Chap. 1 Solomons: Chapter 12 Alcohols from Carbonyl Compounds: Oxidation-Reduction and Organometallic Compounds Chapter 12
t Introduction èSeveral functional groups contain the carbonyl group H Carbonyl groups can be converted into alcohols by various reactions l Structure of the Carbonyl Group èThe carbonyl carbon is sp 2 hybridized and is trigonal planar H All three atoms attached to the carbonyl group lie in one plane Chapter 12 2
èThe carbonyl group is polarized; there is substantial d+ charge on the carbon Chapter 12 3
l Reactions of Carbonyl Compounds with Nucleophiles èCarbonyl groups can undergo nucleophilic addition H The nucleophile adds to the d+ carbon H The p electrons shift to the oxygen H The carbon becomes sp 3 hybridized and therefore tetrahedral H Hydride ions and carbanions are two examples of nucleophiles that react with the carbonyl carbon èCarbonyl groups and alcohols can be interconverted by oxidation and reduction reactions H Alcohols can be oxidized to aldehydes; aldehydes can be reduced to alcohols Chapter 12 4
t Oxidation-Reduction Reactions in Organic Chemistry èReduction: increasing the hydrogen content or decreasing the oxygen content of an organic molecule èOxidation: increasing the oxygen content or decreasing the hydrogen content of an organic molecule H A general symbol for oxidation is [O] H Oxidation can also be defined as a reaction that increases the content of any element more electronegative than carbon Chapter 12 5
t Alcohols by Reduction of Carbonyl Compounds èA variety of carbonyl compounds can be reduced to alcohols èCarboxylic acids can be reduced to primary alcohols H These are difficult reductions and require the use of powerful reducing agents such as lithium aluminum hydride (Li. Al. H 4 also abbreviated LAH) Chapter 12 6
èEsters are also reduced to primary alcohols H LAH or high pressure hydrogenation can accomplish this transformation èAldehydes and ketones are reduced to 1 o and 2 o alcohols respectively H Aldehydes and ketones are reduced relatively easily; the mild reducing agent sodium borohydride (Na. BH 4) is typically used H LAH and hydrogenation with a metal catalyst can also be used Chapter 12 7
èThe key step in the reduction is reaction of hydride with the carbonyl carbon èCarboxylic acids and esters are considerably less reactive to reduction than aldehydes and ketones and require the use of LAH èLithium aluminium hydride is very reactive with water and must be used in an anhydrous solvent such as ether H Sodium borohydride is considerably less reactive and can be used in solvents such as water or an alcohol Chapter 12 8
t Oxidation of Alcohols l Oxidation of Primary Alcohols to Aldehydes èA primary alcohol can be oxidized to an aldehyde or a carboxylic acid H The oxidation is difficult to stop at the aldehyde stage and usually proceeds to the carboxylic acid èA reagent which stops the oxidation at the aldehyde stage is pyridinium chlorochromate (PCC) PCC is made from chromium trioxide under acidic conditions H It is used in organic solvents such as methylene chloride (CH 2 Cl 2) H Chapter 12 9
l Oxidation of Primary Alcohols to Carboxylic Acids èPotassium permanganate (KMn. O 4) is a typical reagent used for oxidation of a primary alcohol to a carboxylic acid H The reaction is generally carried out in aqueous solution; a brown precipitate of Mn. O 2 indicates that oxidation has taken place l Oxidation of Secondary Alcohols to Ketones èOxidation of a secondary alcohol stops at the ketone H Many oxidizing agents can be used, including chromic acid (H 2 Cr. O 4) and Jones reagent (Cr. O 3 in acetone) Chapter 12 10
l A Chemical Test for Primary and Secondary Alcohols èChromium oxide in acid has a clear orange color which changes to greenish opaque if an oxidizable alcohol is present l Spectroscopic Evidence for Alcohols èAlcohol O-H infrared stretching absorptions appear as strong, broad peaks around 3200 -3600 cm-1 èAlcohol 1 H NMR signals for hydroxyl protons are often broad; the signal disappears on treatment with D 2 O H The protons on the hydroxyl carbon appear at d 3. 3 to 4. 0 èAlcohol 13 C NMR signals for the hydroxyl carbon appear between d 50 and d 90 Chapter 12 11
Organometallic Compounds Chapter 12
t Organometallic Compounds èCarbon-metal bonds vary widely in character from mostly covalent to mostly ionic depending on the metal èThe greater the ionic character of the bond, the more reactive the compound H Organopotassium compounds react explosively with water and burst into flame when exposed to air Chapter 12 13
t Preparation of Organolithium and Organo-magnesium Compounds l Organolithium Compounds èOrganolithium compounds can be prepared by reaction of an alkyl halide with lithium metal in an ether solvent H The order of reactivity of halides is R-I > R-Br > R-Cl (R-F is seldom used) Chapter 12 14
l Grignard Reagents èGrignard reagents are prepared by the reaction of organic halides with magnesium turnings H An ether solvent is used because it forms a complex with the Grignard reagent which stabilizes it Chapter 12 15
t Reactions of Organolithium and Organo-magnesium Compounds l Reactions with Compounds Containing Acidic Hydrogen Atoms èOrganolithium and Grignard reagents behave as if they were carbanions and they are therefore very strong bases Chapter 12 16
èOrganolithium and Grignard reagents can be used to form alkynides by acid-base reactions Chapter 12 17
l Reactions of Grignard Reagents with Oxiranes (Epoxides) èGrignard reagents are very powerful nucleophiles and can react with the d+ carbons of oxiranes H The reaction results in ring opening and formation of an alcohol product H Reaction occurs at the least-substituted ring carbon of the oxirane H The net result is carbon-carbon bond formation two carbons away from the alcohol Chapter 12 18
l Reaction of Grignard Reagents with Carbonyl Compounds èNucleophilic attack of Grignard reagents at carbonyl carbons is the most important reaction of Grignard reagents H Reaction of Grignard reagents with aldehydes and ketones yields a new carbon-carbon bond an alcohol Chapter 12 19
t Alcohols from Grignard Reagents Chapter 12 20
èEsters react with two molar equivalents of a Grignard reagent to yield a tertiary alcohol Chapter 12 21
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l Planning a Grignard Synthesis èExample : Synthesis of 3 -phenyl-3 -pentanol Chapter 12 23
èSolved Problem: Synthesize the following compound using an alcohol of not more than 4 carbons as the only organic starting material Chapter 12 24
l Restrictions on the Use of Grignard Reagents èGrignard reagents are very powerful nucleophiles and bases H They react as if they were carbanions èGrignard reagents cannot be made from halides which contain acidic groups or electrophilic sites elsewhere in the molecule è substrate for reaction with the Grignard reagent cannot contain any The acidic hydrogen atoms HThe acidic hydrogens will react first and will quench the Grignard reagent HTwo equivalents of Grignard reagent could be used． Chapter 12 25
l The Use of Lithium Reagents èOrganolithium reagents react similarly to Grignard reagents l The Use of Sodium Alkynides èSodium alkynides react with carbonyl compounds such as aldehydes and ketones to form new carbon-carbon bonds Chapter 12 26
l Solved Problem èSynthesize the following compounds using reagents of 6 carbons or less Chapter 12 27
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t Lithium Dialkylcuprates: The Corey-Posner, Whitesides- House Synthesis èThis is an alternative formation of carbon-carbon bonds which, in effect, couples two alkyl halides èOne of the halides is converted to a lithium dialkylcuprate by a two step sequence èTreatment of the lithium dialkylcuprate with the other halide results in coupling of the two organic groups Chapter 12 29
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1: Alcohols from Carbonyl Compounds (Chap 12)12. 4, 12. 5, 12. 9, 12. 10 ※ Due: Sept. 18 （Thr) Chapter 12 31
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