Lewis Structures|Reactivity #2: Reactivity of acrylonitrile

Dot structures and Reactivity - Example: Reactivity of acrylonitrile  

The reactivity of acrylonitrile can be explained using the corresponding Lewis electron dot structures.

The dot structures of acrylonitrile (C₃H₃N) were derived in a previous article named “Writing Lewis Structures a step by step approach: Acrylonitrile C3H3N" using a simple procedure.  These Lewis dot structures are shown below, Figure 1:

  

Fig. 1: Dot structures of  acrylonitrile (C₃H₃N)

Structures 1 and 2 are resonance structures since they differ only in the location of double and triple bonds and nonbonding electrons – they do not differ in the location of single bonds, i.e. in which atoms are bound to other atoms.

Many molecules that contain double and/or triple bonds and/or unshared electron pairs have structures that can best be understood in terms of a combination of two or more Lewis structures.

The resonance explanation for a molecule with two or more resonance structures is that the  structure of the molecule is a hybrid of the two (or more) possible resonance forms.

Therefore, the carbon-carbon bonds of the acrylonitrile are a hybrid of a single and a double bond – they have a partial double bond character.

Resonance stabilizes molecules and ions, and the corresponding Lewis structures can be used to explain the chemical reactivity of these species.

However, at this point it should be mentioned that not all resonance structures of a molecule have equal importance or contribution to the hybrid structure. The most stable structures contribute more than the less stable structures.

What are the rules for estimating stability of resonance structures?

1. Structures with complete octets are more stable
2. Structures with no charge separation are more stable
3. A structure with a negative charge on the most electronegative atom will be more stable
4. A structure with a positive charge on the most electropositive atom will be more stable
5. Resonance forms that are equivalent have no difference in stability and contribute equally to the hybrid structure.

Using these rules, it can be easily seen that structure #1 of acrylonitrile (Fig. 1) is more stable than structure #2 since there is no charge separation. This does not mean that structure #2 has no contribution to the hybrid structure of the compound. As a matter of fact, often a minor resonance structure such as structure #2 gives an insight into the reactivity of a compound.

The reaction of acrylonitrile with Si surface has been studied by M. Schwartz et al¹ . The proposed mechanism for the reaction involves structure #2 (the more unstable Lewis electron dot structure) where the N atom acts as a nucleophile and attacks the Si atom according to the following scheme (Scheme 1):

References

1. M. Schwartz et al., Surface Science, Vol. 515, 75-86, 2002