Alcohols Notes PDF

Alcohols An Alcohol is an organic compound that has one or more hydroxyl groups (-OH groups) bonded to carbon atom.  Classification 2).Based on the number of carbons bonded to the hydroxyl bearing carbon Monohydric alcohols can be classified as primary (1O), Secondary (20) and Tertiary (30)...

Alcohols An Alcohol is an organic compound that has one or more hydroxyl groups (-OH groups) bonded to carbon atom.  Classification 2).Based on the number of carbons bonded to the hydroxyl bearing carbon Monohydric alcohols can be classified as primary (1O), Secondary (20) and Tertiary (30) alcohols, Based on the number of carbons (Alkyl groups) bonded to the hydroxyl bearing carbon. If one or no C is bonded to the -OH bearing carbon→ 1O Alcohol Two carbons → 20 Alcohol Three carbon → 30 Alcohol Examples of the types of alcohols  Nomenclature of Alcohols Rules 1. Name the longest carbon chain that contains the carbon atom bearing the –OH group. Drop the final –e from the alkane name and add the suffix –ol to give the root name. 2. Number the longest carbon chain starting at the end nearest the hydroxyl group and use the appropriate number to indicate the position of the –OH group. (N.B: The hydroxyl group takes precedence over double and triple bonds). 3. Name all the substituents and give their numbers, as you would for an alkane or alkene.  Names of Diols Alcohols with two –OH groups are called diols or gylcols. They are named like other alcohols except that the alkane name is maintained, and the suffix diol is used. Also, two numbers are needed to tell where the two hydroxyl groups are located Propylene glycol Propane 1,2,3-triol Propane- 1, 2- diol  Synthesis of Alcohols There are two important industrial methods of producing ethanol Fermentation of sugars Hydration of ethene On a smaller scale. Alcohols are prepared by : Hydration of alkenes (catalyzed by acid) Reduction of carbonyl compounds (Aldehydes, ketones and carboxylic acids Using Grignard reagents (Nucleophilic additions to carbonyl compounds) Hydrolysis of halogenoalkanes (Alkyl halides) Hydrolysis of esters  Synthesis of alcohols from Carbonyl compounds Reduction of Carbonyl compounds : -General method both in the laboratory and in living systems. -Reduction of a carbonyl compound adds hydrogen to a C=O bond to give an alcohol. -All kinds of carbonyl compounds can be reduced, including aldehydes, ketones, carboxylic acids, and esters  Reduction of Carboxylic Acids Esters and carboxylic acids are and Esters reduced to give primary alcohols. The reactions are not as rapid as those of aldehydes and ketones, so, a more powerful reducing agent lithium aluminum hydride (LiAlH4) is used rather than NaBH4. (LiAlH4 will also reduce aldehydes and ketones.) N.B: 2 hydrogens are added to the carbonyl carbon during reduction of an ester or carboxylic acid compared to one for aldehydes and ketones  Grignard Reagents and synthesis of Alcohol Grignard Reagents : Formed when Alkyl halides (RX) react with magnesium metal in ether solvent, They yield products called alkyl- magnesium halides (RMgX) also known Grignard reagents. Grignard reagents are examples of organometallic compounds because they contain a carbon–metal bond. Apart from alkyl halides, alkenyl (vinylic) and aryl (aromatic) halides also react with magnesium to give Grignard reagents. The halogen can be Cl, Br, or I, but not F. Grignard Reactions with Carbonyl Compounds to produce Alcohols Physical properties of Alcohols: Boiling point, Solubility, and Hydrogen bonding Physical properties of alcohol continues Physical properties of alcohol continues  Reactions of Alcohol Dehydration to alkene Oxidation to aldehyde, ketone Reduction to alkane Esterification Tosylation Williamson synthesis of ether Substitution to form alkyl halide  Dehydration of Alcohols A chemical process of removing water from a molecule. In the presence of strong acid, alcohol undergoes dehydration to yield the alkene. Dehydration may also be referred to as an elimination reaction. In the dehydration reaction, - OH and -H are eliminated from, the adjacent carbon atoms in the given alcohol to yield the alkene and water (H2O). Examples for dehydration of alcohols  Oxidation of Alcohols The most valuable reaction of alcohols is their oxidation to yield carbonyl compounds This reaction is the opposite of the reduction of carbonyl compounds to give alcohols. Primary alcohols yield aldehydes or carboxylic acids, and secondary alcohols yield ketones, but tertiary alcohols Do NOT normally react with oxidizing agents Many oxidizing agents, such as chromium trioxide (CrO3), sodium dichromate (Na2Cr2O7) in aqueous acid solution, and Pyridinium Chlorochromate (PCC) can be used. Swern reaction uses dimethyl sulfoxide (DMSO) as the oxidizing agent along with oxalyl chloride and pyridine. Pyridinium Chlorochromate (PCC) is another weak oxidant that can be used, when ever  Biological Oxidation Catalyzed by alcohol dehydrogenase (ADH). Oxidizing agent is nicotinamide adenine dinucleotide (NAD+). Ethanol oxidizes to acetaldehyde, then acetic acid, which is a normal metabolite. Methanol oxidizes to formaldehyde, then formic acid, which is more toxic than methanol. Ethylene glycol oxidizes to oxalic acid, which is toxic. Treatment for poisoning is excess ethanol. Chapter 11 22  Enzymatic Oxidation Alcohol dehydrogenase catalyzes an oxidation: the removal of two hydrogen atoms from an alcohol molecule. The oxidizing agent is called nicotinamide adenine dinucleotide (NAD +). Chapter 11 23  Reaction of Alcohol with Carboxylic acids (Esterification reaction) Also called Fisher esterification reaction Reaction of an alcohol and a carboxylic acid to produce an ester. Sulfuric acid is a catalyst. The reaction is an equilibrium between starting materials and products, and for this reason, the reaction is seldom used to prepare esters. The reaction with acyl chloride is preferred.  Reaction of Alcohols with Acyl Chlorides The esterification reaction achieves better results by reacting the alcohol with an acyl chloride. The reaction is exothermic and produces the corresponding ester in high yields with only HCl as a by-product.  Phosphate Esters -An ester derived from an alcohol and phosphoric acid -Can form mono-, di-, and triphosphoric Chapter 11 26 esters  Conversion of Alcohols to Ethers (Williamson ether synthesis) Alcohols are converted into ethers by formation of the corresponding alkoxide ion followed by reaction with an alkyl halide, a reaction known as the Williamson ether synthesis. the alkoxide ion needed in the reaction can be prepared either by reaction of an alcohol with an alkali metal or by reaction with a strong base such as sodium hydride, NaH This is an SN2 displacement reaction and as such, works better with primary alkyl halides to facilitate back-side attack. If a secondary or tertiary alkyl halide is used, the alkoxide will act as a base and an elimination will take place.  CONVERSION OF ALCOHOLS TO ALKYL HALIDES (Substitution reaction) 1). Reaction with hydrogen halides (HCl, HBr, HI) C2H5OH + HCl -------------- C2H5Cl + HOH 2). Reactions with Phosphorous halides 3 C3H7OH + PBr3 ---------- 3 C3H7Br + H3PO3 3). Reactions with Thionyl Chloride C4H9OH + SOCl2 -------- C4H9Cl + SO2 + HCl 4). Reaction with tosylate to form tosylate ester.  Why is converting an alcohol into Alkyl halide important? Converting an alcohol (which has a poor leaving group) into an alkyl halide (which has a great leaving group), Allows us to do all kinds of functional group interconversions that were not previously possible. The SN2 is a very useful and powerful reaction, for example, once a primary alcohol has been converted to a primary alkyl halide, we can then treat it with all varieties of nucleophiles to make a multitude of functional groups. A disadvantage is that the strong acids used for the conversion often affect acid-sensitive functional groups. Also, the rearrangements on secondary carbons can lead to undesired products. To avoid this Tosylates And Mesylates are often used.  Qualitative tests for Alcohols The reactions include Sodium metal test, Jone’s oxidation, Lucas reaction and Iodoform test. Sodium metal test: Sodium reacts with alcohols to produce hydrogen gas. Esterification test: Alcohols reacts with carboxylic acids to produce esters Jone’s Test: Sodium dichromate in acid medium (Na2Cr2O7/H2SO4): 1° alcohols gives positive reaction  Orange dichromate ion  greenish blue chromic ion 2 alcohols gives positive reaction Orange dichromate ion  greenish blue chromic ion 3 alcohols do not react. No color change. Lucas test: ZnCl2 in concentrated HCl: 1° alcohols react slowly or not at all. 2 alcohols react in 1-5 minutes. 3 alcohols react in less than 1 minute. Iodoform test (Triiodomethane CHI3): +ve test given by ethanol & 2 alcohols but not 3