lectures 26-32 sp11 - Organic Chemistry I 310/318M...

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Organic Chemistry I 310/318M Pre-Health Professionals and ChemE’s Unique numbers: 52425 (310M) 52585 (318M) Prof. Jonathan L. Sessler NOTES: Homework is due today. A new homework will be posted today.
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Acidity of Alcohols Acidity depends primarily on the degree of stabilization and solvation of the alkoxide ion. – The negatively charged oxygens of methoxide and ethoxide are about as accessible as the oxygen of hydroxide ion for solvation; these alcohol are about as acidic as water. – As the bulk of the alkyl group increases, the ability of water to solvate the alkoxide decreases, the acidity of the alcohol decreases, and the basicity of the alkoxide ion increases.
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Reaction with Metals Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides. Alcohols are also converted to metal alkoxides by reaction with bases stronger than the alkoxide ion. – One such base is sodium hydride. Ethanol Sodium ethoxide CH 3 2 OH 3 2 O Na + + + H 2 + H Sodium hydride Sodium methoxide (MeO + ) + 2CH 3 O Na + H 2 3 + 2Na
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Reaction with HX – 3° alcohols react very rapidly with HCl, HBr, and HI. – Low-molecular-weight 1° and 2° alcohols are unreactive under these room temperature conditions. – 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides. reflux 1-Bromobutane 1-Butanol + + HBr H 2 O H 2 O OH Br + H 2 O + HCl 25°C Cl 2-Methyl-2- propanol 2-Chloro-2- methylpropane
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Reaction with HX – With HBr and HI, 2° alcohols generally give some rearranged product. – 1° alcohols with extensive β -branching give large amounts of rearranged product. 2-Bromopentane 3-Bromopentane (major product) 3-Pentanol heat + + HBr + H 2 O OH Br a product of rearrangement ! " 2-Bromo-2-methylbutane (a product of rearrangement) 2,2-Dimethyl-1- propanol + + HBr H 2 O OH Br
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Reaction with HX Based on – the relative ease of reaction of alcohols with HX (3° > 2° > 1°) and – the occurrence of rearrangements, Chemists propose that reaction of 2° and 3° alcohols with HX – occurs by an S N 1 mechanism, and – involves a carbocation intermediate
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Reaction with HX - S N 1 Step 1: Proton transfer to the OH group gives an oxonium ion. Step 2: Loss of H 2 O gives a carbocation intermediate. CH 3 3 3 -C-OH : H H H O O H H 3 -C 3 3 : H H O + + rapid and reversible + + O H H 3 -C 3 3 3 3 3 -C+ : H H O + A 3° carbocation intermediate slow, rate determining S N 1 +
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Reaction with HX - S N 1 Step 3: Reaction of the carbocation intermediate (an electrophile) with halide ion (a nucleophile) gives the product. CH 3 3 3 -C+ :Cl 3 -C-Cl CH 3 CH 3 2-Chloro-2-methylpropane ( tert- Butyl chloride) fast +
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Reaction with HX - S N 2 1° alcohols react with HX by an S N 2 mechanism. Step 1: Rapid and reversible proton transfer. Step 2: Displacement of HOH by halide ion.
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lectures 26-32 sp11 - Organic Chemistry I 310/318M...

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