Organic Chemistry – Stereochemistry: Alkenes, Label E or Z

Question

Label the following alkenes as E, Z, or neither:

  1. A 5-carbon ring with a line to 1 CH3 group and a 3 carbon chain off the next carbon. The 3 carbon chain has a double bond right off the ring and a single line off the other end of the double bond.
    Alkene A
  2. A chain of 8 carbons with a double bond between carbons 4 and 5. There is a single bond to a Cl on carbon 4, and a single line connected to 2 other lines off carbon 5.
    Alkene B, Alkenes, Label E or Z 1
  3. A chain of 6 carbon atoms with a double bond between carbons 3 and 4. Carbon 3 has a bond to an NH2 group and carbon 4 has a bond to 2 CH-(CH3)2 groups.
    Alkene C

 

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  1. E
  2. Z
  3. Neither

Refer to Section 8.5.2 Sequence Rules – The E,Z Designation (1).

Strategy Map

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Table 1: Strategy Map
Strategy Map Steps 
1. Look to the left side of the double bond. Using the laws for priority, identify if the substituent with the higher priority is on the top or bottom half of the double bond.

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Both substituents may have the same priority.
2. Repeat step 1 with the right side.
3. Identify if the higher priority substituents are both on one side or opposite sides of the double bond.

Solution

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a. Alkene A

The substituents with higher priorities are on opposite sides.

Answer: E

b. Alkene B

The substituents with higher priorities are on the same side.

Answer: Z

c. Alkene C

Both substituents on the right have the same priority.

Answer: Neither

Guided Solution

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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.

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Table 2: Guided Solution
Guided Solution Ideas
This question is a theory type problem where you must identify the geometry of alkene structures by labeling them as either E, Z, or neither.

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Refer to Section 8.5.2 Sequence Rules – The E,Z Designation (1).
First, determine the 2 substituents on each double-bonded carbon separately.

Rank these substituents based on the atom directly attached to the double-bonded carbon. The substituent whose atom has a higher atomic number takes precedence over the substituent whose atom has a lower atomic number. If the atoms are the same, look to the next atom they are bonded to.

(From Section 8.5.2 Sequence Rules – The E,Z Designation (1))
How do you assign substituent priority?

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Refer to Section 8.4: Absolute Configuration- R-S Sequence Rules (2).
What does it mean for an alkene to have a type E geometry?

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If it has a type E geometry, the highest priority substituents are on opposite sides of the double bond (one is on the top and the other on the bottom).
What does it mean for an alkene to have a type Z geometry?

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If it has a type Z geometry, the highest priority substituents are on the same side of the double bond (both on the top or bottom).
What does it mean if an alkene has neither?

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If it has neither of these, it means that the substituents on one side (left or right) have the same priority.
Table 3: Complete Solution
Complete Solution
a. Alkene 1

The priority substituents are highlighted in red, and the higher priorities are on opposite sides of the double bond.

A carbon carbon double bond with red bonds identifying the priority substituents. On the left of the double bond the upper bond is a red bond to a C with two re bonds off it, one to the 5 member C ring and one to a CH3 group. On the right of the double bond the bottom red bond is to a CH2-CH3 group.

Answer: E
b. Alkene 2

The priority substituents are highlighted in red, and the higher priorities are on the same side of the double bond.

A carbon carbon double bond with red bonds identifying the priority substituents. On the left of the double bond the upper bond is red and goes to a Cl group. On the right side of the double bond the upper bond is red and goes to a CH(CH3)2 group.

Answer: Z
c. Alkene 3

Both of the substituents showing on the right side of the double bond above have the same priority, so it is neither E or Z.

A carbon carbon double bond with red bonds identifying the priority substituents. On the left of the double bond the lower bond is red and goes to an NH2 group. On the right side of the double bond both bonds are red. The upper bond is red and goes to a CH(CH3)2 group. The lower bond is red and goes to a CH(CH3)2 group.

Answer: Neither

Check Your Work

Summary of what we would expect based on the related chemistry theory

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Go back and review your priority assignments for double bond substituents. Make sure you go atom by atom when looking for priority.

Does your answer make chemical sense?

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The naming rules are used in order to fully describe these molecules when other naming systems do not work. Your answer should make sense if you are following all guidelines to identify priority and geometry.

PASS Attribution

Media Attributions

All figures are by Farmer et al., from Organic Chemistry (Morsch et al.) (5), and are used under a CC BY-SA 4.0 license.

References

1. Thompson Rivers University. 8.5.2: Sequence Rules – The E,Z Designation. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/08%3A_Organic_Chemistry_II_-_Stereochemistry/8.05%3A_Stereochemistry_of_the_Alkenes/8.5.02%3A_Sequence_Rules_-_The_EZ_Designation.

2. Thompson Rivers University. 8.4: Absolute Configuration- R-S Sequence Rules. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/08%3A_Organic_Chemistry_II_-_Stereochemistry/8.04%3A_Absolute_Configuration-_R-S__Sequence_Rules.

3. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500.

4. Blackstock, L.; Brewer, S.; Jensen, A. 8.1. Question 8.5.2.3 PASS – Alkenes, Label E or Z. In PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500/08%3A_Organic_Chemistry_II_-_Stereochemistry/8.01%3A_Question_8.5.2.3_PASS_-_alkenes_label_E_or_Z.

5. Farmer, S.; Kennepohl, D.; Sharrett, Z.; Cunningham, K.; Roberts, J.; Caserio, M. C.; Bruner, R. 7.6: Sequence Rules – The E,Z Designation. In Organic Chemistry (Morsch et al.); LibreTexts, 2024. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/07%3A_Alkenes-_Structure_and_Reactivity/7.06%3A_Sequence_Rules_-_The_EZ_Designation.

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