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ELECTROPHILIC SUBSTITUTION


Background
Electrophilic substitution happens in many of the reactions of compounds containing benzene rings - the arenes. For simplicity, we'll only look for now at benzene itself.

Note: Before you start it would be a good idea if you had a clear idea about the structure of benzene. Check your syllabus now to find out what you need to know, and then read the page on the modern orbital view of benzene in the organic bonding section of this site. Don't forget to look in the section(s) in your syllabus on bonding as well as organic chemistry.
Haven't got a syllabus? If you are working towards a UK-based exam (A level or its equivalent), follow this link to find out how to get one.

This is what you need to understand for the purposes of the electrophilic substitution mechanisms:

Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached.

There are delocalised electrons above and below the plane of the ring.

The presence of the delocalised electrons makes benzene particularly stable.

Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.

Benzene is represented by this symbol, where the circle represents the delocalised electrons, and each corner of the hexagon has a carbon atom with a hydrogen attached.


Electrophilic substitution reactions involving positive ions
Benzene and electrophiles

Because of the delocalised electrons exposed above and below the plane of the rest of the molecule, benzene is obviously going to be highly attractive to electrophiles - species which seek after electron rich areas in other molecules.

Species: A useful word which can mean any particle you want it to mean - an atom, a molecule, an ion or a free radical.

The electrophile will either be a positive ion, or the slightly positive end of a polar molecule.

Help! If you aren't sure what a polar molecule is, read about electronegativity and polar bonds before you go on.
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The delocalised electrons above and below the plane of the benzene molecule are open to attack in the same way as those above and below the plane of an ethene molecule. However, the end result will be different.

If benzene underwent addition reactions in the same way as ethene, it would need to use some of the delocalised electrons to form bonds with the new atoms or groups. This would break the delocalisation - and this costs energy.

Note: You can read about electrophilic addition to ethene if you are interested.
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Instead, it can maintain the delocalisation if it replaces a hydrogen atom by something else - a substitution reaction. The hydrogen atoms aren't involved in any way with the delocalised electrons.

In most of benzene's reactions, the electrophile is a positive ion, and these reactions all follow a general pattern.

The general mechanism

The first stage

Suppose the electrophile is a positive ion X+.

Two of the electrons in the delocalised system are attracted towards the X+ and form a bond with it. This has the effect of breaking the delocalisation, although not completely.

Note: If you aren't sure about the use of curly arrows in mechanisms, you must follow this link before you go on.
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The ion formed in this step isn't the final product. It immediately goes on to react with something else. It is just an intermediate.

There is still delocalisation in the intermediate formed, but it only covers part of the ion. When you write one of these mechanisms, draw the partial delocalisation to take in all the carbon atoms apart from the one that the X has become attached to.

The intermediate ion carries a positive charge because you are joining together a neutral molecule and a positive ion. This positive charge is spread over the del

The Bohr model was a one-dimensional model that used one quantum number to describe the distribution of electrons in the atom. The only information that was important was the size of the orbit, which was described by thenquantum number. Schr�dinger's model allowed the electron to occupy three-dimensional space. It therefore required three coordinates, or threequantum numbers, to describe the orbitals in which electrons can be found.The three coordinates that come from Schr�dinger's wave equations are the principal (n), angular (l), and magnetic (m) quantum numbers. These quantum numbers describe the size, shape, and orientation in space of the orbitals on an atom.Theprincipal quantum number(n) describes the size of the orbital. Orbitals for whichn= 2 are larger than those for whichn= 1, for example. Because they have opposite electrical charges, electrons are attracted to the nucleus of the atom. Energy must therefore be absorbed to excite an electron from an orbital in which the electron is close to the nucleus (n= 1) into an orbital in which it is further from the nucleus (n= 2). The principal quantum number therefore indirectlydescribes the energy of an orbital.Theangular quantum number(l) describes the shape of the orbital. Orbitals have shapes that are best described as spherical (l= 0), polar (l= 1), or cloverleaf (l= 2). They can even take on more complex shapes as the value of the angular quantum number becomes larger.There is only one way in which a sphere (l= 0) can be oriented in space. Orbitals that have polar (l= 1) or cloverleaf (l= 2) shapes, however, can point in different directions. We therefore need a third quantum number, known as themagnetic quantum number(m), to describe the orientation in space of a particular orbital. (It is called themagneticquantum number because the effect of different orientations of orbitals was first observed in the presence of a magnetic field.)