Chemistry Reference
In-Depth Information
in Scheme 5.4 will not proceed by an S
N
2 mechanism because of the steric bulk of the start-
ing tert-butylbromide. Additional discussions surrounding the influence of steric factors are
presented in Chapters 3 and 4.
5.3 THE CARBOCATION
As defined in the previous sections of this chapter, carbocations are positively charged
carbon ions. However, simply defining this unique species of cations without exploring
its associated properties does little to promote understanding of S
N
1 reactions and the
related side reactions observed for this mechanistic type. Therefore, this section focuses
on the nature, stability, and reactivity of carbocations as explained using arrow pushing.
While the alluded to side reactions include both elimination reactions and rearrange-
ments, only rearrangements are presented in this chapter. Discussions focused on elimin-
ations are found beginning in Chapter 6.
5.3.1 Molecular Structure and Orbitals
Before delving into more details regarding the reactive nature and stability of carbocations,
it is important to understand the structure of these species. Recall that S
N
2 reactions occur at
carbon atoms bearing four substituents. Furthermore, recall that electrophilic carbon centers
participating in S
N
2 reactions are tetrahedral in geometry with all bond angles measuring
approximately 109.5
8
-the tetrahedral bond angle. This equal spacing, illustrated in
Figure 5.1, is only possible if the natures of all four bonds connecting the central carbon
atom to its four substituents are identical.
Since an understanding of orbital theory is critical to understanding organic reaction
mechanisms, review of the material presented in primary organic chemistry textbooks is
essential. For the purposes of the discussions presented herein, recall that ground-state
first-row elements (including C, N, and O) all possess one s orbital and three p orbitals.
Figure 5.2 illustrates the shapes of s and p orbitals.
If we consider methane (CH
4
), we find that not only does the central carbon atom
possess four hydrogen substituents, these four hydrogens are equally spaced in a tetrahedral
Figure 5.1 Fully substituted carbon atoms present substituents in tetrahedral arrangements.
