© Prentice Hall 2001Chapter 31 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

© Prentice Hall 2001Chapter 32 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

© Prentice Hall 2001Chapter 33 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

© Prentice Hall 2001Chapter 34 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 a cyclic alkene C 5 H 8 C n H 2n-2

© Prentice Hall 2001Chapter 35 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

© Prentice Hall 2001Chapter 36 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

© Prentice Hall 2001Chapter 37 Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with “-ane” replaced by “- ene”

© Prentice Hall 2001Chapter 38 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

© Prentice Hall 2001Chapter 39 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

© Prentice Hall 2001Chapter 310 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

© Prentice Hall 2001Chapter 311 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

© Prentice Hall 2001Chapter 312 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

© Prentice Hall 2001Chapter 313 IUPAC Rules for Alkene Nomenclature The sp 2 carbons of an alkene are called vinylic An sp 3 adjacent carbon is called allylic

© Prentice Hall 2001Chapter 314 The Structure of Alkenes Rotation about a double bond breaks the  bond

© Prentice Hall 2001Chapter 315 Cis-Trans Isomerism

© Prentice Hall 2001Chapter 316 The E, Z System of Nomenclature Which isomer is cis and which is trans? A more definitive nomenclature is needed!

© Prentice Hall 2001Chapter 317 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”)

© Prentice Hall 2001Chapter 318 The E, Z Prioritization Rules 1.Relative priorities depend first on the atomic number of the atom (not the formula weight of the group) bonded to the sp 2 carbon 2.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)

© Prentice Hall 2001Chapter 319 The E, Z Prioritization Rules 3.If an atom is doubly bonded to another atom, the system treats it as if it were bonded to two such atoms 4.In the case of isotopes, the isotope with the greater mass number has the higher priority

© Prentice Hall 2001Chapter 320 Reactivity Considerations Electron-rich atoms or molecules are attracted to electron-deficient atoms or molecules

© Prentice Hall 2001Chapter 321 Reactivity Considerations Electron-deficient ions or molecules are called electrophiles (“electron loving” from the Greek philos, “loving”)

© Prentice Hall 2001Chapter 322 Reactivity Considerations Electron-rich species are called nucleophiles

© Prentice Hall 2001Chapter 323 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

© Prentice Hall 2001Chapter 324 Reaction Mechanisms A detailed description of a reaction is called the reaction mechanisms We study reaction mechanisms because it helps us to learn about chemical reactions without having to memorize volumes of seemingly unrelated data

© Prentice Hall 2001Chapter 325 Reaction Mechanisms We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact

© Prentice Hall 2001Chapter 326 Reaction Mechanisms Curved arrows are drawn only from the electron-rich site to the electron deficient site

© Prentice Hall 2001Chapter 327 Reaction Mechanisms An arrow with a single barb represent the movement of a single unpaired electron