Reactivity in Chemistry

Carbonyl Addition

CO2.  Carbonyls are Electrophiles

The carbonyl bond is very polar. There is a partial positive charge on the carbon and a partial negative charge on the oxygen, because oxygen is more electronegative than carbon. This charge separation is intensified because of the double bond between the carbon and oxygen. Rather than just pulling one pair of bonding electrons towards itself, the oxygen pulls two pairs of electrons towards itself.

Sometimes, a resonance structure is drawn to emphasize the charge separation in the carbonyl. The structure has only one bond between the carbon and oxygen. In this structure, oxygen has an octet but carbon does not. This is not really a good Lewis structure, because the other resonance structure satisfies octets on all the atoms. However, this Lewis structure emphasizes the polarity of the bond and is sometimes drawn to reinforce that idea.

Figure CO2.1.  Thinking about charge distribution in a carbonyl group.

Because of the positive charge on the carbonyl carbon, the most important theme in carbonyl chemistry is reaction of the carbonyl as a Lewis acid. Reactions of carbonyls almost always involve addition of an electron donor to the carbonyl carbon.

The electrophilicity of carbonyls is very important in their reactivity.  The goal of this chapter is to develop an understanding of how carbonyls react.  We will learn about a few key factors that will be used in different combinations under different circumstances.  Eventually, you will build an understanding that will allow you to follow both biological reactions and modern synthetic reactions. 

Figure CO2.2.  Reactivity in carbonyl compounds.  The carbonyl in the lower sugar on the left has reacted with the neighbouring molecule.

It is important to realize that biological reactions, such as carbohydrate synthesis, are very complex and can involve many, many steps. For example, the carbohydrate synthesis shown above involves additional acid-base steps as well as a reaction of a carbonyl.  The additional acid base steps may involve proton donors and acceptors as well as more general Lewis acids.

 

Problem CO2.1.

a)  Explain why the carbon in a C=O unit is very electrophilic, but the carbon in a C-O unit is much less so.

b)  Propose other carbon-heteroatom bonds that may make the carbon electrophilic (heteroatom means not carbon or hydrogen).

Problem CO2.2. 

Provide line structures for the following compounds.

 

Problem CO2.3. 

Translate the following condensed formulae into Lewis-Kekule structures (i.e. like Lewis structures, but use lines for bonds).  All of the compounds are adehydes or ketones.

a)  CH3CH2CH2CH2COCH3    b)  ((CH3)2CH2CH2COCH3    c)  CH3CH2CH2CH(CH3)COCH3

d)  CH3CH2CH2COCH2CH3     e)  CH3CH2CH2COCH2CH2CH3     f)  CH3CH2CC(CH3)COCH2CH3

g) CH3CH2CH2CH2CHO       h)   ((CH3)2CH2CHO       i)  CH3CH2CH(CH3)CHO

j) CH3CH2CH2CH(CH2CH3)CHO   k)  ((CH3)3CH2CH2CHO      l)  CH3CH2CH(CH3)CH(CH3)CHO

m)  CH3CH2OCH2CH2CHO    n)  CH3CHClCH2COCH3    o) CH3CH2CH(NH2)COCH2CH

p) CH3CHCHCH2COCH3   q)  CH3CH2CCCH2COCH3  r)  C6H5CHO

Problem CO2.4.

Translate the condensed formulae in the previous problem into line structures.

Problem CO2.5.

Provide IUPAC names for the compounds in the previous problem.  For help, see the functional group section, simple carbonyls.

Problem CO2.6.

Fill in any missing lone pairs in the following structures.

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

Send corrections to cschaller@csbsju.edu

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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