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

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

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

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

Addition Reactions of Alkenes and Alkynes Addition (combination) reactions have the form A + B  AB For alkenes the general reaction has the form R 2 C=CR 2 + A-B  R 2 AC-CBR 2 (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

Hydrogenation of Alkenes and Alkynes H 2 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.

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)

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:

Addition of Water to Alkenes In the presence of a strong acid catalyst (HCl, H 2 SO 4 etc.) alkenes react with H 2 O to form alcohols Recall that acids form H 3 O + in water; it is the H 3 O + that reacts with the alkene Hydration reactions follow Markovnikov’s rule

Mechanism of Acid-Catalyzed Alkene Hydration First, the alkene reacts with H 3 O + to form a carbocation Next an H 2 O quickly reacts with the carbocation to form a protonated alcohol In the last step the proton is removed by an H 2 O to form an alcohol

Halogenation of Alkenes and Alkynes Halogens (Cl 2 or Br 2 ) can add to alkenes or alkynes to form haloalkanes Alkenes form dihaloalkanes; alkynes form tetrahaloalkanes Reaction with cycloalkenes produces a trans product

Mechanism of Bromonation of Ethene First, a Br + is transferred from Br 2 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

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

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 (C 6 H 6 ) 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)

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

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)

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

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

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

Halogenation of Benzene and Toluene Br 2 or Cl 2 can react with benzene, using a catalyst, to form bromobenzene or chlorobenzene Only the monohalogenation product is produced When Br 2 or Cl 2 reacts with toluene, a mixture of isomers is produced - Ortho and para isomers are the major products, and meta isomer is the minor product

Mechanism of Bromonation of Benzene First, a Br + is transferred from Br 2 to benzene, forming a carbocation and a chloride ion Next, the chloride ion removes an H + from the carbocation to form chlorobenzene and HBr

Nitration and Sulfonation of Benzene Nitric acid can react with benzene, using sulfuric acid as a catalyst, to form nitrobenzene plus water First H 2 SO 4 donates a proton to HNO 3, which then decomposes to form H 2 O and NO 2 + (the reactive species) Sulfur trioxide plus sulfuric acid (fuming sulfuric acid) can react with benzene to produce benzenesulfonic acid First H 2 SO 4 donates a proton to SO 3 to produce HSO 3 + (the reactive species)