18.4: Classification of Electrophiles By Their Carbon Hybridization

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There are two major types of carbon electrophiles, those containing sp³ carbon and those containing sp² carbon. We now look at the characteristics of each.

1. ELECTROPHILES CONTAINING sp³ CARBON

To be of use in synthesis, the electrophilic center of substrates containing sp³ carbon must be sterically accessible and contain a good leaving group. This is the type encountered in ch. 6 in Sn2 reactions. Examples are many primary and secondary halides. The outcome of the reaction between a nucleophile and this type of electrophile is a substitution product.

If the nucleophile used is a carbon nucleophile, the product has an expanded carbon chain because a new carbon-carbon bond has been created. For example, the acetylide ion is used as a carbon nucleophile to make internal alkynes from terminal alkynes. The complete sequence is shown below. This type of reaction is discussed in sections 9-6 and 9-7A of the textbook.

2. ELECTROPHILES CONTAINING sp²CARBON

To be of use in organic synthesis, electrophiles containing sp² carbon must contain a polarized C=X bond, where X can be O, N, or S. In this discussion we consider only carbonyl compounds, where X=O. The polarization of the C=O bond leaves the carbon with low electron density. At the same time it enables the displacement of the π-electrons towards the oxygen to make room for the new carbon-carbon bond that forms when the nucleophile attacks. Examples of electrophiles containing sp²carbon are most aldehydes and ketones.

The outcome of the nucleophilic attack is an addition product. The sp² carbon changes hybridization to sp³(tetrahedral), and π-electrons are displaced towards the oxygen atom, resulting in formation of an alkoxide ion, which is the conjugate base of the alcohol. The last step in the sequence then calls for treatment of the alkoxide ion with water or dilute acid to convert it into the alcohol. As long as the alcohol is desired, this last protonation step is taken for granted and sometimes left out in reaction sequences shown in textbooks. This doesn’t mean that the step was actually omitted, but simply that the author assumes awareness on the reader’s part of such obvious step.

Abbreviated synthetic sequence, as shown in most textbooks

An interesting trend can be observed in regards to the type of alcohol obtained as a product. Carbon nuclephiles, when reacted with formaldehyde (as in the first example), yield primary alcohols. When reacted with any other aldehyde (acetaldehyde in the second example) they yield secondary alcohols. Finally, when reacted with ketones (acetone in the third example), they yield tertiary alcohols.

CARBON NUCLEOPHILE + FORMALDEHYDE -------------> PRIMARY ALCOHOL

CARBON NUCLEOPHILE + OTHER ALDEHYDES ---------> SECONDARY ALCOHOL

CARBON NUCLEOPHILE + KETONE --------------------------> TERTIARY ALCOHOL

Refer to pages 379 - 382 of the Wade textbook (5th ed.) for additional examples using alkynide ions as carbon nucleophiles, and to pages 422 - 429 for examples using Grignard reagents as carbon nucleophiles (including the summary on p. 429). Mention should be made that Grignard chemistry is an important topic that gets heavily emphasized in organic chemistry II.