Why Does Cis-Trans Isomerism Occur?

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In summary, the presence of a carbon-carbon double bond prevents free rotation, which results in the observation of both cis and trans isomers. The validity of this answer depends on the specific analysis being asked for, as a more detailed explanation may be needed for certain reactions or mechanisms.
  • #1
denian
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so, i was given a cis-isomer and a trans-isomer. the question is very simple, why this isomer shows cis-trans isomerism.

i answer it this way, "because the free rotation is prevented by the presence of carbon-carbon double bond"

is that a correct answer? or maybe a more valid answer?
 
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  • #2
It is true that rotation about a carbon-carbon double bond is rare, due to the fact that it would have to break its pi bond to occur.

If that is all the analysis this question is asking for, your answer is fine.

However, if this question is related to a reaction mechanism or anything like that, you may need to state more, and explain exactly WHY both the cis and trans isomers CAN be observed.

For example, if I told you that R-C(triple bond)C-R' reacts with H2 in Lindlar's catalyst to produce ONLY cis HRC=CR'H, whereas it reacts with Na in liquid NH3 to produce ONLY trans HRC=CR'H, you'd have to look to the mechanism to explain it.

Point being, your answer depends on the question. What you've stated is correct, but just be aware of exactly what kind of analysis the question is asking for.

Hope that helps!
 
  • #3


Yes, your answer is correct. Cis-trans isomerism occurs due to the presence of a carbon-carbon double bond in the molecule. This double bond restricts the rotation of the atoms attached to it, leading to two different arrangements of the groups around the double bond - cis and trans. This results in the formation of two isomers with different physical and chemical properties.
 
  • #4


Yes, your answer is correct. Cis-trans isomerism occurs because of the restricted rotation around a carbon-carbon double bond. This means that the atoms or groups attached to each carbon cannot freely rotate, resulting in different spatial arrangements of the molecule. This leads to the formation of two distinct isomers, the cis-isomer where the atoms or groups are on the same side of the double bond, and the trans-isomer where they are on opposite sides. This phenomenon is observed in compounds with double bonds such as alkenes and cycloalkenes.
 

FAQ: Why Does Cis-Trans Isomerism Occur?

1. What is cis-trans isomerism?

Cis-trans isomerism, also known as geometric isomerism, is a type of stereoisomerism where molecules have the same chemical formula and connectivity, but differ in the arrangement of atoms or groups around a double bond or ring structure. This results in different physical and chemical properties.

2. Why does cis-trans isomerism occur?

Cis-trans isomerism occurs because of the restricted rotation around a double bond or ring structure. The double bond or ring structure creates a rigid shape, and the groups attached to it cannot freely rotate. This results in different possible spatial arrangements of the atoms, leading to the formation of cis and trans isomers.

3. What is the difference between cis and trans isomers?

The main difference between cis and trans isomers is the spatial arrangement of the atoms or groups around the double bond or ring structure. In cis isomers, the groups are on the same side of the double bond or ring structure, while in trans isomers, the groups are on opposite sides.

4. How does cis-trans isomerism affect the physical properties of molecules?

Cis-trans isomerism can greatly affect the physical properties of molecules. For example, cis isomers tend to have higher boiling points and lower melting points compared to their trans counterparts. This is because the cis isomers have a more symmetrical shape, allowing for stronger intermolecular forces.

5. Can cis-trans isomerism occur in all molecules with double bonds or ring structures?

No, cis-trans isomerism can only occur in molecules with restricted rotation around a double bond or ring structure. This includes molecules such as alkenes, cycloalkanes, and some aromatic compounds. Molecules with free rotation around a double bond or ring structure, such as alkanes, cannot exhibit cis-trans isomerism.

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