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Chapter 5: Problem 37

Bromine adds to cis and trans-2-butene to give different diastereomers of 2,3 -dibromobutane. What does this say about the mode of addition of bromine to this alkene?

Short Answer

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Short Answer: The formation of different diastereomers of 2,3-dibromobutane from the addition of bromine to cis and trans-2-butene indicates that the mode of addition of bromine to the alkene is a stereo-selective and regio-specific electrophilic addition mechanism. The steric hindrance of the methyl groups and the three-dimensional arrangement of the double bond influence the stereochemistry of the products.

Step by step solution

01

Understand the reactants

We have cis-2-butene and trans-2-butene, both of which are alkenes. The difference between these two isomers lies in the arrangement of the substituents around the double bond, with cis having the methyl groups on the same side of the double bond, and trans having the methyl groups on opposite sides. Bromine will be added to these alkenes to form 2,3-dibromobutane, but the resulting products will have different stereochemical configurations as diastereomers.
02

Determine the reaction mechanism

Since we are adding bromine (Br2) to an alkene, the reaction involved is a halogenation reaction. In this case, the alkene reacts with bromine via an electrophilic addition mechanism, where the alkene's double bond acts as a nucleophile and attacks the bromine molecule, breaking the bond between the two bromine atoms. This leads to the formation of a bromonium ion on the alkene and a bromide ion. The bromonium ion is a three-membered ring containing a positively charged bromine atom and two carbon atoms from the alkene.
03

Analyze the addition of bromine to cis-2-butene

When bromine reacts with cis-2-butene, the bromonium ion formed has a methyl group on each carbon atom and the two methyl groups are on the same side. The bromide ion then attacks the bromonium ion from the back side, opposite to the positively charged bromine atom. Due to the steric hindrance of the methyl groups, the bromide ion preferentially attacks the less hindered carbon atom to open the three-membered ring. This results in one diastereomer of 2,3-dibromobutane with a specific stereochemistry.
04

Analyze the addition of bromine to trans-2-butene

Similarly, when bromine reacts with trans-2-butene, the resulting bromonium ion has a methyl group on each carbon atom, but the two methyl groups are on opposite sides of the ring. Now, the bromide ion can attack either of the carbons with more or less equal probability, as there is no significant difference in steric hindrance. Thus, we obtain two possible diastereomers of 2,3-dibromobutane, each with a different stereochemistry than the product from the cis isomer.
05

Conclusion and mode of addition

The formation of different diastereomers of 2,3-dibromobutane from the addition of bromine to cis and trans-2-butene implies that the mode of addition of bromine to the alkene involves a stereo-selective and regio-specific electrophilic addition mechanism. The steric hindrance of the methyl groups and the three-dimensional arrangement of the double bond play crucial roles in determining the stereochemistry of the products.

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Most popular questions from this chapter

Which alkenes exist as pairs of cis, trans isomers? For each that does, draw the trans isomer. (a) \(\mathrm{CH}_{2}=\mathrm{CHBr}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHBr}\) (c) \(\mathrm{BrCH}=\mathrm{CHBr}\) (d) \(\left(\mathrm{CH}_{\mathrm{s}}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) (e) \(\left(\mathrm{CH}_{5}\right)_{2} \mathrm{CHCH}=\mathrm{CHCH}_{3}\)

\(\beta\)-Ocimene, a triene found in the fragrance of cotton blossoms and several essential oils, has the IUPAC name (Z)-3,7-dimethyl- \(1,3,6\)-octatriene. Draw a structural formula for \(\beta\)-ocimene.

Draw the structural formula for at least one bromoalkene with the molecular formula \(\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{Br}\) that shows: (a) Neither \(E, Z\) isomerism nor chirality. (b) \(E, Z\) isomerism but not chirality. (c) Chirality but not \(E, Z\) isomerism. (d) Both chirality and \(E, Z\) isomerism.

Four stereoisomers exist for S-penten-2-ol. (a) Explain how these four stereoisomers arise. (b) Draw the stereoisomer having the \(E\) configuration about the carbon-carbon double bond and the \(R\) configuration at the chiral center.

Which alkenes show cis, trans isomerism? For each alkene that does, draw the transisomer. (a) 2-Pentene (b) 2-Methyl-2-pentene (c) 3-Methyl-2-pentene
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