Question

Number the following compounds in order of their elution from a silica column, using benzene as the solvent. The compound numbered one should be the first compound off of the column.

CH₃CH₂OCH₃       CH₃(CH₂)₃OH       CH₃CO₂H       CH₃(CH₂)₈CH₃  

Answer 1

In silica chromatography the elution depends upon the polarity of the compound. More polar compounds elute last while less polar compounds elute firsr. The order to be in general as follows.

cyclohexane < aromatic compounds < chlorine substituted methane < ethers < esters < alcohol < carboxylic acids = amino acids.

Hence the order is : CH₃CO₂H > CH₃CH₂OCH₃ > CH₃(CH₂)₃OH > CH₃(CH₂)₈CH₃

                                  4                      3                        2                      1

CH₃(CH₂)₈CH₃ ---- 1

CH₃(CH₂)₃OH   ---- 2

CH₃CH₂OCH₃ ----- 3

CH₃CO₂H       ----- 4

The increasing order will be:

Alkanes (least polar) < alkyl halides < alkenes < aromatic hydrocarbons < ethers < esters < ketones < aldehydes < amines < alcohols < carboxylic acids (most polar).

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