Unsaturated Hydrocarbons Chem101 • Contain one or more double or triple General Chemistry carbon-carbon bond. Lecture 11 H H H Unsaturated Hydrocarbons H H C C H C C H H H H H H University of Wisconsin-Eau Claire Chem101 - Lecture 11 2 Unsaturated Hydrocarbons Unsaturated Hydrocarbons • There are three classes of unsaturated • There are three classes of unsaturated hydrocarbons: hydrocarbons: - Alkenes have at least one carbon-carbon - Alkynes have at least one carbon-carbon double bond triple bond H H ethylene C C acetylene H C C H H H (common name) University of Wisconsin-Eau Claire Chem101 - Lecture 11 3 University of Wisconsin-Eau Claire Chem101 - Lecture 11 4 Unsaturated Hydrocarbons Nomenclature of Alkenes • There are three classes of unsaturated • Nomenclature means a systematic set hydrocarbons: of names. - Aromatics are rings with alternating • The presence of double bonds means double and single bonds. that there is even a greater number of H possible isomers than for alkanes. H H • We will use the IUPAC system for benzene naming alkenes and alkynes. H H H University of Wisconsin-Eau Claire Chem101 - Lecture 11 5 University of Wisconsin-Eau Claire Chem101 - Lecture 11 6 1 Nomenclature of Alkenes Nomenclature of Alkenes • IUPAC Rules: • IUPAC Rules (con’d): 1 Count the carbons in the longest chain that 5 Also in the prefix, identify and locate any contains the double bond to determine the substituent groups that are attached to the root name, longest chain containing the double bond. 2 Use the ending -ene to designate the 6 Combine the prefix, root and ending names presence of a double bond. for the attached groups and the longest 3 Number the longest chain so that the chain into the complete name. carbons participating in the double bond 7 If the molecule contains more than one have the lowest possible number. double bond, the prefixes di-, tri-, tetra 4 In the prefix, locate the double bond by before the -ene ending. using the lower of the two carbon numbers. University of Wisconsin-Eau Claire Chem101 - Lecture 11 7 University of Wisconsin-Eau Claire Chem101 - Lecture 11 8 Exercise 12.1(p388) Exercise 12.3 (p388) Select the compounds that can be correctly called Give the IUPAC name for the following compounds: unsaturated and classify each one as an alkene or an a. CH CH CH CH CH CH alkyne: 3 2 3 CH3 a. CH3 CH2 CH3 b. CH3 CH CH CH3 c. H C C CH CH CH CH CH CH CH CH CH 3 d. b. 3 3 CH3 CH3 CH CH CH2 3 f. c. e. University of Wisconsin-Eau Claire Chem101 - Lecture 11 9 University of Wisconsin-Eau Claire Chem101 - Lecture 11 10 sp2 Hybridization sp2 Hybridization • The orbitals containing the valence • The 2s orbital mixes with two of the 2p electrons in alkenes also hybridize orbitals to form three sp2 hybrid orbitals • But it is done differently than for • This leaves the remaining 2p orbital alkanes unhybridized. University of Wisconsin-Eau Claire Chem101 - Lecture 11 11 University of Wisconsin-Eau Claire Chem101 - Lecture 11 12 2 sp2 Hybridization sp2 Hybridization • The three sp2 hybrid orbitals are oriented in the same plane and separated by 120° from one another. • The unhybridized 2p orbital is oriented 2 at right angles to the sp orbitals University of Wisconsin-Eau Claire Chem101 - Lecture 11 13 University of Wisconsin-Eau Claire Chem101 - Lecture 11 14 sp2 Hybridization sp2 Hybridization • The three sp2 hybrid each form and σ bond to another atom. • The 2p orbital forms a second bond to one of the three atoms. - This second bond is called π (pi) bond. University of Wisconsin-Eau Claire Chem101 - Lecture 11 15 University of Wisconsin-Eau Claire Chem101 - Lecture 11 16 σ and π Bonds σ and π Bonds • s and p atomic orbitals: • Two half-filled s orbitals overlapping to form a σ bond: University of Wisconsin-Eau Claire Chem101 - Lecture 11 17 University of Wisconsin-Eau Claire Chem101 - Lecture 11 18 3 σ and π Bonds σ and π Bonds • A half-filled s orbital overlapping with • A half-filled p orbital overlapping with a p orbital to form a σ bond: a p orbital to form a σ bond: University of Wisconsin-Eau Claire Chem101 - Lecture 11 19 University of Wisconsin-Eau Claire Chem101 - Lecture 11 20 σ and π Bonds σ and π Bonds • A half-filled p orbital overlapping with • A double-bond is a combination of a σ a p orbital to form a π bond: bond and a π bond: University of Wisconsin-Eau Claire Chem101 - Lecture 11 21 University of Wisconsin-Eau Claire Chem101 - Lecture 11 22 σ and π Bonds Geometry of Alkenes • A double-bond is a combination of a σ • This places the two carbons, and the bond and a π bond: four other atoms that these two carbons are bonded to, all in the same plane. - The bond angles between the three bonds are all approximately 120°. University of Wisconsin-Eau Claire Chem101 - Lecture 11 23 University of Wisconsin-Eau Claire Chem101 - Lecture 11 24 4 Geometry of Alkenes Geometry of Alkenes • Unlike single bonds, there is no free • The following strategy can be applied rotation about double bonds. to determine if a double bond is • This situation can produce additional producing a pair of stereo isomers isomers. A C - These isomers are called geometric isomers. C C B D - If A and B are different, and C and D are different, then there is a pair of geometric isomers. University of Wisconsin-Eau Claire Chem101 - Lecture 11 25 University of Wisconsin-Eau Claire Chem101 - Lecture 11 26 Geometry of Alkenes Geometry of Alkenes • An example of this is provided by • An example of this is provided by 2–butene 2–butene CH CH CH H 3 3 3 • The cis prefix is used to indicate that C C C C the hydrocarbon chain enters and leaves H H H CH 3 on the same side of the double bond. cis-2-butene trans-2-butene • Whereas the trans prefix is used to - These are two different molecules. indicate that the hydrocarbon chain - They have different physical properties: enters and leaves on opposite sides of Isomer Melting Point {°C} Boiling Point {°C} Density {g/mL} cis-2-butene -139.9 3.7 0.52 the double bond. trans-2-butene -105.6 0.9 0.6 University of Wisconsin-Eau Claire Chem101 - Lecture 11 27 University of Wisconsin-Eau Claire Chem101 - Lecture 11 28 Seeing: The Difference Physical Properties of Alkenes Between cis and trans. • The physical properties of alkenes are very similar to those of alkanes. University of Wisconsin-Eau Claire Chem101 - Lecture 11 29 University of Wisconsin-Eau Claire Chem101 - Lecture 11 30 5 Chemical Properties of Alkenes Chemical Properties of Alkenes • We learned that alkanes are fairly un • They are particularly susceptible to a reactive. type of reaction called the addition - Except combustion reactions, alkanes do not reaction. undergo very many types of reactions. A B • Alkenes can also undergo combustion C C + A B C C reactions. • In addition, unlike the carbon-carbon • Addition reactions have two reactants single bonds found in alkanes, the and one product. double bonds found in alkenes are chemically reactive. University of Wisconsin-Eau Claire Chem101 - Lecture 11 31 University of Wisconsin-Eau Claire Chem101 - Lecture 11 32 Chemical Properties of Alkenes Chemical Properties of Alkenes • We will look at four different examples A B of addition reactions: C C + A B C C - Halogenation - Addition of halogens √ • F-F, Cl-Cl, Br-Br, I-I In this reaction A and B represent two - Hydrogenation - Addition of hydrogen atoms or two groups of atoms that are √ H-H bonded to one another by a single bond. - Addition of Hydrogen halides -A and B can be the same. √ H-F, H-Cl, H-Br, H-I - Hydration - Addition of water √ H-OH University of Wisconsin-Eau Claire Chem101 - Lecture 11 33 University of Wisconsin-Eau Claire Chem101 - Lecture 11 34 Chemical Properties of Alkenes Chemical Properties of Alkenes • Halogenation - Addition of halogens • Hydrogenation - Addition of hydrogen √ F-F, Cl-Cl, Br-Br, I-I - H-H Br Br H H Pt, Pd or Ni C C + C C + Br Br C C H H C C alkene hydrogen alkene bromine haloalkane alkane (halogen) The “Pt, Pd, or Ni” written above the arrow indicates that - This reaction is often used to test for the this reaction is catalyzed by platinum, palladium or presence of alkenes nickel. √ Bromine is brown and the alkene and - This commercially important reaction is haloalkane are colorless used to convert polyunsaturated vegetable √ The loss of bromine’s brown color is a positive oils, which are liquid, to partially test for alkenes hydrogenated oils, which are solids. University of Wisconsin-Eau Claire Chem101 - Lecture 11 35 University of Wisconsin-Eau Claire Chem101 - Lecture 11 36 6 Chemical Properties of Alkenes Chemical Properties of Alkenes • Addition of Hydrogen halides • Addition of Hydrogen halides - H-F, H-Cl, H-Br, H-I - H-F, H-Cl, H-Br, H-I H Cl Br H C C + H Cl C C CH2 CH CH3 1-bromopropane hydrogen alkene haloalkane CH2 CH CH3 + H Br halide or - Because the substituents being added are propene hydrogen H Br bromide different, it is possible to obtain two CH2 CH CH3 different products for the reaction 2-bromopropane University of Wisconsin-Eau Claire Chem101 - Lecture 11 37 University of Wisconsin-Eau Claire Chem101 - Lecture 11 38 Chemical Properties of Alkenes Chemical Properties of Alkenes • Addition of Hydrogen halides • Addition of Hydrogen halides - H-F, H-Cl, H-Br, H-I - H-F, H-Cl, H-Br, H-I • When there are two possible products, - For the previous example, 2-bromopropane one is usually favored greatly over the will be the major product of the reaction other.