Alkene reactions

Preparation of alkenes

• Elimination: prepare alkenes• Addition: reaction of alkenes

Markovnikov rule (electrophilic addition)

• Unsymmetrical substituted alkene gives a single addition product

• Rule: in addition oh HX to an alkene the more highly substituted carbocation is formed as the intermediate rather than the less highly substituted one

• Carbocation stability: 3o>2o>1o>methyl• Measure: energy required to form carbocation by

dissociation of alkyl halide • why? – inductive stabilization

-- hyperconjugation

Reactions of alkenes

• Addition of halogens– Cl2 and Br2

– F2 is too reactive

– I2 does not react– Trans stereoisomer (anti stereochemistry)– Bromonium ion intermediate

Reactions of alkenes

• Addition of hypohalous acid (halohydrin formation)– HOCl, HOBr– rxn = with Br2 or Cl2 in presence of H2O– Additional nucleophile thus intermediate

bromonium ion intercepted by added nucleophile and diverted to different product

Reactions of alkenes

• Addition of H2O (oxymercuration)– yields alcohols– industry: = + H2O in presence of acid– Lab oxymercuration: = + mercury acetate

(Hg(O2CCH3)2) in THF solvent– Hg(Oac) add to = intermediate mercurinium

ion nucleophile addition of water reduce with sodium borohydride (NaBH4)

– markovnikov

Reactions of alkenes

• Addition of H2O (hydroboration)– non-markovnikov– = + borane (BH3) organoborane intermediate oxidize

organoborane with H2O2 alcohol– borane reactive bcoz only 6 electrons– in THF accept an electron pair to complete octet and

form BH3+ - THF complex

– = + BH3+ - THF complex

– Syn stereochemistry– Boron attached to less

– = + BH3+ - THF complex

– Syn stereochemistry– Boron attached to less highly substituted carbon/

less hindered thus less steric crowding in transition state

Reactions of alkenes

• Addition of carbenes ( cyclopropane synthesis)– = + R2C: cyclopropane

– R2C: produced • CHCl3 + KOH Cl2C:

• CH2I2 + Zn(Cu) ICH2-ZnI (iodomethyl zinc iodide)

– stereospecific (cis-edduct cis-product)

Reactions of alkenes

• Reduction reaction– increase electron density on C– platinum or palladium catalyst– heterogeneous process– absorb H2 to catalyst + alkene partially reduced

intermediate alkane + regenerated catalyst

Reactions of alkenes

• Oxidation of alkenes– O3 treated with reducing agent such as Zn in

acetic acid• Form aldehyde or ketone

– KMnO4 in neutral or acidic solution• When H present on = COOH formed• When 2H present on = CO2 formed

Alkynes

Naming alkynes

• Suffix –yne• Position of ≡ indicated by giving number of first

alkyne carbon• Numbering begins at end nearer to ≡• More than 1 ≡ called diynes, triynes….• Double bond and triple bond called enynes• Numbering enynes from first multiple bond (= or ≡)• Choice in numbering = receives lower priority• Alkyl substituent alkynyl

Preparation of alkynes

• Elimination of HX from alkyl halides• Treatment of 1,2-dihaloalkane (vicinal

dihalide) with excess strong base (KOH or NaNH2)

Reactions of alkynes

• Addition of HX and X2

– 1 equivalent of HX alkene– Excess HX dihalide product– markovnikov

Reactions of alkynes

• Hydration of alkynes – Direct addition of H2O catalyzed by mercury (II) ion

(mercury sulfate)– Markovnikov– Product not enol (intermediate) but ketone– Keto-enol tautomerism: constitutional isomers that

interconvert rapidly– Acidic condition enough to replace mercury by hydrogen– Unsymmetrically substituted internal alkyne mixture

produced– Terminal alkyne methyl ketone produced

Reactions of alkynes

• Hydration of alkynes – Hydroboration of alkynes– Non-markovnikov– Borane adds to alkyne resulting vinylic borane

oxidized by H2O2 to yield enol tautomerism gives ketone (internal alkyne) or aldehyde (terminal alkyne)

Reactions of alkynes

• Reduction of alkynes– Alkynes to alkanes by addition of H2 over a metal

catalyst (Pd/C)– Alkynes to alkenes by Lindlar catalyst– Lindlar catalyst: finely divided Pd metal

precipitated onto CaCO3 support and deactivated by treatment with lead acetate and quinoline

– Hydrogenation by syn stereochemistry giving cis alkene product

Reactions of alkynes

• Oxidation alkynes – Alkynes oxidized with oxidizing agents: ozone or

KMnO4

– Internal alkyne carboxylic acid– Terminal alkyne carbon dioxide

Alkyne acidity

• Terminal alkyne weakly acidic• Treat with strong base (Na+-NH2) terminal H

removed and acetylide anion formed• Acidity: Terminal alkyne > alkenes > alkanes• Stability: Acetylide ion > vinylic> alkyl anions• Hybridization: more “s character”. S orbital

nearer to positive nucleus and lower in E than p orbital. Thus –ve charge is more stabilized in orbital with higher s character

Alkylation of acetylide anions

• Negative charge and unshared electron pair on C make acetylide anion strongly nucleophilic

• Thus can react with alkyl halide substitution to yield new alkyne product

• Limited to primary alkyl halides bcoz acetlyide ions are sufficiently strong base to cause dehydrohalogenation instead of substitution when reacted with 2o and 3o alkyl halides