Aldehydes Polarized multiple bonds

Protic reagents such as water, or alcohols, or amines, in which hydrogen is bonded to an electronegative group, will add, often reversibly, to a variety of polar multiple bonded compounds, such as aldehydes, isocyanates, or carbon dioxide, for example ... 

Answer We have two problems translating the typed formula into a recognizable structure and doing a correct classification. The first compound is a carboxylic acid, which should tip us off to put it in the acids, the H-L or H-A class. The next compound is an aldehyde conjugated to a pi bond. Since an aldehyde is a polarized multiple bond and also an ewg, we would put this compound in the conjugate acceptor class, C=C-ewg. 

C = Y / Polarized multiple bond without L Protonated carbonyls R2C=0H Iminium ions R2C=NRj Aldehydes RHC=0 Ketones R2C=0 Imines R2C=NR The larger partial plus carbon is more reactive Addition. Following Substitution or Elimination (8.5.1) Polarized triple bonds R-C=Y without leaving groups... 

In acidic media, polarized multiple bonds often undergo acid catalyzed addition, and a common mode of addition is the Ad 2. Deprotonation of the nucleophile by solvent gives the neutral compound. Common examples of this easily reversible Adg2 reaction are the formation of hydrates (NuH is H2O) and, if NuH is ROH, hemiacetals (from aldehydes) and hemiketals (from ketones). Usually this reaction favors reactants. 

Under equilibrium conditions (thermodynamic control), the allylic source adds to the polarized multiple bond (path AdN). However, the allylic source can also serve as a base and may deprotonate the sink, creating a mixture of sources and sinks and thus a messy statistical mixture of products. Clean products result if the source is just the deprotonated sink or if the sink has no acidic protons. With ketones, the equilibrium of the attack step favors the starting materials, and therefore the reaction goes to completion only if driven by a following elimination. In the next Adisj2 example, the source is the deprotonated sink. The product is an aldehyde-alcohol, or aldol, a name now used for the general process of an enol (acidic media) or enolate (basic) reacting with an aldehyde or ketone. 

Generic process An addition and an elimination have occurred. Medium Definitely basic, predominant anion is hydroxide, plsTabH 15.7, whose pA"a would give a useful proton transfer A"eq up to about p Ta 26. Sources The carbonyl lone pair, water lone pair, and hydroxide anion. Best source Hydroxide anion, a lone pair source can behave as a nucleophile or as a base. Sinks Polarized multiple bond, the aldehyde carbonyl. Acidic Hs Water and the CH2 next to the aldehyde, pA a 16.7, are within range of hydroxide. Leaving groups None. Resonance forms ... 

Hydrogenation of substrates having a polar multiple C-heteroatom bond such as ketones or aldehydes has attracted significant attention because the alcohols obtained by this hydrogenation are important building blocks. Usually ruthenium, rhodium, and iridium catalysts are used in these reactions [32-36]. Nowadays, it is expected that an iron catalyst is becoming an alternative material to these precious-metal catalysts. 

For couplings of metallated cyclopropenes with carbonyl compounds liquid ammonia seems to be generally a less suitable solvent since enolizable carbonyl compounds may easily undergo deprotonation under strongly polar conditions, while aldehydes such as benzaldehyde may react with the solvent. For the lithiation of 1-methylcyclopropene both BuLi THF and LDA THF seem suitable systems. We prefer to use, however, the latter base because of its decreased tendency to add across multiple bonds. The thermodynamic basicity of LDA will be sufficient though in the less polar THF the rate of deprotonation might be lower than that with potassium amide in liquid ammonia. 

Oxo and imido compounds also react as nucleophiles with compounds containing polar C=X bonds, such as aldehydes, ketones, imines, and heterocumulenes. These reactions generally occur by a formal [2+2] cycloaddition process to generate a metala-cycle that breaks down to the more thermodynamically stable combination of products containing metal-ligand and carbon-heteroatom multiple bonds. Three examples of such reactions are shown in Equations 13.90-13.92. In the first two cases, the four-membered metallacycle was isolated or characterized by low-temp6rature NMR spectroscopy. ... 

In the last chapter, we saw how the structure of the carbonyl group—a multiple bond that is also highly polar—gives rise to a characteristic combination of functional behaviors addition reactions mediated by electrophilic attack (usually by protons) on the Lewis basic oxygen and attack by nucleophiles on the carbon. We turn now to a third site of reactivity in aldehydes... 

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