cis-trans and E-Z naming scheme for alkenes | Alkenes and Alkynes | Organic chemistry | Khan Academy

Summary

This transcript explains the naming conventions for alkenes, focusing on but-2-ene. It covers the rigid nature of the carbon-carbon double bond, which prevents rotation and leads to different molecular configurations. The video introduces two main nomenclature systems: cis/trans and (E)/(Z). The cis/trans system is used for simpler molecules with one functional group on each side, while the (E)/(Z) system is more standard and applies to molecules with multiple functional groups. The video emphasizes the importance of these naming conventions in distinguishing chemically distinct molecules with identical molecular formulas.

Takeaways

• 😀 The first step in naming a molecule with a double bond is identifying the longest chain of carbon atoms and its functional group.
• 😀 For a molecule with a double bond starting at the second carbon in a four-carbon chain, the name is 'but-2-ene'.
• 😀 Molecules with the same structure but different arrangements around the double bond are fundamentally different due to restricted rotation of the double bond.
• 😀 The restricted rotation of the double bond means that molecules like but-2-ene can exist in different configurations (cis or trans).
• 😀 In the simpler naming convention, if the functional groups are on opposite sides of the double bond, the molecule is called 'trans' or '(E)', indicating opposite positions.
• 😀 If the functional groups are on the same side of the double bond, the molecule is called 'cis' or '(Z)', indicating the same side.
• 😀 The terms 'cis' and 'trans' come from Latin and are used to indicate the relative positions of the functional groups on either side of a double bond.
• 😀 The 'E' and 'Z' nomenclature (from German entgegen and zusammen, meaning opposite and together) is more commonly used in modern chemistry, especially for molecules with more than one functional group on each carbon.
• 😀 The 'E' and 'Z' notations are more specific and versatile, capable of handling more complex molecules than the cis/trans system.
• 😀 The (E) and (Z) conventions are used universally in chemistry, and the cis/trans system is often used for simpler molecules with just one functional group on each side.

Q & A

What is the first step in naming an alkene like but-2-ene?

The first step is identifying the presence of a double bond, which makes it an alkene, and then determining the longest carbon chain. In this case, the chain has four carbons, and the double bond starts at the second carbon, so it's named but-2-ene.

How does the rigidity of a carbon-carbon double bond affect molecule behavior?

The double bond creates rigidity due to the pi bond, which prevents the molecule from rotating around the bond. This results in distinct configurations, or isomers, that behave chemically differently.

What is the difference between the two molecules described in the script, despite having the same name (but-2-ene)?

The two molecules are different because they have different configurations around the double bond. One has the functional groups on opposite sides (trans), and the other has them on the same side (cis), which leads to different chemical behaviors.

What naming conventions are used for molecules with double bonds and functional groups on opposite sides?

When the functional groups are on opposite sides of the double bond, the molecule is called 'trans-but-2-ene,' or, using the more formal system, '(E)-but-2-ene,' where 'E' stands for 'entgegen' (German for 'opposite').

What is the naming convention when the functional groups are on the same side of the double bond?

When the functional groups are on the same side of the double bond, the molecule is named 'cis-but-2-ene,' or '(Z)-but-2-ene,' where 'Z' comes from 'zusammen' (German for 'together').

Why is the (E)/(Z) notation considered more powerful than cis/trans?

The (E)/(Z) notation is more precise and can be applied to more complex molecules, where multiple functional groups are attached to the carbons of the double bond. This system works regardless of how many functional groups are present on each side of the bond.

What is the significance of the term 'pi bond' in the context of the carbon-carbon double bond?

The pi bond is the second bond in a double bond and is what restricts the rotation around the carbon-carbon bond, leading to the formation of distinct isomers with different spatial arrangements of functional groups.

What are the advantages of using the (E)/(Z) system over the simpler cis/trans system?

The (E)/(Z) system allows for more detailed and accurate descriptions of molecules, especially those with multiple functional groups on each side of the double bond. It can handle more complex molecules and is more universally applicable.

Can the cis/trans system be used for molecules with more than one functional group on each side of the double bond?

No, the cis/trans system is usually limited to simpler molecules where each carbon in the double bond has one functional group. For molecules with multiple functional groups, the (E)/(Z) system should be used.

What is the root name for a molecule with four carbon atoms in the longest chain and a double bond starting at the second carbon?

The root name would be 'butane' (for four carbons), with the suffix '-ene' indicating a double bond at the second carbon, so the full name is 'but-2-ene.'