Introduction: Reaction of a compound with water can result in a splitting, or lysis, of the compound into two parts. Organic molecules containing a group of atoms called an ester can be hydrolyzed by water to form a –COOH group (carboxylic acid) and an HO-- group (alcohol) as follows:
RCOOR’ + H2O ( RCOOH + HOR’
This reaction is spontaneous for almost all esters but can be very slow under typical conditions of temperature and pressure. The reaction occurs at a much faster rate if there is a significant amount of base (OH-) in the solution. In this lab experiment, the rate of this reaction will be studied using an ester called para-nitrophenyl acetate (PNA), which produces an alcohol, para-nitrophenol (PNP).
Question: What is the rate of reaction for the hydrolysis of PNA? What is the rate constant k? How are the rate of reaction and the rate constant k affected by varying (1) substrate PNA concentration, and (2) changes in pH (OH- base concentration) and addition of different nitrogen-containing base compounds (i.e.catalysts)?
Hypothesis: I hypothesize that the rate of reaction and the rate constant for the hydrolysis of PNA can be determined experimentally to be first order. Also, in the reaction, the experiment will develop as follows:
PNA +H2O --> PNP (yellow) + Ac
Materials and Methods:
The following solutions will be used in the experiment.
1. 0.2 M Phosphate Buffer, pH 6.5 (13.6 g KH2PO4 / 0.5 L, adjust to pH 6.5)
2. 0.4 M Phosphate Buffer, pH 6.5 (27.2 g KH2PO4 / 0.5 L, adjust to pH 6.5)
3. 2 M Potassium Hydroxide (11.2 g KOH / 100 ml solution)
4. 4(para)-nitrophenyl acetate (PNA), 0.0005 M (0.045 g /500 ml), store in 4 oC fridge.
5. Imidazole, 0.025 M (0.17 g Imidazole /100 ml);
6. Super 10x Imidazole, 0.25M (1.7 g /100 ml)
7. 2-Methylimidazole, 0.025 M (0.21 g
