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. 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

6. 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. 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

10. 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

12. 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.

13. 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)

14. 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:

16. 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

17. 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

18. 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

19. 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

20. 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

24. 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)

26. 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

27. 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)

28. 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

29. 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

30. 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

31. 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

32. 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

33. Nitration and Sulfonation of Benzene • Nitric acid can react with benzene, using sulfuric acid as a catalyst, to form nitrobenzene plus water • First H2SO4 donates a proton to HNO3, which then decomposes to form H2O and NO2+ (the reactive species) • Sulfur trioxide plus sulfuric acid (fuming sulfuric acid) can react with benzene to produce benzenesulfonic acid • First H2SO4 donates a proton to SO3 to produce HSO3+ (the reactive species)