Why are halogens ortho- para- directors?

Why are halogens ortho-, para- directors?

• All activating groups are also ortho-, para- directors.
• Halogens (F, Cl, Br, I) are notable in that they are deactivating ortho-, para- directors. Why?
• In electrophilic aromatic substitution (EAS), addition at the ortho- or para– position results in a carbocation intermediate with a resonance form containing a carbocation directly adjacent to the directing group.
• Halogens have a lone pair that can form a pi-bond with the adjacent carbocation.
• Even though halogens are deactivating overall, this “pi donation” helps to stabilize the transition state leading to ortho– or para- products, which is why they are ortho-, para- directors.

Table of Contents

1. All “Activating Groups” Are ortho-, para- Directors
2. Most “Deactivating Groups” Are meta- Directors
3. So Why Are Halogens Ortho-,  Para- Directors?
4. The Lone Pair Of Halogens Stabilizes Adjacent Carbocations Formed In The ortho- And para- Intermediates
5. Notes
6. (Advanced) References and Further Reading

1. All “Activating Groups” Are ortho-, para- Directors

The previous post in this series tried to show that the key to understanding why a substituent is an ortho-, para- director or meta– director lies in understanding how it influences the stability of the ortho-, para- and meta- carbocation intermediates.

In this post, we’ll show why halogens are ortho-, para- directors even though they are deactivating.

In ortho- and para- addition, there’s a resonance form where the carbocation ends up directly bonded to the substituent.

In meta– addition, the carbocation ends up on the carbon adjacent to the carbon bonded to the substituent.

Any group that can donate electron density to a carbocation will be an ortho- , para- director. 

We’ve seen that all activating groups  (such as amines, ethers, and alkyl groups) are ortho-, para– directors.

These groups can donate electron-density to an adjacent carbocation through inductive effects (a.k.a. “sigma-donation”, as with alkyl groups) and/or pi-donation, where donation of a lone pair from an attached oxygen or nitrogen provides a key resonance form where all carbons have a full octet. Carbocations, being electron-poor, are stabilized by electron-rich neighbors.

2. Most Deactivating Groups Are meta- Directors

Most deactivating groups are meta- directors.

They withdraw electron-density through an adjacent carbocation through being “sigma-acceptors” (such as the electron-withdrawing CF₃ group, or the ammonium [–NR₃⁺] group) and/or “pi acceptors”, such as nitro, carbonyl, or sulfonyl groups.

Carbocations are destabilized by electron-poor neighbors.

Which brings us to the peculiar case of halogens.

3. So Why Are Halogens Ortho-,  Para- Directors?

Halogens are deactivating substituents, which is to say that the rate of electrophilic aromatic substitution is lowered when a halogen replaces hydrogen (H) as a substituent. [See this earlier post on “activating vs. deactivating substituents“] . This reflects their high electronegativity, withdrawing electron density from the ring. [Note 1]

At first glance, this might seem to preclude them from being ortho-para directors.  But lo,  they are!

How can we rationalize this observation?

Recall that  “activating” vs. “deactivating” just compares how well a substituent stabilizes a carbocation relative to hydrogen.

That’s not the right comparison here. Just like the old joke goes, it’s not about outrunning the bear – it’s about outrunning the other guy.  

The key for a substituent being an ortho-, para- versus meta- director is the stability of the ortho- and para– carbocation intermediates versus the meta- carbocation intermediate.

4. 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. This results in a resonance form where carbon has a full octet.

Note that I didn’t say “predict” – I said “rationalize” : – ) .  Rationalization involves looking backward from a result and trying to understand why something might have happened.  There are several variables at work here that tug in opposite directions, and predicting the magnitude of these individual effects in the absence of a strong computational model is a fool’s errand. That’s why we run experiments!

From these experiments, it seems that a carbocation intermediate which has a pi-donor is more important toward determining whether it is an ortho-, para- director than whether it is a strong electron withdrawing group.

Notes

Note 1.  It is interesting to note, however, that despite having the highest electronegativity, fluorine is actually the most activating of the halogens (the other halogens are relatively similar in their deactivating powers). This can be attributed to the better orbital overlap of the fluorine sp³ orbitals with the 2p orbitals of the pi system. [For similar reasons, BF₃ is a worse Lewis acid than BCl₃ and BBr₃ ,  since the fluorine orbitals overlap much better with the empty boron 2p orbital].

Note 2. 

Are there any other deactivating ortho-, para- directors?

Yes. NO.

NO?

Yes, NO. Nitroso.

Knowing what we now know about halogens, what predictions would you make for the nitroso group, a group that is somewhat electron withdrawing, but also bears a lone pair on the nitrogen.

The yields aren’t great, but there you go.

(Advanced) References and Further Reading

1. A. F. Holleman, Die direkte Einführung von Substituenten in den Benzolkern
   Rec. Trav. Chim. Pays-Bas 1910, 12, 455-456
   DOI: 10.1002/recl.19100291205
   A.F Holleman from 1910 said that ortho–para orientation is associated with activation and meta orientation with deactivation.
2. —The nature of the alternating effect in carbon chains. Part XXII. An attempt further to define the probable mechanism of orientation in aromatic substitution
   Christopher Kelk Ingold and Florence Ruth Shaw
   J. Chem. Soc. 1927, 2918-2926
   DOI: 10.1039/JR9270002918
   An early paper by the influential Physical Organic Chemist, Prof. C. K. Ingold, stating that halogenobenzenes are inductively electron-withdrawing but simultaneously resonance-stabilizing.
3. Influence of directing groups on nuclear reactivity in oriented aromatic substitutions. Part IV. Nitration of the halogenobenzenes
   Marjorie L. Bird and Christopher K. Ingold
   J. Chem. Soc. 1938, 918-929
   DOI: 10.1039/JR9380000918
   The relative rates of nitration for the halobenzenes are determined here, and it is seen that the order of reactivity is PhF>PhI>PhCl, PhBr
4. The Anomalous Reactivity of Fluorobenzene in Electrophilic Aromatic Substitution and Related Phenomena
   Joel Rosenthal and David I. Schuster
   Journal of Chemical Education 2003, 80 (6), 679
   DOI: 1021/ed080p679
   A very interesting paper, suitable for curious undergrads, and discusses something that most practicing organic chemists will know empirically – fluorobenzene is almost as reactive as benzene in EAS or Friedel-Crafts reactions, which is counterintuitive when one considers electronic effects.
5. —A new orientation rule and the anomaly of the nitroso-group
   Dalziel Llewellyn Hammick and Walter S. Illingworth
   J. Chem. Soc. 1930, 2358-2364
   DOI: 10.1039/JR9300002358
6. 93. The orienting power of the nitroso-group
   Dalziel Ll. Hammick, Randal G. A. New, and Leslie E. Sutton
   J. Chem. Soc. 1932, 742-748
   DOI: 10.1039/JR9320000742
   These two papers discuss the electronics of the nitroso substituent. Both papers refer to C. K. Ingold’s experiment where he observed p-substitution of nitrosobenzene from bromination in CS₂. The authors attempt to explain this by suggesting that in certain solvents nitrosobenzene dimerizes, and the dimer prefers o,p-substitution. This is worth reevaluating with modern methods (hint, hint)!