Position Isomerism. Constitutional or positional isomerism

THE flat or constitutional isomerism is one in which two or more compounds have the same molecular formula, but differ by some aspect in their structure. One of these aspects is the position of unsaturation, function and branching.

So, we can say that a positional or positional isomerism it occurs when the isomers have the same carbon chain, but differ by the position of one of the aforementioned factors.

1st Example:unsaturation position

Note that the two compounds below have the same molecular formula, C₅H₁₀, but its unsaturation (double bond) is found between different carbons in each case. In the first molecule, the double bond is between carbon 1 and 2, in the second, it is between carbon 2 and 3.

H₂Ç ═CH CH₂CH₂CH₃ H₃Ç CH₂CH ═CH CH₃
pent-1-not pent-2-no

2nd Example:position of function

Next we have two isomers that have the same function (ketone), with the same molecular formula, C₆H₁₂The but the position of the functional group (carbonyl) is different. In the first, the carbonyl is leaving carbon 2. In the other compound, it is leaving carbon 3.

OO
║║
H₃Ç ─ ÇCH₂CH₂CH₂CH₃ H₃Ç CH₂─ ÇCH₂CH₂CH₃
hexan-2-one hexane-3-one

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It is important to remember that in position isomerism all isomers belong to the same chemical function.

3rd Example:branch position

Both compounds below have the molecular formula C₇H₁₆, but the (methyl) branch of the first compound is coming off carbon 2. In the second compound, it is located at carbon 3.

H₃C ─ CH CH₂ CH₂ CH₂ CH₃  H₃C CH₂ ─ CH CH₂ CH₂ CH₃
||
CH₃CH₃
2-methyl-hexane 3-methyl-hexane

Note: There is also a special type of position isomerism, but this is considered separately. she is called icompensation sum or metameria, because the difference is in the position of the heteroatom (atom that appears in a carbon chain between carbons).

Example: H₃C ─ O CH₂ CH₂ CH₂ CH₃  H₃C CH₂ ─ O CH₂ CH₂ CH₃

By Jennifer Fogaça
Graduated in Chemistry

Would you like to reference this text in a school or academic work? Look:

FOGAÇA, Jennifer Rocha Vargas. "Position Isomerism"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/isomeria-posicao.htm. Accessed on June 27, 2021.