The bromination of phenol is easier than that of benzene
The second substitution
The electrophilic substitution on the benzene ring was discussed on the last page. However, if the benzene ring is already "occupied" with another functional group, with a so-called primary substituent, then this definitely has an influence on the course of the reaction. Such an already existing substitute, for example an OH group as in phenol, an NH2-Group like in aniline or a COOH group like in benzoic acid, can increase or decrease the reactivity of the benzene ring, and it can influence which carbon atoms of the benzene ring the electrophile attacks.
With the influence of primary substituents, the following three effects can be distinguished:
- Inductive effect
- Mesomeric effects
- Steric Effects
These three effects can have two different types of effects:
- Activating or deactivating effects
- Conducting effects
The first step in electrophilic substitution is the formation of the σ complex. Due to the high electron density of the π electrons, electrophiles are attracted and attach to a sp2-hybridized carbon atom of the benzene ring, creating the σ-complex. The high electron density of the benzene ring plays a decisive role here.
If the benzene ring already has functional groups such as -OH, -NH2, -COOH, -CH3 inter alia is connected - with so-called primary substituents - so these have an influence on the electron density in the benzene ring, either through a positive or a negative inductive effect.Effect on the benzene ring
Primary substituents with an -I effect lower the electron density in the benzene ring and therefore reduce the reactivity of the aromatic = deactivating effect. The bromination of nitrobenzene C6H5-NO2 or chlorobenzene C6H5-Cl is therefore slower than the bromination of benzene C.6H5-H.
Primary substituents with a + I effect increase the electron density of the benzene ring and make the formation of the π complex easier = activating effect. The bromination of toluene is therefore faster than the bromination of benzene.
Critical note for experts:
Inductive effects actually only affect the system of σ electrons; for thisE. but the π electrons of the benzene ring are responsible. Nevertheless, the bromination of toluene C takes place6H5-CH3 faster than the bromination of benzene.
On my lexicon page "Inductive Effect" you will also find an overview of which groups have which effect.
Note that some groups can change their charge when the pH of the solvent changes. For example, a neutral COOH group becomes a negative -COO- - Group, if you add a base (proton acceptor). From an NH2-Group can generate an -NH when adding an acid (proton donor)3+-Group become.
I have also designed this topic for my students as a one-page worksheet, which you can download here: "Inductive Effects".
Mesomeric effectsA contradiction ...
When you brominate phenol, you find that the reaction takes place much faster than the bromination of benzene, some authors even speak of a speed 1000 times faster. However, this is not to be understood, since the OH group is one of the substituents with a negative inductive effect. In fact, the bromination of phenol should be slower than the bromination of benzene.... and its clarification
As always in such situations, you should quickly look for other influencing factors. In the case of electrophilic substitution, not only inductive effects play an important role, but also the so-called mesomeric effects. I explained what mesomeric effects are on my lexicon page "Mesomerism". The explanation in the SimpleChemics film on mesomerism is even better.+ M effect in aniline
Let's take a look at the aniline molecule:
Five resonance structures of the aniline molecule
We can draw five different boundary structures or resonance structures of the aniline molecule, with three of them a so-called charge separation occurs. The positive charge is on the nitrogen atom, while the negative charge is concentrated on three carbon atoms of the benzene ring. These three carbon atoms are almost predestined to take up a positive charge when the sigma complex is formed in the course of an electrophilic addition.
The + M effect activates and directs the substitution in the o and p directions
This positive mesomeric effect (caused by energetically favorable resonance structures, also known as the + M effect) not only activates the electrophilic substitution itself (i.e. it reduces the activation energy for the formation of the sigma complex and thereby accelerates the reaction), but it also directs nor the second substitute. In the bromination of aniline, 2-bromo-aminobenzene and 4-bromo-aminobenzene are preferably formed.
The + M effect of a first substituent activates the electrophilic substitution by introducing a negative charge into the benzene ring (increasing the electron density).
At the same time, the + M effect directs the substitution in the para and ortho directions, because the negative charge introduced is concentrated on these carbon atoms.
The five boundary structures of benzoic acid
The C = O double bond of the benzoic acid is responsible for the fact that negative charges are withdrawn from the benzene ring, at least in three of the five boundary structures. This mesomeric effect decreased the electron density in the ring and is therefore referred to as the negative mesomeric or -M effect.
The -M effect ensures that the positive charge is preferentially in the ortho and para position. These positions were special with the + M effect Cheap for the formation of the sigma complex, with the -M effect these positions are special unfavorable. The result: Primary substituents with an -M effect direct the second substituent away from the ortho and para position, leaving only the meta position.
The -M effect of an initial substituent deactivates the electrophilic substitution by introducing a positive charge into the benzene ring (reduction of the electron density).
At the same time, the -M effect directs the substitution in the meta direction, because the positive charge introduced is concentrated at the ortho and para positions.
+ M substituents activate and direct the second substituent into the ortho and para position, -M substituents deactivate and direct the second substituent into the meta position.Teaching material:
I have also designed this topic for my students as a one-page worksheet, which you can download here: "Mesomeric Effects".
The three possible bromophenols: ortho, meta and para
The bromination of phenol takes place fairly quickly, although the -I effect of the OH group itself has a deactivating effect. The + M effect of the OH group is, however, much stronger, so that activation predominates overall. In addition, the + M effect directs the substitution in the ortho and para directions; only about 5% of the meta-product is formed.
The interesting question, however, is why the para product is predominantly formed. Shouldn't the same amount of ortho- and para-products be created?
A new effect comes into play here. In addition to the inductive and the mesomeric effect, there is always a steric effect in chemical reactions. In many simple reactions, the steric effect does not play a particularly important role, but in this example it does. The O atom of the OH group is quite electronegative, as is the Br atom. You have to imagine that both atoms are surrounded by a negative cloud of charge. Then the ortho product is energetically less favorable than the para product, because with the ortho product the two negative charge clouds are closer to each other than with the para product.Teaching material:
I have also designed this topic as a one-page worksheet for my students, which you can download here: "Conducting Effects".
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