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Chapter 19

Aldehydes and Ketones - Lecture and Textbook Notes Chapter 19

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Cecilia Kutas

CHM247H1 Jasmyn Lee Preview of Carbonyl Chemistry The Occurrence of the Carbonyl Group Compounds in Nature  Sugars  D-aldohexoses C=O Chemistry is Central to Biochemistry  General Principles of Reactivity: o Nucleophiles attack electrophiles o π bonds are more easily broken than σ bonds o Good leaving groups leave, taking their electrons with them I. Kinds of Carbonyl Compounds Types of Carbonyl Compounds 1. Only H or C attached to C=O o H or C cant stabilize a negative charge – cant act as a leaving group in a nucleophile substitution reaction 2. An electronegative atom attached to C=O o Electronegative atom can stabilize a negative charge – can act as a leaving group in nucleophilic substitution reaction II. Nature of the Carbonyl Group  Carbon-oxygen double bond of a carbonyl is similar to carbon-carbon double bond of an alkene 1 CHM247H1 Jasmyn Lee  Carbonyl carbon o sp hybridized o Forms 3 σ bonds o Fourth valence electron remains in p orbital and forms a π bond by overlap with oxygen p orbital  Oxygen atom has two non-bonding pairs of electrons, which occupy its remaining two orbitals  Carbonyl compounds are planar about the double bond and have bond angles of ~120° Carbon-oxygen double bond is strongly polarized because of the high electronegativity  Carbonyl carbon is partially positive, electrophilic (Lewis acidic) site – reacts with nucleophiles  Carbonyl oxygen carries partial negative charge, nucleophilic (Lewis basic) site – reacts with electrophiles III. General Reactions of Carbonyl Compounds  Nucleophilic Addition Reactions of Aldehydes and Ketones (Ch 19)  Nucleophilic Acyl Substitution Reactions of Carboxylic Acid Derivatives (Ch 21)  Alpha Substitution Reactions (Ch 22)  Carbonyl Condensation Reactions (Ch 23) 2 CHM247H1 Jasmyn Lee Chapter 19: Aldehydes and Ketones 19.1 Naming Aldehydes and Ketones Nomenclature  Use “al” or “one” as suffix if naming as aldehyde or ketone  Aldehyde is the highest priority (always at C1)  Cyclic aldehydes in which –CHO group is directly attached to ring – use suffix –carbaldehyde  When C=O is a substituent, it is called “oxo”  When R-C=O is a substituent, the name acyl group is used – name ending –yl is attached o Eg/ -COCH 3cyl, -CHO formyl, -COAr aroyl, -CO6 5 benzoyl 19.14 Spectroscopy of Aldehydes and Ketones Infrared Spectroscopy -1  C=O Stretch around 1700 cm (1660 – 1770) o Aldehyde band ~1730 cm -1 o Ketone band ~1715 cm -1 o Specific environment shifts position of band; [conjugated benzene ring lowers it by ~30cm ] 3 CHM247H1 Jasmyn Lee Nuclear Magnetic Resonance Spectroscopy  1H NMR – absorb near ~10δ o Characteristic aldehyde 13  C NMR o C=O signal of aldehydes and ketones ~200 δ 4 CHM247H1 Jasmyn Lee 19.2 Preparing Aldehydes and Ketones Preparation of Aldehydes 1. Oxidation of Primary Alcohols: 2. Ozonolysis of Alkenes (Reductive Workup): 3. Hydration of Terminal Alkynes: 4. Reduction o A terminal Carbon more oxidized than aldehyde can be reduced using a selective, bulky reducing agent o To stop an aldehyde, the reagent had only one “H-“ to contribute, and made sluggish by large substituents a. Partial Reduction of Esters b. Partial Reduction of Nitriles c. Partial Reduction of Acid Halides 5 CHM247H1 Jasmyn Lee Preparation of Ketones 1. Oxidation of Secondary Alcohols 2. Ozonolysis of Alkenes 3. Hydration of Alkynes: 4. Friedel-Crafts Acylation Reaction 5. Organometallic Reagents a. Grignard Addition to Nitrile b. Gilman Addition to Acid Chloride 6 CHM247H1 Jasmyn Lee Reactions of Aldehydes and Ketones 19.3 Oxidation of Aldehydes and Ketones 1. Oxidation o Aldehydes have a –CHO proton that can be abstracted during oxidation – ketones do not o Aldehyde oxidations occur through intermediate 1,1-diols or hydrates which are formed by a reversible nucleophilic addition of water to the carbonyl group  Hydrate reacts like any typical primary or secondary alcohol and is oxidized to a carbonyl compound o (Ketones are inert to most oxidizing agents but undergo a slow cleavage reaction of the C-C bond next to the carbonyl group when treated wit4 hot alkaline KMnO ) 2. Reduction 19.4 Nucleophilic Addition Reactions to Aldehydes and Ketones  Two General Pathways 1. Pathway leading to an alcohol product o Example:4BH Reduction (in water) H _ O H O O H H O H B H C C C H R H R H H R H H 7 CHM247H1 Jasmyn Lee 2. Pathway leading to a C=Nu double bond  Aldehydes are generally more reactive than ketones o Sterically – the presence of only one large substituent bonded to the C=O carbon versus two large substituents in a ketone – nucleophile is able to approach an aldehyde more readily  Transition state leading to the tetrahedral intermediate is less crowded and lower in energy for an aldehyde than for a ketone o Electronically – carbonyl groups in aldehydes have greater polarization than in ketones  Aldehyde has one alkyl group inductively stabilizing the partial positive charge on the carbonyl; ketone has two alkyl groups inductively stabilizing partial positive charge 19.5 Nucleophilic Addition 2f H O: Hydration  Aldehydes and ketones react w2th H O to yield 1,1-diols or geminal (gem) diols o Reaction is reversible  Base-Catalyzed Hydration o Takes place rapidly because water is converted into hydroxide ion – a much better nucleophile  Acid-catalyzed hydration can also occur – by the same mechanism as for acetal formation (refer to text) o Takes place rapidly because the carbonyl compounds is converted by protonation into a much better electrophile O K HO OH + H2O C C R R' R R'  Typical of a reaction with H-Y (Y = electronegative atom that can stabilize negative charge)  Reversible nucleophilic addition favoring carbonyl reactant rather than tetrahedral addition product o Treatment of an aldehyde or ketone w3th CH2OH, H O, HCl2 H4r or H SO does not normally lead to a stable alcohol addition product 8 CHM247H1 Jasmyn Lee 19.6 Nucleophilic Addition of HCN: Cyanohydrin Formation  Aldehydes and unhindered ketones react with HCN to yield cyanohydrins, RCH(OH)C=N  Mechanism of Formation of Cyanohydrins o Addition of HCN is reversible and base-catalyzed, generating nucleophilic cyanide ion, C=N o Addition of C=N to C=O yields a tetrahedral intermediate o Which is then protonated o Equilibrium favors the adduct  Reactions of Cyanohydrins 9 CHM247H1 Jasmyn Lee  Practice: Determine and Draw Structures A-G + O H dil. chromic acid G E F 1. CH3MgI 2. 3 O
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