Question

(c) difluoroethene molecules have many uses including in fridges and in the production of polymers. two structural isomers of difluoroethene are 1,1-difluoroethene, \\(\text{cf}_2=\text{ch}_2\\), and 1,2-difluoroethene, \\(\text{chf}=\text{chf}\\).

(i) circle the formula of the difluoroethene which will form geometric isomers.
\\\text{cf}_2=\text{ch}_2 \qquad \text{chf}=\text{chf}\\

(ii) draw the structural formulae of the two geometric isomers of your chosen isomer in the boxes given below.

(iii) justify your choice.
your answer should include:

• the requirements for geometric isomerism
• links to the structure of both difluoroethene molecules.

Explanation:

Identify the isomer capable of geometric isomerism

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Determine the structures of cis and trans isomers

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Formulate the justification

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Answer:

Question (i)

The formula of the difluoroethene which will form geometric isomers is:

$$\mathrm{CHF=CHF}$$

Question (ii)

The structural formulas of the two geometric isomers are:

• cis isomer:

  F       F
   \     /
    C = C
   /     \
  H       H

• trans isomer:

  F       H
   \     /
    C = C
   /     \
  H       F

Question (iii)

Justification:

• Requirements for geometric isomerism: There must be restricted rotation around the \(\mathrm{C=C}\) double bond, and each carbon atom in the double bond must be bonded to two different atoms or groups of atoms.
• Link to structures:
• In \(\mathrm{CHF=CHF}\), each carbon atom is bonded to two different groups (\(-\mathrm{H}\) and \(-\mathrm{F}\)). This allows for two distinct spatial arrangements (cis and trans).
• In \(\mathrm{CF_2=CH_2}\), one carbon atom is bonded to two identical fluorine atoms, and the other carbon atom is bonded to two identical hydrogen atoms. Therefore, swapping groups on either carbon does not produce a new spatial arrangement, making geometric isomerism impossible.