In: Chemistry
Nucleophilic Substitution Reactions Organic Chemistry Lab:
Explain the order of elution you observed while performing the gas chromatography for this experiment. What property of the product molecules seem to be the most important in determining relative retention times?
In gas GC, the stationary phase is a high boiling point liquid and mobile phase is an inert gas.The retention times is generally measured as the time taken by the analyte liquid mixture (volatilized in the hot chamber where it is injected in minute quantity), to travel from injection junction to the detector. The detector measures this time interval in minutes.
For efficient separation of components of mixture , the rate of travelling through the column is important. This rate depends on several factors=
But in nucleophillic substitution reactions the ,polarity of the products is an important factor that effects the RRT(relative retention time) : Polar compounds move slowly in polar columns.
The products if are alkyl halides then the order of polarity would be –RF>RCl>RBr>RI
So the more polar one will be retained longer, and have Higher RRT.
RRT=tr/tm
Where, tr= retention time of analyte
tm=retention time for standard species
elution order= RF<RCl<RBr<RI
Other factors that influence the RT are
Boiling point difference of compounds in the mixture is the most important factor, the higher the difference the higher will be the relative retention time of the compound.
Volatility of product : More volatile component travels faster through the column
Column specific temperature: Raising the temperature speeds up the rate of all components of the mixture.
Length of column: longer column, the longer will be the retention time,
However the relative retention time does not change.