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.