{"id":5498,"date":"2012-08-15T16:16:52","date_gmt":"2012-08-15T16:16:52","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=5498"},"modified":"2026-04-18T06:16:33","modified_gmt":"2026-04-18T11:16:33","slug":"rearrangement-reactions-1-hydride-shifts","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2012\/08\/15\/rearrangement-reactions-1-hydride-shifts\/","title":{"rendered":"Rearrangement Reactions (1) – Hydride Shifts"},"content":{"rendered":"

Rearrangement Reactions: Substitution Reactions With Hydride Shifts<\/strong><\/p>\n

In this post we cover several examples of reactions where carbocations form… but then a funny thing happens. An adjacent bonding pair of electrons (i.e. a C-H bond) interacts with the empty p-orbital, and before you know it, the C-H bond has moved and a new,\u00a0more stable\u00a0<\/strong>carbocation has formed! The carbocation is then attacked by the nucleophile, giving a substitution reaction (SN<\/sub>1) with rearrangement!<\/p>\n

\"Summary<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. Spotting A “Substitution With Rearrangement”: An Extra Set Of C-H Bonds Forms And Breaks<\/a><\/li>\n
  2. Carbocation Stability: Tertiary > Secondary >> Primary<\/span><\/a><\/li>\n
  3. If A Less Stable Carbocation Can Be Transformed Into A More Stable Carbocation Through The Migration Of A C-H Bond, Then A Rearrangement Is Possible<\/span><\/a><\/li>\n
  4. Examples Of “Allowed” Carbocation Rearrangement Reactions That Occur Through Hydride Shifts<\/span><\/span><\/a><\/li>\n
  5. The SN1 Reaction With Hydride Shift: Arrow Pushing Mechanism<\/a><\/li>\n
  6. Notes<\/a><\/li>\n
  7. Quiz Yourself!\u00a0<\/a><\/li>\n
  8. References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Spotting A “Substitution With Rearrangement”: An Extra Set Of C-H Bonds Forms And Breaks<\/strong><\/h2>\n

    For nucleophilic substitution<\/a>, the pattern of bonds that form and break is pretty straightforward. You break C-(leaving group) and you form C-(nucleophile). A straight swap.\u00a0But every once in awhile you might see a “weird” substitution reaction. If you look closely at the pattern of bonds formed and bonds broken in the second reaction below, there’s an extra set!<\/p>\n

    \"normal<\/p>\n

    In other words it’s a substitution reaction where the hydrogen has moved. We call these movements “rearrangements”, for reasons that will become clear shortly.<\/p>\n

    The big question is, what’s going on? How did this happen?\u00a0<\/strong><\/p>\n

    <\/a>2. Carbocation Stability: Tertiary > Secondary >> Primary<\/strong><\/h2>\n

    As it turns out, reactions that go through carbocations can sometimes undergo rearrangements. <\/strong>And looking back at substitution reactions, recall that theSN<\/sub>1 reaction goes through a carbocation intermediate. <\/strong>(See post: The SN1 Mechanism<\/a><\/em>)<\/p>\n

    In this post we’ll go through when you’ll expect to see a rearrangement reaction.<\/p>\n

    Let’s think back to carbocations. They’re carbon atoms with six electrons bearing a positive charge. In other words, they’re electron deficient – 2 electrons short of a full octet.<\/p>\n

    So it would make sense that carbocations become more stable as you increase the number of electron donating groups attached to them.<\/strong> Alkyl groups are a perfect example. That’s why carbocation stability increases as you go from primary to secondary to tertiary. (See post: Carbocation Stability<\/a><\/em>)<\/p>\n

    \"carbocation<\/p>\n

    (It’s also worth pointing out that carbocations are also stabilized by resonance, which allows the positive charge to be delocalized or “spread out” over a greater area on the molecule.)<\/em><\/span><\/p>\n

    <\/a>3. If A Less Stable Carbocation Can Be Transformed Into A More Stable Carbocation Through The Migration Of A C-H Bond, Then A Rearrangement Is Possible<\/h2>\n

    So what does this have to do with rearrangements? As it turns out, \u00a0if \u00a0a situation exists where an unstable carbocation<\/strong> can be transformed into \u00a0a more stable carbocation. <\/strong>then\u00a0a rearrangement<\/strong> is possible.<\/p>\n

    One rearrangement pathway where an unstable carbocation can be transformed into a more stable carbocation is called a hydride shift. <\/strong>Look at the diagram below.<\/p>\n

    \"rearrangement
    \n<\/a>
    \n<\/a>In this reaction we have a secondary<\/strong> carbocation on the left hand side. In this rearrangement reaction, the pair of electrons in the C-H bond is transferred to the empty p orbital on the carbocation. In the transition state of this reaction, there’s a partial C-H bond on C3 and a partial C-H bond on C2.<\/p>\n

    The transition state here is kind of like that split second in a relay race where one sprinter is passing the baton to another sprinter and they both have their hands on it. <\/strong><\/p>\n

    Then, as the C2-H bond shortens and the C3-H bond weakens, we end up with a carbocation on C3 (a tertiary carbocation) in the product which is more stable.<\/p>\n

    Note that we only need one arrow<\/strong>\u00a0to show this occurring!<\/p>\n

    <\/a>4. Examples Of “Allowed” Carbocation Rearrangement Reactions That Occur Through Hydride Shifts<\/h2>\n

    Here are some examples of “allowed” rearrangement reactions. Notice how we’re always going from a less substituted carbocation to a more substituted carbocation. One exception is at the very bottom; the rearrangement is favorable because the new carbocation is resonance stabilized.<\/p>\n

    \"list<\/p>\n

    <\/a>5. The SN<\/sub>1 Reaction With Hydride Shift: Arrow Pushing Mechanism<\/h2>\n

    Now we’re ready to show how the rearrangement reaction occurs with the SN<\/sub>1. Recall that the first step in the SN<\/sub>1 is that the leaving group leaves<\/strong>\u00a0to give a carbocation.<\/p>\n

    In the case below, the carbocation that is formed is secondary<\/strong>, and there’s a tertiary <\/strong>carbon next door. Therefore, a rearrangement <\/strong>can occur to give the more stable tertiary carbocation, which is then attacked by the nucleophile (water in this case).<\/p>\n

    Finally, the water is deprotonated to give the neutral alcohol. So this is an example of an SN<\/sub>1 reaction with rearrangement<\/strong>.<\/p>\n

    \"rearrangment<\/p>\n

    I’ve given some more examples of SN<\/sub>1 reactions with rearrangements below. See if you can draw the mechanisms! In the next post we’ll talk about a slightly different rearrangement pathway with substitution reactions.<\/p>\n

    \"try<\/p>\n

    Next Post: Rearrangement Reactions (2) – Alkyl Shifts<\/strong><\/a><\/p>\n

    \n

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

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