Reactivity in Chemistry

Carbonyl Addition

CO6. Relative Reactivity of Carbonyls

Sterics

Steric hindrance, or crowdedness around the electrophile, is an important factor that influences reactivity.

Crowdedness affects reactivity simply by preventing nucleophiles from easily approaching the  electrophilic site in the carbonyl.  If the nucleophile hits something other than the carbonyl carbon, it will probably just bounce off.  It needs to collide with the carbonyl carbon in order to deliver its electrons to the right place.

Charge

Amount of positive charge on the electrophile is an important factor that influences reactivity.

Factors that place more positive charge on the carbonyl (electron withdrawing groups nearby) make the carbonyl more positive and more reactive. Factors that place additional electron density on the carbonyl (electron donors nearby) make the carbonyl less reactive.

There is another resonance structure that we can think about that illustrates the electrophilicity of a carbonyl. That structure places a full negative charge on the oxygen and a full positive charge on the carbon. This isnít a good Lewis structure because the carbon does not have an octet. Nevertheless, when taken together with the regular Lewis structure, it suggests something real about the nature of the carbonyl: there is partial positive charge on the carbon and partial negative charge on the oxygen.

There is a general rule about cation stability on carbon atoms: a carbocation with more carbons attached to it is more stable than a carbocation with more hydrogens attached to it.

This observation is sometimes explained as an inductive effect. The positively charged carbon is more electronegative than the uncharged carbons, so it draws electrons away from them. It can polarize the neighbouring carbons, drawing some negative charge towards itself and leaving some positive charge on the other carbons.  In that way, it s charge is delocalized and stabilized.

In a more sophisticated explanation, the cation becomes stabilized by a molecular orbital interaction involving the empty p orbital on the carbocation and C-H bonds on the neighbouring carbons.

A similar situation results in the partially positive carbon in the carbonyl. The carbonyl carbon in the ketone is a little more stable than the carbonyl carbon in the aldehyde.

Problem CO6.1.

Rank the following carbonyl compounds from most reactive to least reactive towards nucleophilic addition.  Explain your reasoning.

Problem CO6.2.

Provide names for the following aldehydes.

Problem CO6.3.

Provide names for the following ketones.

Problem CO6.4.

Provide names for the following ketones.

 

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

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This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

 

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