Cis and trans naming #1.26

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Isha Bagga 3J
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Cis and trans naming #1.26

Postby Isha Bagga 3J » Thu Mar 05, 2015 12:36 am

Can someone help me with #26 in chapter 1 of the organic chemistry workbook? How do you know that it is trans-4-bromo-3-methyl-3-heptene? I thought it was supposed to be cis- because the bromine and the fluorine were on the same side.

Justin Le 2I
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Re: Cis and trans naming #1.26

Postby Justin Le 2I » Thu Mar 05, 2015 1:12 am

There is no fluorine in this problem, but I can tell you why it is trans.

Each of the C of the double bond are sp2 hybridized and you look at what is connected to those C to see what you prioritize. On the left side, you prioritize H3CH2C because if you add up all the atomic numbers, it is higher than the sum of the atomic numbers in H3C. On the right side, we prioritize Br by using the same process (but you don't have to do the math because Br has a pretty big atomic number). Since Br and H3CH2C are on opposite sides of the double bond (remember that this molecule cannot rotate because of the pi bonds), you know it is trans. Let me know if this is confusing.

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Re: Cis and trans naming #1.26

Postby Chem_Mod » Thu Mar 05, 2015 1:20 am

The answer key has a mistake, and the version of the answer using E/Z nomenclature is correct. cis/trans nomenclature can only be used when the two sides of the double bond have at least 1 group the same. When all four groups are different from each other, as is the case in these problems, cis/trans is not usable.

The E/Z system assigns one group from each pair as having priority, and then you see if these priority groups are across (E) or same side (Z). In problem 26, the two pairs are Methyl vs. Ethyl, Bromine vs. Propyl. Priority is assigned by molecular mass of the first atom at the first difference. Ethyl defeats Methyl at the 2nd atom since C is heavier than H. Bromine defeats Propyl at the 1st atom since Br is heavier than C. Then we compare the locations of Bromine and Ethyl, and since they are across, we designate (E).

Just like assigning numbers to a parent carbon chain, please note that in assigning priority you use the atom of first difference, and do NOT sum up anything.

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