Question

What compounds are used in the reaction of ester preparation?
(A) An organic acid and an aldehyde
(B) An alcohol and an aldehyde
(C) An acid and a ketone
(D) Two alcohols
(E) Alcohol and organic acid.

Answer 1

Hint: First we will define organic acids, later we will define alcohols, aldehydes, ketones and esters. Then we will see the ways of methods of preparation of ester and then select the correct answer. We can also eliminate options.

Complete answer:
Organic acid: An organic acid is a substance that has acidic characteristics. The carboxylic acids, whose acidity is linked to their carboxyl group $ -COOH $ , are the most common organic acids. Sulfonic acids are more powerful acids.
Aldehydes: The oxidation of alcohols produces an organic molecule with the $ --CHO $ group. Methanal (formaldehyde) and ethanal are two common aldehydes (acetaldehyde).
Ketones: An organic molecule formed by oxidizing secondary alcohols and contains a carbonyl group $ = C = O $ bonded to two hydrocarbon groups. Acetone is the most basic of these compounds.
Now lets answer this question:
Alcohol and organic acid are used in the reaction of ester preparation. For example, ethyl acetate is prepared by the reaction of acetic acid with ethanol in presence of concentrated sulphuric acid or dry $ HCl $ gas.
Look at the reaction below:
 $ C{H_3}COOH + C{H_3}C{H_2}OH\xrightarrow[{DryHClgas}]{{conc.{H_2}S{O_4}}}C{H_3}COO{C_2}{H_5} $
Hence we see that ester is formed when carboxylic acid reacts with an alcohol.
Hence the correct answer to this question is option E.

Note:
You can also answer this question by eliminating options. Look at option C, clearly it wont give an ester. Next we know that hemiacetals and hemiketals are formed when alcohols react reversibly with aldehydes and ketones. for the reaction of ketones with acid This reaction is catalysed by small quantities of acids and bases. Hence options A and E remain. No specific product forms when organic acid reacts with aldehydes hence by correct observation you can say that the correct answer is option E.