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Magazine Chapter 5: Problem 75
We'll see in Chapter 11 that alkyl halides react with hydrosulfide ion (HS \(^{-}\) ) to give a product whose stereochemistry is inverted from that of the reactant. Draw the reaction of \((S)-2\) -bromobutane with \(\mathrm{HS}^{-}\) ion to yield 2 -butanethiol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{SH}) \mathrm{CH}_{3} .\) Is the stereochemistry of the product \(R\) or \(S ?\)
Short Answer
Expert verified
The product, 2-butanethiol, has an 'R' configuration after inversion.
Step by step solution
01
Understand the Given Reaction
You are asked to draw and identify the stereochemistry of the reaction between \((S)-2\)-bromobutane and \(\text{HS}^{-}\). The product will be 2-butanethiol, \(\text{CH}_3\text{CH}_2\text{CH(SH)}\text{CH}_3\), with inverted stereochemistry.
02
Draw the Reactant (S)-2-Bromobutane
Start by drawing the structure of \((S)-2\)-bromobutane. This molecule has a chiral center at the 2nd carbon, with the configuration that follows the Cahn-Ingold-Prelog priority rules, resulting in an 'S' configuration. This configuration should have bromine (Br) as the highest priority group, followed by the ethyl group, methyl group, and hydrogen with the lowest priority.
03
Understand the Mechanism
The reaction involves a nucleophilic substitution where the \(\text{HS}^{-}\) ion attacks the electrophilic carbon where the bromine is attached. Since it鈥檚 a substitution reaction, the \(\text{Br}^{-}\) is the leaving group.
04
Draw the Reaction Mechanism
In the reaction mechanism, the \(\text{HS}^{-}\) ion attacks from the opposite side of the leaving group \(\text{Br}^{-}\), causing inversion of configuration at the carbon center.
05
Draw the Product
After the substitution, draw the product which is 2-butanethiol. The thiol group \(\text{SH}\) now occupies the position where \(\text{Br}\) was, but because of the inversion, the spatial arrangement will be opposite compared to the original chiral center.
06
Determine the Stereochemistry of the Product
Once the \(\text{HS}\) group has replaced the \(\text{Br}\), assign priorities again for this chiral center. The priority order will be: \(\text{SH} > \text{CH}_{2}\text{CH}_{3} > \text{CH}_{3} > \text{H}\). Determine the configuration by tracing from highest priority to lowest ignoring the lowest (hydrogen); this should result in an 'R' configuration due to inversion from the 'S' reactant.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Nucleophilic substitution
Nucleophilic substitution is a fundamental type of reaction in organic chemistry where a nucleophile (a species rich in electrons) replaces a leaving group in a molecule. In our case, the hydrosulfide ion (\(\mathrm{HS}^{-}\)) acts as the nucleophile. This reaction is common in alkyl halides due to the polarity of the carbon-halogen bond that makes the carbon atom electrophilic, or electron-deficient.
When the \(\mathrm{HS}^{-}\) ion approaches the positively polarized carbon in \((S)-2\)-bromobutane, it initiates a direct attack, kicking out the bromine as a leaving group. This results in the formation of 2-butanethiol.
Key features of nucleophilic substitutions:
When the \(\mathrm{HS}^{-}\) ion approaches the positively polarized carbon in \((S)-2\)-bromobutane, it initiates a direct attack, kicking out the bromine as a leaving group. This results in the formation of 2-butanethiol.
Key features of nucleophilic substitutions:
- They usually follow either an \(\mathrm{S}_\mathrm{N}1\) or \(\mathrm{S}_\mathrm{N}2\) mechanism. In this particular reaction, an \(\mathrm{S}_\mathrm{N}2\) process occurs, which involves a single concerted step.
- The \(\mathrm{S}_\mathrm{N}2\) mechanism leads to an inversion of the stereochemical configuration at the carbon center.
Chirality in organic compounds
Chirality is an important structural feature in organic molecules. A chiral compound has a non-superimposable mirror image, much like left and right hands. In molecules, chirality often arises from a carbon atom bonded to four different substituents, known as a chiral center or stereogenic center.
In the given exercise, \((S)-2\)-bromobutane is chiral because the second carbon atom is bonded to four different groups: bromine, an ethyl group, a methyl group, and hydrogen.
In the given exercise, \((S)-2\)-bromobutane is chiral because the second carbon atom is bonded to four different groups: bromine, an ethyl group, a methyl group, and hydrogen.
- Chirality is crucial because the spatial arrangement of these substituents impacts the molecule's interactions with other chiral entities, crucial in biological systems.
- Chiral molecules like \((S)-2\)-bromobutane can exist as enantiomers, which are pairs of non-superimposable mirror images.
Cahn-Ingold-Prelog priority rules
To assign stereochemistry to chiral centers, chemists use the Cahn-Ingold-Prelog (CIP) priority rules. These rules help determine the absolute configuration鈥攚hether it's \(R\) or \(S\)鈥攐f a chiral molecule.
Here's how CIP rules work:
Here's how CIP rules work:
- Assign priorities to substituents based on atomic number. The higher the atomic number, the higher the priority.
- Consider the groups attached to the chiral center. In \((S)-2\)-bromobutane, bromine has the highest priority, followed by the ethyl group, the methyl group, and hydrogen, which is lowest.
- Rotate the molecule so the lowest priority group (hydrogen) is in the back.
- Trace a path from the highest to the lowest priority group (ignoring the lowest). If the path is clockwise, the configuration is \(R\); if counterclockwise, it's \(S\).
Inversion of configuration
A defining feature of \(\mathrm{S}_\mathrm{N}2\) reactions like our exercise is the inversion of configuration, which can be likened to an umbrella flipping inside-out.
When \(\mathrm{HS}^{-}\) attacks \((S)-2\)-bromobutane, it approaches the electrophilic carbon from the side opposite the leaving group (bromine). The attack leads to the displacement of bromine and the conversion of the \(S\)-configuration of the reactant into the \(R\)-configuration in the product, 2-butanethiol.
Why does inversion occur?
When \(\mathrm{HS}^{-}\) attacks \((S)-2\)-bromobutane, it approaches the electrophilic carbon from the side opposite the leaving group (bromine). The attack leads to the displacement of bromine and the conversion of the \(S\)-configuration of the reactant into the \(R\)-configuration in the product, 2-butanethiol.
Why does inversion occur?
- The \(\mathrm{S}_\mathrm{N}2\) mechanism is a concerted reaction involving only one transition state where bonds are made and broken simultaneously.
- It necessitates the backside attack for effective overlap with the \( \sigma \)-bonding orbital of the carbon-bromine bond, leading to stereochemical inversion.
