Which of the following compounds can exist as pairs of cis–trans isomers? Draw each cis–trans pair, and indicate the geometry of each isomer. (a) CH3CH P CH2 (b) (CH3)2C P CHCH3 (c) CH3CH2CH P CHCH3 (d) (CH3)2C P C(CH3)CH2CH3 (e) ClCH P CHCl (f) BrCH P CHCl

Explanation:

The first step is to know where we have the double bond in each molecule:

a)

b)

c)

d)

e)

f)

Now, we have to keep in mind that we can have isomers Cis or Trans arround the double bond. The first step is to identify the most important group in each carbon in the double bond. If these groups are in the same side we will have a Cis isomer, if these groups are in opposite sides we will have a trans isomer.

The next step is to check one by one, so:

a)

In this molecule in the right carbon in the double bond, we have two hydrogens. So we can not choose the most important one in this carbon because we have two identical substituents in the same carbon. So, it's not possible to have Cis/Trans Isomers on this molecule.

b)

In b molecule, we have the same situation, but in this case, is in the left carbon. In the left carbon, we will have 2 methyl groups. So, we can not choose the most important group on this carbon and therefore we can not have  Cis/Trans Isomers.

c)

In this molecule on the left carbon, we have the ethyl group as the most important one. In the right carbon is the methyl group. So, when the ethyl and the methyl have opposite orientations we will have the Trans isomer, when we have the same orientation we will have the Cis isomer.

d)

In d molecule we have the same substituents on the left carbon, we have 2 methyl groups. So, it's not possible to have Cis/Trans isomers.

e)

In e molecule, we the most important substituent in the left carbon between the Cl and H is the Cl. The same behavior is repeated in the right carbon, where we have the Cl as the most important substituent. Therefore we can have the Cis/Trans isomers.

d)

For d molecule, we will have a similar situation than e molecule. On the left carbon, we will have as more important substituent the Br atom. In the right carbon, we will have Cl as the more important one. So, we can have the Cis/Trans isomers.

To further explanations see the picture attached