Deduction of Equations Showing Functional Group Changes During Oxidation
When organic compounds undergo oxidation, the functional groups present can be altered, leading to changes in the chemical properties of the molecule.
Alcohols to Aldehydes, Ketones, and Carboxylic Acids
Alcohols, depending on their structure, can be oxidised to various products.
- Primary Alcohols:
- Oxidation of primary alcohols first leads to the formation of aldehydes.
- Example: RCH2OH to RCHO
- If further oxidised, these aldehydes can then form carboxylic acids.
- Example: RCHO to RCOOH
- Oxidation of primary alcohols first leads to the formation of aldehydes.
Image courtesy of Gabriel Tojo
- Secondary Alcohols:
- These are oxidised to produce ketones.
- Example: R2CHOH to R2CO
- These are oxidised to produce ketones.
- Tertiary Alcohols:
- Interestingly, tertiary alcohols don't get oxidised under typical conditions due to the lack of a hydrogen atom bonded to the carbon attached to the -OH group.
Image courtesy of Master Organic Chemistry
Mechanism of Oxidation
- Primary Alcohols: The oxidising agent, often acidified potassium dichromate, removes two hydrogen atoms: one from the OH group and one from the adjacent carbon atom.
- Secondary Alcohols: Oxidation involves the removal of the hydrogen from the OH group and another hydrogen from the carbon atom adjacent to it.
Experimental Setups: Distillation and Reflux
Chemical reactions often produce a mixture of compounds, necessitating separation processes to isolate the desired product.
Distillation
Distillation is a versatile technique based on differences in boiling points.
- Apparatus: It includes a round-bottom flask, a heating mantle, a thermometer, a distillation column, a condenser, and a receiving flask.
- Procedure:
- The organic mixture is heated, causing the compound with the lowest boiling point to evaporate first.
- The vapour passes through a cooling condenser where it is condensed back into a liquid.
- The resulting liquid (distillate) is collected separately.
A fractional distillation flask. For the separation of components with significant differences in boiling points, a fractionating column is not used. For components with closer boiling points fractionating column is used allowing better separation.
Image courtesy of Theresa knott
Reflux
Reflux is essential for reactions that need to be heated over an extended period.
- Apparatus: A round-bottom flask connected to a condenser, with the condenser's outlet dripping back into the flask.
- Procedure:
- As the reaction mixture is heated, any vapours produced are cooled and condensed by the condenser.
- The condensed liquid returns to the flask, ensuring that the reaction continues without any loss of reactants or products.
Image courtesy of Tooto
Impact of Functional Groups on Physical Properties
Functional groups play a pivotal role in determining the physical properties of organic compounds.
- Boiling Point:
- Molecules with hydrogen-bonding functional groups, like -OH or -NH2, generally have a higher boiling point due to increased intermolecular forces.
- Solubility:
- Functional groups can introduce polar regions into a molecule, enhancing its solubility in polar solvents.
- Density and Viscosity:
- The presence of heavy atoms in functional groups, especially halogens, can influence these properties significantly.
Differentiating Between Combustion and Oxidation of an Alcohol
These two processes, though involving the reaction of substances with oxygen, are distinct in nature and outcome.
Combustion
- This is a rapid process where an alcohol burns in air or oxygen to produce carbon dioxide, water, and releases energy.
- Example: CH3OH + 1.5O2 -> CO2 + 2H2O
Oxidation
- Oxidation is more controlled and doesn't necessarily involve the direct addition of oxygen. It could involve the loss of hydrogen or electrons.
- For instance, the controlled oxidation of ethanol can yield ethanal (an aldehyde) instead of just carbon dioxide and water.
Colour Change in Oxidation Reactions Involving Transition Elements
Transition elements, with their d-orbitals, can adopt various oxidation states, which often have distinct colours.
- For example, the oxidation of Mn2+ ions in aqueous solutions yields a colour change from pale pink to purple due to the formation of MnO4- ions.
- Another instance is the oxidation of Fe2+ ions (green) to Fe3+ ions, which are brown/yellow in aqueous solutions.
These colour changes not only make redox reactions involving transition metals visually engaging but also offer a practical means of tracking reaction progress. It's especially crucial in titration experiments where the endpoint is determined by a noticeable colour change.
FAQ
The structure of a molecule, specifically the arrangement and connectivity of atoms, determines its oxidation state and reactivity. The oxidation state is defined based on the hypothetical charge an atom would have if all shared electrons were completely transferred to the atom with the greater electronegativity. Hence, molecules with more electronegative elements (like oxygen) often exhibit higher oxidation states. The molecule's structure also affects reactivity. For instance, primary alcohols have the hydroxyl group attached to a carbon, which is also attached to two hydrogen atoms, making them more susceptible to oxidation. In contrast, tertiary alcohols, due to their structural layout, resist similar oxidations.
Pure alcohols, on their own, are generally stable and do not spontaneously oxidise in the presence of air. An external oxidising agent or catalyst is required to facilitate the oxidation process, primarily by providing an alternative pathway with a lower activation energy. Oxidising agents, like acidified potassium dichromate, supply the necessary electron acceptors that pull electrons away from the alcohol molecule, thus promoting oxidation. Catalysts, on the other hand, speed up the reaction without getting consumed, essentially by offering a more energetically favourable route for the reaction to proceed.
Transition elements possess d-orbitals where the distribution of electrons can change upon undergoing redox reactions. When these elements engage in oxidation reactions, the number of d-electrons can change, leading to a shift in their electronic configurations. This shift can alter the wavelengths of light the compound can absorb, resulting in a visible colour change. For instance, when manganese in permanganate ions (purple) gets reduced to manganese(II) ions, the solution changes from purple to colourless. Such colour changes act as a useful visual cue in many chemical reactions involving transition elements.
Acidified potassium dichromate poses significant health and environmental risks. It is highly toxic when ingested, inhaled, or absorbed through the skin. Prolonged exposure can cause serious skin, eye, and respiratory irritation. Furthermore, hexavalent chromium compounds, like potassium dichromate, are classified as carcinogens, which means they can increase the risk of cancer upon prolonged exposure. Additionally, these compounds are environmental pollutants; they can contaminate soil and water sources, posing threats to aquatic life. Therefore, while using acidified potassium dichromate in the laboratory, it is crucial to handle it with care, using appropriate personal protective equipment and ensuring proper disposal.
Potassium dichromate is a powerful oxidising agent, particularly when acidified with dilute sulphuric acid. Its ability to oxidise alcohols is attributed to the presence of the chromium(VI) ion, which can be reduced to chromium(III) ion. During this process, the chromium ion changes colour from orange to green, providing a clear visual indicator of the reaction's progression. Moreover, potassium dichromate is commonly available and can oxidise a variety of organic compounds, making it a favoured choice in organic chemistry labs. Its effectiveness in oxidising primary and secondary alcohols to aldehydes, ketones, and carboxylic acids makes it an essential tool in organic synthesis.
Practice Questions
Primary alcohols, when oxidised using acidified potassium dichromate, initially produce aldehydes. If the oxidation continues, these aldehydes are further oxidised to form carboxylic acids. Secondary alcohols, on the other hand, are oxidised to form ketones. The key distinction between these alcohols is the position of the hydroxyl group and the adjacent carbon atoms, which affects their susceptibility to the oxidising agent. Tertiary alcohols do not undergo oxidation under similar conditions. This is because tertiary alcohols lack a hydrogen atom bonded to the carbon which is attached to the -OH group, preventing the removal of hydrogen needed for the oxidation to proceed.
Combustion of ethanol is a rapid reaction that occurs when ethanol burns in the presence of air or oxygen, producing carbon dioxide and water. The process releases energy in the form of heat and light, typically seen as flames. On the other hand, the oxidation of ethanol is a more controlled process and does not necessarily involve the direct addition of oxygen. Instead, it might involve the loss of hydrogen or electrons. Under controlled oxidation conditions, ethanol can be oxidised to ethanal, which is an aldehyde. The products formed in the oxidation of ethanol differ from combustion, indicating the unique nature of each process.
