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Overview of the Reactions of Carbonyl Compounds

Overview of the Reactions of Carbonyl Compounds. Topical Outline of Coverage I. Kinds of Carbonyl Compounds. II.Polarity of the Carbonyl Functional Group. III.General Reactions of Carbonyl Compounds A. Nucleophilic Addition Reactions B. Nucleophilic Substitution Reactions.

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Overview of the Reactions of Carbonyl Compounds

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  1. Overview of the Reactions of Carbonyl Compounds • Topical Outline of Coverage • I.Kinds of Carbonyl Compounds. • II.Polarity of the Carbonyl Functional Group. • III.General Reactions of Carbonyl Compounds • A.Nucleophilic Addition Reactions • B.Nucleophilic Substitution Reactions

  2. Kinds of Carbonyl Compounds • All carbonyl compounds contain the acyl group • where the (R) residue bonded to the carbonyl maybe alkyl, aryl, alkenyl, or alkynyl. The different kinds of carbonyl compounds arise from the nature of the other residue bonded to the carbonyl group. X X

  3. Kinds of Carbonyl Compounds

  4. Categories of Carbonyl Compounds • Carbonyl Compounds may be grouped into two broad categories based upon whether or not they take part in NucleophilicSubstitution Reactions

  5. Aldehydes and Ketones X R X • Aldehydes and Ketones - X = H and R respectively ; these carbonyl compounds do not undergonucleophilic substitution reactions. That is to say, the H and R groups are never substituted by other groups. Both H- and R- make poor leaving groups.

  6. Carboxylic Acids and their Derivatives X • Carboxylic acids and their derivatives – X = some heteroatom (O, Cl, or N). Nucleophilic substitution reactions are possible for these carbonyl compounds because the electronegative heteroatom can stabilize a negative charge and form good Leaving Groups.

  7. Polarity of the Carbonyl Groups • The carbon-oxygen double bond of the carbonyl group is extremely polarized in the direction of the highly electronegative oxygen. This polarization is responsible for the characteristic reactions of carbonyl compounds - +

  8. General Reactions Of Carbonyl Compounds • Nucleophilic Addition Reactions • Nucleophilic Acyl Substitution

  9. Nucleophilic Addition Reactions – Chapter 09 • There are two different ways in which a nucleophile can add to a carbonyl compound. Each way leads to a different nucleophilic addition reaction but the mechanisms for both reactions involves the same 1st step. • In this step, the nucleophile bonds to the carbonyl carbon and thereby causes a carbon-oxygen bond to break. The carbonyl carbon rehybridizes from sp2 to sp3 and the carbonyl oxygen becomes negatively charged. At this point the tetrahedral intermediate can either be protonated to form an alcohol (NaBH4, LiAlH4, or Grignard Reduction) or a non-bonded e- pair on the nucleophile can be used to form a second bond to the carbonyl carbon. The new bond formation causes expulsion of the carbonyl oxygen as H2O.

  10. First Type of Nucleophilic Addition • Alcohol Formation – Ketones and Aldehydes react with NaBH4, LiAlH4, and Grignard reagents to form alcohols

  11. Second Type of Nucleophilic Addition • Imine formation - Ketones and Aldehydes react with 1o amines to form imines .

  12. Nucleophilic Acyl Substitution – • Theses reactions do not apply to aldehydes and ketones. These reactions involve the substitution of the nucleophile for the X residue of the carbonyl compound.

  13. Nucleophilic Acyl Substitution

  14. Carboxylic Acid Derivatives

  15. Carboxylic Acid Derivatives • These all have an acyl group bonded to Y, an electronegative atom or leaving group • Includes: Y = halide (acid halides), acyloxy (anhydrides), alkoxy (esters), amine (amides).

  16. General Reaction Pattern • Nucleophilic acyl substitution

  17. Nucleophilic Acyl Substitution-The Mechanism • Carboxylic acid derivatives have an acyl carbon bonded to an electronegative group Y that can leave • A tetrahedral intermediate is formed, then the leaving group is expelled to generate a new carbonyl compound, leading to substitution

  18. Found in Nature Substitution in Synthesis • We can readily convert a more reactive acid derivative into a less reactive one • Reactions in the opposite sense are possible but require more complex approaches

  19. Reactions of Acid Halides • Nucleophilic acyl substitution • Halogen replaced by OH, by OR, or by NH2 • Reduction yields a primary alcohol • Grignard reagent yields a tertiary alcohol

  20. Reactions of Acid Anhydrides • Similar to acid chlorides in reactivity

  21. Reactions of Esters • Less reactive toward nucleophiles than are acid chlorides or anhydrides • Cyclic esters are called lactones and react similarly to acyclic esters

  22. Chapter 09. Aldehydes and Ketones: Nucleophilic Addition Reactions

  23. Aldehydes • Aldehydes are carbonyl compounds having at least one hydrogen attached to the carbonyl carbon.

  24. Ketones • Ketones are carbonyl compounds having two alkyl fragments attached to the carbonyl carbon.

  25. Naming Aldehydes and Ketones • Aldehydes are named by replacing the terminal -e of the corresponding alkane name with –al • The parent chain must contain the CHO group • The CHO carbon is numbered as C1 • If the CHO group is attached to a ring, use the suffix carbaldehyde

  26. Names of more Complex Aldehydes

  27. Naming Ketones • Replace the terminal -e of the alkane name with –one • Parent chain is the longest one that contains the ketone group • Numbering begins at the end nearer the carbonyl carbon

  28. Ketones with Common Names • IUPAC retains well-used but unsystematic names for a few ketones

  29. Preparation of Aldehydes and Ketones • Preparing Aldehydes • We have already discussed two of the best methods of aldehyde synthesis. These are oxidation of primary alcohols, and oxidative cleavage of alkenes. Oxidize primary alcohols using pyridinium chlorochromate

  30. Preparing Ketones • Ketones may be prepared by the oxidation of secondary alcohols. A wide range of oxidizing can accomplish this purpose. Some of these are: Jones reagent (CrO3 in aqueous sulfuric acid), sodium chromate (Na2CrO4) and potassium permanganate (KMnO4).

  31. Prep. Of Ketones by Ozonolysis of Alkenes • Ozonolysis of alkenes yields ketones if one of the doubly bonded carbons is itself bonded to two alkyl groups.

  32. Prep. Of Ketones by Hydration of Terminal Alkynes • Methyl ketones can be prepared by the Markovnikov addition of water to a terminal alkyne. The reaction needs to be catalyzed by Hg+2 ion. See Section 4.13 of text.

  33. Aryl Ketones by Acylation • Friedel–Crafts acylation of an aromatic ring with an acid chloride in the presence of AlCl3 catalyst (see Section 5.6)

  34. Oxidation of Aldehydes and Ketones • Aldehydes are readily oxidized to carboxylic acid but ketones are unreactive towards oxidation except under the most vigorous conditions. This difference in reactivity towards oxidation lies in the structural difference between the two types of carbonyl compounds. Aldehydes are more easily oxidized because they posses a hydrogen atom bonded to the carbonyl carbon. This hydrogen atom can be removed as a proton with the final result being the oxidation (loss of hydrogen) from the original aldehyde. Ketones have no expendable carbonyl-hydrogen bond.

  35. Oxidation of Aldehydes and Ketones • Many oxidizing agents will convert aldehydes to carboxylic acids. Some of these are Jones reagent, hot nitric acid and KMnO4. • One drawback to the Jones reagent is that it is acidic. Many sensitive aldehydes would undergo acid - catalyzed decomposition before oxidation if Jones reagent was used

  36. A Milder Oxidizing Agent • For acid sensitive molecules a milder oxidizing agent such as the silver ion (Ag+) may be used. A dilute ammonia solution of silver oxide, Ag2O, (Tollens reagent) oxidizes aldehydes in high yield without harming carbon-carbon double bonds or other functional groups.

  37. Tollens Oxidation • Note; In this reaction the oxidizing agent is Ag+ and it is ultimately reduced to Ag(s). • A shiny mirror of metallic silver is deposited on the inside walls of the flask during a Tollens oxidation: observation of such a mirror forms the basis of an old qualitative test for the presence of an aldehyde functional group in a molecule of unknown structure.

  38. Nucleophilic Addition Reactions of Aldehydes and Ketones • Nu- approaches 45° to the plane of C=O and adds to the Carbonyl Carbon • A tetrahedral alkoxide ion intermediate is produced and ultimately protonated

  39. Nucleophiles • Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu-H) • If neutral, the nucleophile usually carries a hydrogen atom that can subsequently be eliminated and carry away the positive charge.

  40. Relative Reactivity of Aldehydes and Ketones • Aldehydes are generally much more reactive than ketones. There are two reasons for this; • Aldehydes are less sterically hindered than ketones. In other words the carbonyl carbon of aldehydes is more accessibly to attack. The presence of two relatively large substituents in ketone hinders the attacking nucleophile from reaching the carbonyl carbon. • The + on the carbonyl carbon is reduced in ketones because of the ability of the extra alkyl group to stabilize a + charge. This ability is emphasized in the stability order of carbocations.3o>2o>1o

  41. Aldehydes Have A Greater Electrophilicity Than Do Ketones • Aldehyde C=O is more polarized than ketone C=O • As in carbocations, more alkyl groups stabilize + character • Ketone has more alkyl groups, stabilizing the C=O carbon inductively

  42. Addition of H-Y to C=O • Reaction of C=O with H-Y, where Y is electronegative, gives an addition product (“adduct”) and the reaction is readily reversible because the electronegative Y is a good leaving group.

  43. Nucleophilic Addition of Alcohols: Acetal Formation • Two equivalents of ROH in the presence of an acid catalyst add to C=O to yield acetals, R2C(OR)2 • Alcohols, ROH, fall under the category of Y-H and therefore the reaction is reversible.

  44. Mechanism for Formation of Acetals

  45. Uses of Acetals • Acetals can serve as protecting groups for aldehydes and ketones-remember the rxn. is reversible. • It is convenient to use a diol, to form a cyclic acetal (the reaction goes even more readily)

  46. Nucleophilic Addition of Grignard Reagents and Hydride Reagents: Alcohol Formation • Treatment of aldehydes or ketones with Grignard reagents yields an alcohol • Nucleophilic addition of the equivalent of a carbon anion, or carbanion. A carbon–magnesium bond is strongly polarized in the direction of the carbon atom, so a Grignard reagent reacts for all practical purposes as R: and MgX +.

  47. Mechanism of Addition of Grignard Reagents • R- attacks the carbonyl carbon. The alkoxide anion is then protonated by dilute acid. • Grignard additions are irreversible because a carbanion is not a leaving group

  48. Hydride Addition • H- attacks the carbonyl carbon. The alkoxide anion is then protonated by dilute acid. • Hydride additions are irreversible because a hydride is not a good leaving group • LiAlH4 and NaBH4 react as donors of hydride ion (H-)

  49. Nucleophilic Addition of Amines: Imine Formation Primary amines (RNH2) add to C=O to form imines, R2C=NR (after loss of HOH)

  50. Mechanism of Imine Formation

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