10 Reactions of Alkenes PDF

Physical Properties: The physical properties of the alkenes are very similar to those of the alkanes. ** alkenes although nonpolar are slightly more soluble in water than the corresponding alkanes because the pi electrons, which are somewhat exposed, are attracted to the partially positive hydroge...

Physical Properties: The physical properties of the alkenes are very similar to those of the alkanes. ** alkenes although nonpolar are slightly more soluble in water than the corresponding alkanes because the pi electrons, which are somewhat exposed, are attracted to the partially positive hydrogens of water. REACTIONS OF ALKENES 2 V-Reactions of Alkenes: A- Reactions at the C=C bond. B- Reactions not at the C=C, but at certain positions having special relationship to the double bond (Allylic Halogenation):. vinylic carbon allylic carbon C C C H free radical substitution addition, reduction and oxidation A- Reactions at the C=C bond: *The double bond in alkenes consists of a strong  and a weak  bond. *The typical reactions of the double bond are Addition Reactions where the  bond is broken and two strong  bonds are formed in its place. Reduction and oxidation also occur at the C=C bond. Hydrocarbons… Alkene: Reactions Name Preparations Reactions E+ - - - - d+ -Nu 1) Addition of Hydrogen Halides, Hydrohalogenation: General mechanism X slow fast C C + H step C C C C X H H ** dry gaseous hydrogen halide is used to avoid the addition of water to the alkene. The order of reactivity of HX in this reaction is: HI > HBr > HCl > HF ** Unsymmetrical polar reagents add to unsymmetrical alkenes according to Markovnikov’s rule (the positive portion adds to the carbon that has more Hs) “the rich gets richer”. ***The explanation of this rule, in electrophilic addition the reactions proceed via formation of carbocation, so that the most stable carbocation (3>2>1).  CH3 CH3 _ CH3 C CH CH3 CH3 C CH CH3 + Br + H H Br CH3 CH3 CH3 C CH CH3 CH3 C CH CH3 + H Br H _ Br 6  Br CH3CH2CH2 CH3CH2CH2Br H 1° (less stable) (not formed) e.g.1 CH3CH CH2 slow CH3CHCH3 Br CH3CHCH3 2° (more stable) Br (actual product) N.B. The presence of electron withdrawing substituents ( e.g. CF3, NO2..etc) would produce a less stable carbocation and addition would appear to go anti-Markovnikov (anti MR). d d - Br F3C CH2CH2 F3C CH2CH2 Br (more stable) F3C CH CH2 + H -  d d F3C CHCH3 (not formed) (less stable)... The addition of HX to alkenes is a regioselective reaction ???? As the reaction can give 2 isomeric products but one of them predominates. Hydrocarbons… Alkene: Reactions Name Markovnicov's rule: Anti-Markovnicov's addition: the rich gets richer Strong Electron withdrawing groups Preparations Reactions Addition 1870 Charge repulsion Annalen der Pharmacie 1870, 153, 228 http://en.wikipedia.org/wiki/Markovnikov%27s_rule#cite_note-1  Regiospecificity Markovnikov’s Rule: The proton of an acid(the positive part of the reagent) adds to the carbon in the double bond that already has the most H’s. “Rich get richer.” More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate(carbocation). HI> HBr> HCl add to alkenes to form Markovnikov products Chapter 8 9  Anti-Markovnikov addition: Presence of an electronwithdrawing ( - I effect)group adjacent to the olefinc (double bond) e,g CF3 , NO2 ,C≡N and COOH make the addition antimark. δ δ 10 2- Addition of Water to Alkenes: Hydration of Alkenes Alcohol formation + H OH H C C + H2O C C alkene alcohol Reverse of dehydration of alcohol Use very dilute solutions of H2SO4 or H3PO4 to drive equilibrium toward hydration( acid catalyst reaction). 11 Acid catalyst hydration of alkene is seldom (prohibited) used in alcohol preparation because of possibility of rearrangement CH3 CH3 H ~CH3 CH3 C CH CH2 CH3 C CH CH3 rearrangement CH3 CH3 2° carbocation CH3 CH3 CH3 CH3 H2O -H CH3 C CH CH3 CH3 C CH CH3 OH 3° carbocation 12 3-Addition of Halogens(unsaturation test) Cl , Br ,( in CCl )add to a double bond in absence of light 2 2 4 and at room temperature to form a vicinal dibromide. Anti addition, so reaction is stereospecific. Br C C + Br2 C C Br No rearrangement No carbocation 13  Br C C C C + Br Br + Br => Br Br C C C C Br Br Cyclic bromonium cation 14  It is surprising that bromine should act as an electrophile as it is non-polar. Explanation... as a bromine molecule approaches an alkene, electrons in the pi bond of the alkene repel the electron pair in the bromine-bromine bond thus inducing a dipole. NON-POLAR POLAR AS A NON-POLAR BROMINE MOLECULE THE ELECTRON PAIR IS NOW APPROACHES AN ALKENE, ELECTRONS NEARER ONE END SO THE IN THE PI ORBITAL OF THE ALKENE BROMINE MOLECULE IS POLAR REPEL THE SHARED PAIR OF AND BECOMES ELECTROPHILIC. ELECTRONS IN THE Br-Br BOND  ELECTROPHILIC ADDITION OF BROMINE TEST FOR UNSATURATION The addition of bromine dissolved in tetrachloromethane (CCl4) or water (known as bromine water) is used as a test for unsaturation. If the reddish- brown colour is removed from the bromine solution, the substance possesses a C=C bond. A PLACE A SOLUTION OF BROMINE IN A TEST TUBE B ADD THE HYDROCARBON TO BE TESTED AND SHAKE C IF THE BROWN COLOUR DISAPPEARS THEN THE HYDROCARBON IS AN ALKENE A B C Because the bromine adds to the alkene, it no longer exists as molecular bromine and the typical red-brown colour disappears  OXIDATION REACTION 1- Oxidation to 1,2-diols( vicinal diols) (glycols) where the Pi Bond only broken with formation of 2 new σ bond bearing OH groups A- cis or syn diols Alkene is converted to a cis-1,2-diol,Using cold, dilute aqueous potassium permanganate, followed by hydrolysis with base( Bayer test for unsaturation) 17  B- Trans or anti diols ( epoxide formation) Alkene reacts with a peroxyacid(peroxybenzoic) to form an epoxide (also called oxirane)followed by acid hydrolysis O OH CH3COOH H OH H 18 Hydrocarbons… Alkene: Reactions Karl Sharpless Name Epoxide Formation: Preparations Reactions Reading Addition Cycloaddition Oxidation Glycol Nobel Prize in Epoxide Chemistry 2001 Advanced Application: http://www.synarchive.com/named-reactions/Sharpless_Epoxidation C-Oxidative Cleavage Both the pi and sigma bonds break. C=C becomes C=O. Using warm concentrated acidic KMnO4. Used to determine the position of a double bond in an unknown. Permanganate is a strong oxidizing agent. Glycol initially formed is further oxidized. Disubstituted carbons become ketones. Monosubstituted carbons become carboxylic acids. Terminal =CH2 becomes CO2. 20  H CH3 H CH3 KMnO4 C C H3C C C CH3 CH3 CH3 (warm, conc.) OH OH H CH3 H3C C + C CH3 O O OH H3C C 21 O 21 Hydrocarbons… Alkene: Reactions Name Oxidation with σ bond cleavage: Using hot alkaline KMnO4. 2-alkenes oxidized into acids salts Preparations O- KMnO4/OH- Reactions H3CHC CHCH3 2 H3C C heat 2-butene O Addition Terminal CH2 in 1-alkene oxidized into CO2 and H2O Cycloaddition O- Oxidation - H3CHC CH2 KMnO4/OH H3C C + CO2 + H2O Glycol 1-propene heat O Epoxide Alkenes with Disubstituted C atom of a double bond oxidized into ketons σ -cleavage CH3 H3C KMnO4/OH- H3CC CH2 C O + CO2 + H2O heat 2-methyl-1-propene H3C II- Reactions not at the C=C, but at certain positions having special relationship to the double bond (Allylic Halogenation): **Halogenation of alkenes at high temperature in the gaseous state proceeds by a free radical mechanism resulting in substitution reaction. **While ionic addition reaction which occurs at low temperatures and in absence of light and generally in liquid phase. Cl low temp./ CCl4 H3C CH CH2 (Addition) Cl2 Cl H3C HC CH2 500o-600o C CH2 CH CH2 (Free radical substitution) gas phase Cl Hydrogen atoms attached to the carbon atom next to double bond are known as allylic hydrogen, which is easiest hydrogens to be abstracted. Allylic hydrogens > 3o > 2o > 1o > CH4 > Vinylic Ease of abstraction of hydrogen atoms Ease of formation of free radicals As the formed allylic radical are stabilized by resonance. H2C CHCH2 H2C C CH2 HC C CH2 H H The allylic radical are resonance stabilized... order of stability of carbocation Allyl carbocation > 3o C+ > 2o C+ > 1o C+ CH3 Cl2 / 600 oC H3 C C CH CH2 No reaction as no allylic H CH3 Hydrocarbons… Alkdiene diene each double bond react isolated separately each double bond react cumulated separately conjugated