6PPD

6PPD is prepared by reductive amination of methyl isobutyl ketone (which has six carbon atoms, hence the '6' in the name) with phenyl phenylenediamine (PPD).Hans-Wilhelm Engels et al., "Rubber, 4. Chemicals and Additives" in Ullmann's Encyclopedia of Industrial Chemistry, 2007, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a23_365.pub2}} This produces a racemic mixture.

6PPD is a common rubber antiozonant found in vehicle tires. It is mobile within the rubber and slowly migrates to the surface via blooming. On the surface it forms a "scavenger-protective film" that reacts with the ozone more quickly than the ozone can react with the rubber.{{cite journal |last1=Lattimer |first1=R. P. |last2=Hooser |first2=E. R. |last3=Layer |first3=R. W. |last4=Rhee |first4=C. K. |title=Mechanisms of Ozonation of N-(1,3-Dimethylbutyl)-N′-Phenyl-p-Phenylenediamine |journal=Rubber Chemistry and Technology |date=1 May 1983 |volume=56 |issue=2 |pages=431–439 |doi=10.5254/1.3538136}} This process forms aminoxyl radicals{{cite journal |last1=Cataldo |first1=Franco |last2=Faucette |first2=Brad |last3=Huang |first3=Semone |last4=Ebenezer |first4=Warren |title=On the early reaction stages of ozone with N,N′-substituted p-phenylenediamines (6PPD, 77PD) and N,N′,N"-substituted-1,3,5-triazine "Durazone®": An electron spin resonance (ESR) and electronic absorption spectroscopy study |journal=Polymer Degradation and Stability |date=January 2015 |volume=111 |pages=223–231 |doi=10.1016/j.polymdegradstab.2014.11.011}}{{cite journal |last1=Cataldo |first1=Franco |title=Early stages of p-phenylenediamine antiozonants reaction with ozone: Radical cation and nitroxyl radical formation |journal=Polymer Degradation and Stability |date=January 2018 |volume=147 |pages=132–141 |doi=10.1016/j.polymdegradstab.2017.11.020}} and was first thought to degrade only to the quinone diimine, but has since been understood to continue to oxidize to quinones, amongst other products.{{cite journal |last1=Seiwert |first1=Bettina |last2=Nihemaiti |first2=Maolida |last3=Troussier |first3=Mareva |last4=Weyrauch |first4=Steffen |last5=Reemtsma |first5=Thorsten |title=Abiotic oxidative transformation of 6-PPD and 6-PPD quinone from tires and occurrence of their products in snow from urban roads and in municipal wastewater |journal=Water Research |date=April 2022 |volume=212 |page=118122 |doi=10.1016/j.watres.2022.118122|pmid=35101694 |bibcode=2022WatRe.21218122S |s2cid=246336931 |doi-access=free }} Despite 6PPD being used in tires since the mid 1970s, its transformation to quinones was first recognized in 2020.

{{citation|journal=Science|author1=Tian, Zhenyu|author2=Zhao, Haoqi|author3=Peter, Katherine T.|author4=Gonzalez, Melissa|author5=Wetzel, Jill|author6=Wu, Christopher |author7=Hu, Ximin|author8=Prat, Jasmine

|author9=Mudrock, Emma|author10=Hettinger, Rachel|author11=Cortina, Allan E.|author12=Biswas, Rajshree Ghosh |author13=Kock, Flávio Vinicius Crizóstomo|author14=Soong, Ronald|author15=Jenne, Amy|author16=Du, Bowen|author17=Hou, Fan|author18=He, Huan|author19=Lundeen, Rachel|author20=Gilbreath, Alicia|author21=Sutton, Rebecca|author22=Scholz, Nathaniel L.|author23=Davis, Jay W.|author24=Dodd, Michael C.|author25=Simpson, Andre

|author26=McIntyre, Jenifer K.|title=A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon|quote=... existing TWP [tire wear particle] loading, leaching, and toxicity assessments are clearly incomplete. ... Accordingly, the human health effects of such exposures merit evaluation. ... It is unlikely that coho salmon are uniquely sensitive ... ( in print 8 Jan 2021)|doi=10.1126/science.abd6951|date= 3 December 2020|volume=371|issue=6525

|pages=185–189|pmid=33273063|s2cid=227281491|doi-access=free|bibcode=2021Sci...371..185T }}

Also an erratum to this paper published in Science vol. 375, No. 6582, 18 Feb 2022 {{doi|10.1126/science.abo5785}} reporting the updated toxicity estimates, as referenced below.

The oxidized products are not effective antiozonants, meaning that 6PPD is a sacrificial agent.

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The tendency of 6PPD to bloom towards the surface is protective because the surface film of antiozonant is replenished from reserves held within the rubber. However, this same property facilitates the transfer of 6PPD and its oxidation products into the environment as tire-wear debris. The 6PPD-quinone (6PPD-Q, CAS RN: 2754428-18-5) is of particular and increasing concern, due to its toxicity to fish.

6PPD and 6PPD-quinone enter the environment through tire-wear particles and are sufficiently water-soluble to enter river systems via urban runoff. From here they become widely distributed (at decreasing levels) from urban rivers through estuaries, coasts and finally deep-sea areas.{{cite journal |last1=Zeng |first1=Lixi |last2=Li |first2=Yi |last3=Sun |first3=Yuxin |last4=Liu |first4=Liang-Ying |last5=Shen |first5=Mingjie |last6=Du |first6=Bibai |title=Widespread Occurrence and Transport of p -Phenylenediamines and Their Quinones in Sediments across Urban Rivers, Estuaries, Coasts, and Deep-Sea Regions |journal=Environmental Science & Technology |date=31 January 2023 |volume=57 |issue=6 |pages=2393–2403 |doi=10.1021/acs.est.2c07652|pmid=36720114 |bibcode=2023EnST...57.2393Z |s2cid=256458111 }}

6PPD-quinone is of environmental concern because it is toxic to coho salmon, killing them before they spawn in freshwater streams.{{cite news|title=Pollution from car tires is killing off salmon on US west coast, study finds|newspaper=The Guardian|url=https://www.theguardian.com/environment/2020/dec/03/coho-salmon-pollution-car-tires-die-off|date=3 December 2020}}{{cite news|title=Scientists solve mystery of mass coho salmon deaths. The killer? A chemical from car tires|newspaper=Los Angeles Times|url=https://www.latimes.com/california/story/2020-12-03/coho-salmon-tire-chemical|date=3 December 2020}}{{cite journal |last1=Johannessen |first1=Cassandra |last2=Helm |first2=Paul |last3=Lashuk |first3=Brent |last4=Yargeau |first4=Viviane |last5=Metcalfe |first5=Chris D. |title=The Tire Wear Compounds 6PPD-Quinone and 1,3-Diphenylguanidine in an Urban Watershed |journal=Archives of Environmental Contamination and Toxicology |date=February 2022 |volume=82 |issue=2 |pages=171–179 |doi=10.1007/s00244-021-00878-4|pmid=34347118 |pmc=8335451 |bibcode=2022ArECT..82..171J }}

A 2022 study also identified the toxic impact on species like brook trout and rainbow trout.{{citation |author=Markus Brinkmann |author2=David Montgomery |author3=Summer Selinger |author4=Justin G. P. Miller |author5=Eric Stock |periodical=Environmental Science & Technology Letters|title=Acute Toxicity of the Tire Rubber-Derived Chemical 6PPD-quinone to Four Fishes of Commercial, Cultural, and Ecological Importance|date=2022-03-02|volume=9 |issue=4 |pages=333–338 |doi=10.1021/acs.estlett.2c00050 |bibcode=2022EnSTL...9..333B |s2cid=247336687 }}

The published lethal concentrations are:

{{citation

|journal=Environmental Science & Technology Letters

|author1=Tian, Zhenyu

|author2=Gonzalez, Melissa

|author3=Rideout, Craig

|author4=Zhao, Hoaqi Nina

|author5=Hu, Ximin

|author6=Wetzel, Jill

|author7=Mudrock, Emma

|author8=James, C. Andrew

|author9=McIntyre, Jenifer K

|author10=Kolodziej, Edward P

|title=6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard

|doi=10.1021/acs.estlett.1c00910

|date= 11 January 2022

|volume=9

|issue=2

|pages=140–146

|bibcode=2022EnSTL...9..140T

|s2cid=245893533

|doi-access=

}}

• coho salmon: LC₅₀ = 0.095 μg/L
• brook trout: LC₅₀ = 0.59 μg/L
• rainbow trout: LC₅₀ = 1.0 μg/L

It is not known why the ozone-oxidised 6PPD is toxic to coho salmon, but has been suggested that the large differences in lethal dose between species may relate to their ability to rid themselves of 6PPD-Q via glucuronidation.{{cite journal |last1=Montgomery |first1=David |last2=Ji |first2=Xiaowen |last3=Cantin |first3=Jenna |last4=Philibert |first4=Danielle |last5=Foster |first5=Garrett |last6=Selinger |first6=Summer |last7=Jain |first7=Niteesh |last8=Miller |first8=Justin |last9=McIntyre |first9=Jenifer |last10=de Jourdan |first10=Benjamin |last11=Wiseman |first11=Steve |last12=Hecker |first12=Markus |last13=Brinkmann |first13=Markus |title=Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification |journal=Environmental Science & Technology |date=19 December 2023 |volume=57 |issue=50 |pages=21071–21079 |doi=10.1021/acs.est.3c06891|pmid=38048442 |bibcode=2023EnST...5721071M |s2cid=265658590 }} The Nisqually and nonprofit Long Live the Kings installed a mobile stormwater filter at a bridge in the Ohop Valley in 2022. The Washington Department of Ecology, Washington State University and the US Tire Manufacturer's Association are working on regulation and education.{{cite news

|title=Your car is killing coho salmon

|newspaper=The Counter

|url=https://thecounter.org/coho-salmon-pacific-northwest-car-tire-toxins-nisqually-tribe-research/

|author = Lena Beck

|date=17 May 2022

}}

6PPD itself is deadly to rotifers, especially in combination with sodium chloride, though not at the level generally found in the runoff from road salt.{{Cite journal |last1=Klauschies |first1=Toni |last2=Isanta-Navarro |first2=Jana |date=2022-07-10 |title=The joint effects of salt and 6PPD contamination on a freshwater herbivore |url=https://www.dynatrait.de/fileadmin/projects/dynatrait/PDF/2022_Klauschies_and_Isanta-Navarro_STOTEN.pdf |journal=Science of the Total Environment |language=en |volume=829 |page=154675 |doi=10.1016/j.scitotenv.2022.154675 |pmid=35314241 |bibcode=2022ScTEn.82954675K |s2cid=247577987 |via=Dynatrait}} A small-scale biomonitoring study in South China has shown both 6PPD and 6PPDQ to be present in human urine; concentrations were low but the health implications are unknown.{{cite journal |last1=Du |first1=Bibai |last2=Liang |first2=Bowen |last3=Li |first3=Yi |last4=Shen |first4=Mingjie |last5=Liu |first5=Liang-Ying |last6=Zeng |first6=Lixi |title=First Report on the Occurrence of N -(1,3-Dimethylbutyl)- N ′-phenyl- p -phenylenediamine (6PPD) and 6PPD-Quinone as Pervasive Pollutants in Human Urine from South China |journal=Environmental Science & Technology Letters |date=13 December 2022 |volume=9 |issue=12 |pages=1056–1062 |doi=10.1021/acs.estlett.2c00821|bibcode=2022EnSTL...9.1056D |s2cid=253828438 }} A synthetic route to the 6PPD-quinone has been posted on ChemRxiv.{{Cite journal|last1=Agua|first1=Alon|last2=Stanton|first2=Ryan|last3=Pirrung|first3=Michael|date=2021-02-04|title=Preparation of 2-((4-Methylpentan-2-Yl)amino)-5-(Phenylamino)cyclohexa-2,5-Diene-1,4-Dione (6PPD-Quinone), an Environmental Hazard for Salmon|journal=ChemRxiv|language=en-US|doi=10.26434/chemrxiv.13698985.v1|s2cid=234062284 |url=https://chemrxiv.org/engage/api-gateway/chemrxiv/assets/orp/resource/item/60c754ab842e658615db41c8/original/preparation-of-2-4-methylpentan-2-yl-amino-5-phenylamino-cyclohexa-2-5-diene-1-4-dione-6ppd-quinone-an-environmental-hazard-for-salmon.pdf}}

• N-Isopropyl-N'-phenyl-1,4-phenylenediamine (IPPD), a related antiozonant
• N,N'-Di-2-butyl-1,4-phenylenediamine, a phenylenediamine-based antioxidant used as a fuel additive

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