Oxidation (HSC SSCE Chemistry): Revision Notes

Oxidation

Introduction to oxidation of alcohols

Alcohols can readily undergo oxidation reactions when treated with strong oxidising agents. The specific products formed depend on the type of alcohol being oxidised—whether it's primary, secondary, or tertiary. Understanding how different alcohols behave during oxidation is essential for predicting reaction outcomes and identifying unknown alcohols.

The oxidation process involves the loss of hydrogen atoms from the alcohol molecule. Different types of alcohols have different numbers of hydrogen atoms attached to the carbon bearing the hydroxyl group ($\text{-OH}$), which determines their oxidation behaviour.

Common oxidising agents

Two main oxidising agents are commonly used to oxidise alcohols in laboratory conditions:

• Acidified permanganate ion solution ($\text{MnO}_4^-$)
• Acidified dichromate ion solution ($\text{Cr}_2\text{O}_7^{2-}$)

Both of these solutions act as strong oxidising agents, meaning they readily accept electrons and become reduced themselves whilst oxidising the alcohol.

Redox half reactions

When these oxidising agents react, they undergo reduction according to the following half equations:

Permanganate reduction:

$\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{e}^- \rightarrow \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})$

Dichromate reduction:

$\text{Cr}_2\text{O}_7^{2-}(\text{aq}) + 14\text{H}^+(\text{aq}) + 6\text{e}^- \rightarrow 2\text{Cr}^{3+}(\text{aq}) + 7\text{H}_2\text{O}(\text{l})$

Oxidation of primary alcohols

Primary alcohols undergo a two-step oxidation process. First, they are oxidised to form aldehydes, and these aldehydes can then be further oxidised to form carboxylic acids. This progressive oxidation makes primary alcohols quite versatile in organic synthesis.

The general oxidation pathway shows that a primary alcohol ($\text{R-CH}_2\text{-OH}$) first loses hydrogen to become an aldehyde ($\text{R-CHO}$), which can then lose more hydrogen to become a carboxylic acid ($\text{R-COOH}$).

Example: oxidation of ethanol

Writing oxidation equations

In organic chemistry, these reactions can also be written using structural formulas with reaction conditions shown above the arrow. You should be comfortable writing equations in both full balanced form (as shown above) and in condensed organic notation where only the organic molecules are shown with conditions indicated.

Collecting aldehydes by distillation

This is achieved using a distillation setup where the aldehyde is removed as it forms.

The distillation process works as follows:

1. Ethanol is heated in a round-bottom flask using an electric heater until the temperature is just above the boiling point of the aldehyde product (ethanal)
2. Acidified dichromate solution is added drop by drop from a funnel at the top
3. As each drop of dichromate meets the hot ethanol, oxidation occurs immediately to form ethanal
4. Because the mixture temperature is above ethanal's boiling point but below ethanol's boiling point, the ethanal immediately vaporises and rises up through the condenser
5. The ethanal vapour is cooled by the water-cooled condenser and collected in a flask sitting in iced water
6. Since the ethanal is removed immediately from the oxidising agent, it cannot undergo further oxidation to ethanoic acid

Oxidation of secondary alcohols

Secondary alcohols undergo oxidation to form ketones. Unlike aldehydes formed from primary alcohols, ketones cannot be oxidised further under normal laboratory conditions. This means the oxidation of a secondary alcohol is a one-step process with the ketone as the final product.

Both acidified permanganate and dichromate ions can oxidise secondary alcohols effectively. The general reaction shows a secondary alcohol being converted to a ketone, with no further oxidation possible.

Example: oxidation of 2-propanol

Oxidation of tertiary alcohols

Tertiary alcohols cannot be oxidised under normal laboratory conditions. Understanding why this occurs requires examining the structural differences between alcohol types.

Why tertiary alcohols resist oxidation

Extremely harsh conditions (high temperatures and very strong oxidising agents) could potentially break down the carbon skeleton, but this is not a useful synthetic reaction.

Distinguishing types of alcohols using oxidation

The different oxidation behaviours of primary, secondary, and tertiary alcohols provide a practical method for identifying unknown alcohol types in the laboratory. This identification relies on observing colour changes in the oxidising agents used.

Using dichromate ions

When acidified dichromate solution is used as the oxidising agent, a distinctive colour change occurs during oxidation reactions:

• Dichromate ions ($\text{Cr}_2\text{O}_7^{2-}$) are orange in colour
• Chromium(III) ions ($\text{Cr}^{3+}$) formed after reduction are green in colour

Using permanganate ions

Acidified permanganate solution can also be used, though it is somewhat less reliable:

• Permanganate ions ($\text{MnO}_4^-$) are purple in colour
• Manganese(II) ions ($\text{Mn}^{2+}$) formed after reduction are colourless

Under acidic conditions, the purple permanganate solution should turn colourless when it oxidises an alcohol. However, in practice, a brown colour is sometimes seen due to formation of manganese dioxide ($\text{MnO}_2$), which makes this test less reliable than the dichromate test for classroom use.

Investigation: oxidation of alcohols

Students can investigate the oxidation of different alcohol types by testing primary, secondary, and tertiary alcohols with both permanganate and dichromate oxidising agents. This practical investigation allows you to observe the colour changes described above and compare how different alcohols behave.

Safety considerations for this investigation:

The investigation typically involves adding small amounts of acidified oxidising agent to samples of different alcohols in test tubes, warming them gently in a water bath, and observing any colour changes over several minutes.

Remember!