Phencyclidine (PCP) synthesis

[G.Patton]

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Introduction​

There are three fairly direct methods for synthesis of PCP and its derivatives: those employing a nitrile intermediate (Scheme I), those employing an enamine intermediate (Scheme II). The method of choice depends on which particular analog is desired, as well as what reagents are available. There are other promising routes to PCP analogs that have appeared in the literature but not received as much attention by clandestine chemists. Route III yield 1-phenyl-1-cyclohexylamine (PCA), which is an active drug itself, and can also be used as an intermediate in synthesis of PCP and other more potent analogs. Scheme III involve the use of 1-phenyl-1-cyclohexanol (PCOH) as an intermediate. In Scheme III, the PCOH is reacted with NaCN and H2SO4 to give N-formyl PCA (Ritter reaction), which can then be hydrolyzed to PCA with acid or base. The PCOH used for these reactions can be prepared from cyclohexanone and phenylmagnesium bromide or phenyllithium or obtained commercially. Probably the most promising alternative method of synthesis is shown in Scheme IV. In this method, N-benzoyl piperidine is reacted with the lithium or magnesium derivative of 1,5-dibromopentane to give PCP in one step.

Boiling Point: 340 °C at 760 mm Hg;
Melting Point: 233-235 °C (hydrochloride salt);
Molecular Weight: 279.85 g/mole;
Density: 1.013 g/mL (20 °C);
CAS Number: 956-90-1.​

Scheme I. Synthesis of PCC​

The most commonly used method for PCP production in clandestine labs is based on the Bruylants reaction, that is displacement of an alpha-amino nitrile by an organometallic reagent. The general outline of this reaction is shown in Scheme I. There are two steps: preparation of a nitrile intermediate (PCC), and reaction of this intermediate with a Grignard reagent. The PCC intermediate can be synthesized through several routes, two of which are illustrated here. A typical clandestine batch operation might be run on a 3 to 5 molar scale, and is usually limited by the amount of piperidine to be employed (usually a maximum of 500 g). This route has an overall yield of ~60%, with a difficulty rating of 2 out of 10, and a hazard rating of 4 out of 10.

Method 1:
The first method involves reacting cyclohexanone with the hydrochloride salt of piperidine and aqueous NaCN or KCN (ref 11) . This is the most direct method, and is the one most commonly used in clandestine labs. Although it has not been reported, there appears to be some danger of evolving deadly HCN gas when following this procedure. To reduce this danger, the reaction should be done with very good ventilation, and the amount of acid carefully regulated so that the solution is in the correct pH range. If the solution becomes too acidic, the danger of HCN evolution will increase. After the solution has been allowed to stand overnight, the PCC will generally crystallize in beautiful ice-like forms. If the PCC has not crystallized after standing overnight, the common procedure in clandestine labs is to extract the solution with white gasoline (Coleman's fuel) or benzene, and dry the solution by the addition of an anhydrous salt such as magnesium sulfate, calcium chloride, or potassium carbonate. This solution of PCC in solvent may now be used directly in the next step for addition to the phenyl magnesium bromide.

Procedure
Piperidine, 85 g (99 mL, 1 mole) is carefully mixed with 84 ml of conc. HCl and 200 g of ice-water, and the pH is adjusted to 3-4. To this solution, 98 g (104 mL, 1 mole) of cyclohexanone is added, followed by 68 g (1.0 mole) of KCN in 150 mL of H2O (or 116 mL of 40% aqueous NaCN) without external cooling but with efficient stirring. After 2 hr. the solution is allowed to stand overnight, the crystalline precipitate is collected, washed with cold water and dried. The yield of PCC sufficiently pure for the next step is 169-182 g. (88-95%) mp 63-68 °C.​

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Method 2:

The second method of PCC synthesis involves the addition of cyclohexanone to an aqueous solution of sodium bisulfite (NaHSO3), producing the bisulfite adduct. Addition of KCN or NaCN results in formation of PCC. This method is very easy and avoids the possibility of HCN evolution.

Procedure
12.6 g of sodium bisulfite is dissolved in 42 mL of H2O. 10.6 g of cyclohexanone is added with vigorous stirring. The bisulfite adduct forms immediately as a thick white slurry. The slurry is then cooled with an ice bath, and a solution of 7.86 g KCN and 9.48 g piperidine is added. After stirring overnight at room temperature followed by cooling in an ice bath, the PCC will crystallize. The product is then filtered off, washed with water, and dried (in vacuo at 30 °C if possible) to give 10.9 g (86.6%) of material, mp 70-71.5 °C, bp 118 °C (2.5 mm Hg). Distillation is not recommended or necessary. If the PCC fails to crystallize, it can be extracted with solvent and dried, as above.​

1-phenylcyclohexylpiperidine (PCP) via nitrile method​

Procedure

A solution of 39 g (0.203 mole) of PCC is prepared in 50:50 ether:benzene or better solvents such as THF, hexanes/ether, ortoluene/ether. This is added slowly to phenyl magnesium bromide prepared from 79 g (57 mL, 0.53 mole) of bromobenzene and 12.3 g (0.505 mole) of Mg turnings in 200 mL of dry ether. The mixture is then heated and stirred for 3 hrs and cooled. After cooling, 175 mL (0.7 equivalents) of 4 N aqueous HBr is slowly added, followed by overnight cooling in a refrigerator. Precipitated PCP hydrombromide is filtered off, air-dried, and dissolved in a minimum amount of hot ethanol. The hot solution is basified with ethanolic NaOH, which deposits a heavy yellow oil that quickly crystallizes. After cooling, crystals of PCP base with minor amounts of inorganics are filtered off, dried and dissolved in benzene (or toluene). One third of the benzene is distilled off to remove water from the solution via azeotropic drying. After the solution has cooled, it is diluted with 2 volumes of dry ether. Saturation with dry HCl deposits PCP hydrochloride, which is filtered off to yield about 40 g (70 %), mp 243-244 °C.

The physical properties of the pyrrolidine analog (PCPy) prepared by this method are bp 114-123 °C at 0.14 mm Hg, and mp. 44-45 °C after recrystallization from isooctane. The mp of the hydrochloride salt is 235-237 °C.​

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Notes on reaction:​

Formation of the Grignard reagent: The reaction is most conveniently carried out in a two-neck flask, but a single necked flask will work. Magnesium shavings and a magnetic stirring bar are introduced into a previously dried round bottom flask. The flask is then held over a gas flame and rotated until the magnesium is quite hot. This will remove any water from its surface. A condenser and drying tube are attached to the flask, and it is allowed to cool. In a second flask, bromobenzene (or equimolar amount of chlorobenzene) is mixed with THF or ether and poured into an addition funnel. Enough solvent is added to the flask to cover the Mg shavings. Approximately one quarter of the bromobenzene solution is added to the flask with stirring, and the cooling water to the condenser is turned on. If the reaction does not begin within 10 min., steps are taken to initiate it (Note 1). The start of the reaction is apparent by the presence of bubbles, a grayish precipitate forming, and the solvent beginning the reflux.

Once the reaction is progressing smoothly, the ether/bromobenzene is added slowly at a rate sufficient to maintain reflux without external heating. After it has all been added, the flask is gently heated at reflux until almost all the magnesium has disappeared.

Note 1. Initiation of Grignard reaction: If the reaction does not begin within 10 min, there are a number of ways to initiate it. It is important not to add more bromobenzene until the reaction has begun. Otherwise, the reaction may suddenly start and become violently out of control. A dishpan full of ice water should be on hand to cool the flask in case this happens. It should also be noted that the reagent can react violently with water once formed, and possibly ignite. If the flask were to break inside the water bath, the results could be disastrous.

Different techniques used to initiate the reaction:​

• A dry glass rod is inserted into the neck of the flask and used to crush some Mg chips against the bottom.​
• Several grams of Mg shavings are added to a flame dried test tube, followed by several mL each of ether and bromobenzene. A dry glass rod is then inserted into the tube and some of the Mg chips are crushed against its bottom. This small-scale reaction should start almost immediately. Once it is underway, the contents are poured into the reaction vessel.​
• Stirring is stopped, a TINY crystal of Iodine is added to the flask, and the reaction allowed to stand until it starts.​
• The flask is heated gently until the solvent begins the reflux. The heat is then removed, and the flask is watched for signs of reaction.​

Notes on reaction of PCC and the Grignard reagent:
If THF is used for solvent, the PCC is dissolved in it in a small flask. If ether is being used, a co-solvent will be necessary to dissolve the PCC. Suitable solvents are dry hexane, toluene, benzene, naphtha, or white gasoline (distilled). White gas is a solvent commonly used in clandestine labs. A ratio of 1.25 moles of Grignard reagent to 1 mole of PCC is the minimum that should be employed. If the Grignard can be increased to a 2 to 1 ratio, then the yield of the final product can be as high as 65% based on the amount of piperidine employed.

Enough solvent is added to a flask to dissolve the PCC, and approximately half as much ether is added. The solution of PCC is then added via the addition funnel to the reaction slowly and with stirring. When it has all been added, heat is applied to the flask, which is maintained at reflux for at least 3 hrs. Note that use of phenyl lithium instead of phenylmagnesium bromide results in failure via addition to the nitrile rather than displacement. However, in the presence of a Lewis acid, phenyllithium will displace the nitrile group and yield the desired product. Primary amino analogs of PCC, such as N-ethylamino cyclohexanecarbonitrile will produce the desired PCP analogs by reaction with 3 moles of phenyllithium.

Notes on quenching the reaction and isolating the final compound:​

Method 1: This is the simplest method, and is the most common one used in clandestine labs. One drawback is the potential for formation of troublesome emulsions from the Mg salts precipitated at basic pH during extraction. This can especially be a problem if ether/benzene was used to dissolve the PCC in the reaction.​

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Several hundred cm3 of crushed ice are placed in a beaker, along with ~15 g of ammonium chloride and 10 mL of ammonium hydroxide. The ammonium hydroxide can be left out, but it is beneficial. The contents of the reaction flask are slowly poured onto the ice/NH4Cl with stirring. After the bubbling has stopped and the ice has melted, the beaker is poured into a separatory funnel along with 30 mL of solvent such as hexanes, toluene, chloroform, dichloromethane, etc. For the first extraction, the funnel is shaken gently, which helps avoid formation of an emulsion. The aqueous layer is extracted two more times with solvent, and the solvent layers are pooled. The combined organic layers are then extracted 3 times with dilute HCl. The acid layers are basified with NaOH, and the product is extracted with organic solvent. Evaporation of the solvent yields the oily PCP freebase, which may crystallize slowly, possibly taking several days to weeks.​

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If the desired method of administration is smoking, the compound is left as the freebase. If the compound is to be administered nasally, by injection, or orally, the base is crystallized as the HCl salt. To accomplish this, the base is dissolved in ether and HCl gas bubbled in. The HCl salt precipitates, is washed with ether, and allowed to dry. An alternative, low tech and dirty method commonly used in clandestine labs is to add the calculated amount of concentrated HCl followed by evaporation to yield the salt.​

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Method 2: Aqueous HBr can also be used to hydrolyze the reaction mixture. This method is illustrated above. It has the distinct advantage of allowing the separation of any unreacted PCC. It also avoids the possibility of troublesome emulsions from precipitated magnesium salts during workup. However, it may be less suitable if THF has been used as the solvent for the Grignard reaction. In this case, the THF may dissolve some PCP hydrobromide and reduce yields. Also, the HBr salt of PCP can be extracted from the quenched reaction mixture with chloroform.​

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Method 3: The product may also be isolated simply by decantation of the solvent from the reaction mixture followed by addition of concentrated HCl to form the HCl salt, followed by purification via acid/base extraction. This method has been used in large low tech clandestine PCP labs.​

Precursors​

The main stumbling block for clandestine synthesis of PCP in the US is the acquisition of piperidine. Piperidine is a closely watched chemical and is generally obtained by diversion from wholesale manufacturers. It has little legitimate use outside of pharmaceutical manufacturing. A clean bottle (i.e. one that is not watched or traceable) can sell for as much as $1000 a kg on the black market. It can be synthesized by the reduction of pyridine, but it should be noted that pyridine itself is somewhat watched because of its use in methamphetamine synthesis. It also may be obtained by the hydrolysis of piperine, a major constituent of black pepper oil, by hydrolysis with aqueous KOH or by cyclization of 1,5-diaminopentane. Of course, the necessity of piperidine can be eliminated by synthesis of a PCP analog that does not contain the piperidine ring, such as PCPy. The piperidine ring can also be built by alkylation of PCA with 1,5-dibromopentane, as discussed in Scheme V.

Cyclohexanone. Although cyclohexanone is not watched as closely as piperidine, it is well known to be a vital ingredient for PCP manufacture. It is commonly available in bulk in the resin's industry, where it is used as a solvent, and is also used in huge quantities in the manufacture of several polymers. It may also be synthesized on a laboratory scale by the oxidation of cyclohexanol.
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Procedure: Cautiously add 20 mL of conc. sulfuric acid to 60 g of crushed ice and mix well. Add 20 g of cyclohexanol and place a thermometer in the mixture (temp. should be <30 °C). Prepare a solution of 21 g sodium dichromate dihydrate in 10 mL of water. Add about 1 mL of this solution to the reaction flask with vigorous swirling. Add the rest of the dichromate solution with continuous swirling at a rate that keeps the temperature between 25 and 35 °C. After the addition is complete, continue stirring until the temperature falls by 1 or 2 degrees. Add about 1 g of solid oxalic acid to destroy any excess dichromate. Rinse the reaction mixture into a 500 mL distillation flask with 100 mL of water, add a boiling stone, and distill the product rapidly. The cyclohexanone will distill as a mixture with water (an azeotrope) at about 95 C. Continue distillation until 60-100 mL of distillate is obtained. Add about 15 g of sodium chloride to the distillate and swirl until most of it has dissolved. Transfer the mixture to a separatory funnel and discard the lower aqueous layer. Dry the upper layer with 1-2 g of potassium carbonate and decant. The cyclohexanone will now be of sufficient purity to use in PCP synthesis, but if further purification is desired, it can be re-distilled.​

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Scheme 2. Synthesis of PCP​

This method is used less frequently in underground synthesis, but has the advantage of not involving toxic cyanide compounds. The first step of the reaction is the dehydration of piperidine and cyclohexanone to form an enamine. The addition of anhydrous p-toluenesulfonic acid or dry HBr produces an intermediate imminium salt. Reaction of this salt with phenylmagnesium bromide yields PCP. This method is most applicable to cyclic secondary amine analogs such as piperidine, pyrrolidine, or morpholine, rather than an acyclic N-substituent such as ethyl or dimethyl. This route has an overall yield of ~70%, with a difficulty rating of 3 out of 10, and a hazard rating of 2 out of 10.​

Procedure​

Step 1. Preparation of cyclohexenyl piperidine: A solution of 98 g (1.0 mole) of cyclohexanone, 100 g (1.17 mole) of piperidine, and 2 g (0.0105 mole) of p-toluenesulfonic acid in 300 mL of toluene is refluxed under azeotropic distillation conditions until the evolution of water stops (about 13 h). The best method for this is to use a Dean Stark or Barrett water trap, but if one is not available, it may be improvised with a distillation head as described in Vogel's Textbook of Practical Organic Chemistry.​

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Either p-toluenesulfonic acid or dry HBr gas can be used in the next step. The intermediate cyclohexenyl-piperidine (enamine) from the previous step is best used crude, but it can be distilled with adequate vacuum.

Step 2, method A. Use of p-toluenesulfonic acid: 190 g of p-toluenesulfonic acid monohydrate in 250 mL of toluene is heated under a Dean Stark trap until all the water is removed. This is then added to 165 g of cyclohexenyl-piperidine in 500 mL of ether with ice cooling to keep temp at 0 deg C. A solution of 1 mole of phenylmagnesium bromide is prepared as in Scheme I from 157 g of bromobenzene and 24 g of Mg turnings in 750 mL of ether. This is added to the cyclohexenyl piperidine while holding the temp at 0 to 5 °C. The mixture is stirred for an additional 30 min after the dropwise addition is complete.​

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A mixture of saturated aqueous ammonium chloride and ammonium hydroxide is added to neutralize the excess phenylmagnesium bromide (approx. 100 g of NH4Cl and 20 mL of strong NH4OH in enough water to make a saturated solution). The ether layer is then removed in a separatory funnel, dried by the addition of potassium carbonate, filtered, and evaporated to give PCP. This can be converted to the hydrochloride salt by dissolving it in an excess of isopropanol saturated with HCl gas and precipitating with ether, followed by crystallization from a mixture of ether and isopropanol.​

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Step 2, method B. Use of HBr gas: The reaction mixture from step one is diluted to 2 L with dry toluene and dry HBr gas was bubbled through until the solution is acidic. The resulting slurry is added at once to a cold (5 °C) stirrred solution of phenylmagnesium bromide prepared from 236 g (1.51 moles) of bromobenzene and 38 g of Mg (1.56 moles) in 1 L of dry ether. The temperature will rise to 45 °C, and the mixture should be stirred for an additional 30 minutes. 300 mL of 48% aqueous HBr is then added, producing the HBr salt of PCP, which is filtered from the reaction mixture. The crude HBr salt is then basified with NaOH, extracted with hexanes, toluene, etc., and either evaporated to yield PCP freebase or treated with isopropanol saturated with HCL, followed by dilution with ether to give PCP hydrochloride.​

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Scheme 3. Synthesis of PCP​

This method is generally easy, and starts with inexpensive and commercially available 1-phenylcyclohexene, or alternately from1-phenylcyclohexanol (PCOH). PCOH or phenylcyclohexene is reacted with sodium cyanide and H2SO4 to give N-formyl PCA in about 50-60% yield. N-formyl PCA is readily hydrolyzed in either acid or base conditions to give PCA, which can then be alkylated to give PCP and other analogs, as illustrated in Scheme IV. This route has an overall yield of ~30% with a difficulty rating of 1-2 out of 10, and a hazard rating of 3 out of10.
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Procedure​

Preparation of PCA

To a mixture of 15.8 g of 1-phenylcyclohexene (0.1 mole) and 12.2 g of NaCN (0.25 mole) in 50 mL of dibutyl ether, was added 30 mL of H2SO4 over 1 hr. After stirring for an additional 1 hr, the reaction mixture was poured into water and extracted with ether.​

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The solvents were removed in vacuo, 30 mL of HCl was added to the residue, and the mixture refluxed for 3 hrs. After cooling, the aqueous layer was separated, basified with NaOH, and extracted with ether. The HCl salt was prepared by adding dry HCl in isopropanol, followed by evaporation. 20 mL of acetone was then added to the residue, followed by recrystallization twice from methanol/ether to give needles (mp 247-248 °С).​

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Preparation of PCP from PCA
A mixture of 8.69 g of PCA, 11.5 g of 1,5-dibromopentane, and 8.0 g of anhydrous K2CO3 in 50 mL of dry DMF was stirred and heated. At 50-55 °С an exothermic reaction took place and the temperature rose to 95-100 °С. The flask was heated for 1 h on a steam bath, poured into ice-cold water and extracted with ether, followed by distillation and recrystallization to give the final compound.​

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Scheme 4. Synthesis of PCP​

This is a promising underutilized method which takes place in one step. The starting material is N-benzoylpiperidine, which may be easily prepared in ~90% yield from benzoyl chloride and piperidine or purchased commercially. Reaction of N-benzoylpiperidine with the lithium or magnesium organometallic derivative of 1,5-dibromopentane gives PCP. The only difficulty in this route is the preparation of the dibromopentane Grignard reagent, with its strict requirement for anhydrous conditions. This route has an overall yield of ~75% with a difficulty rating of 2 out of 10, and a hazard rating of 2 out of 10. Other analogs could also be prepared with this method by substituting another secondary amine (e.g. dimethylamine or diethylamine) for piperidine in the reaction with benzoyl chloride.​

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Procedure​

A Grignard reagent is prepared from 56 g of magnesium turnings and 230 g of 1,5-dibromopentane in 2 liters of ether, and the mixture stirred and refluxed for 3 h. 151 g of N-benzoyl piperidine is added, and the ether is removed by distilling until the temperature in reaction vessel reaches about 83 °С. The reaction mixture is then stirred at this temperature for sixteen hours, cooled and treated with sufficient ammonium hydroxide and a saturated solution of NH4Cl until the precipitate dissolves. The solution was diluted with 2 L of ether, and the ether layer removed by decantation. The ether layer is then washed, dried over sodium hydroxide, and the ether distilled off. The residue is distilled in vacuo to obtain PCP (bp 128-134 °С/0.8 mm Hg).​

 

[ASheSChem]

a way to obtain piperidine from piperine?

 

[G.Patton]

Why do you need it? Piperidine is not controlled substances and it is worth to buy than synthesize

 

[ASheSChem]

I haven't seen any for sale.. I don't have to go to the right places
(but i have seen piperine)

 

[MethylEthyl]

There’s plenty of tek on clear net for such an extraction/synthesis, but the yields are low. Better to reduce pyridine with sodium metal in methanol.

 

[MethylEthyl]

Pip
Piperidine is closely watched in some countries.

 

[IntrusiveReaction]

Very nice post! I am hoping there will be a lot of posts about Arylcycloheylamines in the future. Very cool category of chemicals! If you were to say synthesise PCP, would the road to creating 3-meo-pcp be long? Or would you need to take a few steps backwards and do a different ending of the synthesis + extra steps with 3 methoxy?
I am new to this so my bad if it is formualted in the wrong way

 

[G.Patton]

It depends on synthesis way. You have to use another precursor.

 

[mxtlrr_chem]

A question about Scheme V -- Is vacuum distillation required here? Could evaporation be done here at normal pressure? Or is this just like LSD, where the compound is sensitive enough that must be done en vacuo?

 

[Batracien]

I don't understand this part, can you explain how exactly with more details ?

 

[G.Patton]

Hello. What exactly don't you understand?

 

[Batracien]

How much potassium carbonate you need/ how can you know it's enought ? And does the evaporation need heat (how much) ? thank you

 

[G.Patton]

K2CO3 is used as a drying reagent. You have to add enough to collect all water from the solution. There is video about application such agents:

Drying of Organic Liquids

• G.Patton
• Sep 11, 2022
• 6

Drying of Organic Liquids...

Heat is needed for sure. Depends on solvent boiling point and atmosphere pressure. Look at Lab FAQ before read such complicated syntheses. There are a lot of useful information for u.

 

[Batracien]

Ok thank you

 

[Batracien]

And for the
And it is possible the use piperidine hydrochloride in this receipt ?

 

[G.Patton]

No. You need to use exactly piperidine.

 

[Metribolone]

I remember a video from Vice a while back detailing o-toludine being used in the process and that it’s highly toxic and causes bladder cancer. Is there any benefit of using o-toludine over the methods listed here?

 

[Rutherford_1]

Can I have more information about this compound? I've never heard of it and there's not much information on Google.

thank you!

 

[tripeep]

in scheme 4, how does it mean to treat with sufficient ammonium hydroxide and a saturated solution of NH4Cl until the precipitate dissolves. and how much weight is needed?

 

[G.Patton]

 

[tripeep]

???

 

[G.Patton]

Sorry, there was a bug.