Alcohols and Ethers

Important Alcohols in Nature (-) Menthol Vanillin

Important Ethers in Nature Eucalyptol

IUPAC Substitutive Nomenclature An IUPAC name may have up to 4 features: locants, prefixes, parent compound and suffixes Numbering generally starts from the end of the chain which is closest to the group named in the suffix

Alcohols R-OH IUPAC Nomenclature of Alcohols Select the longest chain containing the hydroxyl and change the suffix name of the corresponding parent alkane from -ane to -ol Number the parent to give the hydroxyl the lowest possible number The other substituents take their locations accordingly

Common Names of simple alcohols are still often used and are approved by IUPAC

Penta-2,3,4-triol 4-Methyl-hex-1-en-3-ol Pent-2-yne-1,4-diol Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature Alcohols with two or more hydroxyls , use the prefixes (di, tri, tetra,....) to indicate the numbers of hydroxyl group. Penta-2,3,4-triol 4-Methyl-hex-1-en-3-ol Pent-2-yne-1,4-diol

Alcohols R-OH Classification of Alcohols 1° Primary Alcohol 2° Secondary Alcohol 3° Tertiary Alcohol

Ethers R-O-R Nomenclature of Ethers Common Names IUPAC The groups attached to the oxygen are listed in alphabetical order IUPAC Ethers are named as having an alkoxyl substituent on the main chain

Cyclic ethers can be named using the prefix oxa- Three-membered ring ethers can be called oxiranes; Four-membered ring ethers can be called oxetanes Oxacyclopropane Or oxirane (ethylene oxide) Oxacyclobutane Or oxetane Oxacyclopentane (tetrahydrofurane) 1,4-Dioxacyclohexane (1,4-dioxane)

Hydrogen bonding between molecules of Methanol Physical Properties of Alcohols and Ethers Ether boiling points are roughly comparable to hydrocarbons of the same molecular weight Molecules of ethers cannot form hydrogen bond to each other Alcohols have considerably higher boiling points Molecules of alcohols connect with hydrogen bond to each other Both alcohols and ethers can form hydrogen bond to water and have similar solubilities in water Diethyl ether and 1-butanol have solubilites of about 8g /100 mL in water Hydrogen bonding between molecules of Methanol

Physical properties of Alcohol and Ethers Steric factors affect hydrogen bonding. Physical properties of Alcohol and Ethers

Synthesis of Alcohols Via Acid-Catalyzed Hydration of Alkenes Via Reduction of Cabonyl Compounds Via Grignard Reagents

Synthesis of Alcohols Via Acid-Catalyzed Hydration of Alkenes This is a reversible reaction with Markovnikov regioselectivity

Reactions of Carbonyl Compounds with Nucleophiles Carbonyl groups can undergo nucleophilic addition The nucleophile adds to the d+ carbon The p electrons shift to the oxygen The carbon becomes sp3 hybridized and therefore tetrahedral Hydride ions and carbanions are two examples of nucleophiles that react with the carbonyl carbon

Synthesis of Alcohols by Reduction of Carbonyl Compounds A variety of carbonyl compounds can be reduced to alcohols Carboxylic acids can be reduced to primary alcohols These are difficult reductions and require the use of powerful reducing agents such as lithium aluminum hydride (LiAlH4 also abbreviated LAH)

Esters are also reduced to primary alcohols LAH or high pressure hydrogenation can accomplish this transformation

Aldehydes and ketones are reduced to 1o and 2o alcohols respectively Aldehydes and ketones are reduced relatively easily; the mild reducing agent sodium borohydride (NaBH4) is typically used LAH and hydrogenation with a metal catalyst can also be used

Aldehydes and ketones are reduced to 1o and 2o alcohols respectively

Carboxylic acids and esters are considerably less reactive to reduction than aldehydes and ketones and require the use of LAH Lithium aluminium hydride (LiAlH4) is very reactive with water and must be used in an anhydrous solvent such as ether Sodium borohydride (NaBH4) is considerably less reactive and can be used in solvents such as water or an alcohol

Synthesis of Alcohols from Grignard Reagents Organometallic Compounds Carbon-metal bonds vary widely in character from mostly covalent to mostly ionic depending on the metal The greater the ionic character of the bond, the more reactive the compound Organopotassium compounds react explosively with water and burst into flame when exposed to air

Grignard Reagents Grignard reagents are prepared by the reaction of organic halides with magnesium turnings An ether solvent is used because it forms a complex with the Grignard reagent which stabilizes it

Synthesis of Alcohols from Grignard Reagents Aldehydes and ketones react with Grignard reagents to yield different classes of alcohols depending on the starting carbonyl compound

Alcohols as Acids Alcohols have acidities similar to water Sterically hindered alcohols such as tert-butyl alcohol are less acidic (have higher pKa values) Why?: The conjugate base is not well solvated and so is not as stable Alcohols are stronger acids than terminal alkynes and primary or secondary amines An alkoxide can be prepared by the reaction of an alcohol with sodium or potassium metal

Reactions of Alcohols Reactions of R----OH bond The oxygen atom of the hydroxyl group is nucleophilic and weakly basic. The hydroxyl group can be converted to a leaving group so as to allow subsyitution or elimination reactions. Reaction with HX Reaction with PX3 Reaction with SOCl2 Reactions of RO----H bond The hydrogen atom of the hydroxyl group is weakly acidic Combusition Oxidation Esterfication

Reactions of R----OH bond Hydroxyl groups are poor leaving groups, and as such, are often converted to alkyl halides when a good leaving group is needed. Three general methods exist for conversion of alcohols to alkyl halides, depending on the classification of the alcohol and the halogen desired. The most commonly used reagents for for conversion of alcohols to alkyl halides are the following: Hydrogen halides (HCl, HBr, HI). Phosphorus tribromide (PBr3). Thionyl chloride (SOCl2).

Reaction with Hydrogen Halides (HX) The order of reactivity of alcohols is 3° > 2° > 1° > methyl . The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive. The reaction is acid-catalized, but they do not react with nonacidic NaCl, NaBr or NaI.

Reaction with Phosphorus Trihalide (PX3). Primary 1°,secondary 2° and tertiary 3° alcohols react with phosphorus trihalide to yield alkyl halides with inversion (D ↔ L) . Reaction Thionyl Chloride (SOCl2). Primary 1°,secondary 2° and tertiary 3° alcohols react with phosphorus tribromide to yield alkyl chlorides with retention (D ↔ D) or (L ↔ L)

Starting with alcohols, outline a synthesis of each of the following: (a) Benzyl bromide (b) Cyclohexyl chloride (c) Butyl bromide

Reactions of RO----H bond 1- Combusition Reaction of Alcohol 2- Oxidation Reaction of Alcohol Alcohol can oxidized by warming an acidic potassium dichromate(VI) (K2Cr2O7) Ethanol Ethanal Ethanoic acid Primary alcohol 1° - H2 + O Aldehyde Carboxylic acid secondary alcohol 2° - H2 + O Ketone tertiary alcohol 3° - H2

Oxidation of Alcohols Oxidation of 1° Primary Alcohols to Aldehydes A primary alcohol can be oxidized to an aldehyde or a carboxylic acid The oxidation is difficult to stop at the aldehyde stage and usually proceeds to the carboxylic acid A reagent which stops the oxidation at the aldehyde stage is pyridinium chlorochromate (PCC) PCC is made from chromium trioxide under acidic conditions It is used in organic solvents such as methylene chloride (CH2Cl2)

Oxidation of 1° Primary Alcohols to Carboxylic Acids Potassium permanganate (KMnO4) is a typical reagent used for oxidation of a primary alcohol to a carboxylic acid The reaction is generally carried out in aqueous solution; a brown precipitate of MnO2 indicates that oxidation has taken place Oxidation of 2° Secondary Alcohols to Ketones Oxidation of a secondary alcohol stops at the ketone Many oxidizing agents can be used, including chromic acid (H2CrO4) and Jones reagent (CrO3 in acetone)

Esterification Reaction Reaction of alcohols with carboxylic acids in the presence of conc. H2SO4 forms esters as shown in the following general equation: carboxylic acid alcohol ester Example:

A Chemical Test for Primary and Secondary Alcohols Chromium oxide in acid has a clear orange color which changes to greenish opaque if an oxidizable alcohol is present

Synthesis of Ethers 1. Ethers by Intermolecular Dehydration of Alcohol Primary alcohols can dehydrate to ethers This reaction occurs at lower temperature than the competing dehydration to an alkene This method generally does not work with secondary or tertiary alcohols because elimination competes strongly 1° Alcohol

Synthesis of Ethers 2. Williamson Ether Synthesis This is a good route for synthesis of unsymmetrical ethers

Reactions of Ethers Acyclic ethers are generally unreactive, except for cleavage by very strong acids to form the corresponding alkyl halides