For a homologous series of compounds (that is, compounds of similar structure th

Question

For a homologous series of compounds (that is, compounds of similar structure that differ in the number of CH2 groups in a chain) the log of the adjusted retention time is usually a linear function of the number of carbon atoms. A compound was known to be a member of the family (CH3)2CH(CH2)nCH2OSi(CH3)3. For the following gas chromatographic adjusted retention times, prepare a graph of log tr' versus n and perform a least squares analysis using Excel to find the equation of the best-fit line.

1) What are the values of the slope and intercept of this line?
(Round your answer to 3 significant figures. The values do not have units.)

2) What is the value for n in the chemical formula of an unknown compound whose retention time is 12.88 minutes?

3) What would the retention time in minutes be for a compound in the same homologous series where n = 5?
(Round your answer to the nearest 0.1 minute).

Explanation / Answer

1) The plot of Log tr versus n can be drawn as follows.

Log tr = 0.20999*n + (-0.98984)

Compare the above equation with y = m*x + c (Where m = slope of plot, c = y-intercept)

Therefore, slope of the line = 0.210 and y-intercept of the line = -0.990

2) If tr = 12.88 min then Log tr = Log(12.88) = 1.1099

Now, 1.1099 = 0.20999*n -0.98984

i.e. n = 9.9993 ~10

3) n = 5, i.e. Log tr = 0.20999*5 -0.98984

i.e. tr = 1.148 min

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