Halogenation of Alkenes Alkene halogenation reaction Common laboratory reaction Limited primarily to marine organisms in nature Carried out by enzymes called haloperoxidases that oxidize Br or Cl ions to a biological equivalent of Br+or Cl+ 1.“Br+" 2.CI Br B-Ocimene CI Br
Alkene halogenation reaction ▪ Common laboratory reaction ▪ Limited primarily to marine organisms in nature ▪ Carried out by enzymes called haloperoxidases that oxidize Bror Clions to a biological equivalent of Br+ or Cl+ Halogenation of Alkenes
8-3 Halohydrins from Alkenes Halohydrin Formation (electrophilic addition) Reaction of alkenes with hypohalous acids HO-CI or HO-Br to yield 1,2-halo alcohols called halohydrins HX ho An alkene A halohydrin In marine organisms halohydrin formation is carried out by haloperoxidases that oxidize Br or Cl ions to corresponding HOBr or HOCI bonded to a metal atom in the enzyme for subsequent addition to the double bond of substrate H OH CH2OH H202.Br,pH=3 CH2OH Bromoperoxidase
Halohydrin Formation (electrophilic addition) ▪ Reaction of alkenes with hypohalous acids HO-Cl or HO-Br to yield 1,2-halo alcohols called halohydrins ▪ In marine organisms halohydrin formation is carried out by haloperoxidases that oxidize Br- or Clions to corresponding HOBr or HOCl bonded to a metal atom in the enzyme for subsequent addition to the double bond of substrate 8-3 Halohydrins from Alkenes
Halohydrins from Alkenes Br2 reacts with CH3 alkene to give H3 cyclic bromonium Reaction of the alkene with Br2 yields a bromonium ion intermediate,as previously ion intermediate discussed. :Br: Intermediate CH3 bromonium ion is Water acts as a nucleophile,using a lone :QH2 intercepted by pair of electrons to open the bromonium ion ring and form a bond to carbon.Since water nucleophile oxygen donates its electrons in this step,it now has the positive charge Br Oxygen loses proton to give the H:H2 H neutral halohydrin Loss of a proton(H*)from oxygen then gives HO and the neutral bromohydrin product addition product. CHa +H30t H3C OH 3-Bromobutan-2-ol
▪ Br2 reacts with alkene to give cyclic bromonium ion intermediate ▪ Intermediate bromonium ion is intercepted by water nucleophile ▪ Oxygen loses proton to give the neutral halohydrin product Halohydrins from Alkenes
8-4 Hydration of Alkenes Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols Hydration of ethylene is not of much use in the laboratory because of the high temperatures often required H H3PO4 catalyst H20 250C CH3CH2OH Ethanol Ethylene Uncommon in biological pathways
Alkenes undergo an acid catalyzed addition reaction with water to yield alcohols ▪ Hydration of ethylene is not of much use in the laboratory because of the high temperatures often required ▪ Uncommon in biological pathways 8-4 Hydration of Alkenes
Hydration of Alkenes Acid-catalyzed hydration of double bond adjacent to carbonyl group More common in biological pathways Adjacent carbonyl group required for elimination of water Not an electrophilic addition mechanism H20,pH=7.4 HA Fumarase Fumarate Anion intermediate Malate
Acid-catalyzed hydration of double bond adjacent to carbonyl group ▪ More common in biological pathways ▪ Adjacent carbonyl group required for elimination of water ▪ Not an electrophilic addition mechanism Hydration of Alkenes