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Reaction of triphenylphosphine with a primary or secondary alkyl halide produces a phosphonium salt The phosphonium salt is deprotonated by a strong base to form the ylide. Iodides, bromides, and chlorides can be used, as can both aldwidos and alkyl halides, though they cannot contain any functional groups that would react with the Grignard reagent once it is formed.
8.AN a aldeídos e cetonas QUI02015 4
Some diseases found in eastern Nigeria can be traced to long-term consumption of HCN. Aldehyde carbonyl groups are more electron deficient because they have only one electron-donating group attached to the carbonyl carbon. Similar glucosides are found in apple pips and the kernels inside the stones of fruit such as peaches and apricots. Aldeeidos that one cyanohydrin was made by the simplest method—just NaCN and acid—while the other came from the bisulfite route we have just discussed.
An aldehyde or ketone dissolved in an alcohol will form an equilibrium mixture containing the corresponding hemiacetal.
Aldeidos e cetonas (1)
But it can zldeidos to medicinal chemists, who make compounds that need to be com- patible with biological systems. The reason is that they react very rapidly and highly exothermically with water to produce 9. We shall discuss the structure of glucose later in this chapter, but for now, just accept that it stabilizes the cyanohydrin. Enviado por Karine flag Denunciar.
Acetal formation is reversible An excess of water in the presence of an acid catalyst will hydrolyze an acetal to. Eventually 50 mg HCN per g of cassava can be released and this is enough to kill a human being after a meal of unfermented cassava.
The solution is to make a bisulfite compound from it. And in one case, the solubility of bisulfite adduct in water is literally vital. Dapsone is insoluble in water. One readily available monomeric form is the bisulfite compound.
Mild acid facilitates departure of the hydroxyl group from the aminoalcohol intermediate without also protonating the nitrogen of the amine starting co mpound. Aldehydes and ketones react with phosphorous ylides to produce alkenes An ylide alddeidos a neutral molecule with adjacent positive and negative charges. Enviado por Karine flag Denunciar. The hydrate is also called a gem-diol gem i.
AldeÃdos e cetonas
Here is one example of each, but you will have to wait until Chapter 12 for the details and the mechanisms of the reactions. There is also a change in oxidation state of the magnesium, from Mg 0 to Mg II. But there is a problem! The second reason that bisulfite compounds are useful is that they are soluble in water.
The driving force for the last reaction is formation of the very strong phosphorusoxygen double bond in triphenylphosphine oxide. Each inserting reaction requires two atoms of lithium and generates one equivalent of lithium halide salt. Thioacetals can be converted to CH2 groups by hydrogenation using a catalyst such as Raney nickel. Secondary amines cannot form a neutral imine by loss of a second proton on nitrogen An enamine is formed instead. The trick is to use the formaldehyde bisulfite compound and exchange the OH group for one of the amino groups in dap- sone.
You have already met cyanide p. The reaction scheme is easy enough to draw, but what is the mechanism? The equilibrum favors a ketone over its hydrate because the tetrahedral ketone hydrate is sterically crowded. The tetrahedral carbon resulting from addition to an aldehyde is less sterically hindered than the tetrahedral carbon resulting from addition to a ketone. The table lists some of the most important commercially available organolithiums and Grignard reagents.
This does not usually matter to most chemists as we often want to carry out reactions in organic solvents rather than water. Acylic hemiacetals are generally not stable, however, cyclic five-and sixmembered ring hemiacetals are.
Overall it involves an insertion of magnesium into the new carbon—halogen bond. You may ask how this is possible since dap- sone has no aldehyde or ketone—just two amino groups and a sulfone. An aldehyde or ketone in the presence of excess alcohol and an acid catalyst will form an acetal. But most larger more than four or so carbon atoms aldehydes and ketones are not.
Acetal formation from ketones and simple alcohols xldeidos less favorable than formation from aldehydes. Making organometallics How to make Grignard reagents Grignard reagents are made by reacting magnesium turnings with alkyl halides in ether solvents to form solutions of alkylmagnesium halide.
CAPAldehydes and Ketones I [Modo de Compatibilidade] – aldeidos e cetonas
The same arguments hold for the C—Mg bond of Grignard reagents. Earlier in this chapter we mentioned the difficulty of working with formaldehyde because it is either an aqueous solution or a dry polymer. As with Grignard formation, there is really very little limit on the types of organolithium that can be made this way.
Dissolving aldehydes or ketones in water causes formation of an equilibrium between the carbonyl compound and its hydrate. The details of this sort of chemistry will come in Chapter 14 when you will meet imines as intermediates. The true structure of organolithiums and Grignard reagents is cetona more complicated!