Paracetamol Synthesis Experiment

N-(4-hydroxyphenyl)ethanamide, otherwise known as Paracetamol or acetaminophen depending on where you live in the world, is one of the most widely used over the counter drugs. It has the molecular formula C₈H₉NO₂. It is an analgesic (pain reliever) and also an antipyretic (fever reliever). For these reasons it is used to relieve a person of mild to moderate pain, for example; toothache, headaches or symptoms of a cold and to control fever (high temperature, also known as pyrexia). For pain relief it works by interfering with certain chemicals in the body called prostaglandins. Prostaglandins were first discovered in the 1930’s from human semen, thinking the chemicals had come from the prostate gland he named them prostaglandins, but it’s since been established they are synthesised in every cell in the body. They act as chemical messengers like hormones but do not move to other sites, they stay in the cell that they were synthesised in. Prostaglandins have a variety of physiological effects, one being that they are released in response to pain or injury, paracetamol works by inhibiting the production of prostaglandins making the body less aware of the pain or injury. Paracetamol reduces temperature by acting on an area of the brain called the hypothalamus, responsible for regulating body temperature.

The history of paracetamol is an interesting one, at the approach of the 20^th century, the discovery and synthesis of medicines was rather arbitrary, with scientists generally just testing new compounds on humans straight away and then observing if it had positive (or negative) effects. The story of paracetamol starts with the first aniline (also known as phenylamine or aminobenzene) derivative to be found to possess analgesic and antipyretic properties, acetanilide. Aniline is an organic compound with the molecular formula

Aniline (1)

C6H5NH2, shown above, consists of a phenyl group attached to an amino group. The new potential medicine acetanilide had been synthesised simply by the aniline gaining a secondary amide group, by reacting the aniline with ethanoic anhydride, ethanoic acid would also be produced. The reaction is shown below.

C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH

Acetanilide (2)

The discovery was soon published and acetanilide medication was soon in production in 1886, remaining in use for several years due to how cheap it was to produce. But although acetanilide was shown to act as being effective in reducing fever and relieving mild pain, a search for less toxic aniline derivatives started because of some of the awful side effects acetanilide had, for instance cyanosis (appearance of blue or purple coloration of the skin due to tissues near the skin being low in oxygen) caused by it deactivating haemoglobin in erythrocytes.

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The search led to a new derivative that was antipyretic and analgesic and was less toxic than acetanilide called N-(4-Ethoxyphenyl)ethanamide. Marketed in 1887 under the name phenacetin, it has remained in use ever since but has declined in its use due to its adverse affects on the liver. It has the chemical formula C₁₀ H₁₃NO₂.

N-(4-Ethoxyphenyl)ethanamide (3)

In 1893 Joseph von Mering improved on phenacetin producing paracetamol, but mistakenly thought it had the same adverse effects as acetanilide. In the 1940’s it was realised that paracetamol was a major metabolite of phenacetin, it was then considered to quite possibly be the component that caused phenacetin to have the desired effects and that the negative effects were caused by a minor metabolite released. Then in 1953 paracetamol hit the markets, being promoted as superior to aspirin in that it was safe for children and with people with ulcers.

Structural equation showing Phenacitin being turned into its metabolites in the body, as you can see from the diagram, the main metabolite is paracetamol. (4)

Paracetamol is made by many different pharmaceutical manufacturers, each giving their products different brand names. In the UK currently there are more than ninety over the counter products containing paracetamol. Different brands may contain different amounts of paracetamol per dose, it will be stated on the packaging, usually in milligrams. Sometimes it may be combined with other medicines such as decongestants (a type of medicine that provides short term relief for a blocked nose).

While it is a very effective medicine, even small overdoses can be fatal, because it is metabolised into non-toxic and toxic products in the liver. The recommended single dose for adults is 1000mg and up to 4000mg in a day. Paracetamol is hepatoxic, meaning that even in the therapeutic dosages stated previously, it can still harm hepatocytes (liver cells) and in combination with other drugs like alcohol the harmful effects are multiplied. Prolonged daily usage can result in upper gastrointestinal complications such as stomach bleeding. Untreated paracetamol overdoses (which would usually involve taking over the therapeutic dosages for several days) results in a lengthy and painful illness. People who overdose often wrongly assume it will render them unconscious, however this doesn’t happen, rather the process of dying takes around three to five days due to acute liver failure.

Aims:

1. To synthesise paracetamol in one step, starting from 4-aminophenol i.e. amide synthesis
2. To try synthesise paracetamol in a microwave using a similar method to how aspirin is synthesised
3. To recrystallise about half of my samples of paracetamol, leaving the other half crude
4. To calculate the percentage yields of paracetamol, in both methods and compare them
5. To perform analysis of my synthesised samples of paracetamol, both recrystallised and crude using analytical techniques such as

• Melting point test
• Thin layer chromatography
• Back Titration (which will give a quantitative analysis, concentrations)
• Infra-red spectroscopy

6) To then use the results of these analytical techniques to determine which method of synthesis produces

1. The most pure paracetamol sample,
2. The greatest percentage yield

by comparing the percentage yields and purities of both the crude and recrystallised samples of both methods.

7. To extract paracetamol from commercial tablets and compare the purity to my synthesised samples
8. To then use the aims 6 and 7 to finally determine which method of synthesis of an amide, paracetamol, is most efficient.

Chemical theory:

Amines: (5)

Amines are the organic chemistry relatives of Ammonia, they are derive by replacing one, two or all three of the hydrogen atoms with alkyl groups and this determines which type of amine it is. Replacing one of the hydrogen atoms gives a primary amine, replacing two a secondary amine and all three a tertiary amine.

Below shows a primary amine being made from a halogenoalkane with bromine as the halogen, the alkyl group would vary depending on the specific primary amine desired. It is a substitution reaction, with the hydrogen on the ammonia being substituted for the alkyl group on the halogenoalkane.

NH₃ + RBr →RNH₂ + HBr

A primary amine (6) A secondary amine (7)

Amines with low relative molecular masses are gases or volatile liquids, similarly to ammonia they also have strong smells, amines have a “fishy” smell. The properties of amines are quite similar to ammonia due to the fact both have the lone pair of electrons that open up a range of opportunities. Their properties are only slightly modified by their alkyl groups such as the state at room temperature.

4-Aminophenol, the building block of paracetamol (reacting 4-aminophenol with ethanoic anhydride gives paracetamol) is a primary amine.

4-Aminophenol (8)

4-Aminophenol is made by reacting phenol with sulphuric acid and sodium nitrate which gives two products, 1- nitrophenol and 2-nitrophenol. The 2-nitrophenol is then reacted with sodium borohyride, which produces 4-aminophenol.

Step one in synthesis of 4-aminophenol (4)

Step two in synthesis of 4-aminophenol (4)

–Very soluble in water

Similarly to Ammonia, amines can form hydrogen bonds with water due to the highly electronegative nitrogen being bonded to the hydrogen atom; these are attracted to water molecules and vice versa. Amines with small alkyl groups are soluble but those with larger alkyl groups are insoluble because the alkyl groups disrupt the hydrogen bonding in the water. This is significant because 4-aminophenol being a building block of paracetamol it is a common impurity, therefore with the recrystallisation, it should in theory be removed very effectively as it should be very soluble and not reach its limit of solubility. This will be discussed later on.

–Act as a base

Again similarly to ammonia, the lone pair of electrons on the nitrogen can form a dative covalent bond with hydrogen atoms, meaning it acts as a base. In water the presence of hydroxide ions causes it to turn alkaline. If the ammonia/amine is placed with acid, then the acid will donate more protons than water, so the reaction will go on until completion, and therefore many ammonium ions/amine ions are formed and therefore the fishy smell is lost. This can impact on the effectiveness of a chromatogram in thin layer chromatography.

–Acting as a nucleophile:

Ammonia as well as amines can act as nucleophiles, which is why they can form an amide when reacted with an acylating agent like ethanoic anhydride. When ammonia acts as a nucleophile it can react with a halogenoalkane or acylating agent to form an primary amide, the lone pair of electrons on the nitrogen atom attack the positively polarised carbon atom and via a substitution reaction will replace the halogen (e.g. chlorine) or functional group of the acylating agent (e.g. HCL from ethanoyl chloride). This occurs by the electrons in the bond being donated to the halogen or specific functional group of the acylating agent. This breaks off with both electrons and therefore leaves the carbon with a high positive charge, allowing the negative nitrogen to form a dative covalent bond with the carbon. Amines also have a lone pair of electrons on the nitrogen atom and so can also attack electrophiles, such as the delta positive carbon atom on the acylating agent. Similarly to the ammonia reaction, a nucleophillic substitution reaction occurs with the electron movements described above and the appropriate functional group is removed and replaced by the R-N-H forming the secondary amide, with the second hydrogen atom being removed from the primary amine along with the functional group.

(9)

Reaction of an primary amine with ethanoyl chloride an acylating agent, as can be seen the chlorine atom from the ethanoyl chloride is removed as well the hydrogen from the primary amine, producing HCL. This would’ve occurred as result of the nitrogen lone pair attacking the central carbon. The resulting secondary amide is produced when the R-N-H bonds to the carbon.

Synthesis and hydrolysis of an Amide: (10)

All amides contain the functional group CONH

All amides contain this functional group (11)

An amide can either be primary or secondary, primary amides have the general formula R-CONH2, the Nitrogen atom is bonded to two hydrogen atoms and then a carbon atom, which is double bonded to an oxygen, the fourth bond of the carbon is to the R group which can either be an alkyl group (methyl, ethyl etc.) or a benzene.

These can be made by reacting Ammonia with an acylating agent such as an acyl chloride like Ethanoyl chloride. These are carboxylic acid derivatives that are reactive enough to form an amide. Hydrogen from the ammonia breaks off as well as the chlorine of the acyl chloride, forming HCL (g). The first carbon (with the double bond oxygen) then bonds with the Nitrogen this forms the functional group.

The general formula for a primary amide

Secondary amides differ in that the Nitrogen is only bonded to one hydrogen and the third bond goes to another R group, giving secondary amides the general formula R-CONH-R’. The R groups may be the same, or may differ.

Paracetamol (N-(4-hydroxyphenyl)ethanamide) as stated earlier has the molecular formula C₆H₉NO₂, by looking at its structural formula shown below, it can be seen that it comprises of three main parts, starting from the left, in the box is the phenol group, one of the R groups of the amide, this explains the “hydroxyphenyl” part of paracetamols systematic name as it was originally part of the 4-aminophenol amine. Next in the oval, is the actual amide functional group, finally on the far right in the triangle is the other R group (R’) which in paracetamol is simply a methyl group. From all this we can determine that paracetamol is a secondary amide.

(4)

Secondary amides are made by reacting a primary amine with an acylating agent like Ethanoic anhydride, in my investigation, I will use ethanoic anhydride as my acylating agent. This occurs by the reaction mechanism of nucleophillic substitution, which is shown below in a curly arrow diagram, with ammonia being used as the nucleophile, attacking the carbon atom.

Steps in Nucleophillic substitution: (12)

1. The first thing to note is that, as explained earlier, ammonia (which is acting as the nucleophile in the example above) as well as amines can act as nucleophiles, due to the fact they have the lone pair of electrons on the nitrogen atom, they have a partial negative charge which is attracted to an electrophile (has a partial positive charge), in this case the polarised carbon atom (as it is bonded to the highly electronegative oxygen atom) on the ethanoic anhydride.
2. The first thing that happens is that the Nitrogen begins its “attack” on the partially positive, also known as delta positive, carbon. Because of the lone pair, it forms a dative covalent bond with the carbon
3. Because it is dative, the carbon atom has gained an electron therefore at has been reduced, so it then donates an electron within the double bond with oxygen to the oxygen atom, this makes the already partially negatively charged oxygen to become negatively charged. There is now only a single bond between the carbon and oxygen.
4. The carbon atom then donates an electron to the oxygen below it that it is also singly bonded to, releasing an ethanoate ion (CH3COO), this has given the carbon atom that donated the electron a positive charge as it has now had a net loss of one electron from its original electron configuration. This is now a carbocation.
5. The reaction then goes back to the negatively charged oxygen that the central carbon donated its electron to earlier, what occurs now is that the oxygen donates the electron back, now that the central carbon is positively charged, this reforms the double bond between the now partially negative oxygen and partially positive carbon.
6. The nitrogen that has bonded to the carbon then loses the third hydrogen atom as nitrogen can only form three bonds in a neutral organic compound, this happens by the hydrogen donating its electron to the nitrogen. The hydrogen then bonds to the ethanoate ion, forming ethanoic acid (CH3COOH) and ethanamide, ethan- the prefix coming from the two carbon atoms present and the suffix –amide due to the CONH functional group.
7. The ethanoic acid produced then will react with any excess ammonia to form ammonium ethanoate, this is because ammonia and amines can act as bases due to the reasons stated earlier, the hydrogen on the ethanoic acid breaks off and bonds to the nitrogen atom.

The “curly arrow” diagram of this reaction is shown below, the stage number relates to the mechanism diagram shown above it and described above, step 1 is omitted because it is an introduction, the first step of the reaction mechanism, is step 2 i.e. shown below step 2 is the attacking of the nitrogen nucleophile to the