Chapter 4 Notes

Chemistry 331 - Fall 1996 Elements of Organic Chemistry I

Professor Carl C. Wamser

Chapter 4 - Alkenes and Alkynes

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)

Tues, Feb. 13 Carbocations

• structure: trigonal (sp2)
• stability: 3° > 2° > 1°
• more alkyl groups stabilize a cation by electron donation to the electron-deficient (6-electron) carbocation

Markovnikov Addition 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)

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

Hydroxylation

• alkene + KMnO4 --(base)--> 1,2-diol
• addition of two OH groups is syn
• cyclopentene --> cis-1,2-cyclopentanediol

Oxidative Cleavage

• C=C --> C=O + O=C
• acidic KMnO4 causes cleavage
• ozone (O3) causes cleavage
• sometimes useful degradation method to identify unknown compounds

Polymers

• long chains of repeating units (monomers)
• n CH2=CH2 --(init)--> (init)-(CH2-CH2)n-
• n=100-10,000 polyethylene
• has properties like a very long alkane
• many polyalkenes are commercially important materials and plastics
• e.g., PVC, Teflon, Orlon

Chain Reactions

• polymerization occurs by a free radical chain mechanism
• initiation - generation of the first free radical from an initiator
• R-O-O-R --(heat)--> 2 R-O·
• (initiators have one weak bond)

Chain Reactions

• propagation - radical adds to a p bond
• RO· + CH2=CH2 ---> RO-CH2-CH2·
• note that the product is also a radical
• RO-CH2-CH2· + CH2=CH2 ---> RO-CH2-CH2-CH2-CH2· ---> etc.
• typically this occurs hundreds or thousands of times
• (until radicals recombine - termination)

Substituted Monomers

• radical additions follow the Markovnikov Rule:
• add radicals to form the more stable radical intermediate
• radical stability is like cation stability: 3° > 2° > 1°
• this leads to polymers with alternating substituents

Vinyl Polymers

• polyvinyl chloride
• polypropylene
• polystyrene

Elimination Reactions

• alkenes are typically prepared by elimination reactions
• loss of HX from alkyl halides
• (promoted by strong base)
• loss of H2O from alcohols
• (promoted by strong acid)
• eliminations are the reverse of additions

Dehydrohalogenation Dehydration Zaitsev Rule

• predicts regiochemistry
• the major product in an elimination reaction is the more substituted alkene
• (generally more stable)

Conjugated Dienes

• two double bonds separated by one single bond
• overlap their p orbitals into an extended (conjugated) molecular orbital
• more stable than separate pi bonds
• e.g., 1,3-butadiene

1,2- and 1,4-Additions

• electrophilic additions (HX, X2, etc.) often add at opposite ends (1,4) of a conjugated diene

Allylic Carbocations

• initial electrophilic addition occurs at end (not middle) of a conjugated diene
• the resultant cation retains three overlapping p orbitals (stabilized)
• allyl - position next to a C=C bond
• (vinyl - position on a C=C bond)

Allylic Resonance

• allylic cation has two Lewis structures (resonance forms)
• the actual structure is a hybrid
• the allyl cation is more stable than normal alkyl cations (due to resonance)

Resonance Forms

• each resonance form is a correct Lewis structure
• no atoms change (only electrons)
• equivalent resonance forms are most important for stabilization
• nonequivalent resonance forms may also contribute
• actual structure is a resonance hybrid

Alkynes - Structure

• carbon-carbon triple bond
• sp hybridization (linear)
• no cis-trans possibilities
• the two pi bonds are perpendicular
• high electron density
• (usually more reactive than alkenes)

Alkynes - Nomenclature

• -yne suffix (with number)
• rules similar to alkenes
• with both -enes and -ynes, suffix is -enyne and numbering is from the end closer to a multiple bond
• (E)-4-hexen-1-yne

Alkyne Additions

• similar to alkenes but more reactive
• Markovnikov Rule is followed
• excess reagent gives double addition
• single addition is usually possible
• single addition gives alkene product, which may be cis (syn addition) or trans (anti addition) or nonspecific

Reduction of Alkynes

• excess H2 + catalyst gives alkanes
• Lindlar catalyst gives cis-alkenes

Halogenation of Alkynes

• first addition of X2 is anti
• product is trans-dibromoalkene

Hydration of Alkynes

• initial product is an enol, which is typically unstable
• an enol isomerizes to a ketone
• (tautomers - a special kind of isomer, where the only difference is the placement of one hydrogen)

Alkyne Acidity

• terminal alkynes have a C-H bond which is slightly acidic
• pKa ~ 25 (for CH2=CH2 , pKa ~44)
• NaNH2 is a strong enough base to deprotonate 1-alkynes
• anions are called acetylide ions, strong bases and good nucleophiles

Alkyne Syntheses

• acetylide anions are useful in preparing larger alkynes