Alcohols and Thiols Unit 11

Structure - Alcohols The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbon bond angles about the hydroxyl oxygen atom are approximately 109.5° Oxygen is 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-Alcohols IUPAC names the parent chain is the longest chain that contains the OH group number the parent chain to give the OH group the lowest possible number change the suffix -e to -ol Common names name the alkyl group bonded to oxygen followed by the word alcohol

Nomenclature-Alcohols Examples

Nomenclature of Alcohols Compounds containing more than one OH group are named diols, triols, etc. C H 2 C H 3 2 C H 2 O H O H H O O H H O H O O H 1,2-Ethanediol (Ethylene glycol) 1,2-Propanediol (Propylene glycol) 1,2,3-Propanetriol (Glycerol, Glycerine)

Nomenclature of Alcohols Unsaturated alcohols show the double bond by changing the infix from -an- to -en- show the the OH group by the suffix -ol number the chain to give OH the lower number

Physical Properties Alcohols are polar compounds they interact with themselves and with other polar compounds by dipole-dipole interactions Dipole-dipole interaction: the attraction between the positive end of one dipole and the negative end of another

Physical Properties Hydrogen bonding: when the positive end of one dipole is an H bonded to F, O, or N (atoms of high electronegativity) and the other end is F, O, or N the strength of hydrogen bonding in water is approximately 21 kJ (5 kcal)/mol hydrogen bonds are considerably weaker than covalent bonds nonetheless, they can have a significant effect on physical properties

Hydrogen Bonding

Physical Properties Ethanol and dimethyl ether are constitutional isomers. Their boiling points are dramatically different ethanol forms intermolecular hydrogen bonds which increase attractive forces between its molecules resulting in a higher boiling point there is no comparable attractive force between molecules of dimethyl ether C H 3 2 O C H 3 O Ethanol Dimethyl ether bp 78° C bp -24°C

Physical Properties In relation to alkanes of comparable size and molecular weight, alcohols have higher boiling points are more soluble in water The presence of additional -OH groups in a molecule further increases solubility in water and boiling point

Physical Properties

Acidity of Alcohols

Acidity of Alcohols In dilute aqueous solution, alcohols are weakly acidic Acidity depends primarily on the degree of stabilization and solvation of the alkoxide ion the negatively charged oxygens of methanol and ethanol are about as accessible as 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

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

Reaction with HX 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

Reaction with HX with HBr and HI, 2° alcohols generally give some rearranged product 1° alcohols with extensive -branching give large amounts of rearranged product

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 SN1 mechanism, and involves a carbocation intermediate

Alkyl Sulfonates Sulfonyl chlorides are derived from sulfonic acids sulfonic acids, like sulfuric acid, are strong acids O O O R - S C l R - S O H R - S O O O O A sulfonyl chloride A sulfonic acid (a very strong acid) A sulfonate anion (a very weak base and stable anion; a very good leaving group

Alkyl Sulfonates A commonly used sulfonyl chloride is p-toluenesulfonyl chloride (Ts-Cl)

Alkyl Sulfonates Another commonly used sulfonyl chloride is methanesulfonyl chloride (Ms-Cl)

Alkyl Sulfonates Sulfonate anions are very weak bases (the conjugate base of a strong acid) and are very good leaving groups for SN2 reactions Conversion of an alcohol to a sulfonate ester converts HOH, a very poor leaving group, into a sulfonic ester, a very good leaving group

Dehydration of ROH An alcohol can be converted to an alkene by acid-catalyzed dehydration (a type of -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 slightly above room temperature

Dehydration of ROH H S O C H O C H = + O 180°C O H H S O + H O 140° C 2 S O 4 C H 3 2 O C H 2 = + O 180°C O H H 2 S O 4 + H 2 O 140° C Cyclohexanol Cyclohexene C H 3 C H 3 H 2 S O 4 C H 3 O C H 3 = 2 + H 2 O 50° C C H 3 2-Methyl-2-propanol ( tert- Butyl alcohol) 2-Methylpropene (Isobutylene)

Dehydration of ROH where isomeric alkenes are possible, the alkene having the greater number of substituents on the double bond (the more stable alkene) usually predominates (Zaitsev rule)

Dehydration of ROH Dehydration of 1° and 2° alcohols is often accompanied by rearrangement acid-catalyzed dehydration of 1-butanol gives a mixture of three alkenes H 2 S O 4 + O H 140 - 170°C 3,3-Dimethyl- 2-butanol 2,3-Dimethyl- 2-butene (80%) 2,3-Dimethyl- 1-butene (20%)

Dehydration of ROH Based on evidence of ease of dehydration (3° > 2° > 1°) prevalence of rearrangements Chemists propose a three-step mechanism for the dehydration of 2° and 3° alcohols because this mechanism involves formation of a carbocation intermediate in the rate-determining step, it is classified as E1

Dehydration of ROH Acid-catalyzed alcohol dehydration and alkene hydration are competing processes 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

Oxidation: 1° ROH Oxidation of a primary alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions to an aldehyde is a two-electron oxidation to a carboxylic acid is a four-electron oxidation

Oxidation of ROH A common oxidizing agent for this purpose is chromic acid, prepared by dissolving chromium(VI) oxide or potassium dichromate in aqueous sulfuric acid H 2 S O 4 C r O 3 + H 2 O H 2 C r O 4 Chromium(VI) oxide Chromic acid H 2 S O 4 H 2 O K 2 C r O 7 H 2 C r O 7 2 H C r O 4 Potassium dichromate Chromic acid

Oxidation: 1° ROH Oxidation of 1-octanol gives octanoic acid the aldehyde intermediate is not isolated

Oxidation: 2° ROH 2° alcohols are oxidized to ketones by chromic acid

Oxidation of Glycols Glycols are cleaved by oxidation with periodic acid, HIO4

Thiols: Structure The functional group of a thiol is an SH (sulfhydryl) group bonded to an sp3 hybridized carbon The 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 oxygen

Nomenclature IUPAC names: Common 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 sulfanyl group Common names: name the alkyl group bonded to sulfur followed by the word mercaptan

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 the boiling points of ethanethiol and its constitutional isomer dimethyl sulfide are almost identical Thiol bp (° C) Alcohol bp (° C) M ethanethiol 6 M ethanol 65 E thanethiol 35 E thanol 78 1-Butanethiol 98 1-Butanol 117

Thiols: Physical Properties Low-molecular-weight thiols = STENCH the scent of skunks is due primarily to these two thiols a blend of low-molecular weight thiols is added to natural gas as an odorant; the two most common of these are

Thiols: acidity Thiols are stronger acids than alcohols when dissolved an aqueous NaOH, they are converted completely to alkylsulfide salts

Thiols: oxidation The sulfur atom of a thiol can be oxidized to several higher oxidation states the most common reaction of thiols in biological systems in interconversion between thiols and disulfides, -S-S-