Unsaturated Hydrocarbons

1. Unsaturated Hydrocarbons Physical properties – Similar to saturated hydrocarbons Chemical properties - 1.More reactive than saturated hydrocarbons 2.The carbon-carbon double or triple bonds are the reactive sites (In most cases we will be working with double bonds) So, common reactive sites are: Multiple bond sites Functional group sites

2. Multiple Bonds • Carbon-carbon multiple bonds (ex.: C2H4) • There are two types of bonds in carbon-carbon multiple bonds • a. Sigma bonds ()– A covalent bond in which atomic orbital overlap occurs along the axis joining the two bonded carbons • b. Pi bonds ()– A covalent bond in which atomic orbital overlap occurs above and below, but not on, the internuclear axis. • Occurrence of  and  bonds • When a single bond is present between two atoms, that bond is always a -bond. • When a double bond is present between two atoms, that bond consists of one -bond and one -bond. • When a triple bond is present between two atoms, that bond always consists of one -bond and two -bonds. • Importance of -bonds • A carbon-carbon-bond is weaker, consequently more reactive • The presence of the -bond causes the bond to be structurally rigid. There is no free rotation. • The -bond must be broken for rotation to occur.

3. Classes of Unsaturated Hydrocarbons • 1. Alkenes – An acyclic hydrocarbon with one or more carbon-carbon double bonds (with one double bond : CnH2n) • 2. Alkynes – An acyclic hydrocarbon with one or more carbon-carbon triple bonds (with one triple bond : CnH2n-2) • 3. Aromatic – A cyclic hydrocarbon six*-carbon (usually) ring containing three carbon-carbon double bonds. * known as a benzene ring (C6H6).

4. Alkenes • An alkene can be formed by removing a hydrogen atom from two adjacent carbons in a carbon chain. • Ex: Hexane -C—C—C—C—C—C- becomes • Hexene -C—C—C=C—C—C- (3-Hexene) • Ex: Ethane -C-C- becomes • Ethene -C=C- (also known as ethylene) • Ex.:Cycloalkenes • C---C • cyclohexene C C • C---C

5. In ethene, the atoms are in a flat (planar) rather than a tetrahedral arrangement. Ethene is the compound that causes tomatoes to ripen.

6. p Bonding in Ethene

7. p Bonding in Ethene

8. H H C C H H Top View C2H4

9. Nomenclature of Alkenes • Select the parent carbon chain with the longest chain of carbon atoms that contains the double bond. • Replace the alkane suffix –ane with –ene to indicate the presence of a double bond. • Number the carbon chain starting with the end of the chain that has the closest double bond. • Indicate location of the double bond using the lowest carbon number of the carbons associated with the double bond. • If more than one double bond is present use the suffixes diene, triene, tetraene, ect. The associated carbon numbers are used to indicate the position of the double bonds. • Ex.: • 3-Pentene • 1,3-Pentadiene • 2,4,6-Octatriene • 6-Methyl-2,4-octadiene

10. Nomenclature of Cycloalkenes • If there is only one double bond, its position does not need to be indicated. It is assumed to be located between carbons one and two. • If there is more than one double bond in the ring, number the bond locations in a manner that will give the lowest numbers. • In substituted cycloalkenes assign the numbers in a manner that will produce the lowest combination of numbers. • Ex.: • Cyclopentene • 3-Ethylcyclopentene • 1,4-Cyclooctadiene • 6-propyl-1,4-Cyclooctadiene

11. Alkenyl Groups • There are THREE important such groups: • Methylene (CH2=) • methylidene • Vinyl (CH2=CH-) • ethenyl • Ex. Vinyl chloride (CH2=CHCl) • Allyl (CH2=CH-CH2-) • 2-propenyl

12. Structural Isomerism • Structural isomer can occur as they do with alkanes • Positional: 1-butene vs. 2-butene • Skeletal: 1-butene vs. 2-methylpropene • The carbon-carbon double bond allows the formation of two additional types of isomers, Cis-and Trans- isomers (these are also known as stereoisomers) • The double bond restricts rotation around the C atoms. • The carbons must have two different types of groups attached to them * A hydrogen functional group * A carbon containing group or a halogen • To determine whether cis or trans occurs draw the molecule and examine the shape. • Ex.: 2-butene • Ex.: Retinal/Opsin

13. Examples of Structural Isomers • Trans-3-Methyl-3-hexene • Cis-2-Pentene • Trans-2-Pentene CH3CH2—CH3 \ / C=C / \ H H • Cis-1-chloro-1-pentene

14. Occurrence • Natural • Pheromones • Terpenes (plant odors & fragrances) • Contain 2 or more isoprene units (2-methyl-1,3-butadiene) • Synthetic • Dehydrogenation of Alkanes (at high temperature and in absence of O2) • Ethane ---> Ethene + H2

15. Physical Properties • Solubility • Insoluble in water • Soluble in nonpolar solvents • Less dense than water • Lower melting point than alkanes • Physical states similar to alkanes • C1 to C5 = gas • C6 to C17 = liquid • > C17 = solid

16. Chemical Reactions • Addition • Symmetrical: -C=C- + X2 --> X-C-C-X • Hydrogenation - results in formation of alkane • Halogenation* • Asymmetrical: -C=C- + AB --> A-C-C-B • Hydrohalogenation • Hydration - results in formation of alcohol • Markovnikov’s* rule: (“rich get richer”) Hydrogen goes to C with most hydrogens. A bromine in water solution is reddish brown. When a small amount of such a solution is added to an unsaturated hydrocarbon, the added solution is decolorized.

17. Chemical Reactions • Polymerization: multiple simple molecules (monomers) add together to form a single, larger molecule (polymer) • These are usually catalyzed reactions! • Addition polymers • C=C + C=C + C=C --> C-C-C-C-C-C (polyethylene) (C-C)n • Substituted-ethene addition polymers • nC=C-X --> (C-C-X)n (ex.: PVC) • Butadiene-based addition polymers • Ex.: natural rubber (2-methyl-1,3-butadiene; isoprene) • Much more flexible than other polymers • Addition Copolymers (two different monomers) • Ex.: Saran wrap (1953) - polyvinylidene chloride (2004) - polyethylene

18. Alkynes • Formation is similar to that of alkenes (more hydrogens are removed; higher temp.) • Ethyne = Acetylene • Naming: same rules as for alkenes • Isomerism: cis-trans NOT possible • Linear geometry around the triple bond • Properties & Reactions are similar to those of alkenes

19. p Bonding in Acetylene

20. p Bonding in Acetylene

21. p Bonding in Acetylene

22. C2H2 C C H H

23. Alkenynes • Hydrocarbons with both double & triple bonds. • Naming: Double bond has priority • #ing Carbons: from end closest to a multiple bond.

24. Aromatics • Unsaturated cyclic hydrocarbons which do not readily undergo addition reactions. • Benzene: the foundation molecule • Contains both localized and delocalized bonds

25. Naming Benzene Derivatives • One substituent derivatives: • Use IUPAC system • Ex.: methylbenzene; bromobenzene • BUT, several of these are considered new Parent molecules: • Toluene • Styrene • Phenol

26. Naming Benzene Derivatives • Two substituent derivatives: • Use the following prefixes to indicate substituent position: • Ortho (1,2) • Meta (1,3) • Para (1,4) • Xylene (dimethylbenzene) • p-dichlorobenzene

27. Occurances • Coal Tar • Petroleum • Synthetic • Ex.: C7H16 ---> Toluene + 4H2

28. Physical Properties & Chemical Reactions • Good solvent for non-polar molecules! • Alkylation reactions: • Benzene + R-Cl ---> • Halogenation: • Benzene + Cl2 ---> • Polymerization • Styrene --> Polystyrene • Largest Synthetic Molecule

29. Fused-Ring Aromatics • Naphthalene • Carcinogenic Fused-ring aromatics: • 4+ fused rings • Same “angle” in ring series • Form when hydrocarbons are heated to high temperatures

30. What do you need to know? • Structural characteristics (know the functional group) • Alkene • Alkyne • Aromatic • Nomenclature (the rules for naming the molecules) • Physical and Chemical properties (basic/simple) • Occurrence and uses (common) • Preparation (what basic reactions produce the molecules) • Characteristic reactions of the molecules