Chapter 6 Notes

Reaction Types

  • additions A + B --> C H-Cl + CH2=CH2 --> CH3-CH2-Cl
  • eliminations X --> Y + Z CH3-CH2-Cl --> CH2=CH2 + H-Cl
  • substitutions A-B + C-D --> A-C + B-D CH4 + Cl2 --> CH3-Cl + H-Cl
  • rearrangements X --> Y cyclopropane --> propene

HCl plus Ethene

  • CH2=CH2 + H-Cl --> CH3-CH2-Cl
  • an electrophilic addition
  • reaction type: addition
  • reagent type: an electrophile HCl, actually H+, a strong Lewis acid

Addition Mechanism

  • pi bond is relatively reactive, especially towards electrophiles it provides a good source of electrons
  • addition of H+ to CH2=CH2 forms a new C-H sigma bond the electrons for the new bond came from the pi bond the other C is left with only 6 e-

Carbocation Intermediate

  • an intermediate is formed in the reaction mechanism CH2=CH2 + H+ --> CH3-CH2+
  • carbocation: a carbon atom with only 3 bonds (6 e-) and a positive charge
  • structure: sp2 hybridized (trigonal)

Formation of Chloroethane

  • the reaction is completed as chloride anion (a nucleophile) adds to the carbocation (an electrophile) CH3-CH2+ + Cl- ---> CH3-CH2-Cl

Alkene Addition Reactions

  • pi bonds undergo addition reactions CH2=CH2 + HCl --> CH3CH2Cl
  • in general, C=C + HX --> H-C-C-X
  • alkenes react with hydrogen halides to form alkyl halides

Addition of HX to Alkenes

  • cyclohexene + HBr --> bromocyclohexane
  • 1-methylcyclohexene + HBr --> 1-bromo-1-methylcyclohexane (not 1-bromo-2-methylcyclohexane)

Reaction Notation

  • reactants -------> products focus on the organic reactants and products
  • show reagents over the arrow
  • show solvent and conditions under the arrow (or show full balanced reaction)

Orientation of Addition

  • regiochemistry: specific orientation of addition (which C gets H, which gets X?)
  • alkene additions are regioselective: one direction of addition is usually preferred

Markovnikov's Rule

  • the original: add H to the C with more H's (or to the C with fewer alkyl groups)
  • the reason: add H+ to form the more stable cation CH3CH=CH2 + HCl ---> CH3CH+CH3 (not CH3CH2CH2+) ---> CH3CHClCH3 (not CH3CH2CH2Cl)

Carbocations

  • structure: trigonal (sp2)
  • stability: 3° > 2° > 1°
  • more alkyl groups stabilize a cation by electron donation to the electron-deficient (6-electron) carbocation
  • hyperconjugation: electron donation from adjacent C-H sigma bond to empty p orbital of teh carbocation

Markovnikov Addition Carbocation Rearrangements

  • carbocations easily rearrange to more stable forms e.g., 1° --> 2° , 1° --> 3° , or 2° --> 3°

Hydration of Alkenes

  • alkene + water --> alcohol CH2=CH2 + H2O --(H+)--> CH3CH2OH
  • mechanism:
  • step 1: addition of H+ electrophile to pi bond
  • step 2: addition of H2O nucleophile to cation

Hydration Mechanism Halogenation of Alkenes

CH2=CH2 + Cl2 ---> Cl-CH2-CH2-Cl

  • mechanism: Cl2 is an electrophile (adds Cl+) then Cl- is a nucleophile

Anti Addition

  • anti stereochemistry: two new groups are added to opposite sides of the original pi bond

cyclopentene + Br2 ---> trans-1,2-dibromocyclopentane (no cis)

  • anti - describes the process
  • trans - describes the product

Bromonium Ion

  • carbocations can be stabilized by bonding to a neighboring Br (also works with Cl, but less favorable)

Halohydrins

  • addition of halogens in water adds X and OH to the pi bond
  • water is the nucleophile that adds anti to X

Hydroboration/Oxidation

  • addition of BH3 followed by H2O2 adds H and OH
  • "anti-Markovnikov" addition
  • completely syn addition
  • consider B as the electrophile that adds first to the pi bond
  • addition of B and H is concerted (simultaneous)

Reduction of Alkenes

  • reduction - addition of H2 (or removal of O)

CH2=CH2 + H2 ---> CH3-CH3

R-O-H + H2 ---> R-H + H2O

Catalytic Hydrogenation

CH2=CH2 + H2 ---> CH3-CH3

  • requires an active catalyst, typically Pt, Pd, Ni, PtO2
  • reaction occurs on the surface
  • both Hs are delivered to the same side of the pi bond

Syn Addition

  • syn stereochemistry: two new groups are added to the same side of the original pi bond

1,2-dimethylcyclohexene + H2 --(cat)-->cis-1,2-dimethylcyclohexane(no trans)

  • syn - describes the process
  • cis - describes the product

Oxidation of Alkenes

  • oxidation - addition of O (or removal of H2) RCH2OH ---> RCH=O ---> RCOOH
  • there are a wide variety of oxidizing agents: O2, O3, KMnO4, CrO3, Na2Cr2O7 metals in high positive oxidation states

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