Effect of pH on rate of ketal formation between propanone & ethanol

Understanding how scientific principles governs us and regulates us has always been an interesting task for me. Being a skilled IB learner, I have always been intrigued to understand the way various chemical reaction occurs and the mechanism they follow. I have developed a special interest for this especially during my classes for Topic-10 (Organic Chemistry) in my second year of Diploma program. I always wanted to know more examples of organic reactions especially when I understood the concept of synthetic routes. This interest became stronger when I came across a research paper (Wright et al.) that spoke about the use of ‘ketals’ and ‘acetals’ as a precursor to synthesize resins. Exploring some organic chemistry websites and text books, I got to know more about them and could identify a similarity between this reaction and esterification which I studied in my class. The fact that intrigued me the most is that what factors could influence the way this reaction occurs. I realized the importance of this fact more when I understood the importance of knowing the mechanism of an organic reaction. I decided to study how certain conditions at which the reaction takes place would impact the reaction. To narrow down my thoughts into a precise platform, I faced two major challenges – Which parameter should I study to delineate the effect of some conditions on an organic reaction? The simplest choice I could get was rate of reaction. The next challenge was to design a process that allows me to collect the adequate data for this investigation. Going back to the concepts, I studied in Topic-6 (Kinetics), I could recollect about using colorimetry or spectroscopy as an analytical method to study rates of reaction. This led me to the research question given below:

How does the average rate of the reaction (measured in abs s^-1 ) of the intermolecular condensation between propanone and ethanol to produce 2,2-diethoxy propane in presence of HCl as catalyst depends on the pH of the medium, determined using UV-Vis spectrophotometer?

Propanone is an organic liquid having the functional group ketone. It has the displayed formula – CH₃COCH₃. Ethanol is an organic liquid belonging to the functional group of alcohol. This has a displayed formula of CH₃CH₂OH. In presence of dilute HCl as a catalyst, they undergo inter molecular condensation to produce a ketal – 2,2-diethoxy propane. Condensation products of ketones and alcohols are termed as ketals while condensation products of aldehyde and alcohols are known as acetals.

This reaction exemplifies condensation as it joins the two molecules – propanone and ethanol by elimination of a simple molecule-water.

The mechanism of the reaction (MacKENZIE and Stocker) is demonstrated below using skeletal structures of the reactants, intermediates and the products and curly arrows to indicate the movement of the pairs of electron.

Step - 1: The O atom of the propanone donates a lone pair to the Hydrogen (H⁺) ion furnished from dilute HCl and gets protonated. As a result, it gets a positive charge. Here, O atom behaves as a nucleophile and the H⁺ ion behaves as an electrophile.

Step - 2: The O atom of the ethanol molecule attacks the carbonyl C atom of the propanone group by using it’s lone pair. Here, the O atom of the ethanol is a nucleophile and the C atom of the carbonyl group is an electrophile. As a result, the pi bond of C=0 is broken and the electrons are given to the O atom which eventually neutralizes the positive charge acquired by the O atom is Step-1. This is also termed as ‘nucleophilic addition to C = O’ (Dong et al.).

Step - 3: The intermediate thus formed undergoes an intra molecular proton transfer. A H atom from the O atom of the OH group in ethanol moiety is transferred to the O atom of the OH group of the propanone moiety. This is an example of 1-3 proton shift causing the molecule to rearrange itself. The shift is 1-3 as if the O atom the H is shifting from is considered as 1 while the O atom, the H is shifting to is 3.

Step - 4: Another nucleophilic addition is done by the second ethanol molecule in a way similar to that in Step - 2. The product formed finally deprotonates itself to yield the main organic product – 2,2-diethoxy propane and water as a by-product.

A = ∈ cl

If molar absorptivity (∈) and path length are constant, then absorbance and concentration is directly related. Thus, these two terms become interchangeable

As the sample remains same and the path length remains unaltered (because the same spectrophotometer has been used), ∈ and path length (l) are assumed to be constant.

 \(Therefore, rate of reaction =\frac{change\ of\ concentration}{time\ taken}= \frac{change\ of\ absorbance}{time\ taken}\) \(\)

Here, propanone is a reactant and thus,

\(Rate \,of \,disappearance \,of propanone = \frac{change\ is\ concentration\ of\ propanone}{time\ taken\ for\ the\ change} =\frac{change\ is\ concentration\ of\ propanone}{time\ taken\ for\ the\ change}\) \(\)

 \(=\frac{Absorabnce\ of\ propanone\ at\ start-Absorabnce\ of\ propanone\ at\ end}{Time\ for\ which\ the\ reaction\ was\ carried\ out}=\frac{ΔA}{time\ taken}abs s-1\)  \(\) 

There is no correlation between the rate of the reaction and the pH of the medium.

There is no correlation between the rate of the reaction and the pH of the medium. This can be justified based on the fact that H⁺ is acting as a catalyst in this reaction and change in concentration of catalyst has an effect on rate of reaction.

The independent variable is the pH of the medium. The pH will be varied from 1.00 to 7.00. For the values of pH from 1.00 to 6.00, dilute HCl of appropriate concentrations, 1.0 × 10^-1 mol dm^-3, 1.0 × 10^-2 mol dm^-3, 1.0 × 10^-3 mol dm^-3, 1.0 × 10^-4 mol dm^-3, 1.0 × 10^-5 mol dm^-3, 1.0 × 10^-6: mol dm^-3 will be used. The pH will be calculated using the formula:

pH of the medium = - log (molar concentration of HCl used)

It has been assumed that HCl dissociates completely in aqueous solution.

The dependent variable is the rate of the reaction in abs s^-1.

The rate of the reaction will be calculated as the rate of disappearance of propanone. To do this, the absorbance of the propanone will be recorded at the start of the reaction and at the end of the reaction, change of absorbance will be calculated and the rate will be calculated using the formula:

 \(Rate \,of \,the \,reaction =\frac{difference\ in\ absorbance}{time\ taken}=\frac{ΔA}{600.00}abs s-1\) 

A UV-Visible spectrophotometer was used to record the absorbance of the sample at a wavelength at which propanone shows maximum absorbance.

• Propanone – 10 cc
• Ethanol – 10 cc
• Concentrated HCl (11.00 moldm^-3) – 10 cc

• Both propanone and ethanol are volatile organic liquids. If inhaled, they can cause respiratory disorders and even senseless ness. A safety mask was used to ensure that these chemicals are not inhaled.
• Concentrated HCl is a corrosive liquid. If exposed to skin, it may cause severe burns. Inhaling vapors may lead to dizziness and headache. A safety masks, hand gloves and a laboratory coat was used to ensure that this does not happen.

All unused chemicals were first diluted and disposed of into a waste bin for further disposal as per the standard laboratory protocols for safe disposal of chemicals.

The investigation has been performed with minimum possible amount of raw materials.

Determining the wavelength of maximum absorbance for propanone

• A cuvette was taken and filled with 2.00 ± 0.05 cc of propanone.
• Another cuvette was taken and filled with distilled water.
• Both the cuvettes were wiped off with soft tissues and inserted into the spectrophotometer. The cuvette with propanone was in the sample chamber and the cuvette with distilled water was in the blank chamber.
• The wavelength of the spectrophotometer was set at 200 nm.
• The absorbance of the sample was recorded.
• The wavelength was increased by 5 nm and the absorbance was recorded again till 400 nm.

Preparation of HCl solutions:

Concentration of HCl supplied (C₁) = 11.00 moldm^-3

Concentration of HCl to be made (C₂) = 0.10 moldm^-3

Volume of HCl to be made (V₂) = 100 cm³ = 0.10 dm³

Volume of supplied HCl to be taken \( (V_1) = \frac{C_2V_2}{C_1}=\frac{0.10×0.10}{11}= 0.00090 dm^3 = 0.90 cm^3\) 

• A volumetric flask containing 20.00 ± 0.05 cc of distilled water added using a 20 cc pipette was taken.
• A 1.00 cm³ pipette was used to transfer 0.90 ± 0.05 cc supplied HCl to it.
• Distilled water was added till the mark of 100 cc.

The other HCl solutions 1.0 × 10^-2 mol dm^-3, 1.0 × 10^-3 mol dm^-3, 1.0 × 10^-4 mol dm^-3, 1.0 × 10^-5 mol dm^-3, 1.0 × 10^-6 mol dm^-3 of HCl solutions were prepared through serial dilution. For example, 1.0 × 10^-2 mol dm^-3 was diluted 10 times to prepare 1.0 × 10^-3 mol dm^-3. To do this, 10.00 ± 0.05 cc of 1.0 × 10^-3 mol dm^-3 was pipetted out and transferred to a 100 cc volumetric flask. Distilled water was added till the mark following this.

Density of propanone³ = 0.791 g /cc

Molar mass = 58

Number of moles = 0.10

Mass = moles × molar mass = 0.10 × 58 = 5.80 g

Volume \(= \frac{Mass}{Density}= \frac{5.80}{0.791}= 7.33cc ≅ 7.30 cc\) \(\) 

Density of ethanol = 0.789 g /cc

Molar mass = 46

Number of moles = 0.10

Mass = moles × molar mass = 0.01 × 46 = 0.46 g

\(Volume = \frac{Mass}{Density}=\frac{0.46}{0.789}= 5.83 ≅ 5.80 cc\) 

Volume for 0.20 moles = 5.80 cc × 2 = 11.60 cc

In both the cases, the value of volume in the second decimal place has been rounded off to zero as the least count of the apparatus used for measuring the volume (a 10 cc graduated pipette) is 0.10 and thus it is not possible to measure the second decimal place in it. For example, 5.80 cc can be measured using it but not 5.83 cc.

• A 100.00 cc glass beaker was taken and washed with distilled water.
• A 20.00 cc graduated pipette was used to transfer 7.30 ± 0.05 cc (0.10 moles) of propanone.
• A 20.00 cc graduated pipette was used to transfer 11.60 ± 0.05 cc (0.10 moles) of ethanol.
• A graduated measuring cylinder was used to add 1.0 × 10^-1 mol dm^-3 of dilute HCl till the mark of 100.00 cc.
• The stop-watch was started.
• A 1.00 cc graded pipette was taken to transfer 1.00 ± 0.05 cc of the reaction mixture to the cuvette.
• The absorbance of the reaction mixture was measured at 275 nm
• As soon as the stop-watch reads 600.00 s, Step-7 was repeated.
• Steps 1-7 were repeated for two more times to collect data in triplicates.
• Steps 1-9 were repeated for other concentrations 1.0 × 10^-2 mol dm^-3, 1.0 × 10^-3 mol dm^-3, 1.0 × 10^-4 mol dm^-3, 1.0 × 10^-5 mol dm^-3, 1.0 × 10^-6 mol dm^-3 of diluted HCl.
• Steps 1-8 were repeated using distilled water instead of dilute HCl

• Propanone is a colorless liquid with a distinct odor.
• Ethanol is a colorless liquid with a distinct odor.
• Fumes were observed on opening the bottle of concentrated HCl.
• Any visible observation like color change, formation of bubbles was not observed when the reaction was in progress.

Figure - 8 shows that propanone shows maximum absorbance at a wavelength of 275 nm. Thus, this wavelength will be used further in this investigation.

pH of the medium = - log (molar concentration of HCl used)

For pure water, the pH is considered as 7.00

Difference in absorbance (∆A)

= Mean absorbance at the start (t=0.00 s) – Mean absorbance at the end (t=600.00 s)

Rate of the reaction \( =\frac{difference\ in\ absorbance}{time\ taken}=\frac{∆A}{600.00} abs s-1\)

For the pH of the medium = 1.00

Rate of the reaction \( (r) =\frac{difference\ in\ absorbance}{time\ taken}\)

\(\frac{∆r}{r}=\frac{∆(∆A)}{∆A}+\frac{∆t}{t}=\frac{±0.002}{0.104}+\frac{±0.01}{600.00} = 0.01924\)

Percentage error  \(=\frac{∆r}{r}× 100 = 0.01924 × 100 = 1.92474359 ≅ 1.92\) 

Total Percentage error\(\) \( = \frac{∆r}{r}\frac{1.92+2.20+2.38+2.63+3.13+3.92+5.13}{7}= 3.05\) 

The total percentage error is 3.05 and the magnitude is really small. This shows that there is not enough systematic error in the investigation. Thus, the data collected is coherent and the result concluded may be claimed to be reliable and accurate.

Figure 13 indicates that rate of the reaction against the pH of the medium. As the pH of the medium decreases from 1.00 to 7.00, the rate of the reaction decreases from 1.73 × 10^-4 abs s^-1 to 6.50 × 10^-5 abs s^-1. This shows that as pH decreases, the rate at which the propanone reacts with ethanol also reduces. As pH decreases, the medium becomes less acidic and thus the molar concentration of Hydrogen ion (H⁺) in the medium decreases and the rate at which the propanone reacts with ethanol decreases.

The graph also displays an equation of trend line. The trend line indicates that the rate of the reaction between propanone and ethanol follows a linear relationship that follows the equation: y=-0.175 x + 1.9143; where y indicates the rate of the reaction between propanone and ethanol in acidic medium and x represents the pH of the medium.

The gradient of the trend line is -0.175 and it is a negative value. This again confirms the fact that as pH decreases, the medium becomes less acidic and thus the molar concentration of Hydrogen ion (H⁺) in the medium decreases and the rate at which the propanone reacts with ethanol decreases. Thus, a negative correlation is predicted between the pH of the medium and the rate of the reaction between propanone and ethanol carried out in presence of dilute HCl. This in turn implies a positive correlation between the concentration of Hydrogen ions and the rate of the reaction.

The value of the correlation of regression (R² ) is 0.9913; this confirms that there is a strong correlation between the values of rate of the reaction between propanone and ethanol and the pH of the medium.

The discussion above clearly indicates that as the pH decreases, there are less Hydrogen ions (H⁺) in the medium and thus the rate of the reaction between propanone and ethanol decreases. A scientific justification for this trend can be provided if the mechanism of this reaction is referred to.

As indicated in the mechanism, (refer to Page - ), more the number of H⁺ ions, more the number of propanone molecule that gets protonated in Step-1. Consequently, the subsequent steps are also favored. This in turn increases the frequency of successful collision and thus increases the rate of the reaction. In other words, the pH increases, the amount of Hydrogen ions (H⁺) decreases, the rate of the reaction decreases. As a result, less amount of propanone is consumed. Thus, the amount of propanone left in the reaction medium increases. Hence, the absorbance of the reaction mixture after the end of the reaction increases. At all values of pH, the initial concentration of propanone remains the same. With increase in pH, concentration of hydrogen ion (H⁺) decreases, rate of reaction decreases, less propanone is consumed, final concentration of propanone increases, absorbance of reaction mixture after the end of the reaction increases and thus the values of change in absorbance decreases. This is also evident from Figure 10; as the pH increases from 1.00 to 7.00, the value for change in absorbance decreases from 0.104 ± 0.001 abs to 0.039 ± 0.001 abs.

How does the average rate of the reaction (measured in abs s^-1 ) of the intermolecular condensation between propanone and ethanol to produce 2,2-diethoxy propane in presence of HCl as catalyst depends on the pH of the medium, determined using UV-Vis spectrophotometer?

• As the pH of the medium decreases from 1.00 to 7.00, the rate of the reaction decreases from • 1.73 × 10^-4 abs s^-1 to 6.50 × 10^-5 abs s^-1. This shows that as pH decreases, the rate at which the propanone reacts with ethanol also reduces.
• As pH decreases, the medium becomes less acidic and thus the molar concentration of Hydrogen ion (H⁺) in the medium decreases and the rate at which the propanone reacts with ethanol decreases.
• The rate of the reaction between propanone and ethanol follows a linear relationship that follows the equation: y=-0.175 x + 1.9143; where y indicates the rate of the reaction between propanone and ethanol in acidic medium and x represents the pH of the medium.
• With increase in pH, concentration of hydrogen ion (H⁺) decreases, rate of reaction decreases, less propanone is consumed, final concentration of propanone increases, absorbance of reaction mixture after the end of the reaction increases and thus the values of change in absorbance decreases.
• The null hypothesis has been rejected and the alternate hypothesis has been accepted.

• The wavelength of maximum absorbance has been detected experimentally instead of using a literature value. Moreover, the value obtained experimentally is in agreement with the literature value. This indicates that the data collected is reliable and accurate.
• The total percentage error is 3.05 and the magnitude is really small. This shows that there is not enough systematic error in the investigation. Thus, the data collected is coherent and the result concluded may be claimed to be reliable and accurate.
• The magnitude of standard deviation has been shown in Figure-7 and Figure-9. The values are quite less in magnitude in comparison to the data values. For example, in Figure-9, the data values for absorbance are – 0.681,0.681 and 0.680 (at pH =1.00) while the standard deviation is 0.001. Thus, it can be claimed that the raw data collected is precise.

There are many organic reactions that happens in acidic medium. For example, aldol condensation (Zalkow et al.) – reaction between aldehydes to produce a compound that has both aldehyde and alcohol group. This is one of the oldest reactions in organic reaction that made a way to join molecules by making C-C bond. Structural features of a molecule always impact the rate of reaction. Thus, I would like to see if the chain length of the aldehyde is increased from 2 in ethanal to 6 in hexanal, how would the rate of the reaction change. To do this, an absorbance versus wavelength plot can be made for the aldehyde and the wavelength of maximum absorbance can be determined from that. Following this, the absorbance of that aldehyde can be recorded after the reaction is over and the change of absorbance can be calculated. This would allow us to determine the rate of the reaction as done here. Finally, a scatter graph can be plotted for the rate of the reaction versus the chain length of the aldehyde used which can indicate that how does the rate of the reaction depends on the chain length of the aldehyde.

Dong, Jian-Lian, et al. “A Simple and Versatile Method for the Formation of Acetals/Ketals Using Trace Conventional Acids.” ACS Omega, vol. 3, no. 5, May 2018, pp. 4974–85. DOI.org (Crossref), doi:10.1021/acsomega.8b00159.

MacKENZIE, C. A., and J. H. Stocker. “PREPARATION OF KETALS. A REACTION MECHANISM.” The Journal of Organic Chemistry, vol. 20, no. 12, Dec. 1955, pp. 1695–701. DOI.org (Crossref), doi:10.1021/jo01364a015.

Wright, Austin C., et al. “Small-Scale Procedure for Acid-Catalyzed Ketal Formation.” The Journal of Organic Chemistry, vol. 84, no. 17, Sept. 2019, pp. 11258–60. DOI.org (Crossref), doi:10.1021/acs.joc.9b01541.

Zalkow, L. H., et al. “A new circular dichroism study of ketal formation of some steroidal ketones.” Tetrahedron, vol. 26, no. 21, Jan. 1970, pp. 4947–52. ScienceDirect, doi:10.1016/S0040- 4020(01)93146-1.