Edexcel A Level Chemistry:复习笔记3.1.4 Structural Isomerism

Structural Isomerism

 

  • Structural isomers are compounds that have the same molecular formula but different structural formulae
    • E.g. propene and cyclopropane

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Propene-and-Cyclopropane

Both propene and cyclopropane are made up of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

  • There are two different types of structural isomerism you need to be aware of:
    • Chain isomerism
    • Positional isomerism

Chain isomerism

  • Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same
  • This is caused by branching
    • E.g. pentane and 2,2-dimethylpropane

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Chain-Isomerism

Both compounds are made up of the same atoms, however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Positional isomerism

  • Positional isomers arise from differences in the position of a functional group in each isomer
    • The functional group can be located on different carbons
    • For example, butan-1-ol and butan-2-ol

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Positional-Isomerism

Both compounds have an alcohol group and are made up of 4 carbons, 10 hydrogens and one oxygen, however in butanol the functional group is located on the first carbon and in 2-butanol on the second carbon

Worked Example

Isomers of dibromopropane
How many isomers are there of dibromopropane, C3H6Br2?

Answer

Step 1: Draw the structural formula of the compound

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Step-1-Isomers-of-dibromopropane

Step 2: Determine whether it is a stereo or structural isomer

There is no restricted bond rotation around the C-C bond, so it is structural isomerism

Step 3: Determine whether it is a functional group, chain or positional isomerism

      • Functional group? No, as Br is the only functional group possible
      • Chain? No, as the longest chain can only be 3
      • Positional? Yes, as the two bromine atoms can be bonded to different carbon atoms

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Step-3-Isomers-of-dibromopropane

10.1.2-Isomers-of-dibromopropane-Worked-example-answer

Worked Example

Deducing isomers of C4H10
How many isomers are there of the compound with molecular formula C4H10?

Answer

Step 1: Draw the structural formula of the compound

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Step-1-Deducing-isomers-of-C4H10

Step 2: Determine whether it is a stereo or structural isomer.

There is no restricted bond rotation around the C-C bond so it is structural isomerism

Step 3: Determine whether it is a functional group, chain or positional isomerism

      • Functional group? No, as there are no functional groups
      • Positional? No, as there are no functional groups which can be positioned on different carbon atoms
      • Chain? Yes!

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Answer-Deducing-isomers-of-C4H10

 

 

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