching; chains pack well and London dispersion forces between them are strong. • Has higher melting point than LDPE and is stronger • Can be blow molded to squeezable jugs and bottles.

22. Polymerization