Effect of number of C on the percentage yield of carboxylic acid produced from oxidation of primary alcohols.

As a chemistry student since the 9^th grade, entering a lab has always been a memorable and eye-opening experience for me. I have always been intrigued to know more about the chemical processes that I come across in my daily life. Vinegar is one of them. On some more research I came to know that ethanol obtained from fermentation of glucose can be industrially oxidised under aerobic oxidations to ethanoic acid whose dilution finally leads to the production of vinegar. Applying chemistry and various principle of it to understand or elucidate the effect of physicochemical factors like temperature, pressure, surface area on rate of reactions is an important parameter. In Topic-6 of IB, all these factors and their effect are taught but my mind was busy in some other factor which was not introduced in this unit. Does the rate of a reaction depends on the nature of the reactant ? the size of the reactant ? chemical environment of the reactant? I could immediately connect this to homologous series. Properties like melting point, boiling point being physical properties follows a definite pattern while moving down a homologous series. But what about rate of reaction? Percentage yield is a numerically expressed chemical property. Rate of reaction, enthalpy changes of reaction are intensive properties of a chemical reaction. Does this properties also follows a trend while moving down a homologous series? Thus, I decided to choose the reaction of ethanol to ethanoic acid as a template and thought of investigating the effect of chain length on the average rate of oxidation of primary alcohols to mono carboxylic acids.

How does the percentage yield of carboxylic acid produced from oxidation of primary straight chain alcohol(ethanol, propanol, butanol, pentanol and hexanol) using Cu turnings as oxidising agent depends on the number of C in the primary alcohol taken, determined by recording the volume of carboxylic acid produced?

Any category of organic and natural compounds where a carbon (C) atom is bonded to an oxygen (O) atom by a double bond and also to some hydroxyl team (―OH) by one bond. A quarter bond links the carbon atom to some hydrogen (H) atom or even to several more univalent combining team. They are named as alkanoic acid where the term ‘alk’ represents the number of C they contain. The carboxylic acids referred to in this investigation are – ethanoic acid (CH₃COOH), propanoic acid (CH₃CH₂COOH), butanoic acid (CH₃CH₂CH₂COOH),pentanoic acid (CH₃CH₂CH₂CH₂COOH) and hexanoic acid (CH₃CH₂CH₂CH₂CH₂COOH).

Alcohols are class of organic compounds that contain the C-OH bond. They are generally represented as R-OH where R is the alkyl group.

Based on the degree of the C holding the OH group, alcohols are classified into three types-primary, secondary and tertiary.

This investigation deals with primary alcohols; it means that the C atom connected with the OH group is connected to only one alkyl group and two other H atoms. Alcohols are named as ‘alkanols’ where alk represents the number of C in the molecule.

All the alcohols chosen in this investigation are primary alcohols and has the OH group in the terminal C. All of them are straight chain. The alcohols chosen are – ethanol (CH₃CH₂OH), propanol (CH₃CH₂CH₂OH), butanol(CH₃CH₂CH₂CH₂OH),pentanol(CH₃CH₂CH₂CH₂CH₂OH)and hexanol(CH₃CH₂CH₂CH₂CH₂CH₂OH)

Alcohols are forms of organic compounds that consists of the functional O-H group and one of the properties for this functional group is its ability to become a carboxylic acid when oxidized. The addition of the oxygen changes the functional group from O-H to COOH due to the excess of oxygen (Mallat and Baiker). An example of the oxidation of alcohol is shown below.

C₂H₅OH OXIDATION →  CH₃COOH

The formula above describes the oxidation of ethanol which is a primary alcohol to form a compound known as ethanoic acid which is a form of carboxylic acid.

The oxidising agents that can be used for this are acidified potassium dichromate(K₂Cr₂O₇), acidified potassium permanganate (KMnO₄), Cu turnings, pyridinium chlorochromate and many more.

Actual yield is defined to be the yield produced when the experiment is carried out. To calculate percentage yield, we need to determine the theoretical yield based on the limiting reactant (Ranveer and Mistry ). The limiting reactant is the reactant in the chemical reaction that limits the amount of product produced. The ratio between theoretical yield and actual yield multiplied by 100 is the percentage yield for that product. The formula to calculate the percentage yield is shown below:

Percentage Yield = \(\frac{Actual\ Yield}{Theoretical\ Yield}\) × 100%

In organic chemistry, organic compounds with similar chemical properties that differ by the same atomic mass and have the same functional group are known to be homologous organic compounds and a number of these compounds can be characterised into a homologous series. In the case of the homologous series for alcohols, all of them have the functional group O-H and differ by an atomic mass of 14 or by a CH₂  unit.

The actual yield of the product of a chemical reaction differs from the theoretical yield due to a variety of factors such as temperature, concentration, pressure and even the physical state of the reactants.(Sadri et al.)

• Temperature:

The rate of reaction is proportional to the temperature at which it is taking place and can result in an increase or decrease in yield.

• Concentration

An increase in concentration can lead to more vigorous reaction since there are more reactant molecules involved in successful collisions that form the product. The higher rate of successful collisions also results in an increase in the rate of reaction and consequently a higher yield of products.

• Pressure

An increase in pressure means that there is a decrease in the amount of space molecules have to successfully collide resulting in a higher probability of successful collisions. This would increase the yield as well.

• Physical State

​​​​​​​The physical state of a substance such as liquid, gas or solid can have great impacts on the rate of reaction because the molecules behave differently in every state.

The current investigation does not deal with any one of the above factors. All of the above factors will be controlled as the motive of the IA is to understand the effect of structural features like chain length on percentage yield.

The table below shows the boiling points and energy required to break the atoms for the different primary alcohols used in this experiment.

When primary alcohols are oxidized they first form an aldehyde and in the presence of excess oxygen they form carboxylic acids. The excess oxygen is provided by the oxidizing agent known as potassium dichromate (K₂Cr₂O₇). The oxidizing agent should be acidified and to achieve this, we dilute it in sulfuric acid (H₂SO₄). An example of the complete reaction is shown below:

C₂H₅OH PARTIAL OXIDATION → CH₃CHO

C₂H₅OH EXCESS OXIDATION → CH₃COOH

Reaction of ethanol to ethanoic acid:

C₂H₅OH  EXCESS OXIDATION → CH₃COOH

Reaction of propanol to propanoic acid:

C₃H₇OH EXCESS OXIDATION→ CH₃CH₂COOH

Reaction of butanol to butanoic acid:

C₄H₉OH EXCESS OXIDATION → CH₃CH₂CH₂COOH

Reaction of pentanol to pentanoic acid:

C₅H₁₁OH EXCESS OXIDATION → CH₃CH₂CH₂CH₂COOH

Reaction of hexanol to hexanoic acid:

C₆H₁₃OH EXCESS OXIDATION → CH₃CH₂CH₂CH₂CH₂COOH

The experiment involves two stages – oxidation of the alcohol and collection of the carboxylic acid produced from the reaction mixture. The alcohol will be taken in a beaker and oxidised using Cu as the oxidising reagent. The reaction will be continued for a particular duration. Once the reaction is completed, the carboxylic acid produced will be collected by distillation. As the alcohol and the carboxylic acid differs in boiling points for more than 40.0°C or so, the distillation is an effective method (Iinuma et al.)The volume of the distillate produced at the boiling point of the carboxylic acid will be recorded by collecting the carboxylic acid in a graduated measuring cylinder.

The null hypothesis for this experiment is that there is no sort of correlation between the number of carbon in a primary alcohol and the percentage yield of the carboxylic acid formed upon its oxidation.

The alternate hypothesis for this experiment is that there is a strong correlation between the number of carbon in a primary alcohol and the percentage yield of the carboxylic acid formed upon the complete oxidation of the primary alcohol

• When an alcohol is oxidized, it starts to produce acidic vapor which later condenses to form the respective carboxylic acid for that alcohol. However, the process between oxidation and condensation, the acidic vapor is released into the surroundings and even with the presence of a cork, the vapor can still diffuse and make its way through. The vapor in its form, is highly flammable and since my experiment involves the use of a Bunsen burner,  safety goggles and a lab coat was used to prevent any damage from fire.  Along with this, it was made sure that if there was an excess of acidic vapor that was leaking out from the cork, heating the test tube is discontinued and the position of the cork is changed or replace it with a new one.
• Safety goggles to prevent any damage from fire to my eyes.
• Lab coat to prevent any damage from fire to my body.
• Use of gloves to make sure that my hand is safe from the intense heat produced from the Bunsen burner.
• Replacement of cork if there was an excess of acidic vapor leaking out.
• Use of mask to prevent toxic fumes from the acid vapor from entering lungs.

The oxidation of any alcohol first produces acidic vapor which is highly flammable. The vapor is dangerous in high amounts because it can remain in the atmosphere for a prolonged period and with the presence of oxygen, it is capable of igniting a fire. Along with this, since it remains in the environment, it can also be a major cause of acid deposition which is already a major global issue us humans are facing. The presence of acid vapor can lead to breathing problems as well and in extreme cases even death. The experiment was performed near a window so that the vapours can diffuse and the amount of sample chosen was less to ensure that minimum amount of vapours are produced.

• Any chemicals that is prohibited was not used..
• Enough care was taken so that the environment is not harmed and there are no potential risk factors.
• No organisms were harmed in any way, as none were used in conducting this experiment.
• All chemicals that could’ve cause harm through spillover effect from being disposed incorrectly was taken care of and made sure it would not.

• A 100.00 cm³ beaker was taken.
• 10.00 ± 0.05 cm³ of ethanol was transferred to the beaker using a graduated pipette.
• The beaker was placed on the water bath and the temperature was set at 60.0°C and covered with a watch glass.
• 2.00 ± 0.01 g of Copper was weighed on a spatula and transferred to the same beaker.
• The stop-watch was started and the reaction was continued for 1 hour.
• The beaker was taken away from the water bath and kept aside in a wooden stand. It was allowed to cool down and come to room temperature.
• The content of the beaker was filtered out to remove the unreacted copper and the filtrate was collected in a round bottomed flask.
• The round bottomed flask was then fixed with a stand and a retort. It was fixed with a condenser with channels for cold water going in and hot water going out.
• A thermometer was also fixed with the round bottomed flask.
• A Bunsen burner was placed below the round bottom flask.
• The flame of the burner was adjusted to blue color.
• The content in the flask was heated and the temperature was allowed to increase.
• As soon as the temperature reached 117°C (boiling point of ethanoic acid), the liquid obtained was collected in a 10.00 ± 0.50 cm³ graduated measuring cylinder. The graduated measuring cylinder was placed inside a beaker containing ice cold water.
• Steps 1-12 were repeated for four more times to collect the data in repeated trials.

The same procedure was followed for other alcohols and the temperature chosen in Step-12 were 141.0°C for propanoic acid obtained from oxidation of propanol, 163.0°C butanoic acid obtained from oxidation of butanol, 186.0°C pentanoic acid obtained from oxidation of pentanol and 205.0°C hexanoic acid obtained from oxidation of hexanol.

The vapor produced from the beaker had a pungent smell to it. This indicated the formation of carboxylic acid.

Average volume of carboxylic acid obtained = \(\frac{(8.20+8.20+8.30+8.40+8.30)}{5}\) = 8.28±0.05 cm³

Volume of alcohol taken = 10 cm³ ± 0.5

Density of ethanol = 0.789 gcm^-3

Mass of ethanol = (density volume ) = (0.789 X 10) = 7.89 ± 0.05 g

Moles of ethanol = \(\frac{mass}{molar\ mass}=\frac{7.89±0.05cm^3}{46.04}\) = 0.17 ± 0.05 cm³

Maximum moles of carboxylic acid produced = 0.17

(As ethanol and ethanoic acid are in the mole ratio 1:1 )

Maximum mass of carboxylic acid produced = moles X molar mass = (0.17 ± 0.05) 60.00 = 10.28 ± 0.05 g

Density of ethanoic acid = 1.050 gcm^-3

Volume of ethanoic acid = \(\frac{mass}{density}=\frac{10.28±0.05}{1.050}\) = 9.79 ± 0.05 cm³

For ethanol

Percentage yield = \(\frac{Experimental\ yield}{Theoretical\ yield}\) × 100 = \(\frac{8.28}{9.79}\) × 100 = 84.58

For ethanol

Theoretical yield = 9.79 ± 0.05 cm³

The theoretical yield has been calculated based on the consideration that the volume of the alcohol taken is 10.00 ± 0.05 cm³.

Apart from the value of the volume of the alcohol, the value of density, molar mass of the alcohol are also used and these are theoretical values.

Thus, the absolute error in theoretical yield is the absolute error in the volume of the alcohol taken which is ± 0.05 cm³ (as the alcohol was taken using a pipette).

Experimental yield = 8.28 ± 0.05 cm³

Fractional error in percentage yield

= Fractional error in theoretical yield + Fractional error in experimental yield

= \(\frac{±0.05}{9.79}+\frac{±0.05}{8.28}\) = ± 0.0111459 = ±1.11 × 10^-2

The graph above shows the percentage yield of carboxylic acid decrease as the number of primary alcohols increase. Down the homologous series from ethanol to pentanol, the percentage yield of the carboxylic acid produced from oxidation decreases from 84.58 % to 54.27%.

The percentage yield of carboxylic acid produced and the alcohol oxidised bears a quantitative relationship as:

% yield = -7.3937 (number of C in alcohol oxidised) + 97.788

This equation is deduced from the equation of best fit line as displayed in the graph where the y represents the percentage yield of carboxylic acid made and x represents the number of C in the alcohol oxidised.

Using regression technology and a line of best fit, we can calculate the coefficient of determination which is the R² value on the diagram. The coefficient of determination is a good way to measure the relationship between two variables since it predicts the fluctuation or variance of one variable from the other variable. It represents the percentage of data which is closest to the line of best fit. This means that the closer the R² value is to 1, the better the line of best fit is able to explain the data. For this experiment the R²  value is 0.993 which means that the regression line is able to explain the data in the graph very closely.

The steep, negative slope is an indicator that the relationship between number of Carbon in the primary alcohol and the percentage yield is inverse and as the number of carbon in the primary alcohol increases, the percentage yield of the carboxylic acid for that particular primary alcohol decreases.

The negative slope of the line of best fit also indicates that for every unit of carbon increased in the primary alcohol, the percentage yield decreases by approximately 7.2% (as indicated by the magnitude of the gradient in the equation of trend line) . With the help of the regression line, it is also possible to predict the percentage yield for primary alcohols with higher number of carbon atoms.

\(\frac{8376+74.07+67.80+61.14+54.27}{5}\) = 68.208%

As we go down the homologous series from ethanol to hexanol, the molar mass of the alcohol increases and the surface area as well. This increase in surface area and molar mass increases the intermolecular force which is London dispersion force in this case among the molecules. As a result, the molecules gets more close to each other and thus they become less reactive towards the oxygen. This in turn reduces the amount of alcohol that gets converted into carboxylic acid and thus finally reduces the volume of carboxylic acid made. The fact that the strength of inter molecular force increases down the group is revealed in the fact that the boiling point of the alcohol increases as we move down the group.

How does the percentage yield of carboxylic acid produced from oxidation of primary straight chain alcohol using Cu turnings as oxidising agent depends on the number of C in the primary alcohol taken, determined using volume measurement?

• The percentage yield of carboxylic acid decrease as the number of primary alcohols increase. Down the homologous series from ethanol to pentanol, the percentage yield of the carboxylic acid produced from oxidation decreases from 84.58 % to 54.27%.
• The percentage yield of carboxylic acid produced and the alcohol oxidised bears a quantitative relationship as: % yield = -7.3937 (number of C in alcohol oxidised) + 97.788
• As indicated by the value of regression coefficient (R² = 0.9833) and the negative value of the gradient (-7.3937) a negative correlation has been established between the percentage yield of carboxylic acid obtained and the number of C in the alcohol that is oxidised. Thus, the alternate hypothesis stands valid.
• As reported in a research article titled – “Kinetics of Oxidation of Aliphatic Alcohols by Potassium Dichromate in Aqueous and Micellar Media” by Mohammad Hassan(Hassan and Al Hakimi)it has been reported that as we go down the homologous series the rate of oxidation of alcohols decreases. This finding is in support of the results obtained in this investigation. As the chain length increases, the reaction takes more time to complete, the rate of production of carboxylic acid would cease down. Thus, the percentage yield of carboxylic acid decreases.

• The relationship between percentage yield and increasing number of carbon chains in a primary alcohol is very strong and small factors like these would not have affected the result too much but avoiding these errors would have made the result more accurate.
• The independent variable chosen follows a continuous nature. The alcohols taken are in the homologous series and they all differ by 1 C.
• The conclusion made is also in agreement with the literature data.
• The design of the experiment is not according to any standard protocol and involves a delineate creative input.
• The experiment that was carried out proved that the alternate hypothesis is right. After processing the data, it could be concluded that there was a strong negative correlation between the percentage yield and the number of carbon chains in a primary alcohol.
• The determinant is so close to that the relationship is almost perfectly linear. This data shows that the experiment was carried out to a meticulous detail and that the limitations did not affect the accuracy as much as expected.

• The method to achieve the aim of my experiment was pretty straightforward and thus there were not many limitations. However, even in a simple experiment such as this, there were a couple of measures that could have been taken to improve the accuracy as shown in the error table above.
• Along with this, the most important limitation was the fact that while heating the alcohol, acid vapor was released and even with the provision of the cork, there was some vapor that escapes into the surroundings which consequently has a major impact on the yield as this gas will not condense into the container submerged into the cold water.
• The test tube in question was also a limitation as the heat from the Bunsen burner was very intense and could have completely melted the glass. Holding the Bunsen burner is a form a random error as no two alcohols could have been heated the exact same way with the exact same angle.

I would like to study the compare the rate of the oxidation of alcohol into carboxylic acids against the chain length. To do this, the oxidation of alcohols can be carried out using acidified potassium dichromate as the reagent. Addition of this reagent causes the colour to change from orange to green. Thus, the rate can be determined as the reciprocal of the time taken for the color to change. I would like to measure the rate of oxidation for ethanol, propanol, butanol, pentanol and hexanol and see if the rate of oxidation changes along the homologous series or not.

Goodarzi, Mohammad, and Matheus P. Freitas. “Predicting Boiling Points of Aliphatic Alcohols through Multivariate Image Analysis Applied to Quantitative Structure−Property Relationships.” The Journal of Physical Chemistry A, vol. 112, no. 44, Nov. 2008, pp. 11263–65. ACS Publications, doi:10.1021/jp8059085.

Hassan , Mohammad, and Ahmad N. Al Hakimi. “Kinetics of Oxidation of Aliphatic Alcohols by Potassium Dichromate in Aqueous and Micellar Media.” South African Journal of Chemistry , vol. 64. Accessed on June 18, 2020

Iinuma, Masataka, et al. “Oxidation of Alcohols to Aldehydes or Ketones with 1-Acetoxy-1,2-Benziodoxole-3(1H)-One Derivatives.” European Journal of Organic Chemistry, vol. 2014, no. 4, Feb. 2014, pp. 772–80. chemistry-europe.onlinelibrary.wiley.com (Atypon), doi:10.1002/ejoc.201301466.Accessed on May 22, 2020

Mallat, Tamas, and Alfons Baiker. “Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts.” Chemical Reviews, vol. 104, no. 6, June 2004, pp. 3037–58. ACS Publications, doi:10.1021/cr0200116. Accessed on June 24, 2020

OpenStax. “5.3 Enthalpy.” Chemistry, 2016. opentextbc.ca, https://opentextbc.ca/chemistry/chapter/5-3-enthalpy/.

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Sadri, Fariba, et al. “Green Oxidation of Alcohols by Using Hydrogen Peroxide in Water in the Presence of Magnetic Fe3O4 Nanoparticles as Recoverable Catalyst.” Green Chemistry Letters and Reviews, vol. 7, no. 3, July 2014, pp. 257–64. Taylor and Francis+NEJM, doi:10.1080/17518253.2014.939721. Accessed on June 17, 2020