Problem 21
Question
In the hydroboration - oxidation reaction of propene with diborane, \(\mathrm{H}_{2} \mathrm{O}_{2}\) and \(\mathrm{NaOH}\), the organic compound formed is: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{CHOHCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\)
Step-by-Step Solution
Verified Answer
The organic compound formed is (c) \(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{OH}\).
1Step 1: Understand the Reactants
The hydroboration-oxidation reaction involves the addition of borane (BH3) followed by oxidation using hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)) and sodium hydroxide (\(\mathrm{NaOH}\)) to alkenes, like propene.
2Step 2: Determine the Stereochemistry and Regiochemistry
Hydroboration-oxidation adds water across a double bond in a syn-selective and anti-Markovnikov manner, meaning the OH group is added to the less substituted carbon.
3Step 3: Apply Hydroboration Reaction
React propene with diborane (\(\mathrm{B}_2\mathrm{H}_6\)). In this step, a hydrogen atom from borane attacks the less substituted carbon of propene, adding in a syn addition, resulting in an organoborane.
4Step 4: Apply Oxidation Reaction
The organoborane is oxidized using \(\mathrm{H}_2\mathrm{O}_2\) and \(\mathrm{NaOH}\). The \(\mathrm{OH}\) group replaces the boron atom on the least substituted carbon, forming propanol.
5Step 5: Identify the Organic Product
The organic product formed from the reaction is propan-1-ol (\(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{OH}\)) as it follows anti-Markovnikov addition where the OH group attaches to the primary carbon atom.
Key Concepts
Regiochemistry in Hydroboration-OxidationStereochemistry and Syn Addition in HydroborationUnderstanding the Anti-Markovnikov Rule
Regiochemistry in Hydroboration-Oxidation
In the realm of chemistry, regiochemistry refers to the orientation or directionality of chemical bond formation in a reaction. During the hydroboration-oxidation of propene, regiochemistry plays a crucial role in determining the position of the newly formed alcohol group.
When propene reacts with borane, the boron atom attaches to the less substituted carbon atom, following the anti-Markovnikov rule. This results in the addition of borane across the double bond, forming an organoborane intermediate.
It's important to remember that hydroboration-oxidation is a two-step process. Initially, propene undergoes hydroboration. The selectivity of this reaction will dictate where the water molecule is finally incorporated.
In the regiospecific outcome, it’s the attachment of this water molecule (as an OH group) to the least substituted carbon atom that defines the placement of the final alcohol group. And thus, in our propene example, the OH group will end up on the terminal carbon, producing propan-1-ol as the organic product.
When propene reacts with borane, the boron atom attaches to the less substituted carbon atom, following the anti-Markovnikov rule. This results in the addition of borane across the double bond, forming an organoborane intermediate.
It's important to remember that hydroboration-oxidation is a two-step process. Initially, propene undergoes hydroboration. The selectivity of this reaction will dictate where the water molecule is finally incorporated.
In the regiospecific outcome, it’s the attachment of this water molecule (as an OH group) to the least substituted carbon atom that defines the placement of the final alcohol group. And thus, in our propene example, the OH group will end up on the terminal carbon, producing propan-1-ol as the organic product.
Stereochemistry and Syn Addition in Hydroboration
Stereochemistry is the study of the spatial arrangement of atoms in molecules. It is particularly crucial in understanding how molecules interact and form. For hydroboration-oxidation reactions, understanding the stereochemistry helps us grasp how the reaction proceeds and why it produces certain products.
In hydroboration-oxidation, the addition is syn, meaning that both the boron atom and the attacking hydrogen enter from the same side of the double bond in the alkene. This is vital because it ensures a specific three-dimensional orientation of the atoms, leading to a predictably structured alcohol.
Such stereochemical considerations are crucial in complex organic synthesis, where the spatial configuration can dramatically affect the physical and biological properties of the compound. For instance, when propene undergoes syn addition, both the hydrogen and boron add to the same face of the alkene, leading to a specific three-dimensional structure for the intermediate that eventually becomes propan-1-ol.
In hydroboration-oxidation, the addition is syn, meaning that both the boron atom and the attacking hydrogen enter from the same side of the double bond in the alkene. This is vital because it ensures a specific three-dimensional orientation of the atoms, leading to a predictably structured alcohol.
Such stereochemical considerations are crucial in complex organic synthesis, where the spatial configuration can dramatically affect the physical and biological properties of the compound. For instance, when propene undergoes syn addition, both the hydrogen and boron add to the same face of the alkene, leading to a specific three-dimensional structure for the intermediate that eventually becomes propan-1-ol.
Understanding the Anti-Markovnikov Rule
The Anti-Markovnikov rule is an important principle in organic chemistry that guides the addition of elements to alkenes. It specifically describes how certain substituents, like hydroxyl groups, attach themselves to the less substituted end of an alkene double bond.
In most typical addition reactions, the rule followed is Markovnikov's rule, where the more substituted carbon receives the substituent due to the stability of carbocations. However, in reactions like hydroboration-oxidation, boron adds to the less substituted carbon with the hydrogen taking the more substituted position.
This approach is labeled as anti-Markovnikov. Once the initial hydroboration part is done, the oxidation step involves replacing the boron with a hydroxyl group, cementing the product as a primary alcohol, in our propene example, this is propan-1-ol.
Understanding this rule helps chemists predict the outcome of a reaction, particularly in designing synthetic pathways for different organic molecules, where the position of certain functional groups is of keen importance.
In most typical addition reactions, the rule followed is Markovnikov's rule, where the more substituted carbon receives the substituent due to the stability of carbocations. However, in reactions like hydroboration-oxidation, boron adds to the less substituted carbon with the hydrogen taking the more substituted position.
This approach is labeled as anti-Markovnikov. Once the initial hydroboration part is done, the oxidation step involves replacing the boron with a hydroxyl group, cementing the product as a primary alcohol, in our propene example, this is propan-1-ol.
Understanding this rule helps chemists predict the outcome of a reaction, particularly in designing synthetic pathways for different organic molecules, where the position of certain functional groups is of keen importance.
Other exercises in this chapter
Problem 19
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