Write the mechanism for the reactions of
1) (E)-3-hexene and bromine and
2) (Z)-3-hexene and bromine in dichloromethane.
Assign R,S configuration for each of the products and determine whether they are related as enantiomers or identical.
I know how to write the mechanism for #1, but is it similar for #2? Does the reaction in dichloromethane make any difference?
Also I'm not sure about R,S configuration. I'm thinking each set of products in #1 and #2 are enantiomers. And then between #1 and #2, they are diastereomers. Please help verify my thinking.
THANK YOU!! :)
The reaction mechanism for the reactions of (E)-3-hexene and bromine and (Z)-3-hexene and bromine in dichloromethane is indeed similar. The main difference is that in dichloromethane, the reaction proceeds more slowly compared to without a solvent.
For the reaction of (E)-3-hexene and bromine, the mechanism involves the following steps:
1) Initiation: Bromine reacts homolytically to generate two bromine radicals, initiated by UV light.
Br2 -> 2Br. (radicals)
2) Propagation:
- The bromine radical attacks the carbon-carbon double bond of (E)-3-hexene, forming a bromonium ion intermediate.
Br. + (E)-3-hexene -> Br-CH2-CH2-CH2-CH2-CH2.: (bromonium ion)
- A bromide ion then attacks the more substituted carbon of the bromonium ion, opening the ring and forming the product.
Br- + Br-CH2-CH2-CH2-CH2-CH2.: -> Br-CH2-CH2-CH2-CH2-CH2-Br (trans-1,2-dibromohexane)
For the reaction of (Z)-3-hexene and bromine in dichloromethane, the steps are the same, but the reaction is slower due to the solvent.
Now let's discuss the R/S configuration of the products.
In both cases, the addition of bromine to the double bond results in the formation of two chiral centers, one at each carbon that was previously part of the double bond.
For (E)-3-hexene, the R/S configuration of the product is as follows:
- The bromine atom attached to the carbon with the highest priority group (bromine = atomic number 35) will always have an R configuration.
- The bromine atom attached to the other carbon will have an opposite configuration.
Therefore, the product will have one R configuration and one S configuration.
For (Z)-3-hexene, the R/S configuration of the product will be the same as in the (E)-3-hexene case, but the specific R/S labels will be different due to the different stereochemistry of (Z)-3-hexene.
Regarding the relationship between the products of (E)-3-hexene and (Z)-3-hexene reactions with bromine, they are not enantiomers but diastereomers. Diastereomers are stereoisomers that are not mirror images of each other and are often formed by reactions that create two or more chiral centers.
The reaction mechanism for the reactions of (E)-3-hexene and bromine and (Z)-3-hexene and bromine in dichloromethane is similar, but there is a slight difference due to the solvent.
For both reactions, the bromine molecule undergoes electrophilic addition to the double bond of the hexene molecule. The reaction proceeds through a cyclic bromonium ion intermediate.
1) Mechanism for the reaction of (E)-3-hexene and bromine:
- The pi electrons attack the bromine molecule, leading to the formation of a cyclic bromonium ion intermediate.
- The bromide ion attacks the less hindered carbon of the intermediate, resulting in the formation of the anti-addition product.
- The final product is (2R,3R)-3,3-dibromohexane.
2) Mechanism for the reaction of (Z)-3-hexene and bromine in dichloromethane:
- The presence of dichloromethane as a solvent stabilizes the intermediate by solvating the bromine molecule and reducing its reactivity.
- The reaction proceeds through the formation of a bromonium ion intermediate, similar to the reaction with (E)-3-hexene.
- However, in the presence of dichloromethane, the intermediate undergoes a nucleophilic attack by dichloromethane itself.
- This attack occurs on the more substituted carbon of the intermediate, resulting in the formation of the syn-addition product.
- The final product is (2S,3S)-3,3-dibromohexane.
Regarding the configuration assignments and the relationship between the products:
In (1) and (2), the products are stereoisomers, specifically diastereomers, because they have the same connectivity but different three-dimensional arrangements.
In terms of R/S configuration, (2R,3R)-3,3-dibromohexane from the reaction of (E)-3-hexene is an optically inactive molecule because it has a plane of symmetry, making it achiral. It cannot be assigned R or S configuration.
On the other hand, (2S,3S)-3,3-dibromohexane from the reaction of (Z)-3-hexene is chiral, and you can assign R or S configuration to it.
To summarize, the products from the two reactions are diastereomers, and the product from the reaction of (Z)-3-hexene is chiral, allowing for an R/S configuration assignment.