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This question is both historical and chemistry-related, but I'm posting it here because I am more interested in the chemical part:

In 1997, Israel tried to assassinate Khaled Mashal, a high-rank official of Hamas. This was to be done using an extremely potent poison that was supposed to be sprayed on him topically and kill him within a few days without a trace (a "quiet" murder).

I searched for the name of this poison, but I was only able to find out that it was a modified version of the drug Fentanyl (here, page 16). It was probably an internal formulation of the Mossad, but I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact.

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  • $\begingroup$ I don't know which analog was used for the assassination attempt. However, regarding your last question, there are fentanyl analogs 100x the potency of fentanyl itself, which is basically due to increased lipophilicity, allowing more efficient transfer to the CNS. All fentanyl analogs readily cross into the blood stream transdermally. Just some tidbits, sorry I don't have a full answer (it does seem surprisingly hard to find). $\endgroup$
    – airhuff
    Jul 26, 2017 at 20:37
  • $\begingroup$ " I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact." Well, formula is not publicly released but you can increase the lipophilicity substituting one or more C-H bond with C-F bonds or adding a -CF3 group. $\endgroup$ Aug 29, 2022 at 13:55

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According to several sources online the chemical weapon used in the failed assissination of Mashal was levofentanyl.

This Wikipedia article states:

On September 25, 1997, Mashal was injected in the ear with a toxin (thought to have been a derivative of the synthetic opiate Fentanyl called Levofentanyl).

Another source reads:

In September 1997, an Israeli hit squad was sent to Amman to assassinate Khalid Mishal, the Hamas leader. The chosen instrument was levofentanyl, a deadly poison that leaves no traces and produces effects like a heart attack. It was administered by a slight physical touch.

And another:

...The agents entered the city using Canadian passports and waited for Mashal at his office. As he was entering, one agent injected a chemically modified toxin called levofentanyl into his left ear.

Concerning the structure I don't think it was ever publicised considering the case was high profile (perhaps all traces were taken offline or can be found in deep when-no one knows)

I've been trying to figure out how fentanyl could have been modified, but have no clue either.

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    $\begingroup$ Levofentanyl sounds like it's simply one enantiomer of fentanyl (see e.g. levocetirizine) but just a guess on my part. $\endgroup$ Jul 26, 2017 at 22:22
  • $\begingroup$ @orthocresol Pretty much like it, I think it was one enantiomer of fentanyl which was lethal $\endgroup$ Jul 27, 2017 at 5:47
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    $\begingroup$ @orthocresol Fentanyl is achiral. $\endgroup$
    – Karsten
    Aug 29, 2022 at 14:34
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[OP] This was to be done using an extremely potent poison that was supposed to be sprayed on him topically and kill him within a few days without a trace (a "quiet" murder).

Poisons leave a trace. In the case of fentanyl derivatives, which are dangerous in very small doses, it is technically challenging to find the trace, though. The path of entry is described differently by different sources. Some speak of "injection" into the ear while others say the intent was to spray on the back of the neck, but Mashal supposedly turned his head to the side, and the spray went into one ear.

[OP] It was probably an internal formulation of the Mossad,

I did not find any evidence, or even other claims, for this.

[OP ...] but I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact.

In general, fentanyl and its derivatives are contact poisons (see airhuff's comment to the OP's question). As of 2022, I don't think Mossad disclosed which substance was involved. However, this peer-reviewed medical paper comments on reports claiming the use of the fentanyl derivative carfentanyl by Russian safety forces in an anti-terrorist case:

Carfentanyl has also been reported to have applications in anti-terrorist interventions as incapacitating agent, used by Russian safety forces in the hostage rescue operation in October, 2002, in the Dubrovka theatre, Moscow. Carfentanyl and remifentanyl were found on a shirt sample and carfentanyl metabolite norcarfentanyl was found in a urine sample. It is possible that an aerosol containing a mixture of carfentanyl and remifentanyl was used.

If the poison was indeed a fentanyl derivative (targeting the opioid receptors), a possible antidote would have been naloxone, approved in the U.S. since 1971 to treat opioid overdoses. It competes with opioids, so the stronger the opioid binds, the higher the necessary does of naloxone (for the example carfentanyl, 100 mg of naloxone are administered to treat a 1 mg exposure with carfentanyl).

[OP] I've been trying to figure out how fentanyl could have been modified, but have no clue either.

Fentanyl itself is a synthetic product. The derivatives, of which many are known and characterized, are not always obtained by using fentanyl as a starting material. Sometimes, they are made using a different series of synthetic steps. So derivative (or modification) often refers to similarity in structure, not necessarily making a claim about the path of synthesis.

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