What is bromination of propylbenzene

Experiments:
Experiment: Behavior of toluene towards bromine and Baeyers Reagent
Experiment: Obtaining benzoic acid from toluene
Experiment: bromination in the core of the toluene compared to the bromination of the side chain


Toluene and the three xylene isomers are alkylbenzenes. The methyl radical activates the benzene nucleus through a (+ I) effect. It pushes electrons into the ring and increases the electron density, so that secondary substitutions, as in Friedel-Crafts alkylation, take place more quickly.

But the methyl radical itself is also reactive. There is often a competitive situation between the reaction of the aromatic nucleus through substitution and various reactions on the methyl radical outside the ring.


Reactions of the aromatic nucleus
As with the bromination of benzene, no bromine is added to the nucleus in toluene without the aid of a catalyst. In addition, none of the BaeyerSample analogous reaction with cold potassium permanganate solution (-> experiment).


Oxidation of the side chain
However, if a mixture of potassium permanganate and toluene is heated, a reaction can be observed (-> experiment). However, it is not the aromatic nucleus of the toluene that is oxidized, but the carbon atom of the methyl radical.

These reactions can be controlled. How, let's look at the different reactions of toluene with bromine. There are two rules that every chemistry student is familiar with.


The SSS rule: bromination of the methyl group
If the mixture of bromine and toluene is heated, a substitution reaction only takes place on the side chain (-> experiment).

When it is supplied with energy, bromine first breaks down into two radicals, one of which replaces a hydrogen atom in the methyl group. This releases a large amount of gaseous HBr.

The reaction is already known from the halogenation of alkanes. It can also be started by exposure to light (not just from the sun ...).

At elevated temperature or exposure to light, bromination of the side chain takes place.

That says the so-called SSS rule (S.onne (S.radiation) - S.ieden - S.side chain).


The KKK rule: the reactivity of the aromatic nucleus
With fine iron filings as a catalyst, however, the electron system of the nucleus can of course also be attacked electrophilically with toluene (-> experiment). This is the well-known bromination of an aromatic, as described on the page Substitutions - the typical reactions in the aromatic system. Toluene reacts to ortho- and para-bromotoluene.

Due to the influence of the (+ I) effect of the methyl radical, this reaction takes place faster than the substitution on benzene. More details can be found here: Substituents - pacemakers and signal boxes for aromatics.

At low temperatures and in the presence of a catalyst, bromination of the aromatic nucleus takes place.

This includes the KKK rule (KLte-KanalyzerKern).


Further texts on the subject of `` hydrocarbons ''