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Difficulty: MediumFunctional Groups, Homologous Series, and IUPAC Nomenclature

Arrange the following organic compounds in order of INCREASING solubility in water, starting with the least soluble compound:

  1. 1Pentane (CH3CH2CH2CH2CH3\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3)
  2. 2Pentan-1-ol (CH3CH2CH2CH2CH2OH\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{OH})
  3. 3Butane-1,4-diol (HOCH2CH2CH2CH2OH\text{HOCH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{OH})
  4. 4Ethane-1,2-diol (HOCH2CH2OH\text{HOCH}_2\text{CH}_2\text{OH})

Answer

The correct sequence from least soluble to most soluble in water is: Pentane, Pentan-1-ol, Butane-1,4-diol, Ethane-1,2-diol.
Water solubility in organic compounds is determined by the balance between hydrophilic (water-attracting) functional groups and hydrophobic (water-repelling) carbon chains. Pentane is non-polar and cannot form hydrogen bonds, making it the least soluble. Pentan-1-ol has one −OH-\text{OH} group but a large 5-carbon hydrophobic chain, giving it low solubility. Butane-1,4-diol contains two −OH-\text{OH} groups, significantly enhancing hydrogen bonding with water molecules. Ethane-1,2-diol has two −OH-\text{OH} groups and the shortest hydrophobic carbon chain (2 carbons), making it the most soluble in water.

Step-by-Step Solution

1
Analyze the functional groups present in each compound.
Pentane has no polar functional groups. Pentan-1-ol has one −OH-\text{OH} group. Butane-1,4-diol has two −OH-\text{OH} groups. Ethane-1,2-diol has two −OH-\text{OH} groups.
Water is a polar solvent that dissolves compounds capable of forming hydrogen bonds through polar functional groups like −OH-\text{OH}.
2
Compare the hydrophobic alkyl chain lengths for compounds with the same functional group capability.
Ethane-1,2-diol has a smaller non-polar carbon chain (2 carbons) compared to Butane-1,4-diol (4 carbons).
A larger non-polar alkyl region increases hydrophobic interaction, reducing solubility in water.
3
Establish the complete order of increasing solubility.
Pentane < Pentan-1-ol < Butane-1,4-diol < Ethane-1,2-diol.
Solubility increases with an increasing ratio of hydrophilic functional groups (−OH-\text{OH}) to non-polar alkyl chain length.

Key Concept

Solubility of organic compounds based on functional group polarity and hydrophobic alkyl chain length
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Topics

Functional Groups, Homologous Series, and IUPAC Nomenclature

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