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The pinacol rearrangement is the acid catalyzed rearrangement of a 1,2-diol into a ketone. It is named after the molecule pinacol, pictured below, the simplest substrate to undergo the reaction. One of the alcohols is protonated to make the leaving group (water) while the other OH participates as the rearrangement promoting group.
- The Hofmann and Curtius Rearrangements
- The Key Rearrangement Step in The Hofmann and Curtius
- Formation of The Isocyanate
- Putting The Steps Together
- The Fate of The Isocyanate
- Summary: The Hofmann and Curtius Rearrangements
- Notes
- References and Further Reading
The Hofmann and Curtius rearrangements are two examples of a whole family of rearrangement reactions that share a common mechanistic step. [as do the Beckmann and Wolff rearrangements – see bonus topic 2] In the Hofmann rearrangement, anamide is treated with bromine and base (usually NaOH or KOH). Upon heating, an intermediate isocyanate is formed,...
Now we get down to business. The key step in the Hofmann and Curtius rearrangments is migration of a carbon atom to displace a leaving group on an adjacent nitrogen. This requires two curved arrows to draw, which are shown in the structure on the far left (below). 1. In the first curved arrow, the C-C bond breaks, and a new C-N bond forms. 2. In th...
The next step is going from our rearranged species to the isocyanate. For anyone accustomed to drawing resonance forms, this shouldn’t be too difficult. (As we’ll discuss below, studies have determined that isocyanate formation occurs at the same time as migration. But for our teaching purposes, I think it helps to treat this step in isolation. We ...
Studies into the mechanism of the Hofmann and Curtius rearrangements have determined that these two steps do not happen sequentially; they actually happen at the same time! (how do we know this? see Note 1) That is, the migration happens at the same time as isocyanate formation. We need to redraw the mechanisms of the Hofmann and Curtius to reflect...
Both the Hofmann and Curtius give rise to an isocyanate. You likely haven’t encountered isocyanates before. At first glance, they are pretty strange-looking species, but their chemistry is not so different from that of other carbonyl species such as esters, amides, and acyl halides. The isocyanate structure closely resembles that of carbon dioxide,...
The mechanism of these rearrangments is tricky at first glance, but is made considerably easier once you realize that you’ve seen a variant of the migration reaction before (i.e. in hydroboration-oxidation). I highly recommend drawing the ugly version first, because it will help you focus on seeing the bonds that form and break, before trying to re...
Note 1.Concerted or separate steps? If the Curtius was stepwise, the loss of nitrogen would lead to a nitrene. Nitrenes undergo a host of interesting reactions, such as insertion into C-H bonds (!). So one can design an experiment to test this: if one tries the following Curtius rearrangement, one could look for evidence of formation of a five-memb...
Ueber die Einwirkung des Broms in alkalischer Lösung auf Amide W. Hofmann Chem. Ber. 1881, 14, 2725-2736 DOI:10.1002/cber.188101402242 Original paper by A. W. Hofmann (who was a very productive che...A Mild and Efficient Modified Hofmann Rearrangement Xicai Huang, Mehran Seid, and Jeffrey W. Keillor The Journal of Organic Chemistry 1997, 62 (21), 7495-7496 DOI:10.1021/jo9708553 This is a modifi...METHYL CARBAMATE FORMATION VIA MODIFIED HOFMANN REARRANGEMENT REACTIONS: METHYL N-(p-METHOXYPHENYL)CARBAMATE Jeffrey W. Keillor and Xicai Huang Org. Synth. 2002, 78, 234 DOI: 10.15227/orgsyn.078.02...The Hofmann Reaction Wallis, Everett L.; Lane, John F. Org. React. 1946, 3, 267-306 DOI: 10.1002/0471264180.or003.07 An old but comprehensive (for its time) review on the Hofmann reaction (the amid...Aug 15, 2012 · The SN1 Reaction With Hydride Shift: Arrow Pushing Mechanism. 1. Spotting A “Substitution With Rearrangement”: An Extra Set Of C-H Bonds Forms And Breaks. For nucleophilic substitution, the pattern of bonds that form and break is pretty straightforward. You break C- (leaving group) and you form C- (nucleophile).
Oct 17, 2011 · Introduction to Rearrangement Reactions. Reactions that involve a carbocation intermediate may be accompanied by rearrangements where a pair of electrons from a C-H or C-C bond migrates toward the carbocation, resulting in breakage and formation of a C-H or C-C bond, and formation of a new carbocation. The new carbocation (generally more stable ...
Jan 22, 2020 · What happens next (step 2 above) is a process called a carbocation rearrangement, and more specifically, a hydride shift. The electrons in the bond between carbon #3 and a hydrogen are attracted by the positive charge on carbon #2, and they simply shift over to fill the empty p p orbital, pulling the proton over with them.
The term “rearrangement” is used to describe two different types of organic chemical reactions. A rearrangement may involve the one -step migration of an H atom or of a larger molecular fragment within a relatively short lived intermediate. On the other hand, a rearrangement may be a multi-step reaction that includes the migration of an H ...
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Carbocation rearrangements | Organic Chemistry 1: An open textbook. 8.4. Carbocation rearrangements. Carbocation rearrangements are common in organic chemistry and are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural reorganizational “shifts” within the molecule.
