Carbonyls (Ch ketones and aldehydes and carboxylic acids derivatives)
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1 arbonyls (h ketones and aldehydes and carboxylic acids derivatives) +δ -δ ' - sp 2 - trigonal planar (120 0 ) - strongly polarized double bond eactivity? addition nucleophilic 1
2 Nucleophilic Addition Nu Nu ' ' - hange of hybridation - Adding complexity to the molecular structure - onverting a carbonyl to alcohol 2
3 Nucleophilic Addition-Elimination at Acyl arbon Aldehydes and ketones undergo nucleophilic additions to the carbonyl group ' ' + Nu Nu Tetrahedral intermediate Nu- ' -- + Nu - Nu A typical reaction of carboxylic acids and their derivatives is nucleophilic addition-elimination. The first step is nucleophilic addition to the carbonyl to give a tetrahedral intermediate, but the presence of a good leaving group (X) at this site results in an elimination that regenerates the trigonal carbonyl. X Nucleophilic addition + Nu - X - - Nu Elimination Nu + X - Tetrahedral intermediate This reaction mechanism is employed in many biological systems, and biochemists call them acyl transfer reactions.
4 - Example ydrolysis of an Acyl hloride --l Nucleophilic addition + - Nucleophile --l Tetrahedral intermediate --l + - Elimination -l This reaction proceeds well because of the great reactivity of the acyl chloride towards nucleophilic addition and the good leaving group ability of l - in the cleavage step.
5 Important Factors in the Addition-Elimination eaction eactivity of the Acyl arbon Electronic influences that increase the electropositive character of the acyl carbon enhance the rate of nucleophilic addition. Nu - δ+ δ --X Stability of the Leaving Group As the stability of X - increases, it becomes a better leaving group. As the electronegativity of X increases, the rate of nucleophilic addition increases. - --X Nu --Nu + X -
6 Acid atalysis Acid catalysis is important in both the addition and elimination steps. Protonation of the carbonyl --X + -Nu X + Nu - Provides electrophilic catalysis + - of addition --X + -Nu Protonation of leaving group - --X Nu + -Nu Deprotonation - --Nu X- Nu - --X Nu- + deprotonation Nu X Nu Tetrahedral intermediate Electrophilic catalysis of elimination Nu + Nu - --Nu + -Nu Acyl additionelimination product + -X + -Nu Protonated (better) leaving group
7 The Mechanism of Acid-atalyzed Esterification 6 5 AID Protonation Addition ESTE Deprotonation Elimination 6 5 proton transfer 3
8 Quiz Acid catalysis is important in nucleophilic addition-elimination at acyl carbon. Sketch the general mechanism for this reaction, indicating where the catalysis occurs. X + -Nu Nu + -X
9 Protonation of carbonyl group + - X + -Nu X + - X enhanced reactivity towards nucleophiles addition step - + Nu - X Nu tetrahedral intermediate + Nu - acid catalysis Protonation of leaving group - X + -Nu Nu cleavage step X- Nu Nu better leaving Nu - group X- Nu + -X Nu - + Nu + -Nu acid catalysis
10 Tutorial on arrow notation and mechanisms 10
11 Back to ketones and aldehydes now! 11
12 Mechanism of reduction - 3 B- ' ' ' ' ' Ease of reduction 12
13 arbon (formal) oxidation states () +3 -()
14 From the qualitative point of view xidation eduction Gaining (increase ox state) Loosing (decrease ox #) Loosing Gaining 2 3 reduction 2 () oxidation 14
15 Aldehyde alcohol 2 LiAl 4 NaB 4 r Mn - 4 s 4 e 4+ 15
16 -l ydride Transfer Mechanism Li + ( 3 ) - 3 -Al-( 3 ) 3 ( 3 ) 3 - is a nucleophile - -l A specific example l (1) LiAl(-t-Bu) 3, -78 o (2) 2 - -l + l - l - is a good leaving group Note selective reduction of acyl chloride in presence of nitro group. N 2 p-nitrobenzoyl chloride N 2 p-nitrobenzaldehyde
17 eduction of Esters with DIBAL- Esters are selectively reduced to aldehydes with DIBAL- when one equivalent of the reducing agent is used at low temperature. ' Ester (1) 1.0 eq (i-bu) 2 Al, -78 o (2) 2 Aldehyde A Proposed Mechanism DIBAL- is a trivalent aluminum species and is coordinatively unsaturated (i.e., a Lewis acid). The aluminum complexes with a nonbonding electron pair of the carbonyl oxygen i-bu Al - i-bu -' i-bu i-bu + - Al - -' A hydridoaluminate (Lewis complex)
18 ydride Transfer Nucleophilic Addition to the arbonyl i-bu i-bu + - Al - -' hydride transfer reduction -- -' Al(i-Bu) Al(i-Bu) Aldehyde At higher temperature and with excess reagent, the aldehyde products are reduced to primary alcohols.
19 5. Using rganometallic compounds (hemistry of carbanions) -δ +δ M M M M Na, K Mg, Li Pb, Sn, g, Tl explosive Versatile and useful strong base (-abstraction), strong nucleophiles 19
20 Preparation Br + 2 Li Li + LiBr Ethers are the preferred solvents but ethers are slowly attacked and decomposed 20
21 Grignard reagents the most popular Mg X -Mg-X - possible with a great variety of - ethers as solvent are crucial to the success of the reaction Mg X 2 Mg + MgX2 21
22 Grignard reagents the most popular Mg X -Mg-X - possible with a great variety of - ethers as solvent are crucial to the success of the reaction Mg X 2 Mg + MgX2 22
23 -d +d - M [M Mg, Li] The strong polarity of the M- bond and the consequent carbanionic character determines the reactivity 1) superstrong base MgX Mg X MgX --MgX ()- ()-MgX 23
24 Nucleophilic Addition (prep. of ) -MgX MgX ' ' ' Use acid hydrolysis to avoid precipitation of Mg() 2 24
25 Formation of alcohols -MgX Formaldehyde 1 0 -MgX Aldehyde 2 0 -MgX ketone 3 0 ' ' 25
26 eaction with both esters and acyl halides -MgX 3 0 ' -MgX Two identical groups 26
27 Planning a Grignard Synthesis We can synthesize ANY alcohol Et Et PhMgBr Et Et EtMgBr Ph 2 EtMgBr Et Ph Ph 27
28 estrictions Incompatible with any function that can be deprotonated -, -N 2, -N, - 2, -S 3, -S, --, aldehyde, ketone, esters, amide -N 2, -N, epoxide Grignard carbonatation to lenght the chain by one atom -MgX + esterification - 3 LiAl 4 28
29 29
30 30
31 31
32 First General Mechanism ' top δ+ Nu - δ- bottom sp 2 -hybridized carbon trigonal planar ' Nu - (and enantiomer) sp 3 -hybridized carbon tetrahedral E + ' -E Nu Addition product (racemic) sp 3 -hybridized carbon tetrahedral In these additions, E + is often a proton or metal ion.
33 Second General Mechanism ' δ+ δ- + -A acid catalyst Fast and reversible protonation eversible addition (and other enantiomer) ' + Nu Nu A Deprotonation ' + - Nu ' Addition product (racemic) ' + + A Protonated carbonyl (an oxonium cation) is more reactive towards nucleophilic addition + -A Acid catayst is regenerated -
34 elative eactivity Aldehydes versus Ketones Generally, aldehydes are more reactive than ketones because of both steric and electronic factors. Steric Factors When a nucleophile adds to the carbonyl group, the reaction center changes from trigonal planar (sp 2 ) to tetrahedral (sp 3 ) there is a closing of the angle between and '() groups from ~120 o to ~109 o. ()' ~120 o trigonal planar + Nu - ()' ~109 o tetrahedral With ketones, where and ' are alkyl or aryl groups, there is more buildup of steric strain as the groups move closer together than there is with aldehydes, where at least one of the groups being squeezed together is the very small hydrogen atom. - Nu
35 Electronic Factors A ketone carbonyl is stabilized relative to that of an aldehyde by the presence of two electron-releasing alkyl groups. δ δ Less stable, so More stable, more reactive δ+ δ+ so less reactive ' This diminishing of carbon charge in ketones contributes to their lower reactivity compared with that of aldehydes. Alkyl ketones are generally more reactive than aryl ketones because of greater electronic stabilization of the starting state of the latter by π- electron delocalization. δ + - etc. δ+ SUMMAY Any reduction in the partial positive charge on the carbon decreases its reactivity in nucleophilic addition. Any increase, e.g., by the presence of a strongly electron withdrawing group like -F 3, increases reactivity.
36 Addition eactions of Aldehydes and Ketones Addition of Water and Alcohols ydrates and Acetals ydrates gem-diols When aldehydes or ketones are dissolved in aqueous media, an equilibrium is established between the carbonyl compound and its hydrate. The hydrate is a geminal diol ("gem-diol"). ' + 2 K ' The position of equilibrium depends on the size and electronic effects of the groups. For most ketones, K' K[ 2 ] << 1. Most ketones exist essentially 100% in the carbonyl form. Aldehydes hydrate somewhat more extensively. Bulky and/or electron-donating groups stabilize the carbonyl compound in the above equilibrium. Electron-withdrawing groups destabilize the carbonyl compound and promote formation of the hydrate. These factors are illustrated in the table that follows.
37 Mechanism for ydrate Formation + - no acid catalysis water is the nucleophile rapid deprotonation and reprotonation yields the hydrate The rate of hydration is much faster under basic (higher p) or acidic (lower p) conditions than at p 7.
38 Additions of Alcohols emiacetals and Acetals Alcohols (-) add to carbonyl compounds like water does. ne important difference is that, overall, two molecules of can react with one of an aldehyde or ketone. A hemiacetal is formed from the addition of one molecule of. + ' ' emiacetal An acetal results from the addition of two molecules of. ' + 2 ' Acetal ' + 2 If the carbonyl compound is a ketone, the terms hemiketal and ketal are often but not always used in place of hemiacetal and acetal.
39 Typical eaction onditions for Acetal Formation The rate of the addition of alcohol is accelerated by acid and, like the hydration reaction, there is an equilibrium between the carbonyl compound and the tetrahedral product. To promote hemiacetal and acetal formation, the reaction is run by allowing the aldehyde to stand with a large excess of the alcohol with a small amount of anhydrous acid. The water produced can be removed as an azeotrope with benzene to drive the reaction to completion. A Mechanism for Acetal Formation Proton transfer ' ' ' ' emiacetal The hemiacetal usually cannot be isolated because of instability.
40 emiacetals Most acyclic hemiacetals are unstable and cannot be isolated. - ' But cyclic hemiacetals with 5- and 6-membered rings are stable and are widely found in carbohydrates (sugars and polysaccharides like cellulose and starch). Intramolecular hemiacetal formation proton + transfer 4-ydroxybutanal - yclic hemi-acetal stable
41 ydrolysis of Acetals All the steps in acetal formation are reversible. In an aqueous acid solution, acetals hydrolyze back to aldehydes. ' Acetal ' Aldehyde + 2 Mechanism ' - Acetal ' + - ' ' ' - emiacetal ' +
42 Quiz ircle the full acetal function in the compound below and provide the structures of the products from the hydrolysis reaction
43 Quiz For the reaction of ethanal and ethanol, (1) Draw the structure of the hemiacetal. (2) Draw the structure of the full acetal. 3 -Et Et 3 -Et (3) omplete this equation for the reaction of cyclopentanone with 1,2-ethanediol to produce a full acetal
44 Acetals as Protecting Groups While acetals readily hydrolyze back to the carbonyl compound in the presence of acid as a catalyst, they are stable to bases, even strong bases. ' + 2 ' '' '' - 2 no reaction Acetals often are used to protect aldehyde and ketone functions from undesired reactions in the presence of strong bases during syntheses. The acetal function, a geminal (1,1) diether, is not reactive towards nucleophiles, and its hydrogens are very non-acidic.
45 An Example The Selective eduction of an Ester in the Presence of a Ketone This is the desired conversion, but to selectively reduce the ester function it is necessary to protect (or mask) the ketone function. Use of protecting group Formation of cyclic ketal protective group ydrolysis removal of protecting group Ketal-ester 2 (1) LiAl 4, ether (2) 2 Ester reduction (ketone would, 2 if not protected, have been reduced, also)
46 Imines Addition of 1 o Amines '() Aldehyde or ketone + 2 N'' 1 o Amine 3 + N '' 2 + '() Imine, (E) & (Z) isomers Mechanism + N 2 '' + 2 N'' - N'' Amino alcohol N '' Imine 2 N + '' Iminium ion - 2 N'' 2 + Note All steps are reversible, so imines hydrolyze to carbonyl compounds in aqueous acid solution.
47 Imine formation proceeds fastest within a p range where there are optimal concentrations of the species involved in the nucleophilic addition step. This key step involves the free base attacking the more reactive protonated carbonyl compound. The concentrations of these two species depend on the p N [Key species] + 3 N- 2 N- + + p eaction ate Profile xn. rate The maximum reaction rate is found within a p range in which both key reactive species have reasonable concentrations p
48 3 Imine-Type arbonyl Derivatives + 2 N- hydroxylamine - 2 N- 3 an oxime + 2 N-N hydrazine N-N 2-2 a hydrazone N-N phenylhydrazine 2 N N-N- N 2 2,4-dinitrophenylhydrazine + 2 N-NN 2 semicarbazide - 2 N-N N - 2 N-NN a phenylhydrazone N-N- N 2 a 2,4-dinitrophenylhydrazone a semicarbazone
49 Addition of ydrogen yanide yanohydrins ydrogen cyanide (N) adds to aldehydes and unhindered ketones to produce cyanohydrins. + - N - N yanohydrins ' + - N - N (unhindered) ' In the reaction, a mineral acid is added to a mixture of the carbonyl compound and sodium cyanide. Too much acid slows the reaction by tying up the nucleophilic cyanide ion as N. The following sequence describes the mechanism of cyanohydrin formation. ' - + N slow yanide ion (a good nucleophile) - - N ' - N - N ' + - N
50 Synthetic Uses of yanohydrins (1) ydrolysis of the nitrile function gives α-hydroxyacids or α,β-unsaturated acids. N l 2, heat 95% 2 S 4 heat, Methyl-2-butenoic acid (2) eduction with lithium aluminum hydride gives β-aminoalcohols. - 2-ydroxy-2-methylbutanoic acid N N LiAl 4 N 2 1-Aminomethylcyclohexanol
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