Chapter 17: Problem 384

An unknown compound is believed to be one of the following. Describe how you would go about finding out which of the possibilities the unknown actually is. Where possible, use simple chemical tests; where necessary, use more elaborate chemical methods like quantitative hydrogenation, cleavage, etc. Make use of any needed tables of physical constants. (a) n-propyl ether (b.p. \(91^{\circ}\) ) and 2 -methylhexane (b.p. \(91^{\circ}\) ) (b) benzyl ethyl ether (b.p. \(188^{\circ}\) ) and allyl phenyl ether (b.p. \(192^{\circ}\) ) (c) methyl p-tolyl ether (b.p. \(176^{\circ}\) ) and methyl m-tolyl ether (b.p. \(177^{\circ}\) )

Short Answer

Expert verified

To differentiate between the given pairs of unknown compounds, perform the following tests: 1. n-Propyl ether vs. 2-Methylhexane: Use the Lucas test by adding Lucas reagent (zinc chloride and hydrochloric acid) to each unknown compound. n-Propyl ether will not react, while 2-methylhexane will form a cloudy mixture, allowing differentiation. 2. Benzyl ethyl ether vs. Allyl phenyl ether: Perform catalytic hydrogenation under mild conditions using a palladium catalyst. Benzyl ethyl ether will not undergo hydrogenation, while allyl phenyl ether will. Comparing IR spectra or reactivity towards hydrogen allows differentiation. 3. Methyl p-tolyl ether vs. Methyl m-tolyl ether: Use NMR spectroscopy to reveal distinct structures and compare obtained spectra of the unknown compounds with reference spectra of methyl p-tolyl ether and methyl m-tolyl ether, allowing determination of the compound's identity.

Step by step solution

n-Propyl Ether vs. 2-Methylhexane

First, we can use the Lucas test to differentiate these two compounds. The Lucas test is a chemical test that uses the reaction of alcohols with Lucas reagent (a mixture of zinc chloride and hydrochloric acid) in the presence of heat. Alcohols react with Lucas reagent to form alkyl chlorides and water, but ethers do not undergo this reaction. Add a few drops of Lucas reagent to separate samples of the unknown compounds and gently heat them. n-Propyl ether will not react with the Lucas reagent while 2-methylhexane, being an alcohol, will form a cloudy mixture, indicating that a reaction has occurred. This would allow us to differentiate between n-propyl ether and 2-methylhexane.

Benzyl Ethyl Ether vs. Allyl Phenyl Ether

Benzyl ethyl ether and allyl phenyl ether can be distinguished by their reactivity towards hydrogenation (reduction with hydrogen gas). Subject the two unknown compounds to a catalytic hydrogenation reaction under mild conditions, using a palladium catalyst. Benzyl ethyl ether does not have any C=C double bonds and therefore will not undergo hydrogenation under mild conditions, while allyl phenyl ether contains an allyl group (with a C=C double bond) that will easily undergo hydrogenation, forming the saturated alkyl group. By comparing the infrared (IR) spectra of the samples before and after hydrogenation, or monitoring their reactivity towards hydrogen under mild conditions, we can differentiate these two compounds.

Methyl p-tolyl Ether vs. Methyl m-tolyl Ether

These two compounds are structural isomers with very similar boiling points and reactivities, making it challenging to differentiate them using simple chemical tests directly. However, we can employ NMR (Nuclear Magnetic Resonance) spectroscopy to reveal their distinct structures and determine which compound is present in our sample. Methyl p-tolyl ether (para-substituted) and methyl m-tolyl ether (meta-substituted) have distinct NMR spectra, with different chemical shifts and coupling constants related to their structural isomers. By comparing the obtained NMR spectra of the unknown compounds with reference spectra for methyl p-tolyl ether and methyl m-tolyl ether, we can determine which compound is present. In summary, using a combination of chemical tests (Lucas test, hydrogenation) and spectroscopy (NMR), we can differentiate between the given pairs of unknown compounds and determine their identities.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Lucas Test

The Lucas test is a classic experiment in organic chemistry utilized to differentiate between various types of alcohols and ethers. In this test, the compound in question is treated with Lucas reagent, which is a mixture of concentrated hydrochloric acid and zinc chloride. The reagent is aggressive and favors the formation of alkyl chlorides from alcohols through a nucleophilic substitution reaction.

When applied to our textbook problem, n-propyl ether and 2-methylhexane can be distinguished because only the latter, being an alcohol, reacts to form a cloudy mixture. Ethers, such as n-propyl ether, do not react with the Lucas reagent, and consequently, no turbidity is observed. This simple yet effective chemical test allows us to rapidly identify the compound without the need for complex equipment.

Catalytic Hydrogenation

Catalytic hydrogenation is a process in which a compound is treated with hydrogen in the presence of a catalyst, typically a transition metal like palladium, platinum, or nickel. This reaction is widely used in organic chemistry to reduce unsaturated bonds, turning alkenes into alkanes.

In the context of the exercise, catalytic hydrogenation aids in distinguishing benzyl ethyl ether and allyl phenyl ether. The presence of a C=C double bond in the allyl phenyl ether allows it to react under mild conditions during the hydrogenation process, changing its chemical structure. Benzyl ethyl ether, however, lacks this double bond and remains unaffected. This differential reactivity provides a clear method to identify which of the two compounds is present.

NMR Spectroscopy

NMR spectroscopy is an analytical method used to determine the structure of molecules based on the magnetic properties of atomic nuclei. In organic chemistry, ^1H and ^13C NMR spectroscopy are often used to identify the structure of organic compounds by measuring the interaction of nuclei with an external magnetic field.

For distinguishing between methyl p-tolyl ether and methyl m-tolyl ether, NMR spectroscopy can be instrumental. Despite their strikingly similar physical properties, their NMR spectra will exhibit unique chemical shifts and splitting patterns corresponding to the para- and meta- positions of the methyl group in relation to the ether group. This powerful tool enables chemists to pinpoint the exact structure of the compound under investigation.

Boiling Point Differentiation

Boiling point differentiation is a primary method for identifying compounds by their unique physical property: the temperature at which they transition from liquid to gas. Boiling points are influenced by molecular weight, structure, and intermolecular forces, such as hydrogen bonding and van der Waals forces.

In our problem cases, however, boiling point differentiation alone is insufficient due to the close boiling points of the compounds in question. Therefore, while this property is valuable for rough distinctions between substances with markedly different boiling points, it is less useful here and must be complemented with other chemical tests or analytical methods.

Chemical Reactivity

Chemical reactivity refers to the tendency of a substance to undergo a chemical change. The reactivity of a compound can reveal much about its functional groups and structure. In practice, chemists often carry out targeted reactions to test for certain groups, like the Lucas test for alcohols, or to exploit the presence of double bonds with catalytic hydrogenation.

In the exercises we analyze, tests such as the Lucas test and catalytic hydrogenation directly leverage the chemical reactivity of the compounds to provide a clear indication of their identity. These reactive properties are purposefully provoked to give observable and measurable changes in the test samples, such as the formation of a cloudy mixture or the consumption of hydrogen gas.

Isomer Distinction

Isomers are compounds with the same molecular formula but different structural arrangements. Isomer distinction is vital in organic chemistry as different isomers can have different physical, chemical, and biological properties. Techniques like NMR spectroscopy are particularly valuable here, as they can detect even subtle differences in isomer structure.

With methyl p-tolyl ether and methyl m-tolyl ether, their isomeric nature makes them challenging to distinguish, but NMR spectroscopy can reveal the distinct positioning of atoms within the molecules. By scrutinizing the NMR spectra, chemists can determine whether methyl groups are para or meta to the ether group, thus identifying the correct isomer.

Short Answer

Step by step solution

n-Propyl Ether vs. 2-Methylhexane

Benzyl Ethyl Ether vs. Allyl Phenyl Ether

Methyl p-tolyl Ether vs. Methyl m-tolyl Ether

Key Concepts

Lucas Test

Catalytic Hydrogenation

NMR Spectroscopy

Boiling Point Differentiation

Chemical Reactivity

Isomer Distinction

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