Presentation on theme: "Chapter 17: Alcohols and Phenols Dr. Sivappa Rasapalli Chemistry and Biochemistry University of Massachusetts Dartmouth."— Presentation transcript:
Chapter 17: Alcohols and Phenols Dr. Sivappa Rasapalli Chemistry and Biochemistry University of Massachusetts Dartmouth
Coverage and Objectives Coverage: 1. Structure and Bonding; Physical Properties of Alcohols 2. Acidity of Alcohols and Phenols 3.Formation of Alkoxides and Phenoxides 4.Preparation of Alcohols including Review Reactions and Ring-opening Reactions of Epoxides (+ Mechanisms). 5.Reactions of Alcohols + Mechanisms Objectives: 1.Know all mechanisms discussed in class. 2.Know how to predict the relative acidities of substituted alcohols and phenols. 3.Know reactions and reagents for preparation of alcohols including review reactions. 4.Know reactions of alcohols and mechanisms.
Grignard Reagents and Other Functional Groups in the Same Molecule Which Solvents (if any) Would be OK for Handling RMgBr?, Which substrates could be converted into RMgBr, and Subsequently reacted with CH3CHO?
Combining Grignard Reactions with Other Reactions
Two general classes of reaction – At the carbon of the C–O bond – At the proton of the O–H bond Some Reactions of Alcohols
Typical Oxidizing Agents High-oxidation state metal salts are useful oxidizing agents, which are soluble in water. Chromic acid is formed when chromate (CrO 4 - ) or dichromate (Cr 2 O 7 2- ) are exposed to aqueous acid which is an oxidizing agent Mn(VII) Cr(VI)
Mechanism of Chromic Acid Oxidation Alcohol forms a chromate ester followed by elimination with electron transfer to give ketone The mechanism was determined by observing the effects of isotopes on rates
Why doesn’t it stop aldehyde stage? Non-aqueous Sources of Cr(VI)
Oxidation of Primary Alcohols with PCC or PDC To aldehyde: pyridinium chlorochromate (PCC, C 5 H 6 NCrO 3 Cl) in dichloromethane Other reagents produce carboxylic acids
Methods to Protect Alcohols Reaction with chlorotrimethylsilane in the presence of base yields an unreactive trimethylsilyl (TMS) ether The ether can be cleaved with acid or with fluoride ion to regenerate the alcohol
Protection-Deprotection An example of TMS-alcohol protection in a synthesis
Summary - Phenols Much more acidic (pK a 10) than alcohols Substitution of the aromatic ring by an electron-withdrawing group increases phenol acidity Substitution by an electron-donating group decreases acidity Oxidized to quinones Quinones are reduced to hydroquinones
Summary - Alcohols Synthesis: – Reduction of aldehydes and ketones – Addition of Grignard reagents to aldehydes and ketones Protection of OH as TMS) ether: Reactions: – Conversion to alkyl halides – Dehydration – Oxidation
Reactions of Alcohols: A Review Conversion to alkyl halides 1. Reaction with hydrogen halides 2. Reaction with thionyl chloride 3. Reaction with phosphorous trihalides Acid-catalyzed dehydration to alkenes Conversion to p-toluenesulfonate esters Conversion to ethers Conversion to esters Esters of inorganic acids Oxidation to carbonyl compounds Cleavage of vicinal diols to ketones and aldehydes
Important Concepts 1.Alcohols are alkanols (IUPAC) - Names derived from stem prefixed by alkyl and halo substitutents. 2. Alcohols Have Polar and Short O-H Bond - Hydroxy group is hydrophilic (hydrogen bonding) Unusually high boiling points Appreciable water solubility Alkyl part is hydrophobic 3.Alcohols Are Amphoteric – Deprotonation by bases whose conjugate acids are weaker than the alcohol. Protonation yields alkyloxonium ions Acidity: primary > secondary > tertiary alcohol Electron withdrawing substituents increase acidity. 4.Reverse Polarization - ie – Conversion of the alkyl group in a haloalkane, C δ + -X δ -, into its nucleophilic analog in an organometallic compound, C δ - -M δ +. 5.Aldehyde and Ketone Carbonyl Carbons are Electrophilic - C=O carbon is subject to attack by hydride hydrogens or organometallic alkyl groups. Aqueous workup yields alcohols. 6.Oxidation of Alcohols – Yields aldehydes and ketones (Chromium IV reagents). 7.Protection of alcohols is required in order to mask them participating in unwanted reactions. 8.Phenols are acidic in nature, and can be oxidized to quinones. 9. Alcohols upon reaction with HX provide alkyl halides. 10. An alkyl halide = an alkyl group with a halogen 11. Haloalkane Properties – Strongly affected by the C-X bond polarization and the polarizability of X. 12. Alcohols and alkyl halides are produced via each other, and also they participate in some common type of reactions. 13. Nucleophilic substitution and eliminations – When a lone pair of electrons on a reagent attacks a positively polarized (or electrophilic) center. If a substituent is replaced, the reaction is termed a nucleophilic substitution. The substituent replaced is called the leaving group. If the neucleophile picks up a proton and leads to elimination of leaving group on adjacent atom it is called elimination.