Alcohols, Ethers, and Thiols

Alcohols, Ethers, and Thiols2011The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbonbond angles about the hydroxyl oxygen atom are approximately 109.5Oxygen is also sp3 hybridizedtwo sp3 hybrid orbitals form sigma bonds to carbon and hydrogenthe remaining two sp3 hybrid orbitals each contain an unshared pair of electrons

Structure - Alcohols2The functional group of an ether is an oxygen atom bonded to two carbon atomsOxygen is sp3 hybridized with bond angles of approximately 109.5. In dimethyl ether, the C-O-C bond angle is 110.3

Structure - Ethers

3The functional group of a thiol is an -SH (sulfhydryl) group bonded to an sp3 hybridized carbonThe bond angle about sulfur in methanethiol is 100.3, which indicates that there is considerably more p character to the bonding orbitals of divalent sulfur than there is to oxygenStructure - Thiols4Nomenclature-AlcoholsIUPAC namesthe longest chain that contains the -OH group is taken as the parent.the parent chain is numbered to give the -OH group the lowest possible numberthe suffix -e is changed to -ol Common names the alkyl group bonded to oxygen is named followed by the word alcohol

Unsaturated alcoholsthe double bond is shown by the infix -en-the hydroxyl group is shown by the suffix -olnumber the chain to give OH the lower number

Exercise: Write IUPAC names of the alcohols below

Nomenclature - EthersIUPAC: the longest carbon chain is the parent. Name the OR group as an alkoxy substituentCommon names: name the groups attached to oxygen followed by the word ether

Nomenclature-thiolsIUPAC names:the parent is the longest chain that contains the -SH groupchange the suffix -e to -thiolCommon names:name the alkyl group bonded to sulfur followed by the word mercaptan

Physical Prop - AlcoholsAlcohols are polar compounds

Alcohols are associated in the liquid state by hydrogen bonding

Physical Prop - EthersEthers are polar molecules; the difference in electronegativity between oxygen (3.5) and carbon (2.5) is 1.0each C-O bond is polar covalentoxygen bears a partial negative charge and each carbon a partial positive chargeEthers are polar molecules, but because of steric hindrance, only weak attractive forces exist between their molecules in the pure liquid stateEthers hydrogen bond with H2O and are more soluble in H2O than are hydrocarbons

Physical Prop - ThiolsLow-molecular-weight thiols have a STENCHthe scent of skunks is due primarily to these two thiols .

The difference in electronegativity between S (2.5) and H (2.1) is 0.4. Because of the low polarity of the S-H bond, thiolsshow little association by hydrogen bondinghave lower boiling points and are less soluble in water than alcohols of comparable MW

Acidity of alcoholsIn dilute aqueous solution, alcohols are weakly acidic

Basicity of alcoholsIn 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

Reaction with MetalsAlcohols react with Li, Na, K, and other active metals to liberate hydrogen gas and form metal alkoxides

Conversion of alcoholo to alkyl halideConversion of an alcohol to an alkyl halide involves substitution of halogen for -OH at a saturated carbon the most common reagents for this purpose are the halogen acids, HX, and thionyl chloride, SOCl2Water-soluble 3 alcohols react very rapidly with HCl, HBr, and HI. Low-molecular-weight 1 and 2 alcohols are unreactive under these conditions

1 and 2 alcohols require concentrated HBr and HI to form alkyl bromides and iodides

Dehdration of ROHAn alcohol can be converted to an alkene by elimination of H and OH from adjacent carbons (a b-elimination)1 alcohols must be heated at high temperature in the presence of an acid catalyst, such as H2SO4 or H3PO42 alcohols undergo dehydration at somewhat lower temperatures3 alcohols often require temperatures only at or slightly above room temperature

Dehdration of ROHWhere isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond generally predominates (Zaitsev rule)

Mechanism of dehydration1. Step 1: proton transfer from H3O+ to the -OH group to form an oxonium ion

Mechanism of dehydration- Step 2: the C-O bond is broken and water is lost, giving a carbocation intermediate

Mechanism of dehydration Step 3: proton transfer of H+ from a carbon adjacent to the positively charged carbon to water. The sigma electrons of the C-H bond become the pi electrons of the carbon-carbon double bond

Oxidation of alcoholA primary alcohol can be oxidized to an aldehyde or a carboxylic acid, depending on the oxidizing agent and experimental conditions

Oxidation of alcoholOxidation of 1-octanol by chromic acid gives octanoic acidthe aldehyde intermediate is not isolated

Oxidation of alcoholPCC oxidation of a 1 alcohol to an aldehyde

Pyridinium chlorochromate (PCC): a form of Cr(VI) prepared by dissolving CrO3 in aqueous HCl and adding pyridine to precipitate PCC.

Oxidation of alcohol2 alcohols are oxidized to ketones by both chromic acid and and PCC

Reaction of EthersEthers, R-O-R, resemble hydrocarbons in their resistance to chemical reactionthey do not react with strong oxidizing agents such as chromic acid, H2CrO4they are not affected by most acids and bases at moderate temperaturesBecause of their good solvent properties and general inertness to chemical reaction, ethers are excellent solvents in which to carry out organic reactions

Cleavage by Strong acids

Reaction of ThiolsThiols are stronger acids than alcohols

When dissolved an aqueous NaOH, they are converted completely to alkylsulfide salts

Thiols are oxidized to disulfides by a variety of oxidizing agents, including O2. they are so susceptible to this oxidation that they must be protected from air during storage