Ch 10 and 11 Chemistry of alcohols and thiols

2 c 2 r 1 r 2 10 49 alkene epoxidation is not facially

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Unformatted text preview: Synthesis of Epoxides! O PhCO3H H3C H3C CH3 H PhCO3H CH3 +! H H O H O H H3C H H H3C +! H CH3 O Nonsuperimposable! - Unique! CH3 H MCPBA PhCO3H H O +! O H MeCO3H Superimposable! - Same! Same! H O O +! O MeCO3H O +! 10-52! Thiols: Structure!         The functional group of a thiol is an SH (sulfhydryl/mercapto) group bonded to an sp3 hybridized carbon! Sulfur analogues of alcohols.! The bond angle about sulfur in methanethiol is 96° (109° in CH3OH)! Complexes with heavy metals: Hg, As, Au.! ! 10-53! Nomenclature!   IUPAC names:! •  the parent is the longest chain that contains the -SH group! •  change the suffix -e to -thiol! •  when -SH is a substituent, it is named as a mercapto group!   Common names:! •  name the alkyl group bonded to sulfur followed by the word mercaptan! SH SH 1-Butanethiol 2-Methyl-1-propanethiol (Butyl mercaptan) (Isobutyl mercaptan) OH HS 2-Mercaptoethanol (2-Sulfanylethanol) 10-54! Thiols: Physical Properties!   Because of the low polarity of the S-H bond, thiols show little association by hydrogen bonding! •  they have lower boiling points and are less soluble in water than alcohols of comparable MW! Thiol! bp (°C)! ! M! thanethiol! 6! e Ethanethiol! 35! ! 1-Butanethiol! 98! ! Alcohol! bp (°C)! M! thanol! 65! e Ethanol! ! 78! 1-Butanol! 117! •  the boiling points of ethanethiol and its constitutional isomer dimethyl sulfide are almost identical! CH3CH 2SH E thaneth iol (bp 35°C) CH 3SCH3 Dimeth yl s ulfide (b p 37°C) 10-55! Thiols: acidity!   Thiols are stronger acids than alcohols! CH3 CH2OH + H 2 O + CH 3CH2O + H 3O CH3 CH 2SH + H 2O CH3 CH2S - + H3 O+ pK a = 1 5 .9 pK a = 8 . 5 •  when dissolved an aqueous NaOH, they are converted completely to alkylsulfide salts! + CH 3CH2SH + Na OH pK a 8. 5 ( Str ong e r a ci d) CH3CH2 S-N a+ + H2O p Ka 1 5 .7 (W e ak er a cid ) 10-56! Thiols: Preparation!   The most common preparation of thiols depends on the very high nucleophilicity of hydrosulfide ion, HS-! Br 1 eq. NaSH SH ethanol •  Use a large excess of sodium hydrosulfide with unhindered alkyl halide to prevent dialkylation to R-S-R! (excess)! 10-57! Thiols: Preparation from Thiourea!   General Reaction:! H2N C S + OH R C X H2N   H2N O + HS R + X H2N Characteristics:! •  More efficient than simple displacement of X- by alkanethiolate ion, RS-, HS- which can generate elimination products!   Mechanism:! 10-58! Thiols: oxidation!   The sulfur atom of a thiol can be oxidized to several higher oxidation states! [O] R-S-H A thiol [O] R-S-S-R A disulfide R-S-OH A sulfenic acid [O] O R-S-OH [O] A sulfinic acid O R-S-OH O A sulfonic acid •  the most common reaction of thiols in biological systems in interconversion between thiols and disulfides, -S-S-! 2RSH + A thiol 1O 22 R S S R + H2 O A disulfide 10-59! Disulfides! Thiols can produce disulfides just through exposure to air oxygen.!   Chemically disulfides are generally produced with mild oxidants (I2, H2O2, etc.)!   2 CH3CH2SH + I2 + 2 NaOH CH3CH2SSCH2CH3 +...
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This note was uploaded on 01/27/2014 for the course BIOL 241 taught by Professor Henrychang during the Winter '10 term at Purdue University-West Lafayette.

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