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AQA A Level Chemistry復習筆記3.1.4 Structural Isomerism

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月30日下午3:22

Molecules: Straight, Branched & Cyclic

Straight, branched and cyclic organic molecules are also called?aliphatic?compounds as long as they do not contain a?benzene ring

Straight-chain

Straight-chain?organic molecules are those in which the carbon atoms are connected to each other in one continuous chain

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Straight-Chained

Pentane is a straight-chain organic molecule as the carbon atoms are connected in one continuous chain

Branched

Branched?organic molecules have side groups attached to the main chain of carbon atoms

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Branched

2-methylbutane is a branched organic molecule as the main chain (consisting of 4 carbon atoms) has a methyl branch

Cyclic

Cyclic?organic molecules are those in which the carbon atoms are connected to each other in a ring shape

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Cyclic

Cyclopentane is a cyclic organic molecule as the carbons are attached to each other in a ring structure

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 three different types of structural isomerism:
- Chain?isomerism
- Positional?isomerism
- Functional?group 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

Functional group isomerism

When different functional groups result in the same molecular formula,?functional group isomers?arise
The isomers have very different chemical properties as they have different functional groups
- For example, butanol and ethoxyethane

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Functional-Group-Isomerism

Both compounds have the same molecular formula however butanol contains an alcohol functional group and ethoxyethane an ether functional group

Deducing Isomers

You should be able to deduce all possible isomers for organic compounds knowing their molecular formula

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

Worked Example

Deducing isomers of C2H2Cl2

How many isomers are there of the compound with molecular formula C2H2Cl2?

Answer

Step 1:?Draw the possible structural formula of the compound

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Step-1-Deducing-isomers-of-C2H2Cl2

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

The compound has to be unsaturated for it to have molecular formula C2H2Cl2?; Due to the double bond there is restricted rotation about the C-C bond; This compound will therefore display geometrical isomerism

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Answer-Deducing-isomers-of-C2H2Cl2