12-1 Principles and Applications of Inorganic, Organic, and Biological Chemistry Denniston, Topping, and Caret 4 th ed Chapter 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Power Point to Accompany

12.1 Alkenes and Alkynes Alkenes have one or more carbon-carbon double bonds. Alkynes have one or more carbon-carbon triple bonds. Simplest alkene: ethene (ethylene) C 2 H 4 Simplest alkyne: ethyne (acetylene) C 2 H 2

12-3 Alkenes and Alkynes Physical properties of the alkenes and alkynes mirror those of alkanes. They are nonpolar and consequently are not soluble in water but highly soluble in nonpolar solvents. Boiling points rise with molecular weight.

12.2 IUPAC Names Base name from longest chain containing the multiple bond. Change -ane to -ene or -yne. Number from the end that will give the first carbon of the multiple bond the lower number. Prefix the name with the number of the first multiple bond carbon. Prefix branch/substituent names as for alkanes.

IUPAC Names-2 Name 3-ethyl-6-methyl-3-heptene Name 2-bromo-3-hexyne

Names-3 Cyclic alkenes are named like cyclic alkanes. Prefix name with cyclo. Numbering must start at one end of the double bond and go through the bond. Substituents must have the lower possible numbers. 5-chloro-3-methylcyclohexene Name:

12.3 Geometric Isomers, The pi Bond sigma bond between carbons Two p orbitals overlap side-by-side pi bond has two lobes CCC C

Geometric Isomers-cont. 2-butene is the first example of an alkene which can have two different structures - based on restricted rotation about the double bond. trans-2-butene cis-2-butene

Identifying cis/trans Isomers If one end of the C=C has two groups the same, cis/trans isomers (geometric isomers) are not possible. Both carbons of the C=C must have two different groups attached. Find a group common to both ends of the C=C. If the common group is on the same side of the pi bond, the molecule is cis; if on the opposite side, the molecule is trans.

Identifying cis/trans Isomers-2 The common group (at each end) is the methyl group. Both CH 3 s are on the same side of the pi bond. cis-3-methyl-2-pentene Neither ene carbon has two groups the same.

Identifying cis/trans Isomers-3 The common group is the chlorine atom. The chlorines are on opposite sides of the pi bond. trans-1,2-dichloro-1-butene

Quiz: cis/trans Isomers Decide whether each compound is cis, trans, or neither. Click to see the answers. A: methyls are trans B: No c/t. Right C has two isopropyls. C: hydrogens are cis

Alkenes in Nature Alkenes are abundant in nature. Ethene is a fruit ripener and promotes plant growth. Polyenes built from the isoprene skeleton are called isoprenoids. The next slide shows some isoprenoids.

12-14 Isoprenoids Geraniol (rose and geraniums) Limonene (oil of lemon and orange) Farnesol (Lily of the Valley)

Alkene Reactions There are two kinds of reactions typical of alkenes: Addition: two molecules combine to give one new molecule. Redox: oxidation and reduction The two classes are not always mutually exclusive.

12-16 Addition: General A small molecule, AQ, reacts with the pi electrons of the double bond. The pi bond breaks and its electrons are used to bond to the A and Q pieces. Some additions require a catalyst.

12-17 Reagents Adding to Alkenes 1. Symmetrical reagents: H 2 (Pt, Pd, or Ni as catalyst) Br 2, Cl 2 2. Unsymmetrical reagents (acids) HCl, HBr H 2 O (requires strong acid catalyst eg. H 3 O +, H 2 SO 4, H 3 PO 4 ) 3. Self addition or polymerization.

12-18 Symmetrical Reagents: Example Note: the two hydrogens attach to each end of the double bond. There is no double bond in the product! The reaction is called hydrogenation.

12-19 Unsymmetrical Reagents: Example Two products are possible depending how the reagent (as H and OH) adds to the ends of the pi bond.

12-20 Markovnikov’s Rule Dimitri Markovnikov (Russian) observed many acid additions to C=C systems. He noticed that in all cases, the majority of the hydrogen went to a specific end of the double bond. He formulated his rule:

12-21 Markovnikov’s Rule-2 When an acid (H-OH, H-Cl, H-Br) adds to a double bond, the H of the acid usually goes to the end of the double bond with more hydrogens attached initially.

12-22 Major product: HCl + propene major prdt. H goes to carbon with more hydrogens minor prdt.

12-23 Addition Polymers Alkene molecules add “head to tail” using heat, pressure, and a catalyst.

12-24 Addition Polymers: Examples Monomer Polymer Name Polystyrene Polymethyl- methacrylate Lucite Polytetra- fluoroethylene Teflon Polyvinyl Chloride (PVC)

12-25 Alkenes: Reduction Alkenes add two hydrogens to give alkanes. We have already seen this reaction under symmetrical additions. Eg:

Aromatic Hydrocarbons Benzene’s structure was first proposed by Kekule in the 1850s. He proposed a cyclic structure for benzene, C 6 H 6. Kekule realized that there was something special about benzene because, although his structures showed double bonds, the molecule did not react as if it had any unsaturation.

12-27 History-2 The two equivalent structures proposed by Kekule are recognized today as resonance structures. The real benzene molecule is a hybrid with each resonance structure contributing to the true structure.

12-28 Bonding in Benzene-Modern The carbons in benzene are sp 2 hybridized. Two sp 2 orbitals are used to bond to other carbons and one to bond to hydrogen. The ring and all the hydrogens are coplanar. This describes the sigma bonding in the ring. (Picture on next slide.)

12-29 Sigma network on benzene

12-30 Pi bonding on benzene The six p orbitals unused for the sp 2 hybrids are perpendicular to the plane of the benzene ring. They overlap with one another to form the pi cloud, a ring of electrons above and below the ring. The pi cloud electrons are free to move around the ring. They are said to be delocalized. The next slide shows pi cloud formation.

12-31 Pi Cloud Formation in Benzene Magenta lines=pi overlap Insert Fig 12.7 to fill space The current model of the bonding in benzene.

12-32 IUPAC Names: Benzenes Certain groups change the base name of the ring system. E. g.

12-33 IUPAC Names: Benzenes For monosubstituted benzenes, name the group and add “benzene” (unless the group conveys a special name.) Name: chlorobenzeneethylbenzene nitrobenzene

12-34 IUPAC Names: Benzenes-2 For disubstituted benzenes, name the groups in alphabetical order. The first named group is at position 1. If a “special group” is present, it must be number 1 on the ring. An older system of naming uses ortho (o), meta (m), and para (p) to indicate groups that are 1,2, 1,3 and 1,4 on the ring.

12-35 IUPAC Names: Benzenes-3 Name: 1-bromo-2-ethylbenzene o-bromoethylbenzene 3-nitrotoluene m-nitrotoluene 1,4-dichlorobenzene or p-dichlorobenzene

12-36 IUPAC Names: Benzenes-4 A final note: When the benzene ring is a substituent on a chain (C 6 H 5 ), it is called a phenyl group. Note the difference between phenyl and phenol (a functional group). 4-phenyl-1-pentene

12-37 Reactions of Benzene Benzene undergos aromatic substitution reactions: an atom or group substitutes for an H on the ring. All benzene reactions (in our class) require a catalyst. The reactions are: 1. Nitration 2. Halogenation 3. Sulfonation

12-38 Reactions of Benzene-1 Nitration places the nitro group on the ring. Sulfuric acid is needed as a catalyst.

12-39 Reactions of Benzene-2 Halogenation places a Br or Cl on the ring. Fe or FeCl 3 are used as catalysts.

12-40 Reactions of Benzene-3 Sulfonation places an SO 3 H group on the ring.

12-41 Heterocyclic Aromatics Rings with a hetero atom (typically O, N, S) and delocalized electrons are also aromatic. Many have a six membered ring, some have a five membered ring. Eg:

12-42 Heterocyclic Aromatics-cont. Heterocyclic aromatics are similar to benzene in stability. Many are significant biologically. Found in DNA and RNA Found in hemoglobin and chlorophyll

12-43 THE END Unsaturated Hydrocarbons