Use the following linear synthesis for the formation of trimethyl silyl 3, 5-dim

Question

Use the following linear synthesis for the formation of trimethyl silyl 3, 5-dimethyl aniline to answer the questions below. (Molecular weights for each compound are listed below) Calculate the theoretical or expected mass of lithium 3, 5-dimethylaniline for the conditions outline in the first step of the linear synthesis above. (for full credit including work for the determination of a limiting reagent) What is the % yield for the first step of the linear synthesis? (for credit) Determine the overall % yield for the linear synthesis (for credit)

Explanation / Answer

Ans A:

According to given balance equation, 1 mole of dimethyl aniline reacts with 1 mole of n-BuLi.     

Number of moles of dimethyl aniline can be calculated as follows:

Moles of dimethyl aniline = Wt. of dimethyl aniline / molecular weight

Moles of dimethyl aniline = 2.67 g / (121.19 g/mol)

Moles of dimethyl aniline = 0.022

Similarly, moles of n-BuLi can be calculated as follows:

Moles of n-BuLi = (Volume of n-BuLi in L) x (Molarity)

Moles of n-BuLi = (15.15 mL x 1L/1000mL) x (1.6M)

Moles of n-BuLi = 0.02424 mol

Thus, number of moles of dimethyl aniline (0.022mol) is less than number of moles of n-BuLi (0.02424mol). Hence, dimethylaniline is limiting reagent.

Theoretical mass of lithium 3,5-dimethylaniline can be calculated as follows:

Theoretical mass = (molar mass of lithium 3,5-dimethylanline / molar mass of dimethylaniline) x mass of dimethylaniline

Theoretical mass = (127.12 / 121.18) x 2.67g

Theoretical mass = 2.80 g

Theoretical mass of lithium 3,5-dimethylanline for given reaction is 2.80g

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Ans B:

% yield of first step can be calculated using following formula:

% yield = (Practical yield / theoretical yield) x 100

% yield = (2.73 / 2.80) x 100

% yield = 97.5%

Thus, percent yield for first step is 97.5%

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Ans C:

Percent yield of first step is 97.5%.

Percent yield of second step can be calculated as follows:

Theoretical yield of second step = (molar mass of trimethylsilyl 3,5-dimethylanline / molar mass of lithium 3,5-dimethylanline) x mass of lithium 3,5-dimethylanline

Theoretical yield of second step = (193.37 / 127.12) x 2.73g

Theoretical yield of second step = 4.15g

Thus, percent yield of second step = (Practical yield / theoretical yield) x 100

Percent yield of second step = (3.80 / 4.15) x 100

Percent yield of second step = 91.6%

Thus, overall yield over two steps = (% yield of first step x % yield of second step)/ 100

Overall yield over two steps = (97.5 x 91.6 ) / 100

Overall yield over two steps = 89.3%

Therefore, overall percent yield of linear synthesis is 89.3%

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