Suitable reagents for nitration of thiophene

I know that for nitration of benzene we can use concentrated nitric and sulfuric acid. As furan and pyrrole are less aromatic than benzene, they can be protonated more easily and undergo polymerisation. We can use $$\ce{AcONO2}$$ and $$\ce{AcOH}$$ instead.

Give the order of aromaticity: benzene > thiophene > pyrrole > furan, I'm not sure whether thiophene is suitable for use with concentrated nitric and sulfuric acid.

According to this reference by Katritzky et. al (2005) (see below):

Thiophenes are easy to nitrate compared to other five membered heterocycles. They react with mild nitrating agents such as copper nitrate.

The reference further reports the nitration of thiophene to 2-nitrothiophene by nitric acid/Trifluoroacetic anhydride in 78% yield.

The classic $$\ce{c HNO3/c H2SO4}$$ conditions are too strong and likely to result in substrate degradation.

Reference:

Alan R. Katritzky, Eric F. V. Scriven, Suman Majumder, Rena G. Akhmedova, Novruz G. Akhmedov, Anatoliy V. Vakulenko, "Direct nitration of five membered heterocycles," ARKIVOC 2005, (iii), 179-191 (https://doi.org/10.3998/ark.5550190.0006.320).

Suitable reagents for nitration of thiophene have already been addressed by elsewhere here. Therefore, I'd address why concentrated nitric and sulfuric acid are not suitable for the task, since you are not sure whether the traditional mixture is suitable or not for use to nitrate thiophene.

Because of thiophene is much reactive towards electrophilic substitution, compared to benzene, the traditional mixture of concentrated nitric acid and sulfuric acid is not the ideal nitrating reagent for thiophene (Ref.1). That's because it is essential that the nitration of thiophene should be carried out in the absence of nitrous acid as it can lead to an explosive reaction. A part of the abstract of Ref.1 states that:

".....the mechanism of reaction appears to be the same as that for benzene compounds. However, it is not a suitable preparative procedure owing to the high reactivity of thiophen. Nitration of thiophen, with a solution of nitric acid in acetic acid, proceeds with explosive violence owing to the easy nitrosation of thiophen in an autocatalytic reaction, leading to a variety of unidentified products. If nitrous acid is removed by addition of urea, nitration of thiophen at low concentration is of the first order in thiophen and nitrothiophen is obtained in high yield. However, at high thiophen concentration nitrosation again becomes the predominant reaction. The reason for this is discussed. As has been known for some time, the most successful reagent for the nitration of thiophen is nitric acid in acetic anhydride. Acetic anhydride does not greatly increase the rate of nitration but appears to remove complications due to nitrosation."

In addition to use of acety1 nitrate (nitric acid in acetic anhydride; Ref.2) as the nitrating reagent (which prevent the formation of nitrous acid), nitronium tetraflouroborate is also a satisfactory nitrating reagent for thiophenes (Ref.3): $$\ce{ArH + NO2+BF4- -> ArNO2 + HF + BF3}$$

Like nitration of substituted benzenes, nitration of thiophene also gives at least two mono-substituted products: The major product is 2-nitrothiophene, is accompanied by approximately 10% of the 3-nitro isomer (Ref.4).

References:

1. Anthony R. Butler, James B. Hendry, “Electrophilic substitution on the thiophen ring. Part IV. Kinetics and mechanism of the nitration of thiophen,” J. Chem. Soc. B 1971, 102-105 (https://doi.org/10.1039/J29710000102).
2. V. S. Babasinian (Checked by Roger Adams and A. E. Knauf), “2-Nitrothiophene,” Org. Synth. 1934, 14, 76 and Organic Syntheses, Coll. Vol. 1943, 2, 466 (DOI: 10.15227/orgsyn.014.0076).
3. G. Oláh, S. Kuhn, A. Mlinkó, “Aromatic substitution. Part II. Nitration of aromatic compounds with nitronium tetrafluoroborate and other stable nitronium salts,” J. Chem. Soc. 1956, 4257-4258 (DOI: 10.1039/JR9560004257).
4. Börje Östman, “Preparation of 2-Nitrothiophene of High Isomeric Purity,” Acta. Chem. Scand. 1968, 22(5), 1687-1689 (DOI: 10.3891/acta.chem.scand.22-1687).