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Chem 267: Chapter 22 Textbook Exam Review Notes

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CHEM 267
Monica Barra

Chem 267 Chapter 22 Notes Winter 2012 22.1 Introduction to Alpha Carbon Chemistry: Enols and Enolates  The C positions (indicated by Greek letters) are relative to the position of the carbonyl group Enols  In the presence of a catalytic acid or base, a ketone will exist in equilibrium with an enol o Ketone is favoured at equilibrium o NOTE: tautomers are NOT resonance structures! (different arrangement of atoms)  HOWEVER: in some cases, the enol tautomer is more stabilized than keto (2 reasons): 1. The enol has a conjugated π system 2. The enol can form an intramolecular H-bonding interaction between the hydroxyl proton and the nearby carbonyl group (shown with the dotted line)  NOTE: The enol tautomer is very reactive o The α position is very nucleophilic due to resonance Enolates  When treated with a strong base, the α position of a ketone is deprotonated to give a resonance- stabilized intermediate called an enolate  Enolates = “ambident nucleophiles” (they possess 2 nucleophilic sites)  NOTE: C-attack is more common than O-attack! (even though O has the lone pairs)  NOTE: Enolates are more reactive than enols (b/c they possess a full negative charge)  NOTE: only the α protons of an aldehyde/ketone are acidic (lead to a resonance stabilized anion) Choosing a Base for Enolate Formation  When an alkoxy ion is used as the base, an equilibrium is established in which the alkoxide ion and the enolate ion are BOTH present (usually less enolate ions present at equilibrium)  NOTE: Since the enolate is a nucleophile and the aldehyde is an electrophile, the 2 species will react with each other when they are both present  other bases, like sodium hydride (NaH), can irreversibly convert the aldehyde into an enolate o when NaH is used as the base, hydrogen gas (H2) is formed and bubbles out of solution o THEREFORE: the aldehyde and enolate are NOT both present (only the enolate)  Another base that can be used for irreversible enolate formation is LDA 1  In a compound with 2 carbonyl groups that are β to each other, the H’s at the α-position are very acidic (has a pKa of ~9 compared to the pKa of 10-20 for ketones) o This is b/c the deprotonated compound with 2 carbonyl groups is resonance stabilized  NOTE: due to the high acidity of beta diketones, LDA or NaH is NOT required to irreversibly deprotonate these compounds! (hydroxide/alkoxide ions also go to complete enolate formation)  NOTE: the large pKa difference indicates that the reaction will go to complete enolate formation 22.2 Alpha Halogenation of Enols and Enolates  Ketones/aldehydes undergo halogenations at the α position under acid-catalyzed conditions o NOTE: reaction works with chlorine (Cl), bromine (Br), and iodine (I), but NOT fluorine (F)  Mechanism has 2 main steps: 1. Enol formation = ketone undergoes tautomerization to produce an enol (this is the RDS!) 2. Halogenation = enol serves as a nucleophile and the double bond attacks the halogen a. The halogen adds to the α position  NOTE: when an unsymmetrical ketone is used, bromination occurs primarily at the more substituted side of the ketone o This is b/c the reaction proceeds faster through the more substituted enol (in step 1)  The halogenated product can react further and undergo elimination when treated with a base Alpha Halogenation in Basic Conditions: The Haloform Reaction  Ketones/aldehydes also undergo halogenations at the α position under basic conditions  NOTE: when more than one α proton is present, it is difficult to achieve monobromination b/c the brominated product is more reactive and rapidly undergoes further bromination (polybromination occurs)  NOTE: when a methyl ketone is treated with excess base and excess halogen, a reaction occurs in which a carboxylic acid is produced after acidic workup  After polybromination, the tribromomethyl group can function as a leaving group, resulting in a nucleophilic acyl substitution reaction (where OH attacks the electrophilic carbon)  NOTE: Even though carbanions almost never act as leaving groups, the negative charge on the carbon is stabilized in this case by the electron-withdrawing effects of the 3 bromine atoms  the resulting carboxylic acid is then deprotonated, producing a carboxylate ion and C3Br (bromoform) ← therefore, called a haloform reaction (can also occur with Cl and I) 2  in order to complete the reaction, it is treated with a proton source (H3O+) to protonate the carboxylate ion and form the carboxylic acid o useful for converting methyl ketones into carboxylic acids! 22.3 Aldol Reactions Aldol Additions  When an aldehyde is treated with sodium hydroxide (NaOH), both the aldehyde and the enolate will be present at equilibrium (therefore, these 2 species can react with each other!) o Called an aldol (contains an aldehydic group and a hydroxyl group) o NOTE: the hydroxyl group is specifically located at the β position relative to the carbonyl group (the product of an aldol addition reaction is always a β-hydroxy aldehyde/ketone)  Since both the aldehyde and enolate are present at equilibrium, once the aldehyde is deprotonated, the resulting enolate attacks another aldehyde  For most simple aldehydes, equilibrium favours the aldol product  For most ketones, the aldol product is NOT favoured (poor yield) o The reverse process (aldol converted back into ketone) is called a retro-aldol reaction o NOTE: The steps of a retro-aldol reaction are exactly the opposite of an aldol addition! Aldol Condensations  When heated in acidic or basic conditions, the product of an aldol addition reaction will undergo elimination to produce unsaturation between the α and β positions Normally, alcohols do NOT undergo dehydration in the presence of a strong base, but the presence of the carbonyl group enables the dehydration reaction to occur  The elimination reaction is a 2-step process called an E1cb mechanism o The leaving group only leaves AFTER deprotonation occurs (unlike an E1 reaction where the leaving group leaves, then deprotonation occurs)  An aldol condensation reaction results in the formation of a highly conjugated π system  NOTE: the yields for condensation reactions are often greater those of addition reactions Crossed Aldol Reactions  Crossed/mixed aldol reactions = aldol reactions that occur between different partners 3  DISADVANTAGE: a mixture of products will result o there are several ways to prevent a mixture of products: 1. If one of the aldehydes lacks α protons and possesses an unhindered carbonyl group a. Without any α protons, the aldehyde cannot form an enolate 2. Directed aldol addition = Crossed aldol reactions are performed using LDA as a base a. Since LDA causes irreversible enolate formation (will not have any aldehyde in solution; therefore,
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