Question

Suggest a simple test to distinguish \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\) from \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHOHCH}_{2} \mathrm{CH}_{3}\).

Short answer

The test involves the use of bromine water. Bromine water will decolourize when mixed with the compound CH3CH=CHCH3 (2-propene) because of the addition reaction but will remain reddish-brown when mixed with CH3CH2CHOHCH2CH3 (pentan-2-ol). Therefore, observing a change in color (or no change) enables you to distinguish between the two compounds.

Step-by-step solution

Test Preparation

Prepare a sample of each compound in separate test tubes. It's important to make sure you know which tube contains which compound, so label them accurately.

Bromine Water Test

Add bromine water (Br2 in H2O), a reddish-brown liquid, to each test tube. Alkenes like 2-propene react with bromine water and decolourize it.

Observing Results

If the test compound is an alkene, the bromine water will lose its reddish-brown colour, confirming the presence of a carbon-carbon double bond in the molecule. If the test compound is an alcohol, the bromine water will retain its colour.

Key concepts

Bromine Water Test

Identifying the structure of different organic compounds is a fundamental skill in chemistry, and the bromine water test is a classic method used for this purpose. In essence, the bromine water test is utilized to determine whether a compound contains a carbon-carbon double bond, characteristic of alkenes.

Bromine water, with its distinct reddish-brown color, acts as an indicator. When an alkene is present, it will readily react with bromine, leading to the addition of bromine atoms across the double bond. This reaction results in a colorless dibromo compound. Therefore, if you add bromine water to a sample and the color disappears, it signifies the presence of an alkene.

Reaction with Alkenes

The chemical reaction that underpins the bromine water test for alkenes can be represented as follows:
An alkene (C=C) + Br2 → Dibromoalkane (colorless).
In practical terms, when conducting the test, a careful observation is essential. Even a slight loss of color is an indication of an alkene. A good tip is to always compare the test sample against a reference or control to confirm the change.

Organic Compound Differentiation

The differentiation of organic compounds is key in chemistry and is crucial for scientists in a range of fields, from materials science to pharmaceuticals. A range of tests exists to differentiate between various organic compounds, with the bromine water test being one among them.

Differentiating between alkenes and alcohols can be challenging because of their similar molecular structures, yet, they have distinctly different reactivities. While the bromine water test involves a simple addition reaction for alkenes, alcohols require a substitution reaction to react with bromine, often under more vigorous conditions which are not met in a simple bromine water test.

Considering Reactivity

It's crucial to consider the functional group reactivity when differentiating between compounds. Alkenes, with their double bond, are often more reactive than the singly bonded hydroxyl group of alcohols. The specific reactivities enable us to design experiments that can clearly show which compound is which, as with the bromine water test.

Alkenes and Alcohols

Alkenes and alcohols are two classes of organic compounds foundational to organic chemistry. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, which imparts reactivity that differentiates them from other hydrocarbon classes, such as alkanes.

Alcohols, on the other hand, are characterized by the presence of a hydroxyl (-OH) group attached to a carbon atom. The hydroxyl group makes alcohols polar and able to engage in hydrogen bonding, which in turn influences their physical properties like boiling points and solubility.

Physical and Chemical Properties

Even though alkenes and alcohols may have similar molecular formulas, their physical and chemical properties differ due to their functional groups. For instance, alcohols generally have higher boiling points compared to alkenes with similar molar mass due to hydrogen bonding. Chemical tests like the bromine water test capitalize on these differences in chemical properties to distinguish between these two classes of compounds.