Calculate the overall percent yield of the synthesis to produce crude lidocaine,

Question

Calculate the overall percent yield of the synthesis to produce crude lidocaine, based on the amount of crude lidocaine you received in Part 3. So I did the following:

Part 3 Preparation of Lidocaine

Limiting Reagent

-chloro-2,6-dimethylacetanilide

(5.641g)/( 197.7g/mol)= 0.0285 mol

Diethylamine

Grams used= (Density)(Volume used)=

(0.7074g/mL)(7.5mL)

5.3055g used

(5.32g)/(73.14g/mol)=0.073mol

Toluene

Grams used= (Density)(Volume used)

(0.87g/mL)(30mL)

26.1g used

(26.1g)/(92.14g/mol)=0.283mol

Theoretical Yield

(0.0285mol)*(234.3g/mol)= 6.685g

% Yield= (Actual/Theoretical)(100%)

= (6.74g/6.68g)(100%)

= 100.9% (which does not make sense since i ended up more than I started with)

What am I doing wrong?

Thanks

Explanation / Answer

Synthesis of Lidocaine is a multi-step process. Hence the overall percent yield is the product of percent yield obtained in the individual steps.

% Yield = (Actual/Theoretical)*100-----------(1)

Step 1: Find the the theoretical yield in each step

1) Calculate the number of moles of all reagents used in that particular step. The limiting species is the one with the least number of moles.

2) Multiply the moles with the molecular weight of the desired product to obtain the theoretical yield in grams

Step 2: Note the actual yield of the desired product obtained experimentally

Step 3: Find the % Yield using equation 1 for every step.

Step 4: Overall % yield = Product of % yield from every step

Synthesis of lidocaine generally involves 3 steps:

1) Preparation of 2,6 dimethylaniline: Let % yield = X1

2) preparation of alpha chloro 2,6 dimethyl acetanilide = X2

3) Preparation of Lidocaine= X3

Over All yield = X1 * X2 * X3

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