Chapter 18. Reactions of Aldehydes and Ketones

Transcription

1 hapter 18. Reactions of 1 Aldehydes and Ketones Reaction of a nucleophile with an aldehyde or ketone gives an alkoxide, and subsequent hydrolysis leads to an alcohol. This chapter will define differences in nucleophiles that lead to different acyl addition products. ther reactions of the carbonyl group, including extensions of the fundamental acyl addition chemistry will also be discussed.

2 To begin, you should know: 2 The structure and rules of nomenclature for aldehydes and ketones. (chapter 5, section 5.9.B and chapter 16, section 16.2) The fundamental properties of a π-bond. hapter 5, section 5.1) The structure of a carbonyl and that a carbonyl group is polarized. (chapter 5, section 5.9) The concept of a nucleophile. (chapter 6, section 6.7 and chapter 11, sections 11.2, 11.3) The concept of a leaving group. (chapter 11, section 11.2.D) How to name bifunctional molecules containing a carbonyl group and an alkene or alkyne, or two carbonyl groups. (chapter 16, section 16.2.) The structure and reactivity of carbocations (carbenium ions). (chapter 10, section 10.2 and chapter 11, section 11.4) The structure and reactivity of an oxocarbenium ion. (chapter 6, section 6.4.D) The carbonyl of an aldehyde or ketone reacts with protonic acids as Brønsted- Lowry bases to generate an oxocarbenium ion. (chapter 6, section 6.4.D) Understand rotamers in acyclic compounds and conformations of cyclic and cyclic compounds. (chapter 8, sections 8.1, 8.4, 8.5, 8.6, 8.7) Understand mechanism. (chapter 7, section 7.8)

3 To begin, you should know: 3 Understand competing reactions. (chapter 7, sections 7.7, and 7.10) Understand transition states. (chapter 7, sections 7.3, 7.6) The carbonyl of an aldehydes or ketone reacts with Lewis acids as a Lewis base to generate an oxocarbenium ion. (chapter 6, section 6.5) Nucleophiles react with aldehydes or ketones by acyl addition to generate an alkoxide product, which is the product of acyl addition. (chapter 16, section 16.3) Alkoxides react with aqueous acid to give an alcohol. (chapter 6, section 6.4.B) A Grignard reagent is an organomagnesium compound formed by the reaction of an alkyl halide with magnesium metal. (chapter 15, sections 15.1, 15.2) rganolithium reagents are formed by the reaction of alkyl-, aryl-, or vinyl halides and lithium metal. (chapter 15, section 15.5) rganolithium reagents react with alkyl halides to give a new organolithium reagent via metal-halogen exchange. ( chapter 15, section 15.5.A) rganolithium reagents are strong bases and good nucleophiles. (chapter 15, section 15.5) Identify and assign R.S configuration to stereogenic centers. (chapter 9, sections, 9.1, 9.3) Understand diastereomers. (chapter 9, section 9.5)

4 When completed, you should know: 4 Ketones and aldehydes reaction with nucleophiles to give substituted alkoxides by acyl addition to the carbonyl to give alcohols in a two-step process: (1) acyl addition (2) hydrolysis. Nucleophilic acyl addition involves forming a new bond between the nucleophile and the acyl carbon, breaking the π-bond of the carbonyl l c with transfer of those electrons to the oxygen to give an alkoxide. Addition of an acid catalyst leads to an oxocarbenium ion that facilitates acyl addition. Grignard reagents and organolithium reagents react as carbanions with aldehydes and ketones to give alcohols in a two-step process: (1) acyl addition (2) hydrolysis. The organometallic reagent reacts as a nucleophile with epoxides at the less substituted carbon atom. Water adds reversibly to aldehydes and ketones to give an unstable hydrate. Alcohols adds reversibly to aldehydes and ketones to give a transient hemi-acetal or hemi-ketal, which then reacts with more alcohol to give an acetal or a ketal.

5 When completed, you should know: 5 Acetal and ketal formation is reversible. An excess of alcohol drives the reaction towards the acetal or ketal, but an excess of water will convert the acetal or ketal back to the aldehyde or ketone. 1,2-Diols react with aldehydes and ketones to give 1,3-dioxolane derivatives and 1,3-diols react to give 1,3-dioxane derivatives. l c Thiols react with aldehydes and ketones to give dithioacetals or dithioketals. 1,2-Dithiols react with aldehydes and ketones to give 1,3- dithiolane derivatives and 1,3-dithiols reaction to give 1,3-dithiane derivatives. Primary amines react with aldehydes and ketones to give imines. Secondary amines react with ketones and sometimes with aldehydes to give enamines. Hydrazine reacts with aldehydes and ketones to give hydrazones. Hydrazone derivatives react with aldehydes and ketone to give N- substituted hydrazones. Hydroxylamine reacts with aldehydes and ketones to give oximes. Semicarbazone reacts with aldehydes and ketones to give semicarbazides.

6 = is a Base 6 The reaction of a generic carbonyl compound 1 with a Brønsted- Lowry acid (sulfuric acid). The π-bond donates two electrons as a base to the acidic proton to give the conjugate acid, oxocarbenium ion 2, along with the conjugate base (the hydrogen sulfate anion). The protonated oxygen has a formal charge of +1, but it is only one resonance contributor to the oxocarbenium ion. An aldehyde or a ketone reacts as a Brønsted-Lowry base with a Brønsted-Lowry acid (sulfuric acid, Hl, nitric acid, p- toluenesulfonic acid) to form the corresponding oxocarbenium ion. R 1 R 1 H S H R 1 R 2 H HS 4 R 1 R H

7 Nucleophilic Acyl Addition 7 The polarization of = group, where the oxygen is δ and the carbon in δ+, is responsible for the acid-base reactions, and also for the reaction of aldehydes and ketones with nucleophiles. Nucleophilic acyl addition occurs when a nucleophile X donates electrons to the carbonyl carbon (the acyl carbon), breaking the π-bond. The product of this reaction with an aldehyde or ketones is an alkoxide, 4. Several different nucleophiles react with aldehydes or ketones at the acyl carbon to generate an alkoxide product. Hydrolysis of the alkoxide leads to the isolated alcohol product, 5. X: + R R!+!" R R X X cat H + H H 2 R 4 R 5

8 Weak Nucleophiles: chloride ion 8 If acetone (2-propanone, 6) reacts with chloride ion via acyl addition, electron donation to the acyl carbon in the usual manner gives 7. When the reaction is examined for products, only acetone and chloride ion are detected, but no 7 is detected. Either (1) chloride ion did not add to the carbonyl, or, (2) acyl addition occurred but the reaction is reversible and loss of chloride ion from 7 regenerates the starting material. The equilibrium must lie far to the left as the reaction is written (K is small). If 7 forms, the new -l bond is polarized and relatively weak. hloride is recognized as a leaving group. The reverse reaction occurs (from right to left as written) by transfer of excess electron density on the oxygen atom in 7 towards the δ+ carbon to regenerate the =, with expulsion of chloride ion as a leaving group. This means that the alkoxide unit donates electrons to the δ+ carbon to form = with loss of chloride ion. This, of course, is the reverse of the initial acyl addition. Na + :l l Na 6 7

9 Weak Nucleophiles: chloride ion 9 Since the experimental results of this reaction show that acetone and Nal did not give the acyl addition product 7, chloride ion is classified as a weak nucleophile in this reaction. Note that the definition of a weak nucleophile is based on the fact that reaction with an aldehyde or a ketone does not give an isolable acyl addition product. Acyl addition should be reversible only if the newly formed bond in the product is connected to a good leaving group, and such nucleophiles are classified a weak nucleophiles. Na + :l l Na 6 7

10 Strong Nucleophiles: cyanide ion? 10 yanide is classified as a moderate nucleophile. Sodium cyanide is composed of two ions, the sodium cation and the cyanide anion. In 8, both carbon and nitrogen have an unshared pair of electrons. Since both atoms have excess electrons, each may react as a nucleophile. A nucleophile with two nucleophilic centers is called a bidentate nucleophile, but one atom is usually the dominant nucleophilic center. The formal charge on the cyanide ion is 1, and it is calculated to be on the carbon. These criteria suggest that carbon is the better nucleophilic center and, in general, most reactions of cyanide occur at the carbon when NaN or KN are used. nucleophilic carbon nucleophilic nitrogen Na : N: 8

11 Strong Nucleophiles: cyanide ion? 11 In both AgN and un, is more covalent, the nitrogen atom is more nucleophilic and reaction with an alkyl halide R-X leads to a molecule called an isocyanide (or isonitrile, R- + N ). Isocyanides and the reaction of such compounds are not discussed in this book. Both sodium and potassium ions form more ionic bonds, and the NaN and KN bonds are essentially ionic. To be certain that reaction occurs at the acyl carbon, all reactions in this book will use sodium cyanide (NaN) or potassium cyanide (KN) as the reagent. nucleophilic carbon nucleophilic nitrogen Na : N: 8

12 Strong Nucleophiles: cyanide ion? 12 yclopentanone (9) reacts with potassium cyanide to give a low yield of 10, consistent with cyanide as a weak nucleophile. ompounds that have both a N unit and an H unit are called cyanohydrins. The direct acyl addition of cyanide to 9 gives a poor yield of 10, suggesting that the reaction may be reversible or that cyanide is a relatively poor electron donor (weak nucleophile), or both. n the other hand, 10 is formed, so cyanide must have some ability to react as a nucleophile, which means that cyanide is a stronger nucleophile than chloride ion. Note: N is weak nucleophile with =; strong with -Br K + : N:!N K 9 10

13 Strong Nucleophiles: cyanide ion? 9 is treated with KN and sulfuric acid at 0 to give 1- hydroxycyclopentane-carbonitrile, 11, isolated in 96% yield. It is known that a carbonyl reacts with an acid to give an oxocarbenium ion (see 12). nce formed, it is quite reasonable that an oxocarbenium ion will react with cyanide ion to give H 2 ; H 2 S 4 + KN 0, overnight 96% H!N 9 11 K H S 3 H : N: 9 H 12 H 11!N

14 Strong Nucleophiles: cyanide ion? 14 yanide ion is a moderately strong nucleophile and acyl addition is facilitated by the presence of an acid catalyst. K + : N:!N K 9 10 Low Yield H 2 ; H 2 S 4 + KN 0, overnight 96% H!N 9 11 Good Yield

15 Strong Nucleophiles: alkyne anions 15 The hydrogen atom of a terminal alkyne (marked in red in 13) ) is weakly acidic, and it reacts with a suitable base to generate a conjugate base (the alkyne anion, 14). Alkyne anions are classified as strong nucleophiles. A terminal alkyne (i.e., - -H) is a carbon-acid. An alkyne anion is a carbanion (a carbon nucleophile). Alkyne anions are strong nucleophiles in acyl addition reactions, generating a carbon-carbon bond irreversibly. 13 H Na:NH 2 : Na + H:NH 2 14

16 Strong Nucleophiles: alkyne anions 16 A specialized base called sodium hydride (NaH) is sometimes used for reaction of alkynes since the conjugate acid is hydrogen gas. An aprotic organic solvent like diethyl ether can be used. This is a particularly useful reagent since the byproduct (H 2 ) escapes from the medium, driving the reaction to the right. H 15 NaH ether : Na H

17 Strong Nucleophiles: alkyne anions 17 Sodium acetylide (17) reacts with 2-butanone (19) to give 20 via acyl addition. An 86% yield of 21 is obtained after a second chemical step that treats 20 with aqueous acid (known as an acidic workup). Examination of 20 reveals that a strong - bond is formed by acyl addition, and the irreversibility of the reaction suggests that the alkyne unit is a rather poor leaving group. 20 is an alkoxide, and the conjugate base of an alcohol, treatment with dilute aqueous acid gives the alcohol, 21. H : Na 17 H H Et Et H H Et + H 2

18 Strong Nucleophiles: alkyne anions 18 Alkyne alcohol 21 contains a stereogenic carbon (shown in green) but the starting materials 2- butanone and propyne do not contain a stereogenic carbon. The acyl addition reaction created a stereogenic center, but the experiment shows that alkyne-alcohol 21 is racemic. Et 1. : Na H 3 + H Et 86%

19 Strong Nucleophiles: alkyne anions 19 If 17 approaches 19 from the "top" (path a), the oxygen is pushed "down" and one enantiomer is formed (S-21). If 17 approaches from the "bottom (path b), however, the oxygen is pushed to the "top" and the opposite enantiomer is formed (R-21). Since there is nothing to bias one side from the other as 17 approaches 19, both enantiomers are formed in equal amounts: a racemic mixture. Nucleophilic acyl addition to aldehydes or ketones is assumed to generate racemic alcohol products in the absence of any additional information. : : a b a b (S) Na + S-21 (R) R-21 Na +