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Transcript
Alcohols, Ethers and Thiols Structure of Alcohols • The functional group of an alcohol is an OH (hydroxyl) group bonded to an sp3 hybridized carbon. • Bond angles about the hydroxyl oxygen atom are approximately 109.5°. • Oxygen is also sp3 hybridized. • Two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen. • The remaining two sp3 hybrid orbitals each contain an unshared pair of electrons. Nomenclature of Alcohols • IUPAC names • The parent chain is the longest chain that contains the -OH group. • Number the parent chain in the direction that gives the -OH group the lower number. • Change the suffix -e to -ol. • Common names • Name the alkyl group bonded to oxygen followed by the word alcohol. • Examples: • Compounds containing • two -OH groups are named as diols, • three -OH groups are named as triols. • -OH groups on adjacent carbons are called glycols. • Ethylene glycol (toxic) and propylene glycol (nontoxic) are used in antifreeze. • Glycerin is used in moisturizers and play bubbles. • Unsaturated alcohols • • • • The double bond is shown by the infix -en-. The hydroxyl group is shown by the suffix -ol. Number the chain to give OH the lower number. Alcohol has higher priority than alkene. Hex-3-en-1-ol Physical Properties • Alcohols are polar solvents. • Polarity of an alcohol originates in the C-O-H bond in methanol. a) Partial positive charges on carbon and hydrogen and a partial negative charge on oxygen. b) An electron density map showing the partial negative charge in red and the partial positive charge in blue. Hydrogen Bonding in Alcohols • Alcohols condense to the liquid state because of hydrogen bonding. • The strength of hydrogen bonding in alcohols is approximately 2 to 5 kcal/mol. • Hydrogen bonds are considerably weaker than covalent bonds (for example, 110 kcal/mol for an O–H bond). • Nonetheless, hydrogen bonding can have a significant effect on physical properties. • Alcohols have relatively high boiling points and melting points because of hydrogen bonding. The association of ethanol molecules in the liquid state. • Note the huge difference in boiling points and solubility between analogous alkanes and alcohols. • Boiling points increase with chain size. • Water solubility of alcohols decreases with increasing chain size. (molecules get more hydrophobic) Acidity of Alcohols • Most alcohols are about the same or slightly weaker acids than water. Basicity of Alcohols • In the presence of strong acids, the oxygen atom of an alcohol behaves as a weak base. • Proton transfer from the strong acid forms an oxonium ion. • Thus, alcohols can function as both weak acids and weak bases. Overview of Reactions of Alcohols • Reactions with active metals • Product is the conjugate base of the alcohol. • Substitution of halogen for hydroxyl group. • Reaction can be with HX or SOCl2. • Dehydration • Acid-catalyzed elimination to form alkene. • Oxidation • Use chromic acid, H2CrO4, (CrO3/H2SO4) to convert 1 alcohol to aldehyde or carboxylic acid • Use chromic acid to convert 2 alcohol to ketone Reaction with Active Metals • Alcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides. • Na is oxidized to Na+ and H+ is reduced to H2. • Alkoxides are somewhat stronger bases that OH–. • Alkoxides can be used as bases in -elimination reactions. • They can be used also as nucleophiles in nucleophilic substitution reactions. Conversion of ROH to RX with HX • Water-soluble 3° alcohols react very rapidly with moderate concentrations of HCl, HBr, and HI. • Low-molecular-weight 1° and 2° alcohols are unreactive under these conditions. • Water-insoluble 3° alcohols react by bubbling gaseous HCl through a solution of the alcohol dissolved in diethyl ether or THF. • 1° and 2° alcohols require concentrated HBr and HI to form alkyl bromides and iodides. • 3° Alcohols react with HX by an SN1 mechanism • Step 1: Add a proton. Rapid and reversible proton transfer from the acid to the —OH group. • This proton transfer converts the OH– from a poor leaving group, to H2O, a much better leaving group. • Step 2: Break a bond to form a stable molecule or ion. Loss of H2O gives a 3° carbocation. • Step 3: Reaction of an electrophile and a nucleophile to form a new covalent bond completes the reaction. • 1° alcohols react with HX by an SN2 mechanism. • Step 1: Add a proton. Proton transfer to OH converts OH–, a poor leaving group, to H2O a better leaving group. • Step 2: Reaction of a nucleophile and an electrophile to form a new covalent bond and break a bond. • Substitutions of –OH for –X are governed by a combination of electronic and steric effects Conversion of ROH to RX with SOCl2 • Thionyl chloride, SOCl2, is the most widely used reagent for conversion of primary and secondary alcohols to alkyl chlorides. Acid-Catalyzed Dehydration of Alcohols • An alcohol can be converted to an alkene by elimination of H and OH from adjacent carbons (a -elimination). • 1° alcohols must be heated at high temperature in the presence of an acid catalyst, such as H2SO4 or H3PO4. • 2° alcohols undergo dehydration at somewhat lower temperatures. • 3° alcohols often require temperatures at or only slightly above room temperature. • When isomeric alkenes are obtained, the more stable alkene (the one with the greater number of substituents on the double bond) generally predominates (Zaitsev’s rule). Dehydration of a 2° Alcohol • A three-step mechanism • Step 1: Add a proton. Proton transfer from H3O+ to the –OH group converts OH–, a poor leaving group, into H2O, a much better leaving group. • Step 2: Break a bond to form a stable molecule or ion. Loss of H2O gives a carbocation intermediate. • Step 3: Take a proton away. Proton transfer from an adjacent carbon to H2O gives the alkene and regenerates the acid catalyst. Dehydration of a 1° Alcohol • A two-step mechanism • Step 1: Add a proton. Proton transfer from the acid gives an oxonium ion. • Step 2: Take a proton away and loss of H2O gives the alkene and regenerates the acid catalyst. Hydration-Dehydration Equilibrium • Acid-catalyzed hydration of an alkene and dehydration of an alcohol are competing processes. • Large amounts of water favor alcohol formation. • Scarcity of water or experimental conditions where water is removed (like high temperatures to evaporate water) favor alkene formation. Oxidation of Alcohols • Oxidation of a 1° alcohol gives an aldehyde or a carboxylic acid, depending on the oxidizing agent and experimental conditions. • The most common oxidizing agent is chromic acid. • Chromic acid oxidation of 1-octanol gives octanoic acid. • To oxidize a 1° alcohol to an aldehyde, use a combination of chromium(VI) oxide, hydrochloric acid and pyridine to form pyridinium chlorochromate, (PCC). • PCC oxidation of geraniol gives geranial. • Oxidation of a 2° alcohol gives a ketone only. • Tertiary alcohols are not oxidized by either of these reagents; they are resistant to oxidation. Structure of Ethers • The functional group of an ether is an oxygen atom bonded to two carbon atoms. • Oxygen is sp3 hybridized with bond angles of approximately 109.5°. • In dimethyl ether, the C–O–C bond angle is 110.3°. Nomenclature of Ethers • IUPAC • The longest carbon chain is the parent alkane. • Name the -OR group as an alkoxy substituent. • Hydroxyl group has priority over alkoxy group. • Common names: • Name the two alkyl branches bonded to oxygen followed by the word ether. • Although cyclic ethers have IUPAC names, their common names are more widely used. Physical Properties of Ethers • Ethers are polar molecules. • Each C-O bond is polar covalent. • However, only weak polar forces exist between ether molecules in the pure liquid. • Ethers are commonly used as polar aprotic solvents. • Ethers are also used as anesthetics. • Ethers have much lower boiling points than their isomeric cousins, alcohols. • Ethers are less soluble in water than alcohols. Reactions of Ethers • Ethers resemble hydrocarbons in their resistance to chemical reaction. • They do not react with strong oxidizing agents such as chromic acid, H2CrO4. • They are not affected by most acids and bases at moderate temperatures. • Because of their good solvent properties and general inertness to chemical reaction, ethers are excellent solvents in which to perform organic reactions. Epoxides • Epoxide: A cyclic ether in which oxygen is one atom of a threemembered ring. • Ethylene oxide is synthesized from ethylene and O2. • Other epoxides can be synthesized from an alkene by oxidation with a peroxycarboxylic acid, RCO3H. O CH3COOOH 2,3-epoxybutane cis-2-butene CH3COOOH cyclohexene O 1,2-epoxycyclohexane Acid-catalyzed Ring Opening of an Epoxide • • • • • • • Epoxides react readily because of the angle strain in the three-membered ring. Acid catalyst protonates epoxide oxygen atom. Water (acting as a nucleophile) attacks least hindered carbon. Reaction of an epoxide with aqueous acid gives a glycol. The addition of the water is anti. Nucleophiles other than water (NH3, OH-, OR-, SH-) can be used. Such reactivity is in contrast to the lack of reactivity generally in ethers. Structure of Thiols • The functional group of a thiol is an -SH (sulfhydryl) group bonded to an sp3 hybridized carbon. Nomenclature of Thiols • IUPAC names • The parent chain is the longest chain containing the -SH group. • Add -thiol to the name of the parent chain. • Sulfhydryl group has lower priority than hydroxyl. • Alternatively, indicate a lower priority -SH by the prefix mercapto. • Common names • Name the alkyl group bonded to sulfur followed by the word mercaptan. Physical Properties of Thiols • Low-molecular-weight thiols have a STENCH! • Odorants are added to natural gas at a concentration of about 1 ppm. • Thiols are less polar than alcohols. • The difference in electronegativity between S and H is 2.5 – 2.1 = 0.4. • Intermolecular forces of thiols are much different than that of alcohols because of their low polarity. • Thiols show little association by hydrogen bonding. • Thiols have lower boiling points and are less soluble in water than alcohols of comparable MW. Chemical Properties of Thiols • Thiols are stronger acids than alcohols. • Thiols react with strong bases to form salts. • Conjugate bases of thiols make good nucleophiles. Reactions of Thiols • Thiols are oxidized by a variety of oxidizing agents, including O2, to disulfides. • Disulfides, in turn, are easily reduced to thiols by several reagents. • This easy interconversion between thiols and disulfides is very important in protein chemistry (the amino acid, cysteine, is a thiol).