Why halogens are deactivating

Thus; stabilizing the intermediate step, speeds up the reaction; and this is due to the decrease of the activating energy. On the other hand, the deactivating groups, withdraw the electrons away from the carbocation formed in the intermediate step, thus; the activation energy is increased which slows down the reaction. The nitration of aniline is going to be faster than the nitration of nitrobenzene, since the aniline is a ring with NH 2 substituent and nitrobenzene is a ring with NO 2 substiuent.

As described above NH 2 is an activating group which speeds up the reaction and NO 2 is deactivating group that slows down the reaction. The direction of the reaction The activating group directs the reaction to the ortho or para position, which means the electrophile substitute the hydrogen that is on carbon 2 or carbon 4.

Activating groups ortho or para directors When the substituents like -OH have an unshared pair of electrons, the resonance effect is stronger than the inductive effect which make these substituents stronger activators, since this resonance effect direct the electron toward the ring. Deactivating group meta directors The deactivating groups deactivate the ring by the inductive effect in the presence of an electronegative atom that withdraws the electrons away from the ring.

Substituents determine the reactivity of rings The reaction of a substituted ring with an activating group is faster than benzene. References Schore, N. Organic Chemistry, structure and function, 5th ed.

New York,NY: W. Freeman and Company. Fryhle, C. Organic Chemistry, 9th ed. Danvers,MA: Wiley. This can be attributed to the better orbital overlap of the fluorine sp 3 orbitals with the 2p orbitals of the pi system. It is mostly an article about the first curved arrows ever drawn.

But near the end there is an interesting discussion about how geometry of the molecule changes between ground state and transition state. You alluded to it in your pictures.

The left structure in the figure with the Cl acting as an e-donor on the benzene ring during EArS has too many e around the Cl.

One of the lone pairs has been used in the resonance interaction…. In the fifth figure from top, chlorine is shown to suffer positive charge. With 3 electron pairs as well as a double bond, how chlorine can become electron deficient? Chlorine is still more electronegative than carbon, and is still partially negative.

Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Which brings us to the peculiar case of halogens. How can we rationalize this observation?

The Lone Pair Of Halogens Stabilizes Adjacent Carbocations Formed In The ortho- And para- Intermediates We can rationalize the ortho- , para- directing ability of halogens by noting that these atoms have attached lone pairs, and can albeit poorly act as pi-donors. Are there any other deactivating ortho-, para- directors? Yes, NO. Advanced References and Further Reading A.

The resonance effect is observed when the atom connected to the aromatic ring has a lone pair s of electrons, which can donate electron density by the conjugated electron system thus stabilizing the resonance contributor during the substitution reaction.

This is the inductive effect and we do count it. For example, phenol and aniline are very reactive towards halogenation and do not require a Lewis acid like AlCl 3 or FeBr 3. In fact, they are so reactive that the reaction does not have any selectivity for producing a mono-halogenated product; all the 3 ortho-para positions react:.

Like the nitrogen and oxygen, the halogens F, Cl, Br, I, are all more electronegative than carbon and they withdraw the electron density from the ring by the inductive effect. Fluorine donates the nonbonded pair of electrons to the ring by resonance as well. However, it is so electronegative that this effect is shaded, and overall fluorine makes the ring electron-poor and deactivates it towards electrophilic substitutions.

The other halogens are not as electronegative. However, they do not show a strong resonance effect either. The reason for this is the lack of a good overlap between the 2p orbital of the carbon and 4p and 5p orbitals of chlorine and bromine. Remember, the conjugation and bond-making depend on the efficiency of the orbital overlapping.

Now, as to why they are ortho- para directors, you need remember that they are still electron-donators by resonance which does stabilize the transition state by an additional resonance contributor whether it is significant or not. To keep this in nutshell, remember that halogens deactivate the ring, but they still orient the substitution to the ortho and para positions because of the resonance effect.

Halogens are deactivating but ortho and para directing ,They increase acidic strength or decrease acidic strength???