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# Synthetic Routes

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We will discuss many synthetic routes elaborating on how to make a chemical compound from another chemical compound. We will learn as well how to go about the chemical reactions, and what reagents and catalysts to use.

You will have to recollect what you learnt in Organic Chemistry and Organic Synthesis.

A synthetic route is a series of steps to be followed in order to make a chemical compound from smaller and less complex chemicals.

• In this article, you will learn what a synthetic route is.
• You will learn how to map a synthetic route when given a starting material and a target compound.
• You will look at the overview of synthetic routes for aliphatic compounds and aromatic compounds.
• The synthetic route overview will help map the steps to take when converting one organic compound into another - the reagents, catalysts, and conditions required for each reaction.
• We will discuss and map some examples of synthetic routes to help you understand how to use the information given in this article.

## Synthetic routes - aliphatic compounds

Let us start by listing out all the functional groups we know. We are familiar with:

That's quite a few! We can draw a flow chart of which functional groups can be converted into other groups. This will make it easier to understand and remember.

Organic synthesis routes, Olive [Odagbu] StudySmarter

The arrows represent which groups can be synthesised from other groups using the right reagents, catalysts, and conditions.

### Synthetic route: 1-bromopropane to propanoic acid

Let us consider the synthesis of propanoic acid from 1-bromopropane. Propanoic acid has a carboxylic acid functional group, while 1-bromopropane is a haloalkane.

Propanoic acid, Kanishk Singh, StudySmarter Originals

1-bromopropane, Kanishk Singh, StudySmarter Originals

Looking at the flowchart, the easiest route to take from haloalkane to carboxylic acid is through an alcohol. Thus, we can convert 1-bromopropane into an alcohol and then into a carboxylic acid. Let's go through the steps of this reaction.

1. Haloalkanes can be converted to alcohols through hydrolysis. Hydrolysis of haloalkanes is a nucleophilic substitution reaction. In this reaction, the nucleophile is H2O. Since H2O alone is a weak nucleophile, this reaction is slow. Therefore, NaOH is added and the solution is heated under reflux. The halogen is replaced by the -OH group.
2. To convert alcohol to carboxylic acid, it needs to be oxidised. For this reaction, acidified Potassium Dichromate (K2Cr2O7/H+) is added. K2Cr2O7 is an orange-coloured substance which is a strong oxidising agent. The reaction needs to take place with heat under reflux.

Since we're starting with 1-bromopropane, this molecule would be called the starting material in this particular route, while propanoic acid is the target compound. All compounds formed in between the starting material and the target compound are called intermediate compounds. Propanol is called the intermediate compound.

The synthetic route of Propanoic acid from 1-bromopropane can be described like this -

Synthetic Route of 1-bromopropane to Propanoic acid, Kanishk Singh, StudySmarter Originals

### Synthetic route: ethene to propylamine

Let us consider another synthetic route. We can synthesise propylamine from ethene. Ethene is an alkene with a double bond between 2 carbon atoms. Propylamine has an amine group attached at the end of a 3-Carbon chain.

Ethene, Kanishk Singh StudySmarter Originals

Propylamine, Kanishk Singh, StudySmarter Originals

Look at the flowchart again. To get to an Amine from an Alkene, you can take the route of alkene → haloalkane → amine. But if we take this route, the end product will be ethylamine, which is a 2- carbon compound. Where do we get an extra carbon from? We will have to take a longer route through the nitrile. So, the final route would be alkene → haloalkane → nitrile → amine. The nitrile group (CN) will give us the extra carbon we need. Let us go through the steps of this synthesis route:

1. Alkenes can be converted to haloalkanes by adding a hydrogen halide at 20oC. Thus, adding a Hydrogen halide (HBr for example) to ethene and holding the reaction at 20oC will give us 1-bromoethane. The double bond of ethene is replaced by hydrogen on one side and bromine on the other side.
2. We will now replace the halogen with a nitrile group. Haloalkanes can be converted to nitrile by reacting them with a solution of sodium cyanide (NaCN) or potassium cyanide (KCN) in ethanol, and heating under reflux. With this reaction, 1-bromoethane will be converted to propanenitrile.
3. Next, we need to convert the nitrile group to an amine group. The nitrile group consists of a triple bond between carbon and nitrogen ( $\mathrm{C}\equiv \mathrm{N}$). This can be reduced by catalytic hydrogenation (reaction with hydrogen in the presence of a catalyst). The catalyst used is usually a metal catalyst such as palladium, platinum, or nickel. Through this reaction, $\mathrm{C}\equiv \mathrm{N}$ will give us $\mathrm{C}-{\mathrm{NH}}_{2}$.

Propanenitrile is also called ethyl cyanide, and propionitrile.

This diagram summarises the reactions discussed above, and represents the synthetic route of ethene to propylamine.

Synthetic route of Ethene to Propylamine, Kanishk Singh, StudySmarter Originals

### Synthetic route: ethene to ethanol (hydration of alkene)

Let us now consider a simple synthetic route. We shall synthesise ethanol from ethene. Ethene is an alkene with a double bond between 2 carbon atoms. Ethanol is an alcohol with an -OH group attached to a 2-carbon chain.

Ethene, Kanishk Singh, StudySmarter Originals

Ethanol | Kanishk Singh, StudySmarter Originals

Looking at the flowchart, we can convert an alkene to an alcohol in a single step i.e. single reaction. This reaction is called hydration and is the direct, simpler method. In hydration, the alkene is treated with steam at high temperature and pressure, in the presence of a catalyst. The catalyst used in this process is phosphoric acid (H3PO4).

But there is also a two-step hydration process of alkenes. Let us see the reactions of the two-step hydration of ethene.

1. In the first step, ethene is treated with sulphuric acid (H2SO4). This reaction yields Ethyl Hydrogen Sulphate. It is also called the 'Ethyl Ester of Sulphuric Acid'.
2. Then, ethyl hydrogen sulphate is hydrolysed i.e. reacted with water. This reaction produces ethanol, and also regenerates the sulphuric acid.

Synthetic Route of Ethene to Ethanol, Kanishk Singh, StudySmarter Originals

This two-step hydration of alkenes is traditionally used in the industrial production of ethanol.

### Synthetic routes for all aliphatic compounds

The synthetic routes overview diagram given below shows all possible synthetic routes between functional groups found in aliphatic organic compounds. The diagram shows the reagents and conditions required for each conversion. You can map any synthetic route between any starting organic material to any target organic compound. To map a synthetic route between a starting material and a target compound, check for common intermediate functional groups from the flowchart below. Then, list the reactions required to be done, as well as the reagents, catalysts, and conditions required for each intermediate compound.

Synthetic Route Overview for Aliphatic Compounds | StuDocu

Looking at the synthetic routes overview diagram, there are always multiple routes to get from a starting material to a target compound. You should always try to minimise the steps it takes to get from the starting material to the target compound, to maximise the product yield.

## Synthetic routes - aromatic compounds

Similar to the synthetic routes overview diagram for aliphatic compounds, we can draw a synthetic routes overview diagram for aromatic compounds. Thankfully, it is much smaller than for aliphatic compounds, and much easier to remember.

Synthetic Routes Overview for Aromatic Compounds | StuDocu

You know that the parent member of all aromatic compounds is benzene. All aromatic compounds can be synthesised from benzene, and that already makes it easier to remember. As an example, let us try to synthesise one aromatic compound from benzene.

### Synthetic route: 4-bromo-3-nitroacetophenone from benzene

Let us draw the starting material and the target compound first.

Benzene as the starting material, 4-bromo-3-nitroacetophenone as the target compound, Kanishk Singh, StudySmarter Originals

In the target compound, there are 3 functional groups attached to the parent compound (benzene). Looking at the overview of the synthetic routes, we can see that there is no direct route to get from our starting material to the target compound. So, this will be a 3-step route, in which each step will be a reaction to add 1 functional group to the parent compound. To map this route, we will employ the technique of retrosynthesis.

Retrosynthesis is the process of figuring out the synthetic route in the reverse direction - from target compound to the starting material.

We look at the target compound, thinking about which reaction was done last to obtain this compound. To determine this, we have to look at the functional groups.

1. We know that Br is an ortho-, para-director because of the lone pairs of electrons on it.
2. We also know that the nitro group is a meta-director because of the +1 formal charge.
3. We also know that the acyl group is a meta-director because of the partial positive charge on the carbon double-bonded to oxygen.

Considering these points, it makes sense that the nitro group was added last because:

1. It is in the ortho position to the ortho-director bromine.
2. It is meta to the meta-director acyl group.

So, the precursor to the target compound was 4-bromoacetophenone, on which the nitration reaction was done. We can write the nitration reaction as below:

Nitration of 4-bromoacetophenone, Kanishk Singh, StudySmarter Originals

Following the same procedure, we know that bromine is an ortho-, para-director and the acyl group is a meta-director. Therefore, it makes sense that the precursor to 4-bromoacetophenone was bromobenzene and the acyl group was added to the ortho position by the ortho-director bromine. The reaction to add an acyl group is called Friedel–Crafts acylation reaction.

Friedel-Crafts Acylationof Bromobenzene, Kanishk Singh, StudySmarter Originals

You might be wondering; since bromine is a deactivation group on the benzene ring, how can we add an acyl group? The answer to this is that bromine is only a weakly deactivating group. We can't do a Friedel-Crafts reaction when there is a moderately or highly deactivating group present on the ring.

Finally, the only group left is bromine. The reaction is a bromination reaction:

Bromination of Benzene, Kanishk Singh, StudySmarter Originals

We have discussed the individual steps of the synthetic route of 4-bromo-3-nitroacetophenone from benzene. This diagram shows the complete synthetic route.

Synthetic Route of 4-bromo-3-nitroacetophenone from Benzene, Kanishk Singh, StudySmarter Originals

For your exam, you are expected to know the synthetic route for any starting material to any target compound, and the reagents, catalysts, and conditions required. There are numerous possibilities! The easiest way is to remember the two synthetic route overview diagrams given in this article (one for aliphatic compounds and the other for aromatic compounds) and construct the route for any given starting and target compound. You'll need a lot of practice, but you can handle this!

## Synthetic Routes - Key takeaways

• A synthetic route is a series of steps to be followed in order to make a chemical compound from smaller and less complex chemicals.
• In a synthetic route, each step is a chemical reaction.
• The final chemical in the route i.e., the chemical compound to be made is called the target compound. The chemical from which you start the route is called the starting material.
• All chemical compounds formed between the starting material and the target compound are called intermediate compounds.
• While mapping synthetic routes, always try to minimise the number of steps to maximise the product yield.
• Retrosynthesis is the process of figuring out the synthetic route in the reverse direction - from target compound to the starting material.
• For your exam, you are expected to know the synthetic routes - reagents, catalysts, and the conditions required for each reaction - for any given starting and target compound.

## Synthetic Routes

To make a synthetic route between a starting material and the target compound -

1. List all the molecules that can be made from the starting material.
2. List all the molecules from which the target molecule can be made.
3. Check for common intermediate compounds between the starting material and the target compound.

Synthetic Route is a series of steps to be followed in order to make a chemical compound from smaller and less complex chemicals.

The easiest way to map synthetic routes on your own is to remember the two synthetic route overview diagrams given in this article (one for aliphatic compounds and the other from aromatic compounds), and construct the route for any given starting and target compound from there.

## Final Synthetic Routes Quiz

Question

How do you carry out hydration of alkenes?

Alkenes are treated with steam at 300oC and 60-70 atm in the presence of phosphoric acid.

Show question

Question

What is the product of hydration of alkenes?

Alcohol

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Question

What do you get when you oxidize an alcohol?

Carboxylic acid

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Question

To convert 1-bromopropane to propaneamine, which reaction do you require?

Reaction with excess ethanolic ammonia and heat.

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Question

What do you get when you hydrolyse haloalkanes?

Alcohol

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Question

Which of the following chemicals is not the same as the others?

Propanenitril

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Question

How many steps would it take to synthesize propylamine from ethene?

3 steps

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Question

How many intermediate compounds will be formed during the synthesis of propylamine from ethene?

2 intermediate compounds.

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Question

How many steps would it take to synthesize propanoic acid from propanenitrile?

1 step

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Question

How do you convert propanenitrile to propanoic acid?

Reaction with dil. HCl and water, and heat under reflux

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Question

How do you convert an alcohol to an alkene?

Reaction of alcohol with concentrated phosphoric acid and heat at 170oC.

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Question

How do you convert carboxylic acid to a primary amide?

1. First, convert carboxylic acid to acyl chloride by reaction with SOCl2.
2. Then, add ammonia at 20oC to get the primary amide.

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Question

What do get after reduction of an alkene?

Alkane

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Question

How can you reduce an alkene to get an alkane?

Reaction of alkene with hydrogen in the presence of a metal catalyst (Pd, Pt, or Ni) at 1500

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Question

How do you convert an aldehyde group to a hydroxynitrile group?

Reaction with hydrogen cyanide (HCN)

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