Structure of Alkenes

1. Structure of Alkenes • Alkenes (and alkynes) are unsaturated hydrocarbons • Alkenes have one or more double bonds • The two bonds in a double bond are different: - one bond is a sigma () bond; these are cylindrical in shape and are very strong - the other is a pi (π) bond; these involve sideways overlap of p-orbitals and are weaker than  bonds • Alkenes are flat and have a trigonal planar shape around each of the two C’s in a double bond

4. Alkenes • Structure: • the VSEPR model predicts bond angles of 120° about each carbon of a double bond • in ethylene, the actual angles are close to 120° • in substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion of alkyl groups bonded to the double bond

5. Structure of Alkynes • Alkynes have one or more triple bonds • A triple bond consists of one  bond and two π bonds - the two π bonds are orthogonal (perpendicular) • Alkynes are linear around each of the two C’s in the triple bond • Because alkenes and alkynes have π bonds, which are much weaker than  bonds, they are far more chemically reactive than alkanes

7. Naming Alkenes and Alkynes • Parent name ends in -ene or -yne • Find longest chain containing double or triple bond • Number C’s starting at end nearest multiple bond • Locate and number substituents and give full name - use a number to indicate position of multiple bond - cycloalkenes have cyclo- before the parent name; numbering begins at double bond, giving substituents lowest possible numbers - use a prefix (di-, tri-) to indicate multiple double bonds in a compound

8. Alkenes - IUPAC Names

9. Cycloalkenes • 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 • number and list substituents in alphabetical order

10. Dienes, Trienes, Polyenes • alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so on • those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many)

11. Alkynes - IUPAC Names

13. Common Names • Common names are still used for some alkenes and alkynes, particularly those of low molecular weight

14. Cis-Trans Isomers of Alkenes • The π bond gives an alkene a rigid structure • Free rotation around the C-C bond is not possible because the π bond would have to break and re-form • So, groups attached to the double bond are fixed on one side or the other • If each C in the double bond has two different groups attached, then cis-trans isomers are possible: - Cis = 2 groups attached to the same side of the double bond - Trans = 2 groups attached to opposite sides of the double bond

16. Physical Properties • alkenes and alkynes are nonpolar compounds • the only attractive forces between their molecules are London dispersion forces • their physical properties are similar to those of alkanes with the same carbon skeletons • alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids • alkenes and alkynes that are liquid or solid at room temperature have densities less than 1 g/mL; they float on water

17. Addition Reactions of Alkenes and Alkynes • Addition (combination) reactions have the form A + B  AB • For alkenes the general reaction has the form R2C=CR2 + A-B  R2AC-CBR2 (where R = any alkyl group or H) • Addition reactions are the most common types of reactions for alkenes and alkynes • The π bonds are easily broken, and that pair of electrons can form a new  bond • The reactions are favorable because the products (all  bonds) are more stable than the reactants

19. Hydrogenation of Alkenes and Alkynes • H2 can be added to alkenes or alkynes to form alkanes • Usually a metal catalyst (Pt, Pd or Ni) is used to speed up the reaction (the reaction generally doesn’t work without a catalyst) • Because these reactions take place on a surface, hydrogenation of substituted cycloalkenes produces cis products.

20. Addition of H2 - Reduction • Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni

21. Hydrohalogenation of Alkenes • Hydrogen halides (HCl, HBr or HI) can add to alkenes to form haloalkanes • When a hydrogen halide adds to a substituted alkene, the halide goes to the more substituted C (Markovnikov’s rule)

22. Addition of HX • reaction is regioselective • Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon

23. Mechanism of hydrohalogenation • Hydrohalogenation takes place in two steps • In the first step, H+ is transferred from HBr to the alkene to form a carbocation and bromide ion • Second, Br- reacts with the carbocation to form a bromoalkane Example:

25. Addition of Water to Alkenes • In the presence of a strong acid catalyst (HCl, H2SO4 etc.) alkenes react with H2O to form alcohols • Recall that acids form H3O+ in water; it is the H3O+ that reacts with the alkene • Hydration reactions follow Markovnikov’s rule

26. Mechanism of Acid-Catalyzed Alkene Hydration • First, the alkene reacts with H3O+ to form a carbocation • Next an H2O quickly reacts with the carbocation to form a protonated alcohol • In the last step the proton is removed by an H2O to form an alcohol

27. Halogenation of Alkenes and Alkynes • Halogens (Cl2 or Br2) can add to alkenes or alkynes to form haloalkanes • Alkenes form dihaloalkanes; alkynes form tetrahaloalkanes • Reaction with cycloalkenes produces a trans product

28. Addition of Cl2 and Br2 • Addition takes place readily at room temperature • reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent • addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond • Br2 has a deep red color; dibromoalkanes are colorless

29. Mechanism of Bromonation of Ethene • First, a Br+ is transferred from Br2 to the alkene to form a bromonium ion and a bromide ion • Next, the bromide ion reacts with the bromonium ion to form the product

30. Polymers • A polymer is a long chain of repeating subunits called monomers - examples of natural polymers: DNA, protein, starch - example of synthetic polymers: polyethylene • Many synthetic polymers are made from alkenes, although other functional groups are also used • The monomers are added to the chain through a series of addition reactions • Polymerization reactions usually require high temperature and pressure and are often radical reactions carried out with a catalyst

31. Polymerization • From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization • polymer: Greek: poly, many and meros, part • monomer: Greek: mono, single and meros, part

32. Polymerization • show the structure of a polymer by placing parentheses around the repeating monomer unit • place a subscript, n, outside the parentheses to indicate that this unit repeats n times • the structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions • following a section of polypropene (polypropylene)

36. Polyethylene • 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% 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 then are strong • has higher melting point than LDPE and is stronger • can be blow molded to squeezable jugs and bottles

37. Aromatic Compounds • Aromatic compound: a hydrocarbon that contains one or more benzene-like rings • arene: a term used to describe aromatic compounds • Ar-: a symbol for an aromatic group derived by removing an -H from an arene • Kekulé structure for benzene (1872)

38. Conjugated Alkenes and Aromatic Compounds • Recall that a double bond consists of one  bond and one  bond; a  bond is formed by sideways overlap of two p orbitals (one electron comes from each orbital) • A conjugated alkene has alternating double and single bonds • The p orbitals overlap in a conjugated system (the  electrons are “delocalized” throughout the system), making conjugated alkenes more stable than non-conjugated alkenes • An aromatic hydrocarbon consists of alternating double and single bonds in a flat ring system • Benzene (C6H6) is the most common aromatic hydrocarbon • In benzene all the double bonds are conjugated, and so the  electrons can circulate around the ring, making benzene more stable than 1,3,5-hexatriene (the p orbitals on the end of a chain can not overlap)

40. Resonance Structures • There are two ways to write the structure of benzene • These are called “resonance structures” • However, neither of these represents the true structure of benzene since benzene has only one structure, with all C-C bonds being equivalent • The true structure is a hybrid of the the two resonance structures; this can be represented by drawing the  bonds as a circle • We use the individual resonance structures when we write reaction mechanisms involving benzene to show more clearly the bond formation and bond breaking in the reaction

41. Benzene Delocalized electrons are not confined between two adjacent bonding atoms, but actually extend over three or more atoms.

42. Naming Monosubstituted Benzene Compounds • Benzene compounds with a single substituent are named by writing the substituent name followed by benzene • Many of these compounds also have common names that are accepted by IUPAC (you should know those listed here)

43. Naming Multisubstituted Benzene Compounds • When there are 2 or more substituents, they are numbered to give the lowest numbers (alphabetical if same both ways) • Disubsituted benzenes are also named by the common prefixes ortho, meta and para

44. Nomenclature • For three or more substituents: • if one of the substituents imparts a special name, name the molecule as a derivative of that parent • if none of the substituents imparts a special name, number the substituents to give the smallest set of numbers, and list them in alphabetical order before the ending "benzene"

45. Nomenclature • phenyl group (C6H5- or Ph-): the substituent group derived by loss of an H from benzene

46. PAHs • Polynuclear aromatic hydrocarbon (PAH) • a hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

47. Physical Properties of Aromatic Compounds • Because aromatic compounds (like benzene) are flat, they stack well, and so have higher melting and boiling points than corresponding alkanes and alkenes (similar to cycloalkanes) • Substituted aromatic compounds can have higher or lower melting and boiling points than benzene - para-xylene has a higher m.p. than benzene - ortho and meta-xylene have lower m.p.’s than benzene • Aromatic compounds are more dense than other hydrocarbons, but less dense than water (halogenated aromatics can be more dense than water, as can haloalkanes) • Aromatic compounds are insoluble in water, and are commonly used as solvents for organic reactions • Aromatic compounds are also flammable, and many are carcinogenic

48. Chemical Reactivity of Aromatic Compounds • Aromatic compounds do not undergo addition reactions because they would lose their special stability (aromaticity) • Instead, they undergo substitution reactions, which allow them to retain their aromaticity • We will study three types of substitution reactions of benzene: halogenation, nitration and sulfonation

49. Halogenation of Benzene and Toluene • Br2 or Cl2 can react with benzene, using a catalyst, to form bromobenzene or chlorobenzene • Only the monohalogenation product is produced • When Br2 or Cl2 reacts with toluene, a mixture of isomers is produced - Ortho and para isomers are the major products, and meta isomer is the minor product

50. Mechanism of Bromonation of Benzene • First, a Br+ is transferred from Br2 to benzene, forming a carbocation and a chloride ion • Next, the chloride ion removes an H+ from the carbocation to form chlorobenzene and HBr