Chapter 15: Carboxylic Acids & Derivatives
Carboxylic Acids & Derivatives
Firstly, let's take a look at some of the important physical
properties of both carboxylic acids and acyl chlorides.
|Carboxylic acid||(i) Carboxylic acids have higher boiling points than its corresponding alkanes
(ii) Carboxylic acids have higher boiling points than its corresponding alcohol
(iii) Boiling point increases with increasing length of alkyl chain of the carboxylic acid
|(i) The first four members of the aliphatic acids are completely miscible in water
(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
||(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
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|
Cl-CH2COOH is a stronger acid than CH3-CH2COOH due to the presence of Cl which is an electron withdrawing group
CCl3COOH is the most acidic, followed by CHCl2COOH, then CH2ClCOOH
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.
(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.
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)
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.
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
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: