They all involve a nucleophile attacking the partially positively charged carbon atom to create a tetrahedral intermediate. Different reaction mechanisms are distinguished by the timing of protonation of the oxygen atom as well as the presence or absense of a leaving group attached to the carbonyl.
Grignard reagents add to aldehydes and ketones to give tetrahedral alkoxide intermediates can be isolated. Most commercial polyacetals are produced by anionic polymerization. However, formaldehyde and many other carbonyl compounds can also be readily produced by cationic polymerization. The mechanism is very similar to that for anionic polymerization, however, high molecular weight polymers can only be prepared with strong cationic initiators that form very stable cations because cationic growth centers readily undergo chain termination by recombination with anion fragments.
Some other monomers that readily undergo ionic chain polymerization include acetaldehyde and higher aliphatic aldehydes 4 , haloaldehydes chloral, fluoral, bromal , methyl glyoxal as well as dialdehydes with carbon atom spacing such as glutaraldehyde, succinaldehyde, and phthalaldehyde. Glyoxal yields highly crosslinked polymers whereas most other dialdehydes cyclopolymerize in the presence of acid initiators into relatively low molecular weight polymers with five- and six-membered rings.
Most aldehydes and ketones have a low ceiling temperature T c which is either at or noticeably below ambient temperature which means that these compounds do not polymerize at or above room temperature.
The low T c values are due to the low molar free enthalpy of polymerization of carbonyl double bonds which is appreciably lower than that of vinyl monomers whereas their entropy of polymerization is similar. Carbonyl compounds, like olefins, have a double bond that can undergo many addition reactions.
The double bond of carbonyl compounds is highly polarized and thus susceptible to nucleophilic and electrophilic attack. Many aldehydes and ketones can be polymerized in the presence of a base or a strong acid catalyst. We expect the reactivity of carbonyl groups in addition processes to be influenced by the size of the substituents thereon, because when addition occurs the substituent groups are pushed back closer to one another.
In fact, reactivity and equilibrium constant decrease with increasing bulkiness of substituents, as in the following series also see Table :. Strain effects also contribute to reactivity of cyclic carbonyl compounds. This change in the angle strain means that a sizable enhancement of both the reactivity and equilibrium constant for addition is expected. In practice, the strain effect is so large that cyclopropanone reacts rapidly with methanol to give a stable hemiketal from which the ketone cannot be recovered.
Cyclobutanone is less reactive than cyclopropanone, but more reactive than cyclohexanone or cyclopentanone. Electrical effects also are important in influencing the ease of addition to carbonyl groups. Electron-attracting groups facilitate the addition of nucleophilic reagents to carbon by increasing its positive character:. One should be cautious in interpreting dipole moments in terms of the ionic character of bonds.
Carbonyl-containing functional groups are also found in the structures of many drugs. These groups can also make direct contact with the target, forming hydrogen bonds and other intermolecular interactions as we have seen in Chapter 2.
This chapter examines many important classes of carbonyl- containing functional groups and reviews the biologically relevant chemistry of the carbonyl. Structures and stick models of carbonyl groups in formaldehyde, acetaldehyde, and acetone.
Organic Chemistry. The bonding between carbon and oxygen in a carbonyl is analogous to that between the carbon atoms of ethylene. The greater electronegativity of oxygen as compared to carbon, however, means that electron density in the carbonyl function is polarized. This polarization can also be understood in resonance terms, the resonance forms shown below implying partial positive character at carbon and partial negative character at oxygen.
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