# Effect of chloromethyl substituent vs methyl substituent on yields of nitration

The effects of $$\ce {-CH3}$$ and $$\ce {-CCl3}$$ substituents on the product distribution of an electrophilic aromatic substitution reaction are unambiguous. The $$\ce {-CH3}$$ group exhibits a clear electron-donating effect and directs electrophiles to the 2- and 4- positions. Similarly, the $$\ce {-CCl3}$$ group exhibits a clear electron-withdrawing effect and directs to the 3- position. Additionally, due to the steric bulk of the three chlorine atoms, there is significant steric hindrance that electrophiles would face when they attack the 2- position. Hence, the proportion of ortho nitration products would be very low.

Some data on the production distributions for nitrations of particular substituted benzenes, taken from Carey & Sundberg (2007), p. 786, is shown below. Observe that the ratios given are consistent with what we have discussed above.

$$\begin{array}{|c|c|c|c|} \hline \text{Substituent } & ortho & meta & para \\ \hline \ce{\ce {-CH3}} & 56-63 & 2-4 & 34-41 \\ \hline \ce{\ce {-CCl3}} & 7 & 64 & 29\\ \hline \end{array}$$

However, we are interested in the $$\ce {-CH2Cl}$$ substituent, not the $$\ce {-CCl3}$$ substituent. We would, of course, expect the steric effect to be lesser than that of the $$\ce {-CCl3}$$ substituent and we would also expect different electronic effects exerted on the ring. If we were to look at more data from p. 786, as shown below, we would observe that for a substituent like $$\ce {-CHCl2}$$, it still remains as an electron-donating group as the proportion of meta nitration is still lower than that of ortho and para nitration. It seems that even with one $$\ce {C-H}$$ bond, there is still sufficient hyperconjugation with the benzene ring to outweigh the inductive pull of the two electronegative chlorine atoms.

I have no data for the $$\ce {-CH2Cl}$$ substituent but I believe that based on the above analysis, we predict that $$\ce {-CH2Cl}$$ would also be an electron-donating group that is more electron-donating than $$\ce {-CHCl2}$$ but less so than $$\ce {-CH3}$$. Additionally, we would expect to also display directing effects to the 2- and 4- positions However, I have great uncertainty regarding the steric effect of this substituent. How would the ratio of ortho to para nitration be like?

Reference

Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry Part A. Structure and Mechanisms (5th ed.). Springer, 2007.

The selective nitration of benzyl chloride and its application in the synthesis of 4-nitrobenzaldehyde were studied. Nitration of benzyl chloride with $$\ce{HNO3}$$ in the presence of $$\ce{Nb2O5}$$(6% weight percent) at 0-5℃ for 1.5 hours gave mononitro products in 94% yield with 77-80% para selectivty. $$\ce{Nb2O5}$$ was recovered by filtration and reused in the nitration. The yield of mononitro products and para selectivity were above 90% and 76% respectively for 5 times repeated use of $$\ce{Nb2O5}$$. Nitration of benzyl chloride with urea nitrate(UN) in $$\ce{H2SO4}$$ was also studied. The nitration was carried out at 25℃ for 24 hours and gave mononitro products in 98% yield with 90% para selectivity. Base catalyzed hydrolysis of 4-nitrobenzyl chloride at 75-80℃ for 22 hours gave 4-nitrobenzyl alcohol in 77% yield with 98% purity. Oxidation of 4-nitrobenzyl alcohol in dilute nitric acid at 55℃ for 3 hours gave 86% 4-nitrobenzaldehyde, compared with industrial oxidation of 4-nitrobenzyl alcohol,whichwas carried out at 70℃for 4 hours and gave 70% 4-nitrobenzaldehyde. 4-nitrobenzaldehyde was also obtained in 86% yield with 92% purity via ionic liquid immobilized $$\ce{TEMPO/CuCl}$$ catalyzed oxidation of 4-nitrobenzyl alcohol with molecular oxygen in ionic liquid $$\ce{[bmim]PF6}$$. The reaction condition is mild, the selectivity is high, and no 4-nitrobenzoic acid is detected in any case.