Question

Explain why the name but-3-ene is incorrect. What is the proper name for this molecule?

Short answer

The correct name for the molecule might be 'but-1-ene' or 'but-2-ene' since 'but-3-ene' does not exist.

Step-by-step solution

Understand the Prefix

The prefix 'but-' indicates that the hydrocarbon has four carbon atoms in its longest chain. This is a linear chain designation in organic chemistry nomenclature.

Understand the Position of Double Bonds

The number '3' in the name 'but-3-ene' suggests that there is a double bond starting at the third carbon. However, in a four-carbon chain, numbering from either end, there's no third carbon where a double bond can start. The longest carbon chain should be numbered to give the first double bond the lowest possible number.

Identify the Correct Position

For a four-carbon chain, the possible positions for a double bond are between the first and second carbon or the second and third carbon. Thus, the correct designation is between C-1 and C-2 or C-2 and C-3.

Determine the Proper Name

With the double bond between the second and third carbons, following IUPAC rules, the molecule should be named with the lowest possible number. Therefore, it should be 'but-1-ene' or 'but-2-ene' depending on the actual structure.

Key concepts

Hybridization and Molecular Geometry

In organic chemistry, hybridization is a key concept that explains the bonding situation in molecules. When atoms form bonds, their electron orbitals mix to create new hybrid orbitals.
In the case of carbon, when it's part of a double bond as in an alkene, sp² hybridization occurs. Here, one s orbital and two p orbitals from carbon combine to form three equivalent sp² hybrid orbitals.
This results in a trigonal planar molecular geometry, which means the molecule is flat with bond angles of approximately 120 degrees. Since double bonds restrict rotation, the spatial arrangement is fixed, contributing to the rigorous naming rules in IUPAC nomenclature.

• sp² hybridization: associated with double bonds.
• Trigonal planar geometry: leads to flat structures.
• Fixed spatial arrangement: impacts molecule behavior and naming.

Carbon Chain

Carbon chains are the backbone of organic molecules. They can vary in length and structure, impacting the properties and the nomenclature of the compounds.
In IUPAC nomenclature, the longest continuous chain of carbon atoms determines the base name of the molecule. For example, 'but-' in 'but-3-ene' indicates a chain of four carbon atoms.
The numbering of the carbon chain is crucial and is done to assign the lowest possible numbers to the substituents and multiple bonds.

Longest chain: determines base name.

Numbering: ensures lowest numbers for key groups.

Structural considerations: straight vs. branched chains.

Structural Isomers

Structural isomers are molecules with the same molecular formula but different structural arrangements. This variance leads to different physical and chemical properties.
For example, with a four-carbon chain, moving a double bond or arranging the chain differently results in different isomers. 'But-1-ene' and 'but-2-ene' are structural isomers of butene, each differing in the position of the double bond.
Recognizing and naming these isomers follows strict IUPAC nomenclature rules to clearly communicate structural differences.

• Same formula: different structures.
• IUPAC rules: standardize naming.
• Influence properties: different isomers have unique characteristics.