Electrophilic addition

The Electrophilic Addition (AE ) is a chemical reaction in organic chemistry, react with the unsaturated hydrocarbons (alkenes or alkynes ) with different classes of substances. A common feature of the electrophilic additions is that the reaction by the attack of an electron- loving particle, the electrophile is introduced to the double or triple bond, more precisely to the π - bond.

  • 2.1 Addition of water to alkenes
  • 2.2 Synthesis of diols

Addition to the C = C double bond

Addition of halogens to alkenes

Molecular halogens may be added in a two-step mechanism of the double bond of alkenes. The halogen molecule occurs here in interaction with the double bond of the alkene, thereby the halogen molecule is polarized, the electrons split heterolytic bond. In the first step the addition of bromonium ion (Br ) at the double bond of the carbon atom at which it forms a short-lived cyclic cation to cleave the double bond takes place. In the second step, the bromide ion attacks nucleophilic at the positivised carbon atom to form the saturated α, β - brominated product.

The addition of halogens is limited to the elements chlorine, bromine and iodine. Molecular fluorine, however, is highly reactive and unselective would CC and CH bonds attack. Chlorine is more electronegative and poor polarizable than bromine and iodine. For this reason, the addition of chlorine over a carbocation intermediate which is not stabilized by the formation of a cyclic Bromoniumspezies runs.

The formation of stabilized cations, as occurs for the addition of bromine and iodine, also determines the stereochemistry of the dihalide formed. This one side of the molecule is effectively shielded for nucleophilic attack, so the attack can only take place from the opposite side. This is known as an anti -addition. This means that during the addition of bromine, and iodine, with high selectivity, the anti- product is formed.

Addition of hydrogen halides

Also, hydrogen halides can be added to alkenes, which haloalkanes are formed. This addition also proceeds in two stages. In the first step the proton of the acid used is added to the double bond. Halonium ions as opposed to the proton does not have the ability to stabilize the positive charge and therefore a carbocation is formed. At this now added in the second step the anion of the acid.

In this addition, two different products may be formed that differ in the position of the halide. Which of the products is preferably formed depends on the stabilization of the intermediate carbocation and is described by the Markovnikov rule, which states that the hydrogen atom is always bound to the already hydrogen -rich carbon atom. Preferably, the product is formed which has a better stabilized carbocation. Generally the more stable carbocation is alkylated higher. Depending on the starting material high regioselectivity can be achieved in this reaction.

Alcohols from alkenes

Addition of water to alkenes

Water is a poor nucleophile, which is why the reaction of alkenes with water is usually not the expected reaction product, an alcohol leads. However, the reaction proceeds under acid catalysis. As previously described herein is added in the first step, a proton of the acid to the double bond. The carbocation formed is now sufficiently electrophilic for nucleophilic attack of a water molecule. Cleaves after the addition of a proton to give the desired alcohol is produced.

Is the anion of the acid itself a nucleophile, so the anion competes with water occurs by the addition to the carbocation (see also previous section).

Synthesis of diols

The addition of water to the double bonds yields alcohols, diols, however, can not be synthesized in this way. These are made ​​possible by the addition of inorganic oxygen compounds, such as potassium permanganate or osmium tetroxide. In the first step here added to the oxygen carrier to the alkene in the second step, the resulting intermediate is hydrolyzed so that the diol is released.

Among other things, the development of a stereoselective dihydroxylation of the Nobel Prize in Chemistry 2001 was awarded to Barry Sharpless.

→ see also Main article: dihydroxylation

Addition to conjugated double bond

The addition to conjugated double bonds follows the same rules as the addition to isolated double bonds described above. However, it should be noted that the carbocation formed in the first step can be mesomeric stabilized. This stabilization of the positive charge is distributed on a plurality of carbon atoms, whereby the nucleophile can attack at different positions. Usually in such reactions, a mixture of two products is obtained.

Addition to alkynes

Analogously to the addition of alkenes and additions may be made to the triple bond of alkynes. Mechanistically run this analogy to the addition to alkenes. In the first step attacks a electrophile of the triple bond, thereby forming a vinyl cation formed. In the second step then attacks an existing nucleophile on the cationic position. Here, a substituted alkene. This can react further in a second addition reaction to the corresponding alkane.

Introduced by the first addition of electron- withdrawing groups such as halogen atoms, the second addition proceeds much more slowly as the electron density of double bond is reduced. The reaction can so often be stopped after the first addition to level of the alkene.

The acid-catalyzed addition of water to alkynes provides enols. This tautomerization to ketone ( keto -enol tautomerism ), so the addition of water to alkynes is a possibility for the synthesis of ketones.

Addition to carbonyls

Carbonyl addition reactions can also be performed. In contrast to the addition of carbon-carbon bonds, the C-O bond due to the higher electronegativity compared to carbon is always polarized. Nucleophiles thus always access to the electron-deficient carbon atom, electrophiles react consequently to the oxygen atom. An example of an electrophilic addition of a carbonyl group, the formation of acetals from ketones and aldehydes. In the first step of this carbonyl oxygen is protonated by an acid. This results in a positive charge on the carbonyl carbon, an alcohol nucleophile to attack the second step. By splitting off of the proton at the former alcohol hemiacetal is released.