Reaction Pathways and Atom Economy (VCE SSCE Chemistry): Revision Notes

Reaction Pathways and Atom Economy

Introduction to reaction pathways

Organic chemists are skilled at creating compounds with specific properties for particular purposes, such as pharmaceuticals and polymers. Once a desired compound has been identified, chemists must develop an effective method for synthesizing it. The goal is to design an efficient route for converting easily accessible starting materials, often alkenes or alkanes, into more complex products.

A reaction pathway is a series of one or more steps that transforms a reactant containing certain functional groups into a desired product with different functional groups. Modern chemists aim to design environmentally friendly synthetic routes that minimize waste, use 'greener' solvents, require less energy, and help preserve natural resources.

Simple reaction pathways

Simple reaction pathways involve converting basic organic compounds into products through one or a few steps. Understanding these fundamental transformations allows chemists to build more complex synthesis routes.

Ethane and ethene-based conversions

Starting from ethane ($CH_{3}CH_{3}$) or ethene ($CH_{2}=CH_{2}$) , we can create various organic compounds through different reaction types. The same principles and reaction conditions can be applied to other members of the alkane and alkene families.

The diagram above shows several important transformations:

• Ethane to chloroethane: Free radical halogenation using chlorine gas ($Cl_{2}$) under UV light
• Chloroethane to ethanol: Nucleophilic substitution with aqueous sodium hydroxide ($NaOH(aq)$)
• Ethanol to ethanoic acid: Oxidation using dichromate ions ($Cr_{2}O_{7}^{2-}$) in acidic conditions
• Ethanoic acid to alkyl ethanoate: Esterification with an alcohol ($ROH$) using sulfuric acid catalyst
• Ethene to ethanol: Direct hydration or via chloroethane intermediate
• Ethanol to ethene: Elimination reaction using concentrated $H_{3}PO_{4}$ or $H_{2}SO_{4}$ and heat

Making ethanol from ethene

Ethanol ($CH_{3}CH_{2}OH$) is an important two-carbon alcohol that can be synthesized from ethene in two different ways.

Pathway 1 (direct route): Ethene reacts directly with water in the presence of a phosphoric acid ($H_{3}PO_{4}$) catalyst to produce ethanol. This is the more efficient route as it requires only one step.

Pathway 2 (two-step route): Ethene first reacts with hydrogen chloride ($HCl$) to form chloroethane as an intermediate compound. The chloroethane then undergoes substitution with hydroxide ions ($OH^{-}$) to produce ethanol.

Complex reaction pathways

More complex syntheses often require multiple steps and careful planning. Let's examine the synthesis of ethyl propanoate, an ester used in fragrances and flavorings.

Making propanoic acid from propane

Propanoic acid ($CH_{3}CH_{2}COOH$) is a three-carbon carboxylic acid. Working backwards from the final product helps us plan the synthesis:

• Propanoic acid can be formed by oxidizing the primary alcohol propan-1-ol
• Propan-1-ol can be made by reacting 1-chloropropane with sodium hydroxide
• 1-chloropropane can be prepared by reacting propane with chlorine under UV light

Making ethyl propanoate

Ethyl propanoate is an ester with the structure $CH_{3}CH_{2}COOCH_{2}CH_{3}$. This compound requires two separate precursors: ethanol and propanoic acid.

The complete synthesis pathway combines the routes we've already discussed:

This multi-step synthesis demonstrates how chemists combine simpler reactions to build more complex molecules. The final esterification step involves condensing propanoic acid and ethanol using sulfuric acid as a catalyst, producing the ester and water.

Summary of reaction pathways

The following comprehensive diagram shows the interconnected pathways for producing various organic compounds from alkenes:

This diagram is useful when planning new syntheses because it shows:

• Multiple routes to the same product
• Which functional groups can be interconverted
• The reagents and conditions needed for each transformation

Planning a synthesis

When designing a reaction pathway, chemists must consider several factors beyond simply identifying a possible sequence of reactions:

The planning process involves studying the structure of the target molecule, identifying functional groups, devising a synthetic pathway using known reactions, considering by-products and separation methods, evaluating purity, and calculating expected yields.

Yield calculations

Understanding and calculating yields is essential for evaluating the efficiency of chemical reactions and industrial processes.

Theoretical and actual yields

Theoretical yield is the maximum amount of product that could be formed if the limiting reactant reacts completely according to the stoichiometric ratios in the balanced equation. It assumes 100% conversion and is calculated using mole ratios.

Actual yield is the amount of desired product actually obtained at the end of the reaction. The actual yield is typically less than the theoretical yield due to several factors:

• Reactions reaching equilibrium before completion
• Slow reaction rates preventing full conversion
• Loss of material during transfers and purification
• Competing reactions producing unwanted by-products

Percentage yield

Percentage yield compares the actual yield to the theoretical yield, providing a measure of reaction efficiency:

$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$

A higher percentage yield indicates greater efficiency in converting reactants to products.

Multi-step synthesis yields

When a synthesis involves multiple steps, the overall percentage yield is the product of the individual yields at each step. Each intermediate reaction reduces the final amount of product obtained.

Atom economy

An important consideration in modern chemistry is atom economy, which measures how efficiently the atoms in reactants are incorporated into the desired product.

Understanding atom economy

Atom economy is the percentage of atoms from the reactants that end up in the desired product. A high atom economy means fewer waste products are generated, making the reaction more sustainable and environmentally friendly.

The diagram illustrates the difference between high and low atom economy reactions. In a high atom economy reaction, all reactant atoms become part of the desired product. In a low atom economy reaction, some reactant atoms become waste by-products.

Calculating atom economy

The atom economy can be calculated using either of these formulas:

$\text{atom economy} = \frac{\text{molar mass of desired product}}{\text{molar mass of all reactants}} \times 100$

or alternatively:

$\text{atom economy} = \frac{\text{mass of desired product}}{\text{mass of all products}} \times 100$

Advantages of high atom economy

The choice of synthetic route can dramatically affect atom economy and environmental impact. Consider these two pathways for oxidizing 1-phenylethanol to acetophenone:

Pathway 1 (traditional method): Uses Jones reagent (chromium trioxide and sulfuric acid) with an atom economy of 44%. This produces significant amounts of chromium(III) sulfate waste, which must be disposed of safely. The process requires expensive reagents and consumes more of Earth's resources.

Pathway 2 (modern method): Uses catalytic oxidation with oxygen, achieving an atom economy of 91%. The only by-product is water, making it much more environmentally friendly. The catalyst also reduces the energy needed for the reaction.

Case study: Aspirin synthesis

Pharmaceutical products are often developed from substances found in plants used as traditional medicines. Aspirin provides an excellent example of this process.

Historical background

The origin of aspirin traces back to salicin, a compound found in willow tree leaves and bark. Ancient civilizations, including the Sumerians and Egyptians (3000-1500 BCE), used willow as a medicine. Around 400 BCE, the Greek physician Hippocrates recommended willow leaf infusions to relieve pain.

In 1829, salicin was identified and isolated as the active ingredient. Scientists discovered that the body converts salicin to salicylic acid, which reduces fever and acts as a painkiller. However, pure salicylic acid is harsh and causes stomach irritation.

Aspirin synthesis reaction

Aspirin is produced through a condensation reaction between salicylic acid and ethanoic acid (acetic acid):

In this reaction:

• The hydroxyl group of salicylic acid reacts with the carboxyl group of ethanoic acid
• An ester link is formed, creating acetylsalicylic acid (aspirin)
• Water is eliminated as a by-product
• Sulfuric acid acts as a catalyst

Although this reaction can be performed in a school laboratory, the yield is quite low. Industrial production uses more complex methods to achieve better yields.

Remember!