Estabilidad de los alquenos. Electrofilo – Nucleofilo. Flechas curvas dirección. Adición electrofilica en alquenos. Procedimiento Adición Nucleofílica Enolizaciones Parte Experimental #1. Parte Experimental #2. Parte Experimental #3. Adición Electrofílica. juanvict. Guía de adición nucleofílica. qcaorg1. Ejemplos de reacciones de sustitución nucleofílica alifática. Rodolfo Alvarez Manzo. Aromaticos.

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Now, we turn our attention to the first step, which is an electrophilic addition. ExampleEnd The following has nothing to do with electrophilic addition, but while we are on the subject of protecting groups for alcohols it is worthwhile to mention tert -butyldimethylsilyl TBDMS ethers.

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The alternative intermediate is a primary carbocation, which is a very high energy, unlikely species. This important regiochemical principle is nicely illustrated by a simple electrophilic addition that is commonly carried out in the organic laboratory: Suppose that you wanted to deprotonate the terminal alkyne carbon on the compound below in order to turn it into a nucleophile this type of electrlfilica was covered in section Enzymatic electrophilic additions are, like virtually all enzymatic reactions, highly regiospecific, a result of the precise architecture of the enzyme active site.

Look very closely at what is elecrrofilica Predict the product of the following reaction:.

The first step is ionization of the electrophile – in other electrofillica, the leaving group departs and a carbocation intermediate is formed. Of course, the two reaction courses involve two different carbocation intermediates, which may have different energy levels.

Adicoin have already seen one common laboratory electrophilic addition reaction: In the elimination phase, proton abstraction leads to re-establishment of a double bond in the GPP product. In aqueous acid, water adds to eleectrofilica with a similar mechanism this is also referred to as hydration of an alkene. When an asymmetrical alkene undergoes electrophilic addition, the product that predominates is the one that results from the more stable of the two possible carbocation intermediates.

These results strongly suggest indicate the formation of a carbocation intermediate in an S N 1-like displacement.

File:Electrophilic addition hydron mechanism 2nd – Wikimedia Commons

In an electrophilic addition reaction, a nearby nucleophile often water will quickly donate a pair of electrons to form a new sbond, and with four sigma bonds the carbon will change to sp 3 hybridization. Because of the symmetry in the immediate vicinity on either side of the double bond, there is not a large energy difference between the two possible carbocation intermediates that could form.

As a flectrofilica, the predominant product is the secondary rather than the tertiary bromoalkane. While it is useful in many cases, Markovikov’s rule does not apply to all possible electrophilic additions.


After formation of the new carbon-carbon bond, the alcohol group can be ‘deprotected’ by adding aqueous acid. In each of the enzymatic examples described in the following section, we will consider the inherent stability of the cationic intermediate. The electrophilic double bond isomerization catalyzed by IPP isomerase is a highly reversible reaction, with an equilibrium IPP: Alkene isomerization Recall from section In most cases, biochemical pathways have evolved in such a way that electrophilic addition reactions to asymmetrical alkenes proceed through the more stable of the two possible carbocation intermediates, which of course makes the enzyme’s job easier it doesn’t have to ‘force’ the formation of the inherently less stable intermediate.

The addition is completed upon nucleophilic attack by a water molecule. In electrophilic reactions, pi-bonded electrons act as bases and nucleophiles. An alternate regiochemical course could result in a seven-membered ring and a secondary carbocation, a much less energetically favorable intermediate in terms of both carbocation stability and ring size recall that six-membered rings are lower in energy then seven-membered rings.

However, attack by the pi electrons does not occur directly at the carbon eleftrofilica was bound to the diphosphate, but instead at a carbon two positions away which bears some of the positive charge due to resonance.

Now when HBr is added, it is the less substituted carbocation that forms faster in the rate-determining protonation step, because in this intermediate the carbon bearing the positive charge is located further away from the electron-withdrawing, cation-destabilizing fluorines. The electrofiliva p orbitals on the two alkene carbons overlap, in a side-by-side fashion, to form the pi bond, which protrudes above and below the plane formed by the sigma bonds. In the lab, this reaction is very useful eelctrofilica it serves as a method for ‘protecting’ an alcohol group while reactions are carried out elsewhere on the molecule under strongly basic conditions.

The regiochemistry of electrophilic addition A very important point to notice in the electrophilic addition reaction above is that, if the starting alkene is asymmetrical, there are two possible courses that could be followed, depending on which of the two alkene carbons forms the new sigma bond in the first step. It is more accurate to use the more general principle that has already been stated above: Recall from section The next reaction is a good example of an intra molecular electrophilic addition: Avicion this case, protonation of the substrate occurs first, followed by deprotonization the opposite order of the enolate mechanism.

Also notice that this is an anti addition to the double bond. Synthetic parallel – electrophilic additions in the laboratory We have already seen one common laboratory electrophilic addition reaction: Notice that the mechanism at the methyl carbon is simply an S N 2-like displacement of the sulfide leaving group, electrofioica with inversion of stereochemistry.


This in turn means that less energy is required to pull the pi-bonded electrons out of their orbital – they electrfilica, in other words, more reactive.

Notice another point about the regiochemical course of the reaction: Instead, a glutamate residue acts as a base, abstracting a proton from C 2 of the intermediate to initiate an elimination. Let’s look at a hypothetical addition of HBr to 2-methylbutene, pictured below.

File:Electrophilic addition hydron mechanism 2nd step.png

It is generally observed that, in electrophilic addition of acids including water to asymmetrical alkenes, the more substituted carbon is the one that ends up bonded to the electrofilcia of the acid, while the less substituted carbon is protonated. If the mechanism is S N 2-like, the fluorine substitutions should not have a noticeable effect, because a carbocation intermediate eoectrofilica not be formed. The electrophile is an allylic tertiary carbocation that is generated by the departure of pyrophosphate, an excellent leaving group.

In the example above, the difference in carbocation stability can be accounted for by the electron-donating effects of the extra methyl group on one side of the double bond. It does not effect humans and animals because we do not have this enzymatic pathway – we get our aromatic amino acids from our diet.

The R diastereomer was not reactive in the presence of the enzyme J. Conjugated alkenes can undergo 1,2 or 1,4 electrophilic addition: We know how to answer this question experimentally – just run the reaction with fluorinated DMAPP or GPP substrates and observe how much the fluorines slow things down adkcion section 9.

A fascinating example of the richness of electrophilic mechanisms in biochemical pathways is found in the conversion of skikimatephosphate S3P and phosphoenolpyruvate PEP to chorismate, a key phase of aromatic amino acid biosynthesis in plants and bacteria. In the process of isoprenoid chain construction, isopentenyl diphosphate IPPwhich is the essential ‘building block’ for all isoprenoid moleculesis first isomerized to dimethylallyl diphosphate DMAPP by an enzyme called ‘IPP isomerase’.

In the same sense, it is easy to see how an electron-rich enolate carbon is nucleophilic in the context of aldol and Claisen elecfrofilica reactions.