Question

Treatment of \(\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{Br}\) with strong base gave an alkene mixture that was shown by careful gas chromatographic analysis and separation to consist of three alkenes, \(\mathrm{C}_{7} \mathrm{H}_{14}\), A, B, and C. Catalytic hydrogenation of each alkene gave 2-methythexane. Reaction of A with \(\mathrm{B}_{2} \mathrm{H}_{6}\) followed by \(\mathrm{H}_{2} \mathrm{O}_{2}\) and \(\mathrm{OH}^{-}\) gave mostly an alcohol, D. Similar reaction of \(\mathrm{B}\) or C gave approximately equal amounts of \(\mathrm{D}\) and an isomeric alcohol \(\mathrm{E}\). What structural assignments can be made for \(\mathrm{A}\) to \(E\) on the basis of these observations? What structural element is left undetermined by these data alone?

Short answer

The following structural assignments can be made for A to E based on the observations: A: 1-Methylcyclohexene B: 3-Methylcyclohexene C: 4-Methylcyclohexene D: 2-Methylcyclohexanol E: 4-Methylcyclohexanol The undetermined structural element is the stereochemistry of the alcohols D and E due to the presence of chirality centers and the syn addition nature of the hydroboration-oxidation reaction.

Step-by-step solution

Identify Potential Alkene Structures

Based on the information that all alkenes A, B, and C produce 2-methylhexane upon hydrogenation, we can deduce that each of the alkenes must differ by the position of the double bond. To maintain a seven-carbon structure (C7H14), we can write out the three possible alkene structures as follows: 1. 1-Methylcyclohexene 2. 3-Methylcyclohexene 3. 4-Methylcyclohexene

Analyzing Reaction with B2H6, H2O2, and OH-

Upon treatment with B2H6 followed by H2O2 and OH-, alkene A produces only alcohol D, while alkenes B and C yield an approximately equal mixture of alcohols D and E. This reaction sequence is an example of a hydroboration-oxidation reaction, which results in an anti-Markovnikov addition of water to the alkene, forming an alcohol.

Assign Structures to Alkenes A, B, and C

Since A produces only one alcohol product (D) upon hydroboration-oxidation, it indicates that the double bond in A is less substituted than the double bonds in B and C, so we can assign: A: 1-Methylcyclohexene For alkenes B and C, since they produce an equal mixture of alcohols D and E upon hydroboration-oxidation, it implies that they have double bonds with similar substitution. Thus, we can assign: B: 3-Methylcyclohexene C: 4-Methylcyclohexene (an isomer of B)

Identify Alcohols D and E

Based on the above assignments, we know that the hydroboration-oxidation of A, B, and C yield the following alcohols: From A: Alcohol D From B: Alcohols D and E (equal mixture) From C: Alcohols D and E (equal mixture) Since A (1-Methylcyclohexene) produces only alcohol D, we can determine D's structure to be 2-Methylcyclohexanol. For alcohol E, since it is an isomeric alcohol produced alongside D from B and C (3-Methylcyclohexene and 4-Methylcyclohexene), E could be 4-methylcyclohexanol (isomer of D).

Undetermined Structural Element

The structural element left undetermined by this data alone is the stereochemistry of the alcohols D and E. The hydroboration-oxidation reaction typically results in syn addition and forms chirality centers. Due to the presence of chirality centers in D and E, the stereochemistry of the alcohols remains undetermined, as we do not have enough information to deduce the relative configurations of D and E.

Key concepts

Organic Chemistry

Organic chemistry is the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds, which include not only hydrocarbons but also compounds with any number of other elements, including hydrogen, nitrogen, oxygen, halogens, phosphorus, silicon, and sulfur.

Within organic chemistry, understanding the behavior of different functional groups is essential. This field is fundamental in the development of new molecules for various industries, ranging from pharmaceuticals to polymers. Important operations include the ability to predict the outcome of chemical reactions involving organic compounds and the synthesis of these compounds.

An intriguing area of study within organic chemistry is how various chemical reactions can be manipulated to produce desired outcomes. One such reaction is the hydroboration-oxidation, which plays a vital role in transforming alkenes into alcohols. In the context of the exercise, understanding the principles of organic reactivity and the nuances of reaction mechanisms allows for the deduction of the structure of unknown alkenes and their resulting alcohols.

Alkene Reactions

Alkenes, characterized by the presence of a carbon-carbon double bond, are a versatile group of molecules in organic chemistry that undergo a variety of chemical reactions. The double bond is a region of high electron density, which makes alkenes susceptible to electrophilic addition reactions, wherein electron-seeking reagents (electrophiles) add across the double bond.

One prominent electrophilic addition reaction is the hydroboration-oxidation reaction. This two-step process involves the addition of borane (BH3 or B2H6) to the double bond, followed by oxidation with hydrogen peroxide (H2O2) in the presence of a hydroxide ion (OH-). The result is the formation of an alcohol, with the peculiarity that the reaction follows anti-Markovnikov's rule; that is, the hydrogen atom adds to the more substituted carbon, leading to the formation of the more substituted alcohol.

Significance in Synthesis

The hydroboration-oxidation reaction's regioselectivity plays a significant role in synthetic organic chemistry, allowing for the production of alcohols that may be difficult to obtain through other methods. As exemplified in the textbook exercise, the selectivity and outcomes of this reaction enable the determination of alkene structure, showcasing the importance of understanding reaction mechanisms within the broader scope of alkene chemistry.

Structural Isomerism

Structural isomerism, or constitutional isomerism, is a concept where molecules with the same molecular formula have their atoms connected in different orders, leading to distinct structures. This form of isomerism creates molecules that may vary in physical and chemical properties despite having the same numbers of atoms of each element.

The textbook exercise depicts a scenario in which the treatment of a compound leads to the formation of various structural isomers, namely alkenes A, B, and C. Understanding structural isomerism is critical in deciphering the nature of these different alkenes. It also influences the reactivity patterns, as seen in the hydroboration-oxidation reaction, where the position of a double bond within a cycloalkene ring determines the regiochemistry of the resultant alcohol.

Understanding Isomers

In the context of chemistry education, grasping the concept of structural isomerism equips students with the ability to predict and explain the existence of various isomeric compounds that arise from seemingly similar substrates. Students can learn to distinguish between molecules like 3-Methylcyclohexene and 4-Methylcyclohexene, which are constitutional isomers and differ in the position of their double bonds and substituents, intimately affecting their chemical reactions and resulting products.