A demonstration of bromine substitution and addition reactions is helpful at this point, and a virtual demonstration may be initiated by clicking here. Many other substitution reactions of benzene have been observed, the five most useful are listed below chlorination and bromination are the most common halogenation reactions. Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution.
The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution. The specific electrophile believed to function in each type of reaction is listed in the right hand column. A two-step mechanism has been proposed for these electrophilic substitution reactions. In the first, slow or rate-determining, step the electrophile forms a sigma-bond to the benzene ring, generating a positively charged benzenonium intermediate.
In the second, fast step, a proton is removed from this intermediate, yielding a substituted benzene ring. The following four-part illustration shows this mechanism for the bromination reaction. Also, an animated diagram may be viewed. These may be viewed repeatedly by continued clicking of the "Next Slide" button. This mechanism for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates.
To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes:.
The cation may bond to a nucleophile to give a substitution or addition product. The cation may transfer a proton to a base, giving a double bond product. The cation may rearrange to a more stable carbocation, and then react by mode 1 or 2. S N 1 and E1 reactions are respective examples of the first two modes of reaction. The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed.
The carbocation intermediate in electrophilic aromatic substitution the benzenonium ion is stabilized by charge delocalization resonance so it is not subject to rearrangement. In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 ie. When substituted benzene compounds undergo electrophilic substitution reactions of the kind discussed above, two related features must be considered:. The first is the relative reactivity of the compound compared with benzene itself.
Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner. For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated by the case of anisole in the virtual demonstration above.
In contrast, a nitro substituent decreases the ring's reactivity by roughly a million. This activation or deactivation of the benzene ring toward electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, as measured by molecular dipole moments. In the following diagram we see that electron donating substituents blue dipoles activate the benzene ring toward electrophilic attack, and electron withdrawing substituents red dipoles deactivate the ring make it less reactive to electrophilic attack.
The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:. The first is the inductive effect of the substituent.
Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Q9 Draw the cis and trans structures of hexene. Which isomer will have higher b. Q10 Why is benzene extra ordinarily stable though it contains three doublebonds? Q11 What are the necessary conditions for any system to be aromatic? Q12 Explain why the following systems are not aromatic? Q13 How will you convert benzene into i p - nitrobromobenzene ii m - nitrochlorobenzene iii p - nitrot Q15 What effect does branching of an alkane chain has on its boiling point?
Q16 Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yi Q17 Write down the products of ozonolysis of 1,2-dimethylbenzene o-xylene.
How does the result support Kekule struc Q18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour Q19 Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficul 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.
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. If you aren't sure what a polar molecule is, read about electronegativity and polar bonds before you go on. 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. 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. This has the effect of breaking the delocalisation, although not completely. In able to decide the point of attack in molecule, inductive affects changes the electron density in a molecule. Mesomeric effect The mesomeric effect If all of the energy levels in the atom are full populated with electrons, it is said to be stable, and in most cases, is therefore unreactive.
Examples of this include the noble or inert gases such as neon or argon. However if the outer energy level of the atom is not stable, it will automatically try to either gain or lose electrons to become stable. This is achieved by an ionic reaction. Ionic bonding occurs when the outer atoms of on material changes orbit and joins another material for example: Sodium chloride As you can see, sodium is a group one metal it has one electron on its outer energy level so is therefore unstable.
Ionic Bonds Ions are atoms with an extra electron or a missing electron. But a normal atom would be neutral because it has the same number of electrons as the atomic number. If you are an atom and you are missing one electron, it does not mean that you are another atom, but you are not a complete atom either. You are something new, an ion. The goal of an atom is to be happy. Also the polarity of drug is partially since there are no charges.
It seems like its partition happens more in lipid because of the presence many non-polar hydrocarbons than polar. It can be ionized in H2O since there is acidic functional groups phenol and amide. So in simpler words, it means that one of the two atoms bonded together contributes all of the shared electrons.
The condition for the formation of dative bonding is that the donor atom should have lone of pair electrons meaning it should be an electron rich species. On the other hand, the acceptor atom must have the ability of holding that pair of electrons, in other words, it should be an electron deficient species.
Most polyatomic cations and anions contain covalent and coordinate covalent bonds.
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