Possible Mechanisms for Substitution Reactions

The four arrow-pushing patterns for ionic processes. 

Every substitution reaction exhibits at least two of the four patterns— nucleophilic attack 

But consider the order of these events. Do they occur simultaneously (in a concerted fashion), as shown above, or do they occur in a stepwise fashion, as shown below  

In the stepwise mechanism, the leaving group leaves, generating an intermediate carbocation, which is then attacked by the nucleophile. The nucleophile cannot attack before the leaving group leaves, because that would violate the octet rule  

Therefore, there are only two possible mechanisms for a substitution reaction  

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