Chapter 7 Lecture Alkenes I. Structure & Properties Organic Chemistry, 8 th Edition L. G. Wade, Jr.

Lecture 18: Overview Introduction to Alkenes & Alkynes Bonding of Alkenes & Alkynes Nomenclature of Alkenes & Alkynes Ring Systems with Multiple Bonds E-Z Nomenclature (More Stereoisomerism) Commercial Uses of Alkenes Physical Properties of Alkenes 2

Introduction to Alkenes Alkenes are hydrocarbons with carbon–carbon double bonds. Alkenes are also called olefins, meaning “oil- forming gas.” The functional group of alkenes is the carbon– carbon double bond, which gives this group its reactivity. Chapter 73

Introduction to Alkynes Alkynes are hydrocarbons with carbon–carbon triple bonds. Alkynes have General Formula C n H 2n-2. Some reactions resemble reactions of alkenes, like addition and oxidation. Other reactions are specific to alkynes. Chapter 74

Sigma Bonds of Ethylene Chapter 75

Orbital Description Sigma bonds around the double-bonded carbon are sp 2 hybridized. Angles are approximately 120º and the molecular geometry is trigonal planar. Unhybridized p orbitals with one electron will overlap, forming the double bond (pi bond). Chapter 76

Bond Lengths and Angles sp 2 hybrid orbitals have more s character than the sp 3 hybrid orbitals. Pi overlap brings carbon atoms closer, shortening the C—C bond from 1.54 Å in alkanes down to 1.33 Å in alkenes. Chapter 77

Pi Bonding in Ethylene The pi bond in ethylene is formed by overlap of the unhybridized p orbitals of the sp 2 hybrid carbon atoms. Each carbon has one unpaired electron in the p orbital. This overlap requires the two ends of the molecule to be coplanar. Chapter 78

Cis-Trans Interconversion Cis and trans isomers cannot be interconverted. No rotation around the carbon–carbon bond is possible without breaking the pi bond (264 kJ/mole). Chapter 79

Molecular Structure of Acetylene Triple-bonded carbons have sp hybrid orbitals. A sigma bond is formed between the carbons by overlap of the sp orbitals. Sigma bonds to the hydrogens are formed by using the second sp orbital. Since the sp orbitals are linear, acetylene will be a linear molecule.

Overlap of the p Orbitals of Acetylene Each carbon in acetylene has two unhybridized p orbitals with one nonbonded electron. It is the overlap of the parallel p orbitals that forms the triple bond (two pi orbitals).

Bond Lengths Triple bonds are shorter than double or single bonds because of the two pi overlapping orbitals.

IUPAC Nomenclature of Alkenes Find the longest continuous carbon chain that includes the double-bonded carbons. Ending -ane changes to -ene. Number the chain so that the double bond has the lowest possible number. In a ring, the double bond is assumed to be between carbon 1 and carbon 2. Chapter 713

IUPAC and New IUPAC Chapter 714

Nomenclature: IUPAC Find the longest chain containing the triple bond. Change -ane ending to -yne. Number the chain, starting at the end closest to the triple bond. Give branches or other substituents a number to locate their position.

Examples of Nomenclature All other functional groups, except ethers and halides, have a higher priority than alkynes.

Common Names Named as substituted acetylene. methylacetylene (terminal alkyne) isobutylisopropylacetylene (internal alkyne) CH 3 CH CH 3 CH 2 CCCH CH 3 CH 3 CH 3 CCH

Ring Nomenclature 1-methylcyclopentene In a ring, the double bond is assumed to be between carbon 1 and carbon methylcyclopentene Chapter 718

Multiple Double Bonds Give the double bonds the lowest numbers possible. Use di-, tri-, tetra- before the ending -ene to specify how many double bonds are present. Chapter 719

E-Z Nomenclature Use the Cahn–Ingold–Prelog rules to assign priorities to groups attached to each carbon in the double bond. If high-priority groups are on the same side, the name is Z (for zusammen). If high-priority groups are on opposite sides, the name is E (for entgegen). Chapter 720

Example Assign priority to the substituents according to their atomic number (1 is highest priority). If the highest priority groups are on opposite sides, the isomer is E. If the highest priority groups are on the same side, the isomer is Z E-1-bromo-1-chloropropene Chapter 721

Cyclic Stereoisomers Double bonds outside the ring can show stereoisomerism. Chapter 722

Stereochemistry in Dienes If there is more than one double bond in the molecule, the stereochemistry of all the double bonds should be specified. Chapter 723

© 2013 Pearson Education, Inc. Chapter 924 Commercial Uses of Ethylene Chapter 724

© 2013 Pearson Education, Inc. Chapter 925 Addition Polymers Chapter 725

Ethyne Commonly called acetylene. It is used in welding torches. The oxyacetylene flame reaches temperatures as high as 2800 °C. Thermodynamically unstable. When it decomposes to its elements, it releases 234 kJ (56 kcal) of energy per mole.

Stability of Cycloalkene Cis isomer is more stable than trans in small cycloalkenes. Small rings have additional ring strain. Must have at least eight carbons to form a stable trans double bond. For cyclodecene (and larger), the trans double bond is almost as stable as the cis. Chapter 727

Physical Properties of Alkenes Low boiling points, increasing with mass. Branched alkenes have lower boiling points. Less dense than water. Slightly polar: ◦ Pi bond is polarizable, so instantaneous dipole–dipole interactions occur. ◦ Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. Chapter 728

Physical Properties of Alkynes Nonpolar, insoluble in water. Soluble in most organic solvents. Boiling points are similar to alkane of same size. Less dense than water. Up to four carbons, gas at room temperature.