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
