Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 7 - Structure and Synthesis of Alkenes; Elimination

Problem 6c,d

Chapter 7, Problem 6c,d

1. Determine which of the following compounds show cis-trans isomerism.
2. Draw and name the cis and trans (or Z and E) isomers of those that do.
c. hexa-2,4-diene
d. 3-methylpent-2-ene

Verified step by step guidance

Step 1: Understand the concept of cis-trans isomerism, also known as geometric isomerism. This occurs in alkenes where there is restricted rotation around the double bond, and each carbon in the double bond has two different groups attached.

Step 2: Analyze compound c, hexa-2,4-diene. This compound has two double bonds. Check each double bond to see if it can exhibit cis-trans isomerism. For cis-trans isomerism to occur, each carbon in the double bond must have two different substituents.

Step 3: Analyze compound d, 3-methylpent-2-ene. Look at the double bond between the second and third carbon atoms. Determine if each carbon in the double bond has two different groups attached, which is necessary for cis-trans isomerism.

Step 4: For compounds that exhibit cis-trans isomerism, draw the structural formulas for both the cis and trans isomers. In cis isomers, similar groups are on the same side of the double bond, while in trans isomers, they are on opposite sides.

Step 5: Name the isomers using the IUPAC naming system. For compounds with more complex substituents, use the E/Z nomenclature, where Z (zusammen) indicates the higher priority groups are on the same side, and E (entgegen) indicates they are on opposite sides.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in compounds with restricted rotation around a bond, typically a double bond. It requires two different groups attached to each carbon of the double bond. Cis isomers have similar groups on the same side, while trans isomers have them on opposite sides.

Z and E Nomenclature

Z and E nomenclature is used to describe the stereochemistry of double bonds when there are more than two substituents. Z (zusammen) indicates that the higher priority groups are on the same side, while E (entgegen) indicates they are on opposite sides. Priority is determined using Cahn-Ingold-Prelog rules.

Identifying Isomerism in Alkenes

To determine if an alkene can exhibit cis-trans isomerism, check for a double bond with two different substituents on each carbon. In hexa-2,4-diene, the double bonds allow for potential isomerism, while in 3-methylpent-2-ene, the substituents on the double bond carbons must be evaluated for differences.

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Related Practice

Textbook Question

Give the systematic (IUPAC) names of the following alkenes.

(d)

(e)

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Textbook Question

The following names are all incorrect. Draw the structure represented by the incorrect name (or a consistent structure if the name is ambiguous), and give your drawing the correct name.

a. cis-dimethylpent-2-ene

b. 3-vinylhex-4-ene

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Textbook Question

For each reaction, decide whether substitution or elimination (or both) is possible, and predict the products you expect. Label the major products.

b. 1-bromo-1-methylcyclohexane + triethylamine(Et₃N:)

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Textbook Question

Give the systematic (IUPAC) names of the following alkenes.

(f)