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

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.

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. ene 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.

Chapter 12: Alkenes and Alkynes To name an alkyne: yne 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.

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.

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

Chapter 12: Alkenes and Alkynes Terpene: 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.

Chapter 12: Terpenes

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

Chapter 12: Reactions 1. Combustion (but we already spoke about it in chp.11) 2. Addition to the double bond. 3. Polymerization

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.

+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. regioselective. Reaction is regioselective. One direction of bond forming (or bond breaking) occurs in preference to all other directions. Markovnikov’s rule: Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

reaction mechanism. 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. +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. +HX

hydration. Addition of water is called hydration. Hydration is acid catalyzed, most commonly by H 2 SO 4. Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon. +H 2 0

Step 1 Step 2 Step 3 +H 2 0

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 Br 2 is a useful qualitative test for the presence of a carbon-carbon double bond. Br 2 has a deep red color; dibromoalkanes are colorless. +Cl 2 / +Br 2

Virtually all alkenes add H 2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni. +H 2

? < Wilhelm Normann invented 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

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

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.

Polymerization