Reactions of Alkenes. Thermodynamics and Kinetics

Reactions of Alkenes. Thermodynamics and Kinetics

General Molecular Formula for Alkenes General molecular formula for acyclic alkanes is CnH2n+2 CH3CH2CH2CH2CH3 an alkane C5H12 CnH2n+2 Prentice Hall 2001 Chapter 3

1 General Molecular Formula for Alkenes Each bond introduced, reduces the H content by 2 CH3CH2CH2CH=CH 2 an alkene C5H10

CnH2n Prentice Hall 2001 Chapter 3 2 General Molecular Formula for Alkenes Each ring also reduces the H content by 2 a cyclic alkane

C5H10 CnH2n Prentice Hall 2001 Chapter 3 3 General Molecular Formula for Alkenes Generalization: The molecular formula for a hydrocarbon is CnH2n+2 minus 2 hydrogens for every bond and/or ring present in the molecule

a cyclic alkene C5H8 CnH2n-2 Prentice Hall 2001 Chapter 3 4 Saturated and Unsaturated Hydrocarbons Alkanes or saturated hydrocarbons contain the maximum number of

carbon-hydrogen bonds CH3CH2CH2CH2CH3 a saturated hydrocarbon Prentice Hall 2001 Chapter 3 5 Saturated and Unsaturated Hydrocarbons Alkenes contain fewer than the maximum number of carbon-hydrogen

bonds and are therefore referred to as unsaturated hydrocarbons CH3CH2CH2CH=CH2 an unsaturated hydrocarbon Prentice Hall 2001 Chapter 3 6 Nomenclature of Alkenes IUPAC names of alkenes are based on the corresponding alkane with -ane replaced by ene

Prentice Hall 2001 Chapter 3 7 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 2001 Chapter 3

8 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 2001 Chapter 3 9 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 2001 Chapter 3 10 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 2001 Chapter 3 11 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 2001 Chapter 3 12 IUPAC Rules for Alkene Nomenclature The sp2 carbons of an alkene are called

vinylic An sp3 adjacent carbon is called allylic Prentice Hall 2001 Chapter 3 13 The Structure of Alkenes Rotation about a double bond breaks the bond

Prentice Hall 2001 Chapter 3 14 Cis-Trans Isomerism Prentice Hall 2001 Chapter 3 15 The E, Z System of

Nomenclature Br Cl C Br C C H CH3 CH3

H C Cl Which isomer is cis and which is trans? A more definitive nomenclature is needed! Prentice Hall 2001 Chapter 3 16 The E, Z System of Nomenclature

First prioritize the groups bonded to the two sp2 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 2001

Chapter 3 17 The E, Z Prioritization Rules 1. 2. Relative priorities depend first on the atomic number of the atom (not the formula weight of the group) bonded to the sp2 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) Prentice Hall 2001 Chapter 3 18 The E, Z Prioritization Rules 3. 4. 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 Prentice Hall 2001 Chapter 3 19 Reactivity Considerations Electron-rich atoms or molecules are

attracted to electron-deficient atoms or molecules Prentice Hall 2001 Chapter 3 20 Reactivity Considerations Electron-deficient ions or molecules are called electrophiles (electron loving from the Greek philos, loving)

Prentice Hall 2001 Chapter 3 21 Reactivity Considerations Electron-rich species are called nucleophiles Prentice Hall 2001 Chapter 3

22 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 2001 Chapter 3 23 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 2001 Chapter 3 24 Reaction Mechanisms We use curved arrows to indicate the movement of pairs of electrons as two molecules, ions or atoms interact

Prentice Hall 2001 Chapter 3 25 Reaction Mechanisms Curved arrows are drawn only from the electron-rich site to the electron deficient site Prentice Hall 2001 Chapter 3

26 Reaction Mechanisms An arrow with a single barb represent the movement of a single unpaired electron Prentice Hall 2001 Chapter 3 27

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