Fundamentals of Organic Chemistry Alcohols, Phenols and ..., Summaries of Organic Chemistry

Download Fundamentals of Organic Chemistry Alcohols, Phenols and ... and more Summaries Organic Chemistry in PDF only on Docsity! 1/27/2019 1 Fundamentals of Organic Chemistry CHEM 109 For Students of Health Colleges Credit hrs.: (2+1) King Saud University College of Science, Chemistry Department CHEM 109 CHAPTER 4. ALCOHOLS, PHENOLS AND ETHERS 2 Alcohols, Phenols and Ethers o Alcohols and phenols have a common functional group, the hydroxyl group, -OH. o Alcohols are compounds whose molecules have a hydroxyl group attached to a saturated carbon atom. o Phenols are compounds that have a hydroxyl group attached directly to a benzene ring . o Ethers are compounds whose molecules have an oxygen atom bonded to two carbon atom. R-O-R R-O-Ar Ar-O-Ar Ether Ar-O-H Phenol R-O-H Alcohol H-O-H Water o Alcohols, phenols and ethers may be viewed as organic derivatives of water. 1/27/2019 2 3 Classification of Alcohols and Ethers o Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°), depending on whether one, two, or three organic groups are connected to the hydroxyl-bearing carbon atom.  Methyl alcohol, which is not strictly covered by this classification, is usually grouped with the primary alcohols. o Ethers are classified as  Symmetrical ethers; When the organic groups attached to the oxygen are identical.  Unsymmetrical ethers (mixed ethers); When the organic groups attached to the oxygen are different. 4 Nomenclature of Alcohols o In the IUPAC system, alcohols are named according to the following rules. 1. Select the longest continuous carbon chain that contains the -OH group. Drop the –e ending of the parent alkane and replace it by the suffix -ol: Alkanol 2. When isomers are possible, the chain is numbered so as to give the functional group (-OH) the lowest possible number. o The common names for the simplest alcohols consist of alkyl group attached to the hydroxyl function followed by the word alcohol: Alkyl alcohol . 1/27/2019 5 9 Nomenclature of Ethers o IUPAC system Ethers are usually named by giving the name of each alkyl or aryl group, in alphabetical order, followed by the word ether. o Common Names  For ethers with more complex structures, it may be necessary to name the -OR group as an alkoxy group.  In the IUPAC system, the smaller alkoxy group is named as a substituent. 10 Physical Properties of Alcohols and Ethers • Ethers are colorless compounds with characteristic, relatively pleasant odors. o Physical State • Ethers have lower boiling points (bp,s) than alcohols with an equal number of carbon atoms. o Boiling Points • Ether has nearly the same b.p. as the corresponding hydrocarbon in which a -CH2- group replaces the ether’s oxygen. Because of their structures (no O-H bonds), ether molecules cannot form hydrogen bonds with one another. • The simplest alcohol, methanol, is a liquid at room temperature. In contrast, alkanes from methane to butane are gases. • Phenol is a colorless, crystalline, and low-melting solid and other phenols also are solids, . 1/27/2019 6 11 Physical Properties of Alcohols and Ethers o Boiling Points • Series of normal alcohols; The boiling points increase with increasing molecular weights. • A comparison of boiling points among isomeric alcohols; The boiling points decrease as the number of alkyl branches from the carbinol group increases. • Phenol and most other phenols have high boiling points. 12 Physical Properties of Alcohols and Ethers o Solubility • Low-molecular-weight ethers, such as dimethyl ether, are quite soluble in water. Ether molecules can form hydrogen bonds to water. • The lower alcohols are completely miscible with water. • As the number of carbons in the alcohol increases, the solubility in water decreases. • Phenol and most other phenols are slightly soluble in water . 1/27/2019 7 13 Hydrogen Bonding in Alcohols and Ethers o The boiling points (bp’s) of alcohols are much higher than those of ethers or hydrocarbons with similar molecular weights. Why? Two or more alcohol molecules thus become loosely bonded to one another through hydrogen bonds. o Hydrogen bonds are weaker than ordinary covalent bonds. o Consequently, alcohols have relatively high boiling points because they must supply enough heat to break the hydrogen bonds before each molecule. The O-H bond is polarized by the high electronegativity of the oxygen atom and places a partial positive charge on the hydrogen atom and a partial negative charge on the oxygen atom. Because alcohols form hydrogen bonds with one another. 14 Hydrogen Bonding in Alcohols and Ethers o This accounts for the complete miscibility of the lower alcohols and ethers with water. o The lower molecular-weight alcohols and ethers can form H-bond with water molecules. o However, as the organic chain lengthens and the alcohol becomes relatively more hydrocarbon like, its water solubility decreases. 1/27/2019 10 19 Preparation of Alcohols 1. Hydration of Alkenes a. Addition of water to a double bond in the presence of an acid catalyst, H+. b. The addition follows Markovnikov’s rule. c. It is not possible to prepare primary alcohols except Ethanol. 2. Oxidation of Cycloalkenes Alkenes react with alkaline potassium permanganate to form glycols. 20 o Nucleophilic Substitution of Alkyl Halide o Reduction of Ketones, and Aldehydes Aldehydes and ketones are easily reduced to primary and secondary alcohols, respectively. Preparation of Alcohols 1/27/2019 11 21 o Addition of Grignard’s Reagent to Aldehydes and Ketones Preparation of Alcohols 22 o The Alkali Fusion of Sulfonates 1. Sulfonation of an aromatic ring. 2. Melting (fusion) of the aromatic sulfonic acid with sodium hydroxide to give a phenoxide salt. 3. Acidification of the phenoxide with HCl to produce the phenol. The alkali fusion of sulfonates involves the following steps; Preparation of Phenols 1/27/2019 12 23 o It takes place in the presence of acid catalysts (H2SO4, H3PO4) (intermolecular reaction) o Example; The most important commercial ether is diethyl ether. It is prepared from ethanol and sulfuric acid. o When ethyl alcohol is dehydrated by sulfuric acid at 180° C, the dominant product is ethylene. 1) Dehydration of Alcohols Preparation of Ethers 24 • This method has two steps; 1) An alcohol is converted to its alkoxide by treatment with a reactive metal (sodium or potassium). 2) Displacement is carried out between the alkoxide and an alkyl halide. • To obtain the best yields of mixed dialkyl ethers, we select a 1 rather than a 2or 3alkyl halide and react it with a sodium alkoxide • To prepare an alkyl aryl ether, we must be careful not to pick a combination in which one of the reagents has a halogen directly attached to an aromatic ring. 2) Williamson Synthesis Preparation of Ethers 1/27/2019 15 29 1) Nucleophilic Substitution Reaction; The Reaction of Alcohols with Hydrogen Halides: Alkyl Halides A) Those that involve the rupture of the carbon-oxygen bond (C-OH). 2) Dehydration of Alcohols: Formation of Alkenes Alcohols react with hydrogen halides (HCl, HBr and HI) to give alkyl halides. Alcohols can be dehydrated by heating them with strong acid. Reactions of Alcohols 30 Reactions of Alcohols B) Oxidation Reactions Alcohols with at least one hydrogen attached to the hydroxyl-bearing carbon can be oxidized to carbonyl compounds. o Primary alcohols give aldehydes, which may be further oxidized to carboxylic acids.  Primary alcohols, oxidation can be stopped at aldehyde stage by special reagents, such as “pyridinium chlorochromate (PCC)”. 1/27/2019 16 31 Reactions of Alcohols B) Oxidation Reactions  Primary alcohols yield aldehydes when treated with mild oxidizing agents such as hot metallic copper or CrO3 in pyridine.  Primary alcohols; when treated with stronger oxidizing agents, such as chromic acid, H2Cr2O7, or neutral potassium permanganate, KMnO4, the intermediate aldehydes formed initially are oxidized further to carboxylic acids. 32 Reactions of Alcohols B) Oxidation Reactions o Secondary alcohols, when treated with any of the oxidizing agents mentioned previously, yield ketones. o Tertiary alcohols, having no hydrogen atom on hydroxyl-bearing carbon, do not undergo oxidation. 1/27/2019 17 33 Reactions of Phenols o Halogenation takes place without catalyst.  The products depend on the solvent used.  In aprotic solvents (solvents that do not release protons) (CCl4, CS2)- bromination gives a mixture of o- and p-bromophenol.  In protic solvents (solvents that can release protons) (H2O)-halogenation gives a trisubstituted phenol is produced. 34  When ethers are heated in concentrated acid solutions, the ether linkage is broken.  The acids most often used in this reaction are HI, HBr, and HCl.  If an excess of acid is present, the alcohol initially produced is converted into an alkyl halide by the reaction. Reactions of Ethers oCleavage of Ethers by Hot Concentrated Acids