Noah Vinter
Several reagents can be used to distinguish between primary and secondary alcohols. One commonly used reagent is Schiff's reagent, a fuchsia dye that changes colour when sulfur dioxide is passed through it. When heated, the vapours produced by Schiff's reagent can be passed through a reaction mixture containing a primary alcohol, which will turn magenta due to the presence of aldehyde. However, a simple trace of pink or no colour change indicates the absence of a primary alcohol. Acidified potassium dichromate (VI) is another reagent used to identify secondary alcohols. The colour change from orange to green indicates the presence of secondary alcohols, while primary alcohols are oxidised to aldehydes, forming a silver mirror with Tollens reagent. The Lucas test, which treats the alcohol with concentrated HCl and ZnCl2, can also differentiate between primary and secondary alcohols based on the time taken for turbidity to form. Finally, the Jones test uses chromium trioxide as a powerful oxidising agent in the presence of sulfuric acid to distinguish between primary and secondary alcohols.
| Characteristics | Values |
|---|---|
| Lucas Test | Primary alcohols do not form turbidity at room temperature. An oily layer forms when heated. Secondary alcohols form an oily layer in 5-6 minutes. |
| Jones Test | Primary alcohol is converted to an aldehyde and then to a carboxylic acid. Secondary alcohol is oxidized to a ketone. |
| Number of carbon atoms attached to alpha-carbon | Primary alcohols have one carbon atom attached to the alpha-carbon. |
| Hydrogen atom attached to hydroxyl group | Secondary alcohols have one hydrogen atom attached to the hydroxyl group. |
| Number of R groups | Primary alcohols have one R group. Secondary alcohols have two R groups. |
| Reaction with Tollen's reagent | Primary alcohol is oxidized to an aldehyde, which forms a silver mirror with Tollen's reagent. Secondary alcohol is oxidized to a ketone, which does not react with Tollen's reagent. |
| Reaction with Schiff's reagent | No color change in Schiff's reagent indicates no primary alcohol. |
| Reaction with acidified potassium dichromate | Acidified potassium dichromate oxidizes primary alcohol to an aldehyde. |
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What You'll Learn
Schiff's reagent
The Schiff's test is a simple and fairly reliable method to distinguish between primary and secondary alcohols. It involves the use of Schiff's reagent, a fuchsine dye decolorized by passing sulfur dioxide through it.
In the presence of a primary alcohol, the Schiff's reagent quickly turns magenta, indicating the production of an aldehyde. This colour change occurs within a minute or so of adding the reagent. On the other hand, if there is no colour change or only a slight pink tint, it suggests that no aldehyde was formed, indicating the absence of a primary alcohol.
Secondary alcohols, on the other hand, are identified by their interaction with acidified potassium dichromate(VI) solution. When secondary alcohols are oxidised, they turn the orange solution of dichromate(VI) ions into a green solution containing chromium(III) ions. This colour change is distinct from the magenta observed with primary alcohols.
It is important to note that the Schiff's reagent should be checked promptly after the colour change in the potassium dichromate(VI) solution, as leaving it too long may result in a colour change in the secondary alcohol case as well, leading to potential confusion.
Other tests, such as the Lucas test and the Jones test, also help distinguish between primary and secondary alcohols. The Lucas test involves treating the alcohol with concentrated HCl and ZnCl2, while the Jones test utilises chromium trioxide as a powerful oxidising agent in the presence of sulfuric acid. However, these tests may be more complex and time-consuming than the Schiff's test.
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Lucas test
The Lucas test is a chemical test used to distinguish primary, secondary, and tertiary alcohols. It was developed by Howard Lucas in 1930 and is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an SN1 reaction.
The reagent used in the Lucas test is known as Lucas reagent, a solution of anhydrous zinc chloride in concentrated hydrochloric acid. The test works by adding the Lucas reagent to the alcohol and observing the rate of turbidity formation to identify the alcohol's class. The key difference between the three classes of alcohols is how quickly they turn the solution turbid when reacted with the Lucas reagent.
Tertiary alcohols react the fastest with Lucas reagent, forming a cloudy solution almost immediately. This is due to the low solubility of the organic chloride in the aqueous mixture and the higher stability of the intermediate tertiary carbocation. Secondary alcohols take a few minutes (usually 2-5 minutes) to turn cloudy, while primary alcohols either react very slowly or not at all at room temperature and may require heating to form an oily layer.
The Lucas test is widely used in organic chemistry labs to quickly determine the type of alcohol present in a sample. It is a valuable skill for any chemistry student to be able to identify alcohol types in lab tests. However, it is important to note that the Lucas test is qualitative and not quantitative, meaning it provides information about the presence or absence of certain functional groups rather than precise measurements.
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Jones test
The Jones test is a chemical test used to identify primary and secondary aliphatic alcohols. It involves the use of Jones' reagent, a mixture of chromium trioxide and sulfuric acid in water, which forms chromic acid (H2CrO4). This reagent is a strong oxidizing agent and is added to the alcohol. The oxidation state of chromium is key to this test—in the reagent, chromium is in the +6 oxidation state, giving it a reddish-orange colour.
Jones' reagent reacts with primary alcohols to form aldehydes, which then further react to form carboxylic acids. With secondary alcohols, the reagent reacts to form ketones. The addition of the reagent to primary and secondary alcohols causes the solution to change colour from orange to dark green. Tertiary alcohols, on the other hand, do not react with Jones' reagent as they are resistant to oxidation. Thus, the solution remains orange. This colour change can be used to distinguish between primary and secondary alcohols and tertiary alcohols.
In a laboratory setting, the Jones test is performed by adding a few drops of known aliphatic alcohols and the unknown alcohol to separate tubes. Jones' reagent is then carefully added to each tube, and the solutions are mixed well. The solutions are allowed to react for a set time, typically around 10 minutes, before the unknown alcohol is compared to the known alcohols. If the unknown solution turns green, the test is positive for a primary or secondary alcohol. If it remains orange, the test is negative, indicating a tertiary alcohol.
It is important to note that the Jones test does not differentiate between primary and secondary alcohols. To identify between these two types of alcohols, a subsequent test, such as the Lucas test, is required. The Lucas test utilises zinc(II) chloride in the presence of hydrochloric acid. This reagent reacts with secondary and tertiary alcohols, forming an insoluble product in aqueous solutions. The reaction rate depends on the formation of a carbocation, which is influenced by the stability of the carbocation. Tertiary alcohols form stable carbocations, resulting in a rapid reaction and a visible second phase in the mixture. Secondary alcohols exhibit a slower reaction, taking a few seconds to a few minutes to show a positive result. Primary alcohols, on the other hand, form unstable carbocations, leading to no observable reaction at room temperature.
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Oxidation of alcohols
Alcohols are organic compounds with a hydroxyl group attached to an alkyl or aryl group (ROH). The number of other substituent groups (R) on the carbon atom determines whether an alcohol is primary (RCH2OH), secondary (R2CHOH), or tertiary (R3COH). A hydroxyl carbon with only one R group is a primary alcohol, while a hydroxyl carbon with two R groups is a secondary alcohol.
The oxidation of alcohols is a reaction that can be used to distinguish between primary, secondary, and tertiary alcohols. The oxidizing agent used in these reactions is typically a solution of sodium or potassium dichromate(VI) acidified with dilute sulfuric acid. If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.
Primary alcohols can be oxidized to either aldehydes or carboxylic acids, depending on the reaction conditions. In the presence of the Jones reagent, a primary alcohol is converted to an aldehyde and then to a carboxylic acid. The aldehyde formed as the halfway product should remain in the mixture. The full equation for the oxidation of ethanol to ethanoic acid is as follows:
$$3CH_3CH_2OH + 2Cr_2O_7^{2-} + 16H+ \rightarrow 3CH_3COOH + 4Cr^{3+} + 11H_2O$$
The simplified version of this equation is:
$$CH_3CH_2OH + 2 [O] \rightarrow CH_3COOH + H_2O$$
Secondary alcohols, on the other hand, are oxidized to ketones. This is a useful differentiating factor between primary and secondary alcohols, as primary alcohols can be further oxidized to carboxylic acids, while secondary alcohols cannot. An example of this is the oxidation of the secondary alcohol propan-2-ol with acidified sodium or potassium dichromate(VI) solution, which forms propanone.
Tertiary alcohols do not react with chromium, and no precipitate is formed, resulting in an orange solution. They resist oxidation reactions due to the absence of hydrogen atoms on the carbon holding the hydroxyl group, and they cannot be oxidized further under normal conditions.
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Distinguishing characteristics
Alcohols are organic compounds with a hydroxyl group (OH) attached to an alkyl or aryl group (ROH). The number of other substituent groups (R) on the carbon atom attached to the hydroxyl group distinguishes primary, secondary, and tertiary alcohols.
A primary alcohol has only one substituent group (R) attached to the carbon atom bonded to the hydroxyl group. This carbon atom will only be attached to the alpha-carbon if the hydroxyl group is at the end of the molecule chain. Examples of primary alcohols include ethanol, propanol, and butanol. When no carbon atoms are bonded, methanol is the primary alcohol.
A secondary alcohol, on the other hand, has two substituent groups (R) attached to the carbon atom bonded to the hydroxyl group. This carbon atom is typically bonded to two other carbon atoms and one hydrogen atom. An example of a secondary alcohol is isopropanol (CH3CHOHCH3), where the central carbon is linked to two methyl groups and one hydrogen atom.
The Lucas test is a method used to distinguish between primary, secondary, and tertiary alcohols. It involves treating the alcohol with Lucas reagent (concentrated HCl and ZnCl2). The time taken for turbidity to form is then observed. In the case of a primary alcohol, no turbidity forms at room temperature, but an oily layer forms when heated. With a secondary alcohol, an oily layer forms after 5-6 minutes, indicating a slower reaction. For tertiary alcohols, turbidity is immediate due to the ease of forming halides.
Another distinguishing test involves oxidizing the alcohol with acidified potassium dichromate and then testing with Tollens reagent. Primary alcohols will be oxidized to aldehydes, forming a silver mirror with Tollens reagent. Secondary alcohols, on the other hand, will be oxidized to ketones, which do not react with Tollens reagent.
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Frequently asked questions
A primary alcohol has only one carbon atom attached to the alpha-carbon. This occurs when the hydroxyl group is at the end of the molecule chain. Examples include ethanol, propanol, and butanol.
One way to identify a primary alcohol is through the Jones test, which uses chromium trioxide as a powerful oxidizing agent in the presence of sulfuric acid. A primary alcohol will be converted into an aldehyde in the presence of Jones' reagent.
A secondary alcohol has only one hydrogen atom attached to the hydroxyl group (-OH). This can occur anywhere along a carbon chain.
One method to distinguish between primary and secondary alcohols is the Schiff's test. The Schiff's reagent is a fuchsia dye that turns bright magenta in the presence of aldehyde. A simple trace of pink colour or no colour change indicates that no aldehyde was formed and, therefore, no primary alcohol is present. The secondary alcohol will be oxidised to a ketone, which does not react with the Schiff's reagent.
Another method is the Lucas test, which compares the reactivity of primary and secondary alcohols to hydrogen chloride. The time taken for turbidity to form is recorded. Turbidity does not form at room temperature for primary alcohols, but an oily layer forms when heated. For secondary alcohols, an oily layer forms within 5-6 minutes.
