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