Chlorination of (R)-2-Chlorobutane: Meso vs Racemic Isomers

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In summary, this reaction is carried out in the presence of light and chlorine to form 2-chlorobutane, which has a 71% meso and 29% racemic mixture. The racemic mixture is less likely to form than the meso mixture, and this is because the C-Cl bond is exchanged more quickly than the chlorination of the 3 carbon.
  • #1
xcube
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When (R)-2-chlorobutane is chlorinated, we obtain some 2,3-dichlorobutane. It consists of 71% meso isomer and 29% racemic isomers. Explain why the mixture need not be 50:50 meso and (2R,3R)-2,3-dichlorobutane
 
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  • #2
Not an easy question

Hello,

I have been doing quite a lot of brainstorm after I saw your question. The mixture is supposed to be a 71% meso-product and RR and SS products, 14,5% each.

Please view the attachment for easy understanding.

We may explain it by relatively large volume of chlorine atom. Chlorine molecule tends to approach to the alfa carbon atom from the other side of methyl group because of the steric hindrance; this is the cause of formation of 2,3-disubstituted product. Two 14,5% mixtures of racemic products are formed also. The yield of meso product is quite higher than the racemic ones, and this is because symmetrical diastereomers are less likely to occur than the asymmetric ones; thus "symmetric" racemic mixture is expected to be formed less than the "asymmetric" meso-product.

My approach is not the only explanation; I will read other comments also.

Regards
chem_tr
 
  • #3
thanks for your answer. but the question use (R) isomer not a racemic.
 
  • #4
Hello,

You may be right, but it is not important in my opinion whether you take 2R- or 2S-chlorobutane as a start; the resultant compounds are always the same.

2R-chlorobutane or 2S-chlorobutane react similarly with chlorine to give the same products, regardless of stereochemistry of the starting material.

Let's speculate this in terms of stereochemistry and reaction mechanism.

Regards
chem_tr
 
  • #5
If the conditions for the chlorination are Cl2 and light, then it's possible that the chlorine atom in the 2-chlorobutane is exchanging. The light can cleave the relatively weak Cl-C bond forming a secondary radical, which has no stereochemical information. That radical can then combine with a chlorine radical (or chlorine molecule) to for racemic 2-chlorobutane. Then the second reaction can take place at the 3 position of the butane to give the observed racemic product.

For reference, a C-Cl bond is worth about 330 kJ/mol and a C-H bond is worth about 413 kJ/mol. From those figures, the C-Cl bond should exchange much faster than the chlorination of the 3 position. (energies from this website: http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html )

It is possible, however, that this chlorine doesn't exchange. If I were really going to run this reaction, I would expect it to however.

The reason the meso is more prevalent than the racemic is because the addition of the chlorine radical to the 3 carbon is diastereoselective (it depends on the stereochemistry of the 2 carbon, to an extent). I think that's essentially what chem_tr is saying too.
 
  • #6
That is remember that diastereomers have different properties, as with the meso and the racemic product of this reaction, unlike enantiomers. This will correspond to different transition states.


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  • #7
GCT: precisely. Thanks for distilling that down.
 

1. What is the purpose of chlorination of (R)-2-Chlorobutane?

The purpose of chlorination of (R)-2-Chlorobutane is to introduce a chlorine atom onto the carbon chain, which can have various applications in organic synthesis and industrial processes.

2. What is the difference between meso and racemic isomers?

Meso isomers are molecules that have an internal plane of symmetry, while racemic isomers are non-superimposable mirror images of each other. In other words, meso isomers are identical on both sides, while racemic isomers are different on both sides.

3. How does the chlorination process impact the formation of meso and racemic isomers?

The chlorination process can lead to the formation of both meso and racemic isomers, depending on the starting material and reaction conditions. The stereochemistry of the starting compound, as well as the type of chlorination reagent used, can affect the ratio of meso and racemic isomers produced.

4. What factors influence the selectivity of meso and racemic isomer formation during chlorination?

The selectivity of meso and racemic isomer formation during chlorination can be influenced by several factors, including the temperature, solvent, type of catalyst, and concentration of reactants. The reactivity of the starting compound and the strength of the C-Cl bond formed can also impact the selectivity of isomers.

5. How can the ratio of meso and racemic isomers be controlled during chlorination?

The ratio of meso and racemic isomers during chlorination can be controlled by varying the reaction conditions, such as temperature and solvent, and by using specific catalysts that can promote the formation of a desired isomer. Additionally, the use of chiral reagents or additives can also influence the selectivity of isomers formed.

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