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# Which is higher in stereochemical priority?

If a CH2CH3 is a substituent and CH3 is a substituent, which is higher in priority?

Also, It would help if someone could explain why when naming some alkene compounds, the E-Z system must be implemented, while in some it is not required. Is this because two of the substituents are the same on a single carbon?

• In assigning priorities, start from a carbon in the C=C and follow the other two bonds on that carbon toward the groups attached to them. In the case you asked about, you would arrive at a carbon on each of the two bonds.
.......C (we will say this C is from the CH3)
....../
C=C
......\
.......C (we will say this carbon is from the CH2CH3)
Since the two C's have the same priorities, but the methyl and ethyl are clearly different, you must now follow the bonds leading away from each of the two carbons already found.
....H...H
......\../....(the next atoms in the methyl are the 3 H's
.......C---H
....../
C=C
......\
.......C---C
....../..\...The next atoms in the ethyl are 2H's and a C
....H....H

The C in the H,H,C group is higher priority than is any atom in the H,H,H group. For that reason, the ethyl group is higher priority than the CH3.

Any single higher priority atom gives that group a higher priority. For example, a C with H,H,O on it is higher priority than a C with C,C,C on it.

Your answer to the second part is correct. For E/Z (or cis/trans) stereochemistry to exist, the two groups on each carbon in the C=C must be different. They do not need to be different from one end to the other, however. For example, CH3CH=C(CH3)2 has a CH3 and an H at one end, so they are different, but at the other there are two CH3's which are identical, so there is no E/Z. However, for
CH3CH2CH=CHCH2CH3, at one end of the C=C is an H and a CH2CH3 (different) while at the other there is also an H and a CH2CH3 (also different from each other), so E/Z can exist. When the structure is written in linear form like this, however, you cannot tell which it is, E or Z.
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