Groups on the number three carbon, one, two, three. We have one, two, three,įour, five carbons. a reaction where a strong base steals a hydrogen, causing the remaining electron density to push out the leaving group is an E2. a reaction where the strong nucleophile edges its way in and forces out the leaving group, thereby replacing it is SN2. a reaction that only depends on the leaving group leaving, but NOT being replaced by the weak base, is E1. a reaction that only depends on the the leaving group leaving (and being replaced by a weak nucleophile) is SN1 This causes an SN2 reaction, because the rate depends on BOTH the leaving group, and the nucleophile.
A STRONG nucleophile, on the other hand, TAKES what it wants, when it wants it (so to speak) and PUSHES the leaving group out, taking its spot. It therefore needs to wait until the leaving group "decides" it's ready to go, and THEN the nucleophile swoops in and enjoys the positive charge left behind. This is a lot like SN1! With SN1, again, the nucleophile just isn't strong enough to kick the leaving group out. A weak base just isn't strong enough to participate- if it was, it'd be a strong base, and all of the sudden the rate-determining step would depend on TWO things (the Leaving Group leaving AND the base entering), which would make it E2. This is why it's called an E1 reaction- the reaction is entirely dependent on one thing to move forward- the leaving group going.
In the video, Sal makes a point to mention that Ethanol, the weak base, just wasn't strong enough to push its way in and MAKE the bromine leave (as would happen in an E2).
In an E1 reaction, the base needs to wait around for the halide to leave of its own accord. An E1 reaction requires a weak base, because a strong one would butt-in and cause an E2 reaction.
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