Textbook Question
Draw skeletal structures for the compounds in Problem 3, including any cis–trans isomers.
1.
4. Alkanes and Cycloalkanes
Cis vs Trans
3:42 minutesProblem 2b
Textbook Question
Draw the cis and trans isomers for the following:
b. 1-ethyl-3-methylcyclobutane
Verified step by step guidance1
Step 1: Understand the structure of the compound. The base structure is cyclobutane, which is a four-membered ring. The substituents are an ethyl group (-CH2CH3) at position 1 and a methyl group (-CH3) at position 3.
Step 2: Recall the concept of cis and trans isomerism in cyclic compounds. In cis isomers, both substituents are on the same side of the ring (either both above or both below the plane of the ring). In trans isomers, the substituents are on opposite sides of the ring (one above and one below the plane).
Step 3: Draw the cyclobutane ring as a square. Place the ethyl group (-CH2CH3) at position 1 and the methyl group (-CH3) at position 3. For the cis isomer, ensure both substituents are either above or below the plane of the ring. Use wedge bonds (solid triangles) to indicate groups above the plane and dashed bonds to indicate groups below the plane.
Step 4: For the trans isomer, place the ethyl group (-CH2CH3) at position 1 above the plane of the ring (using a wedge bond) and the methyl group (-CH3) at position 3 below the plane (using a dashed bond), or vice versa. This ensures the substituents are on opposite sides of the ring.
Step 5: Verify your drawings by checking that the positions of the substituents match the naming (1-ethyl and 3-methyl) and that the spatial arrangement corresponds to cis (same side) or trans (opposite sides) isomerism.
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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 or a ring structure. In this type of isomerism, the spatial arrangement of substituents differs, leading to distinct isomers. 'Cis' refers to substituents on the same side, while 'trans' indicates they are on opposite sides, affecting the physical and chemical properties of the molecules.
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Cyclobutane Structure
Cyclobutane is a four-membered carbon ring that adopts a non-planar conformation to minimize angle strain. The ring's structure allows for the possibility of substituents being positioned differently, which is crucial for identifying cis and trans isomers. The presence of substituents on the ring can lead to different spatial arrangements, influencing the overall stability and reactivity of the isomers.
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Substituent Positioning
In the context of cycloalkanes, the positioning of substituents is vital for determining the type of isomer. For 1-ethyl-3-methylcyclobutane, the ethyl group is at the first carbon and the methyl group at the third carbon. The relative positions of these groups in the ring will dictate whether the isomer is classified as cis or trans, impacting the molecule's steric interactions and overall stability.
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