{"id":8893,"date":"2015-04-21T12:21:27","date_gmt":"2015-04-21T16:21:27","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8893"},"modified":"2022-11-29T22:23:23","modified_gmt":"2022-11-30T04:23:23","slug":"carbocations-and-the-sn1-e1-and-alkene-addition-reactions","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2015\/04\/21\/carbocations-and-the-sn1-e1-and-alkene-addition-reactions\/","title":{"rendered":"The SN1, E1, and Alkene Addition Reactions All Pass Through A Carbocation Intermediate"},"content":{"rendered":"

The Carbocation Intermediate That Connects The SN<\/sub>1, E1, And Alkene Addition Reactions<\/strong><\/p>\n

Was going to include this in my last post but it was getting too big.<\/p>\n

Note how the SN<\/sub>1, E1, and alkene addition reactions all pass through a carbocation intermediate.<\/strong> Here’s how they’re related.<\/p>\n

\"united<\/p>\n

1. The Carbocation Intermediate From Alkene Addition<\/strong><\/h2>\n

Adding strong acid to an alkene gets us to a carbocation (A–>C).<\/p>\n

If the counterion to that acid is a decent nucleophile (think Cl-, Br-, or I-) then it will then add to it, giving us the addition product D. Alternatively if we use an acid like H2<\/sub>SO4<\/sub> (which has a poorly nucleophilic counterion) in the presence of water or another nucleophilic solvent, we can also get addition products.<\/p>\n

The pathway A –> C –> D is an example of alkene addition.\u00a0<\/strong><\/p>\n

2. The Carbocation Intermediate From The SN<\/sub>1 Reaction<\/strong><\/h2>\n

Alternatively the carbocation can be generated through loss of a leaving group from an alkyl halide<\/strong> (B–> C) . Attack of that carbocation by a nucleophile (e.g. a nucleophilic solvent, again, like H2<\/sub>O or CH3<\/sub>OH) will give us a new product.<\/p>\n

The pathway B –> C –> D is what we call the SN<\/sub>1 reaction.\u00a0<\/strong><\/p>\n

3. The Carbocation Intermediate From The E1 Reaction<\/h2>\n

Finally, if the carbocation is generated through loss of a leaving group from an alkyl halide but there isn’t any reasonably good nucleophile present, elimination may occur to give the alkene. This is particularly favored by heat<\/a>.<\/p>\n

The pathway B –> C –> A is what we call the E1 reaction.\u00a0<\/strong><\/p>\n

So there you have three very important reactions all intersecting through a common intermediate.<\/p>\n

[Remember that leaving groups (LG) are just nucleophiles (Nu) acting in reverse.<\/a> That’s why there aren’t double [equilibrium] arrows going between B, C and D]<\/p>\n

4. What About Rearrangements?<\/h2>\n

One complication that’s left out here is carbocation rearrangements<\/strong>, which can arise when a less stable carbocation (often secondary) can rearrange to a more stable carbocation (often tertiary) through a hydride or alkyl shift. [See post: Rearrangement Reactions – Hydride Shifts<\/a>]<\/p>\n

Important to remember that they can occur but I couldn’t think of a way to put them in while keeping the diagram neat and tidy. :- )<\/p>\n

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<\/a>Notes<\/h2>\n

Related Articles<\/strong><\/p>