Ch05_Handout3 - Addition of H2O(Hydration Addition of water...

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1 Addition of H Addition of H 2 O (Hydration) O (Hydration) Addition of water is called hydration Acid-catalyzed hydration of an alkene is regioselective; hydrogen adds preferentially to the less substituted carbon of the double bond HOH adds in accordance with Markovnikov s rule CH 3 CH=CH 2 H 2 O H 2 SO 4 Propene 2-Propanol + CH 3 C=CH 2 CH 3 H 2 O H 2 SO 4 2-Methyl-2-propanol 2-Methylpropene + Hydration Hydration Step 1: proton transfer from H 3 O + to the alkene Step 2: reaction of the carbocation (an electrophile) with water (a nucleophile) gives an oxonium ion oxonium ion Step 3: proton transfer to water gives the alcohol + + + intermediate A 2 o carbocation + H O H H O H H CH 3 CH=CH 2 CH 3 CHCH 3 slow, rate determining : : : : : + + + An oxonium ion H O H H CH 3 CHCH 3 O-H CH 3 CHCH 3 fast : : : + + + O H H O H H H H O H C H 3 CHCH 3 C H 3 CHCH 3 fast O H : : : :
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2 Hydration Hydration Rearrangements are possible Propose a mechanism: Principle of Microscopic Reversibility Principle of Microscopic Reversibility Dehydration : Loss of H 2 O from an alcohol Acid-catalyzed alcohol dehydration and alkene hydration are competing processes C C H OH C C + H 2 O acid catalyst Direction depends on Le Chatelier s principle : equilibrium reacts to offset any disturbance to it Principle of microscopic reversibility: Principle of microscopic reversibility: the sequence of transition states and reactive intermediates in the mechanism of a reversible reaction must be the same, but in reverse order, for the reverse reaction as for the forward reaction
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3 Addition of Cl Addition of Cl 2 and Br and Br 2 Cl 2 , Br 2 , and I 2 can add to a double bond to form a vicinal dibromide. carried out with either the pure reagents or in an inert solvent such as CH 2 Cl 2 addition of bromine or chlorine to a cycloalkene gives a trans -dihalocycloalkane addition occurs with anti stereoselectivity anti stereoselectivity ; halogen atoms add from the opposite face of the double bond Br 2 CH 2 Cl 2 Br Br Br Br + trans- 1,2-Dibromocyclohexane (a racemic mixture) Cyclohexene + CH 3 CH=CHCH 3 Br 2 CH 2 Cl 2 CH 3 CH-CHCH 3 Br Br 2,3-Dibromobutane 2-Butene + C C Br Br C C Br C C Br C C Br Br - The bridged bromonium ion retains the geometry These carbocations are contributing structures + Addition of Cl Addition of Cl 2 and Br and Br 2 Step 1: formation of a bridged bromonium ion intermediate Pi electrons attack the bromine molecule (or vice versa).
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