Chapter 7 Outline alkenes are unsaturated meaning it can react with more Hydrogen heteroatoms hetero different are any atoms other than C and H when calculating elements of unsaturation Halogens count as Hydrogens would don t count oxygens at all nitrogen counts as half of a carbon atom when an alkene chain reaches more than 3 carbon atoms a nuber is used to give the location of the double bond the lower of the two carbons that it is attached to cycloalkenes are assumed to have the double bond in the number 1 position diene compound with two double bonds triene compound with three double bonds tetraene compound with four double bonds for all of these numbers are still used to specify which carbons the double bonds are on alkenyl groups is the name given to alkenes as substituents common ones are vinyl allyl methylene phenyl phenyl is different because it s aromatic and doesn t react the same way common names in organic chem 2 methylpropene is isobutylene 2 methylbuta 1 3 diene is isoprene ethenylbenzene is called styrene or vinylbenzene once again cis trans isomerism is geometric isomerism E Z system if the two first priority groups are on the same size it s the Z isomer zusammen together if the two first priority atoms are on opposite sides trans of the double bond you have the E isomer entgegen opposite the use of E Z names is required whenever a double bond is not clearly cis or trans most trisubstituted and tetrasubstituted double bonds are more clearly named E or Z rather than cis or trans the C C bond in alkenes is readily converted into other functional groups making it an important intermediate in the synthesis of polymers drugs pesticides and other valuable chemicals polymer is a large molecule made up of many monomers an alkene monomer can polymerize by a chain reaction where additional alkene molecules add to the end of the growing polymer chain addition polymers polyolefins polymers made from monofunctional single functional group alkenes such as ethylene and propylene heat of hydrogenation how we compare different alkenes energies Zaitsev s rule more substituted double bonds are usually more stable trans isomers are more stable than cis because alkyl substituents are separated farther away from each other Bredt s rule a bridged bicyclic compound cannot have a double bond at it s bridgehead position unless one of the rings contains at least 8 C atoms if there is a double bond at the bridgehead carbon of a bridged bicyclic system one of the two rings contains a cis double bond and the other must contain a trans double bond compounds that violate Bredt s rule are unstable at room temperature and can sometimes usually with 7 C atoms in the larger ring be synthesized at low temperatures although alkanes and alkenes are both generally nonpolar alkenes tend to be a little more polar because 1 the more weakly held electrons in the pi bond are more polarizable contributing to instantaneous dipole moments 2 the vinylic bonds tend to be slightly polar contributing to a permanent dipole moment dehydrohalogenation the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene E2 dehydrohalogenation takes place in 1 step in which a strong base abstracts a proton from one carbon atom as the leaving group leaves the adjacent carbon a strong base forces second order elimination by abstracting a proton the molecule s bulkiness hinders second order substitution SN2 tertiary halides are the best E2 substrates because they are prone to elimination and cannot undergo SN2 reactions Hofmann product the least highly substituted alkene product Zaitsev product the most substituted alkene product E2 reaction is stereospecific meaning different stereoisomers of the reactant give different stereoisomers of the product it is stereospecific because it normally goes through an anti and coplanar transition state trans diaxial an anti and coplanar arrangement allowing E2 elimination of 2 adjacent substituents on a cyclohexane ring The substituents must be trans to each other and both must be in axial positions on the ring E2 elimination can take place on the chair conformation only if the proton and the leaving group can get into a trans diaxial arrangement vicinal dibromides two bromines on adjacent carbon atoms E1 dehydrohalogenation usually takes place in good ionizing solvent alcohol or water without strong nucleophile or base to force 2nd order kinetics substrate is usually secondary or tertiary alkyl halide requires ionization to form a carbocation which loses a proton to a weak base usually the solvent generally accompanied by SN1 substitution because the nucleophilic solvent can also attack the carbocation directly to form the substitution product common way to make alkenes is dehydration of alcohols concentrated sulfuric acid and or concentrated phosphoric acid are often used as reagents for dehydration because these acids act both as acidic catalysts and as dehydrating agents tertiary alcohols react faster than secondary and so on the major product is usually the one with the most substituted double bond catalytic cracking the heating of petroleum products in the presence of a catalyst usually an aluminosilicate mineral causing bond cleavage to form alkenes and alkanes of lower molecular weight least expensive way to make alkenes on a large scale unsuitable for laboratory synthesis of alkenes because the products are always mixtures dehydrogenation is the removal of H2 from a molecule dehydrogenation of an alkane gives an alkene endothermic similar to catalytic cracking in that a catalyst lowers the activation energy and both reactions use high temperatures to increase a favorable entropy term and overcome and unfavorable enthalpy term they also both produce a mixture of products which is unfavorable in the laboratory hydrogenation of alkenes is therefore exothermic