01 Free Studying Resources 08 Junior College 2 Chemistry

Chapter 15: Carboxylic Acids & Derivatives

July 7, 2021 by Admin

Chapter 16

Carboxylic Acids & Derivatives

Firstly, let's take a look at some of the important physical
properties of both carboxylic acids and acyl chlorides.

	Volatility	Solubility
Carboxylic acid	(i) Carboxylic acids have higher boiling points than its corresponding alkanes - The hydrogen bonds between carboxylic acid molecules are stronger and requires more energy to break than the instantaneous dipole induced dipole interactions between alkane molecules (ii) Carboxylic acids have higher boiling points than its corresponding alcohol - The hydrogen bonds between carboxylic acid molecules are stronger and require more energy to break because the -OH group in carboxylic acid is more polarised due to the presence of the electron withdrawing C=O group (iii) Boiling point increases with increasing length of alkyl chain of the carboxylic acid - An increase in electron cloud size will increase the polarisability of the electron cloud of each carboxylic acid, leading to stronger id-id interactions which requires more energy to overcome	(i) The first four members of the aliphatic acids are completely miscible in water - -COOH group can form hydrogen bonds with water molecules - Carboxylic acids are partially dissociate in water to form ions which are capable of forming ion-dipole interactions with water molecules (ii) As the length of the non polar hydrocarbon chain increases, solubility in water decreases
Acyl chlorides and esters	(i) Acyl chlorides and esters have lower boiling points a than the parents carboxylic acid - Unable to form intermolecular hydrogen bonds with their own molecules as they lack a H atom attached to a highly electronegative O atom - Only possesses permanent dipole-permanent dipole interactions which are weaker than hydrogen bonds hence requiring less energy to overcome	(i) For both acyl chlorides and esters, as size of the hydrocarbon chain increases, solubility in water decreases (ii) Esters are generally soluble in non-polar solvents (iii) Acyl chlorides are soluble in non-polar solvents and soluble in water - They hydrolyse rapidly in water to form HCL and RCOOH, which then ionises

Volatility

Solubility

Carboxylic acid

(i) Carboxylic acids have higher boiling points than its corresponding alkanes

- The hydrogen bonds between carboxylic acid molecules are stronger and requires more energy to break than the instantaneous dipole induced dipole interactions between alkane molecules

(ii) Carboxylic acids have higher boiling points than its corresponding alcohol

- The hydrogen bonds between carboxylic acid molecules are stronger and require more energy to break because the -OH group in carboxylic acid is more polarised due to the presence of the electron withdrawing C=O group

(iii) Boiling point increases with increasing length of alkyl chain of the carboxylic acid

- An increase in electron cloud size will increase the polarisability of the electron cloud of each carboxylic acid, leading to stronger id-id interactions which requires more energy to overcome

(i) The first four members of the aliphatic acids are completely miscible in water

- -COOH group can form hydrogen bonds with water molecules
- Carboxylic acids are partially dissociate in water to form ions which are capable of forming ion-dipole interactions with water molecules

(ii) As the length of the non polar hydrocarbon chain increases, solubility in water decreases

Acyl chlorides and esters

(i) Acyl chlorides and esters have lower boiling points a than the parents carboxylic acid

- Unable to form intermolecular hydrogen bonds with their own molecules as they lack a H atom attached to a highly electronegative O atom
- Only possesses permanent dipole-permanent dipole interactions which are weaker than hydrogen bonds hence requiring less energy to overcome

(i) For both acyl chlorides and esters, as size of the hydrocarbon chain increases, solubility in water decreases

(ii) Esters are generally soluble in non-polar solvents

(iii) Acyl chlorides are soluble in non-polar solvents and soluble in water

- They hydrolyse rapidly in water to form HCL and RCOOH, which then ionises

Carboxylic acid undergoes several chemical reactions. Students should know the workings of each reaction well. The diagram below will link the reactions and products, helping students to better recall the chemical reactions undertaken by carboxylic acids.

Carboxylic acids are weak acids due to partial dissociation in water. However, they are more acidic than alcohols and phenols.

There are mainly 3 factors affecting the acid strength of a carboxylic acid:

Nature of R group	Number of substituents	Position of substituents
Electron donating substituents such as alkyl groups intensify the negative charge on the carboxylate ion This destabilizes the conjugate base, decreasing acidity Conversely, electron withdrawing substituents such as F and Cl disperses the negative charge on the carboxylate ion This stabilizes the conjugate base, increasing acidity	The greater the number of electron withdrawing substituent groups, the greater the extent of negative charge dispersal on RCOO⁻. The conjugate base is hence stabilised and the acid is more acidic Conversely, the greater the number of electron donating substituent groups, the more intensified the negative charge on RCOO⁻ is. Hence the conjugate base is destabilise and the acid is less acidic	The closer the electron withdrawing substituents are to the –COOH group, the more acidic the carboxylic acid is
Example: Cl-CH₂COOH is a stronger acid than CH₃-CH₂COOH due to the presence of Cl which is an electron withdrawing group	Example: CCl₃COOH is the most acidic, followed by CHCl₂COOH, then CH₂ClCOOH	Example: CH₃-CH₂-CHCl-COOH is a stronger acid than CH₂Cl-CH₂-CH₂-COOH as the electron withdrawing Cl is closer to the carboxylate ion in the former acid, dispersing the negative charge to a greater extent, causing CH₃-CH₂-CHCl-COOH to be a stronger acid

Let's look at a worked example below:

Worked Example 1:

When compounds W, X and Z are added to separate portions of water, solutions are formed with pH 0.5, 2.5 and 3.0 (not exactly in that order). When aqueous silver nitrate is added to these three solutions, two show no reaction but the third one produces a thick white precipitate.

CH₃CO₂H
W

CH₂ClCO₂H
X

CHCl₂CO₂H
Y

CH₃COCl
Z

(i) Suggest, with explanations, which pH value is associated with each of W, X and Z. Explain the formation of the white precipitate.

Answer: pH of W is 3.0, X is 2.5, Z is 0.5.

For Z, the presence of the electron withdrawing Cl group bonded directly to the C atom makes the C atom more electron deficient. Hence Z is the most reactive and likely to give up a proton, the C-Cl bond undergoes hydrolysis in water to form HCl, a strong acid. The white ppt of AgCl is formed when AgNO₃ is added.

W and X are both R-COOH, hence are weakly acidic and have a higher pH than Z

For X, the presence of an electron withdrawing Cl group further disperses the negative charge
on the ion of X, hence stabilizing the conjugate base further. POE of X’s dissociate lies further to
the right and it is more acidic than W.

(ii) Predict with a reason the likely pH value of an aqueous solution of compound Y.

Answer: 2.0

Y had one more Cl atom than X. The presence of another electron withdrawing group further disperses the negative charge on Y’s ion, hence stabilising it's conjugate base further. POE of Y’s dissociation lies further to the right than that of X, thus it is more acidic than X.

Reactions of Alcohol, Phenol and Carboxylic Acid: (Recall this summary table)

	Na	NaOH	Na₂CO₃
Alcohol	✔	✖	✖
Phenol	✔	✔	✖
Carboxylic Acid	✔	✔	✔

Now, let’s look at a worked example below:

Worked Example 2:

Suggest three distinguishing tests to differentiate between:

Each test must only react positively with one of the compounds.
You should also state what you would observe for each compound in each test.

Solution:

Firstly, compare and note down the differences between the 3 compounds. Recall the distinguishing tests that are unique to deduce certain functional groups attached to a compound. Tip: Drawing out the structural form of A would help you to visualise better and identify the functional groups present clearly.

Reagents & Conditions	Observations for A	Observations for B	Observations for C
Na₂CO₃ (aq)	No effervescence is observed.	No effervescence is observed.	Effervescence is observed. Colourless gas evolved forms white ppt with Ca(OH)₂ (aq). C is confirmed.
Br₂ (aq) OR neutral aqueous FeCl₃	Orange solution remained. OR No violet colouration.	Orange solution decolourised. B is confirmed. OR Violet colouration is observed.	Orange solution remained. OR No violet colouration
I₂ , NaOH (aq), warm OR K₂Cr₂O₇ (aq) + H₂SO₄ (aq), heat	Yellow ppt formed. A is confirmed. OR Orange dichromate solution turned green.	No yellow ppt. OR Orange dichromate solution remained.	No yellow ppt. OR Orange dichromate solution remained.

We have previously provided a diagram detailing the chemical reactions of carboxylic acids. Acyl chlorides similarly has various reactions it can take part in. The diagram below shows such:

Hydrolysis of Acyl Chlorides

The following is a concise summary of the hydrolysis reagents & conditions, reasons for rate of hydrolysis of the various types of organic compounds, and the different distinguishing tests to differentiate the organic compounds from each other.

Gist of Hydrolysis process: Breaking of C-Cl bond

	Acyl Chloride	Alkyl Chloride	Aryl Chloride
Hydrolysis Reagents & Conditions	H₂O(l)	NaOH(aq) or KOH(aq), heat under reflux	Hydrolysis cannot take place. However, if higher pressure & temperature are present, it is possible.
Rate of Hydrolysis
Explanation for the rate of hydrolysis	C=O carbon is bonded to 2 highly electronegative atoms O and Cl. There is a higher 𝛿+ charge on C, hence making it more susceptible to attack by nucleophiles.	C-Cl carbon is bonded to only 1 highly electronegative atom, Cl. There is a lower 𝛿+ charge on C hence making it less susceptible to attack by nucleophiles (as compared to RCOCl).	C-Cl bond will not break. Lone pair electrons on Cl atom will delocalise into the benzene ring, resulting in partial double bond character in C-Cl bond. The C-Cl bond is now more difficult to cleave. There is also steric hindrance with the nucleophiles as the bulky benzene ring hinders nucleophilic attack.
Distinguishing test with AgNO₃ , at rtp	White precipitate (AgCl) will form immediately	No precipitate is observed.	No precipitate is observed.

Esters

Last but not least, esters are sweet-smelling compounds that can be formed from both carboxylic acids and acyl chlorides. The main reaction undertaken by esters is hydrolysis (be it acidic or alkaline). The diagram below will sum up the various hydrolysis reactions: