4- Alkanes and Alkenes lll Lecture version.pdf

Organic Chemistry 4: Alkenes Unsaturated Hydrocarbons Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Chemical Properties of Alkenes Addition Reactions of Alkenes Addition at the C=C bond is the most common reaction of alkenes. H2 Example: Br2 and Cl2 H H HBr, HCl H2SO4 H3C...

Organic Chemistry 4: Alkenes Unsaturated Hydrocarbons Emdadul Haque, Ph.D. Department of Biochemistry & Molecular Biology Chemical Properties of Alkenes Addition Reactions of Alkenes Addition at the C=C bond is the most common reaction of alkenes. H2 Example: Br2 and Cl2 H H HBr, HCl H2SO4 H3C C C CH3 + H2O Atom or group of atoms is added to carbon atoms of the double bond. Br2 Addition of H2 Hydrogenation In the presence of an appropriate catalyst (such as platinum, palladium or nickel) hydrogen adds to alkenes and converts them into corresponding alkanes. This type of reaction is called hydrogenation. H H3C C H C 2-Butene CH3 + Pt, 25 C H2 1 atm H3C H H C C H H CH3 Butane Addition of Halogen (X2) Halogenation is one of the most common addition reactions. When bromine (halogen), is added to alkene, the double bond reacts, and only a carbon-carbon single bond remains in the product. The product is known as haloalkane or alkyl halide. Bromination of 2butene produces 2,3-dibromobutane. The addition of Br2 to double bond is a simple laboratory test for unsaturation. In this test, the characteristic red-brown color of the added solution fades as it is used up and colorless dibromoalkane forms. Bromination (Br2) or Chlorination (Cl2) H3C H H C C 2-Butene CH3 + Br2 H3C H H C C Br Br CH3 2,3-Dibromobutane Addition of X2 Halogenation is one of the most common addition reactions. When bromine, a halogen, is added to alkene, the double bond reacts, and only a carbon-carbon single bond remains in the product. The product is known as haloalkane or alkyl halide. Bromination of 2-butene produces 2,3-dibromobutane. The addition of Br2 to double bond is a simple laboratory test for unsaturation. In this test, the characteristic red-brown color of the added fades as it is used up and colorless dibromoalkane forms. Bromination (Br2) or Chlorination (Cl2) H3C H H C C 2-Butene CH3 + Br2 H3C H H C C Br Br CH3 2,3-Dibromobutane Addition of HX Hydrobromination (HBr) or Hydrochlorination (HCl) H3C H H C C 2-Butene CH3 + HCl H3C H H C C H Cl CH3 2-Chlorobutane Addition of H2SO4 H3C H H C C 2-Butene CH3 + HOSO3H H3C H H C C H OSO3H CH3 2-butane hydrogen sulphate Addition of H2O to Alkenes An alkene does not react with pure water since water is not acidic enough to react with alkenes. However, with the presence of a small amount of an acid, the reaction does occur with a water molecule added to the double bond of alkene, and the product is an alcohol. H H3C C H C CH3 + H2O (H-OH) 2-Butene H+ H3C H H C C H OH 2-Butanol CH3 Addition of HX to an Unsymmetrical Alkene 2-Chloropropane H H3C CH2 C H3C H C CH2 + H-Cl Cl H (about 100% yield) H3C H C CH2 H Cl (trace) 1-Chloropropane Reaction Mechanism 1. A proton (H+) from HCl bonds to carbon 1 of propene by utilizing the pi bond electrons. The intermediate formed is a positively charged alkyl group, or carbocation. The positive charge is localized on carbon 2 of this carbocation. H3C 3 H C 2 CH2 + H-Cl 1 H3C H C CH2 + Cl - H isopropyl carbocation Reaction Mechanism 2. The chloride ion then adds to the positively charged carbon atom to form a molecule of 2chloropropane. H3C H C CH2 + Cl H - H3C H C CH2 Cl H 2-chloropropane Carbocation An ion in which a carbon atom has a positive charge is known as a carbocation. H H H C H methyl carbocation C C C C C H C C C H o C o primary (1 ) secondary (2 ) carbocation carbocation tertiary (3o) carbocation The order of stability of carbocation and hence the ease with which they are formed is: H H H C H methyl carbocation C C C C C H C C C H o C o primary (1 ) secondary (2 ) carbocation carbocation o tertiary (3 ) carbocation increasing stability When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms. H3C H3C H C CH2 + H-Cl H C CH2 Cl H (about 100% yield) H3C H C CH2 H Cl (trace) This reaction proceeds via the formation of the most stable carbocation intermediate (2°). Markovnikov addition (mechanism) R=CH3 1 1 Secondary carbocation 2 2 1 2 Primary carbocation Write formulas for the organic products formed when 2-methyl-1-butene reacts with: H2, Pt/25°C Cl2 HCl H2O, H+ 2-methyl-1-butene + H2, Pt/25 °C CH3 CH3 Pt H2C C CH2CH3 + 2-methyl-1-butene H2 H2C C H H CH2CH3 2-methylbutane 2-methyl-1-butene + Cl2 CH3 H2C C CH3 CH2CH3 + Cl2 2-methyl-1-butene H2C C Cl Cl CH2CH3 1,2-dichloro-2-methylbutane 2-methyl-1-butene + HCl CH3 H2C C CH3 CH2CH3 + HCl H2C C H Cl Follow Markovnikov Rule CH2CH3 2-methyl-1-butene + H2O CH3 CH3 H2C H+ C CH2CH3 + H2O H2C H Follow Markovnikov Rule C OH CH2CH3 Anti-Markovnikov addition In an addition reaction of HX to an alkene, the hydrogen atom of HX becomes bonded to the carbon atom that had the least number of hydrogen atoms in the alkene if hydrogen peroxide is present in the reaction. Anti-Markovnikov addition (if hydrogen Peroxide is added) Hydrogen peroxide Markovnikov addition (if hydrogen Peroxide is absent) Oxidation of alkenes Alkenes undergo oxidation reaction. When a purple alkaline solution of the oxidizing agent KMnO4 is added to an alkene, the alkene is oxidized to a diol and the KMnO4 is converted to brown MnO2. Baeyer Test H2C CH2 + KMnO4 (aq) + H2O ethene (purple) (ethylene) H2C OH CH2 + MnO2 + KOH OH (brown) 1,2-ethanediol (ethylene glycol) Positive Test for unsaturated hydrocarbon (Alkene): The disappearance of the KMnO4's purple color and the appearance of a brown suspension of MnO2 is a positive test for unsaturation. Alkanes are non-reactive to KMnO4 (no change). Write the name of the products? trans-2-hexene HBr ? Functional Groups The members of each class of compounds contain a characteristic atom or group of atoms called a functional group.