Chapter 11: Problem 13
Show how will you synthesise: (i) 1 -phenylethanol from a suitable alkene. (ii) cyclohexylmethanol using an alkyl halide by an \(\mathrm{S}_{\mathrm{N}} 2\) reaction. (iii) pentan-1-ol using a suitable alkyl halide?
Short Answer
Expert verified
1. Use styrene and hydroboration-oxidation for 1-phenylethanol.
2. Employ cyclohexyl bromide and S_N2 nucleophilic substitution for cyclohexylmethanol.
3. Use 1-bromopentane and S_N2 reaction to synthesize pentan-1-ol.
Step by step solution
01
Identify the Alkene for 1-phenylethanol Synthesis
To synthesize 1-phenylethanol, we begin by identifying the suitable alkene. In this case, we can use styrene (phenylethene) as it has a phenyl group attached to an ethene moiety, necessary for our target alcohol.
02
Add Hydroxyl Group via Hydroboration-Oxidation
Perform hydroboration-oxidation on the styrene. First, add diborane (BH3) to the alkene, followed by oxidation with hydrogen peroxide (H2O2) and a base (such as NaOH). This method adds a hydroxyl group to the less substituted carbon, forming 1-phenylethanol.
03
Starting Alkyl Halide for Cyclohexylmethanol
For synthesizing cyclohexylmethanol, start with cyclohexyl bromide as the alkyl halide. This setup is appropriate because it allows for the substitution reaction required to form the alcohol.
04
Execute S_N2 Reaction
Use a strong nucleophile like hydroxide ion (OH-) to displace the bromide ion from cyclohexyl bromide. The nucleophile attacks the less hindered position, substituting the bromide and forming cyclohexylmethanol through an S_N2 reaction.
05
Identify Alkyl Halide for Pentan-1-ol
To synthesize pentan-1-ol, start with 1-bromopentane as the alkyl halide. The linear structure allows easy substitution on the primary carbon.
06
Conduct S_N2 Reaction for Pentan-1-ol
Again, use a strong base like hydroxide ion (OH-). This nucleophile will attack the primary carbon, replacing the bromide ion and yielding pentan-1-ol as the product via an S_N2 mechanism.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Hydroboration-Oxidation
Hydroboration-oxidation is a two-step chemical reaction that transforms alkenes into alcohols. The process involves adding a hydroxyl group to an alkene, and it's especially useful for converting unsaturated hydrocarbons into alcohols with a specific orientation of the hydroxyl group.
In the first step, called hydroboration, an alkene reacts with diborane (\(\text{BH}_3\)). This part of the reaction involves the addition of the boron atom and a hydrogen atom across the double bond of the alkene. Importantly, the boron adds to the less hindered carbon, creating a trialkylborane intermediate.
Next comes the oxidation step, where the trialkylborane is reacted with hydrogen peroxide (\(\text{H}_2\text{O}_2\)) and a base like sodium hydroxide (NaOH). This converts the alkylborane into an alcohol, placing the hydroxyl group on the less substituted carbon atom. This is especially helpful for synthesizing alcohols like 1-phenylethanol from styrene, as the process adds a hydroxyl group precisely where needed.
In the first step, called hydroboration, an alkene reacts with diborane (\(\text{BH}_3\)). This part of the reaction involves the addition of the boron atom and a hydrogen atom across the double bond of the alkene. Importantly, the boron adds to the less hindered carbon, creating a trialkylborane intermediate.
Next comes the oxidation step, where the trialkylborane is reacted with hydrogen peroxide (\(\text{H}_2\text{O}_2\)) and a base like sodium hydroxide (NaOH). This converts the alkylborane into an alcohol, placing the hydroxyl group on the less substituted carbon atom. This is especially helpful for synthesizing alcohols like 1-phenylethanol from styrene, as the process adds a hydroxyl group precisely where needed.
SN2 Reaction
The SN2 reaction mechanism is a common and simple way to convert an alkyl halide into an alcohol. It stands for substitution nucleophilic bimolecular, which means it's a type of nucleophilic substitution reaction that involves two molecules participating in the rate-determining step.
The nucleophile, often a hydroxide ion (OH-), attacks the carbon atom of the alkyl halide complex from the back side. This pushes out the leaving group, usually a halogen atom like bromide or chloride, creating an inverted product. This inversion is similar to an umbrella turning inside out in the wind!
This mechanism works well for synthesizing alcohols like cyclohexylmethanol and pentan-1-ol, especially when starting with less hindered alkyl halides. Because the nucleophile approaches the carbon atom directly, steric hindrance greatly affects the reaction outcome. Hence, primary carbons are more favorable for SN2 reactions.
The nucleophile, often a hydroxide ion (OH-), attacks the carbon atom of the alkyl halide complex from the back side. This pushes out the leaving group, usually a halogen atom like bromide or chloride, creating an inverted product. This inversion is similar to an umbrella turning inside out in the wind!
This mechanism works well for synthesizing alcohols like cyclohexylmethanol and pentan-1-ol, especially when starting with less hindered alkyl halides. Because the nucleophile approaches the carbon atom directly, steric hindrance greatly affects the reaction outcome. Hence, primary carbons are more favorable for SN2 reactions.
Alkyl Halide
Alkyl halides are organic compounds that consist of an alkane with one or more halogen atoms (like fluorine, chlorine, bromine, or iodine) replacing hydrogen atoms. They are often key starting materials in organic synthesis due to their reactive nature.
Their reactivity stems from the carbon-halogen bond, where the carbon atom is electrophilic, making it susceptible to attack by nucleophiles. The strength and length of this bond vary with the halogen, affecting how easily the halogen can be replaced in reactions like SN2.
In synthesis exercises, choosing the right alkyl halide is crucial. For cyclohexylmethanol, cyclohexyl bromide is a wise choice because it's reactive enough for efficient substitution. Similarly, 1-bromopentane serves as a convenient starting point for making pentan-1-ol because it's linear and undergoes nucleophilic attack readily.
Their reactivity stems from the carbon-halogen bond, where the carbon atom is electrophilic, making it susceptible to attack by nucleophiles. The strength and length of this bond vary with the halogen, affecting how easily the halogen can be replaced in reactions like SN2.
In synthesis exercises, choosing the right alkyl halide is crucial. For cyclohexylmethanol, cyclohexyl bromide is a wise choice because it's reactive enough for efficient substitution. Similarly, 1-bromopentane serves as a convenient starting point for making pentan-1-ol because it's linear and undergoes nucleophilic attack readily.
Nucleophilic Substitution
Nucleophilic substitution reactions involve the replacement of a leaving group by a nucleophile on a substrate molecule. This fundamental concept in organic chemistry enables the transformation of simple organic compounds into more complex structures.
The leaving group, often a halide ion, departs during the process, and the nucleophile forms a new bond with the substrate. Depending on the conditions, the substitution can occur via two primary mechanisms: SN1 or SN2.
In SN2 reactions, both the nucleophile and substrate are involved in the rate-determining step, leading to a direct replacement. This mechanism works best with substrates that are less sterically hindered, such as primary and some secondary alkyl halides. For many alcohol syntheses, SN2 reactions are preferred due to their predictable outcome and tendency to proceed with complete inversion of stereochemistry at the reactive carbon.
The leaving group, often a halide ion, departs during the process, and the nucleophile forms a new bond with the substrate. Depending on the conditions, the substitution can occur via two primary mechanisms: SN1 or SN2.
In SN2 reactions, both the nucleophile and substrate are involved in the rate-determining step, leading to a direct replacement. This mechanism works best with substrates that are less sterically hindered, such as primary and some secondary alkyl halides. For many alcohol syntheses, SN2 reactions are preferred due to their predictable outcome and tendency to proceed with complete inversion of stereochemistry at the reactive carbon.
Organic Chemistry Synthesis
Organic chemistry synthesis involves constructing complex chemical compounds from simpler ones. It’s like building with Lego blocks where you put together smaller pieces to create something new and functional.
For students, mastering synthesis is crucial as it combines knowledge of reaction mechanisms and reactivity principles to achieve desired compounds. Understanding synthesis allows chemists to not only replicate natural compounds but also create novel ones.
In practical exercises, the challenge is often in selecting the right pathways and reagents. For example, converting an alkene into an alcohol requires knowing the best method, like hydroboration-oxidation for 1-phenylethanol. Similarly, crafting specific alcohols from alkyl halides might involve choosing effective nucleophiles for SN2 reactions. Therefore, a solid grasp of different reactions and their mechanisms underpins successful synthesis in organic chemistry.
For students, mastering synthesis is crucial as it combines knowledge of reaction mechanisms and reactivity principles to achieve desired compounds. Understanding synthesis allows chemists to not only replicate natural compounds but also create novel ones.
In practical exercises, the challenge is often in selecting the right pathways and reagents. For example, converting an alkene into an alcohol requires knowing the best method, like hydroboration-oxidation for 1-phenylethanol. Similarly, crafting specific alcohols from alkyl halides might involve choosing effective nucleophiles for SN2 reactions. Therefore, a solid grasp of different reactions and their mechanisms underpins successful synthesis in organic chemistry.
