Chapter 41 Alkenes. Chapter 4

Chapter 42 Contents of Chapter 3 General Formulae and Nomenclature of Alkenes General Formulae and Nomenclature of Alkenes Cis-Trans Isomerism Cis-Trans Isomerism Reactivity Considerations Reactivity Considerations Thermodynamics and Kinetics Thermodynamics and Kinetics

Chapter 43 General Molecular Formula for Alkenes General molecular formula for acyclic alkanes is C n H 2n+2 CH 3 CH 2 CH 2 CH 2 CH 3 an alkane C 5 H 12 C n H 2n+2

Chapter 44 General Molecular Formula for Alkenes Each  bond introduced, reduces the H content by 2 CH 3 CH 2 CH 2 CH=CH 2 an alkene C 5 H 10 C n H 2n

Chapter 45 General Molecular Formula for Alkenes Each ring also reduces the H content by 2 a cyclic alkane C 5 H 10 C n H 2n

Chapter 46 General Molecular Formula for Alkenes Generalization: The molecular formula for a hydrocarbon is C n H 2n+2 minus 2 hydrogens for every  bond and/or ring present in the molecule Each  bond or ring is considered a unit of unsaturation. a cyclic alkene with 2 units of unsaturation C 5 H 8 C n H 2n-2

Chapter 47 Saturated and Unsaturated Hydrocarbons Alkanes or saturated hydrocarbons contain the maximum number of carbon-hydrogen bonds CH 3 CH 2 CH 2 CH 2 CH 3 a saturated hydrocarbon

Chapter 48 Saturated and Unsaturated Hydrocarbons Alkenes contain fewer than the maximum number of carbon-hydrogen bonds and are therefore referred to as unsaturated hydrocarbons CH 3 CH 2 CH 2 CH=CH 2 an unsaturated hydrocarbon

Chapter 49 Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with “ane” replaced by “ene”

Chapter 410 IUPAC Rules for Alkene Nomenclature 1.The longest chain containing the functional group (the double bond) is numbered such that the double bond is the lowest possible number

Chapter 411 IUPAC Rules for Alkene Nomenclature 2.If there are substituents, the chain is still numbered in a direction that gives the double bond the lowest number

Chapter 412 IUPAC Rules for Alkene Nomenclature 3.If chain has more than one substituent, they are cited in alphabetical (not numerical) order. Rules for alphabetizing are the same as for alkanes

Chapter 413 IUPAC Rules for Alkene Nomenclature 4.If the same number for the double bond is obtained in both directions, number in the direction that gives lowest number to a substituent.

Chapter 414 IUPAC Rules for Alkene Nomenclature 5.In cyclic compounds, a number is not needed to denote the position of the functional group The double bond is assumed to be between carbons 1 and 2

Chapter 415 IUPAC Rules for Alkene Nomenclature 6.If both directions yield same low number for a functional group and for one substituent, number in the direction which yields the lower number for one of the remaining substituents

Chapter 416 IUPAC Rules for Alkene Nomenclature Two groups containing double bonds that are used as names for substituents are the vinyl group and the allyl group

Chapter 417 IUPAC Rules for Alkene Nomenclature The sp 2 carbons of an alkene are called vinylic An sp 3 adjacent carbon is called allylic

Chapter 418 IUPAC Nomenclature of Dienes Find the longest chain containing both double bonds butyl-1,4-pentadiene

Chapter 419 IUPAC Nomenclature of Dienes Use corresponding alkane name but replace the “ne” ending with “diene” “pentane” changed to “pentadiene” 3-butyl-1,4-pentadiene

Chapter 420 IUPAC Nomenclature of Dienes Number in the direction that gives the lowest number to a double bond 1,5-heptadiene not 2,6-heptadiene

Chapter 421 IUPAC Nomenclature of Dienes List substituents in alphabetical order 5-ethyl-2-methyl-2,4-heptadiene

Chapter 422 IUPAC Nomenclature of Dienes Place numbers indicating the double bond positions either in front of the parent compound or in the middle of the name immediately before the diene suffix 5-ethyl-2-methyl-2,4-heptadiene or 5-ethyl-2-methyl-hepta-2,4-diene

Chapter 423 The E, Z System of Nomenclature Which isomer is cis and which is trans? A more definitive nomenclature is needed!

Chapter 424 The E, Z System of Nomenclature First prioritize the groups bonded to the two sp 2 carbons If the higher priority group for each carbon is on the same side of the double bond, it is the Z isomer (for Zusammen, German for “together”) If the higher priority group for each carbon is on the opposite side of the double bond, it is the E isomer (for Entgegen, German for “opposite”)

Chapter 425 The E, Z Prioritization Rules Relative priorities depend first on the atomic number of the atom (not the formula weight of the group) bonded to the sp 2 carbon In the case of a tie, the atomic numbers of the atoms bonded to the tied atoms are considered next (e.g. C, C, & H beats C, H, & H)

Chapter 426 The E, Z Prioritization Rules If an atom is doubly bonded to another atom, the system treats it as if it were bonded to two such atoms In the case of isotopes, the isotope with the greater mass number has the higher priority

Chapter 427 Relative Stabilities of Alkenes

Chapter 428 Relative Stabilities of Alkenes The more alkyl substituents attached to a double bond the more stable the double bond. Trans alkenes more stable than cis alkenes Not difficult concepts but should be learned now in order to understand Chapter 9 later.

Chapter 429 Reactivity Considerations Electrophiles react with nucleophiles An alkene has electron density above and below the  bond making it electron-rich and therefore a nucleophile Therefore alkenes react with electrophiles

Chapter 430 Reaction Mechanisms We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact

Chapter 431 Reaction Mechanisms Curved arrows are drawn only from the electron-rich site to the electron deficient site

Chapter 432 Thermodynamics When  G° is negative the reaction is exergonic

Chapter 433 Thermodynamics When  G° is positive the reaction is endergonic

Chapter 434 Kinetics Knowing the  G° of a reaction will not tell us how fast it will occur or if it will occur at all We need to know the rate of reaction The rate of a reaction is related to the height of the energy barrier for the reaction,  G ‡, the free energy of activation

Chapter 435 Free Energy of Activation

Chapter 436 Rate-Determining Step Formation of the carbocation intermediate is the slower of the two steps It is the rate-determining step

Chapter 437 Rate-Determining Step Carbocation intermediates are consumed by bromide ions as fast as they are formed The rate of the overall reaction is determined by the slow first step

Chapter 438 Transition States and Intermediates It is important to distinguish between a transition state and a reaction intermediate A transition state is a local maximum in the reaction coordinate diagram has partially formed and partially broken bonds has only fleeting existence

Chapter 439 Transition States and Intermediates An intermediate is at a local minimum energy in the reaction coordinate diagram may be isolated in some cases