Question

State the number and kind of stereoisomers formed when \((R)-3\)-methyl-1-pentene is treated with these reagents. Assume that the starting alkene is enantiomerically pure and optically active. Will each product be optically active or inactive? (a) \(\mathrm{Hg}(\mathrm{OAc})_{2} \mathrm{H}_{2} \mathrm{O}\) followed by \(\mathrm{NaBH}_{4}\) (b) \(\mathrm{H}_{2} / \mathrm{Pt}\) (c) \(\mathrm{BH}_{3}\) followed by \(\mathrm{H}_{2} \mathrm{O}_{2}\) in \(\mathrm{NaOH}\) (d) \(\mathrm{Br}_{2}\) in \(\mathrm{CCl}_{4}\)

Short answer

Question: Determine the number and kind of stereoisomers formed and the optical activity of the products when (R)-3-methyl-1-pentene is treated with the following reagents: a) Hg(OAc)2, H2O followed by NaBH4 b) H2/Pt c) BH3 followed by H2O2 in NaOH d) Br2 in CCl4 Answer: a) 2 stereoisomers (racemic mixture), optically inactive b) 1 stereoisomer, optically active c) 1 stereoisomer, optically active d) 2 stereoisomers (diastereomers), optically inactive due to the meso compound

Step-by-step solution

Draw the initial molecule

First, draw the enantiomerically pure \((R)-3\)-methyl-1-pentene molecule.

Reaction (a) Hg(OAc)2 H2O followed by NaBH4

In this reaction, we have an oxymercuration followed by a reduction. The reaction proceeds with Markovnikov's rule, and the intermediate formed is a mercurinium ion, which leads to the formation of one product, a racemic mixture of alcohols. This reaction will form two stereoisomers, but since it's a racemic mixture, it will not be optically active. Number of stereoisomers formed: 2 (both enantiomers) Optical activity: Inactive (racemic mixture)

Reaction (b) H2 / Pt

In this reaction, we have a hydrogenation reaction with the alkene, which breaks the double bond and adds two hydrogens to it. The reaction proceeds with syn addition, forming no new stereocenters and no new stereoisomers. The product will be optically active as its enantiomer has not been produced. Number of stereoisomers formed: 1 Optical activity: Active

Reaction (c) BH3 followed by H2O2 in NaOH

In this reaction, we have an anti-Markovnikov hydroboration where the boron adds to the less substituted carbon of the alkene and the hydrogen adds to the more substituted carbon. The reaction proceeds with syn addition, and we will have a single product. The product, in this case, will be optically active. Number of stereoisomers formed: 1 Optical activity: Active

Reaction (d) Br2 in CCl4

In this reaction, a halogenation reaction takes place with the alkene, which breaks the double bond and adds two bromine atoms to it. The reaction proceeds with anti-addition, forming a bromonium ion intermediate. In this case, we will have two products (diastereomers) with three stereocenters. Because of the presence of a meso compound among the products, the reaction will be optically inactive overall. Number of stereoisomers formed: 2 (diastereomers) Optical activity: Inactive (meso compound) In summary, we have determined the number of stereoisomers formed and the optical activity for each reaction as follows: (a) 2 stereoisomers (racemic mixture), optically inactive (b) 1 stereoisomer, optically active (c) 1 stereoisomer, optically active (d) 2 stereoisomers (diastereomers), optically inactive due to the meso compound

Key concepts

Stereoisomerism

Stereoisomerism is a form of isomerism where molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This key concept is paramount in understanding many aspects of organic chemistry, as it affects the physical and chemical properties of compounds.

Stereoisomers are categorized into two main types: enantiomers, which are mirror images of each other and diastereomers, which are not. For example, when (R)-3-methyl-1-pentene undergoes various reactions, it can lead to the formation of enantiomers or diastereomers, depending on the specific reaction conditions.

In enantiomers, like the ones formed in reaction (a) with Hg(OAc)2 followed by NaBH4, they are non-superimposable and have a property called chirality, which we associate with optical activity. Diastereomers, on the other hand, have different arrangements at one or more of the stereogenic centers but are not mirror images of each other, as seen in reaction (d) with Br2 in CCl4 resulting in a meso compound.

Understanding Reaction Outcomes

The outcome of these reactions in terms of stereoisomerism can often be predicted by rules like Markovnikov's or anti-Markovnikov's rules, or by considering the addition patterns to alkenes (syn- or anti-addition). For instance, syn addition leads to the formation of only one stereoisomer, which retains the original optical activity of the starting material if no new stereocenters are created.

Optical Activity

Optical activity is a remarkable phenomenon observed in chiral materials, where the presence of asymmetric carbon atoms causes the rotation of plane-polarized light. A substance is considered optically active if it can rotate the plane of polarization of light, a property that is measured using a polarimeter. This property is intrinsic to chiral substances that exist as two enantiomers.

However, when equal amounts of both enantiomers are present in a mixture—known as a racemic mixture—the mixture is optically inactive because the rotations caused by each enantiomer cancel each other out. The reaction in step 2 nicely illustrates this, where treatment of (R)-3-methyl-1-pentene with Hg(OAc)2 and NaBH4 produces a racemic mixture and hence, no net rotation of plane-polarized light results, rendering the compound optically inactive.

Racemic Mixtures and Meso Compounds

Similarly, meso compounds, despite having multiple chiral centers, are achiral due to an internal plane of symmetry, also resulting in no net optical activity. This is observed in reaction (d), where a meso compound is formed. It's critical to recognize that not all stereoisomers exhibit optical activity, highlighting the importance of complete stereoisomer analysis during structural elucidation.

Reaction Mechanisms

Reaction mechanisms provide a detailed step-by-step account of the process by which chemical reactions occur. Understanding the reaction mechanism is essential for predicting the reactivity and stereochemical outcomes of organic reactions.

In the context of the given exercise, each reaction follows a distinct mechanism: oxymercuration-reduction, hydrogenation, hydroboration-oxidation, and halogenation. For example, oxymercuration in reaction (a) involves a Markovnikov addition of a mercurinium ion, followed by reduction, yielding a racemic product. Hydrogenation in reaction (b), specifically, does not introduce new stereoisomers due to syn addition of hydrogen atoms.

Guiding Principles for Reaction Mechanisms

Familiarizing oneself with mechanistic principles, such as the regioselectivity seen in Markovnikov-like or anti-Markovnikov-like additions, or the stereochemistry that results from syn or anti additions, is crucial. These principles, once understood, allow students to predict the types of stereoisomers that might arise from a given reaction and whether a reaction will yield an optically active or inactive product, as in the case of reactions (b) and (c), both leading to optically active products.