Chemistry Objective Questions and Answers for Class 12 on “Carboxylic Acids Chemical Reactions – 2”.
1. How many molecules of acetic acid react with H2SO4 on heating to give one molecule of acetic anhydride?
a) 1
b) 2
c) 3
d) 4
Answer: b
Clarification: Two molecules combine with the loss of one water molecule (H atom from one molecule and OH group from another molecule) when heated with dehydrating agents like H2SO4 to give acetic anhydride.
2. Benzoic acid reacts with phosphorus pentaoxide in the presence of heat to give ________
a) C6H5OC6H5
b) C6H5COC6H5
c) C6H5COOC6H5
d) C6H5COOCOC6H5
Answer: d
Clarification: Benzoic acid on heating with a strong dehydrating agent like P2O5 forms its corresponding anhydride, i.e., benzoic anhydride, which consists of two C6H5C=O groups bridged by a (-O-) group.
3. Esterification of carboxylic acids is a/an __________ reaction.
a) irreversible
b) nucleophilic addition
c) electrophilic substitution
d) nucleophilic substitution
Answer: d
Clarification: When carboxylic acids are heated with alcohols with suitable catalyst, esters are formed. This is a reversible reaction due to all the individual steps involved in the mechanism of this reaction are reversible in nature. It is a type of nucleophilic acyl substitution reaction.
4. Which of the following is the most suitable catalyst for the esterification of acetic acid with ethanol?
a) P2O5
b) HCl gas
c) KMnO4-KOH
d) Pyridine
Answer: b
Clarification: HCl gas and concentrated H2SO4 are catalyst which provide a proton for the protonic attack on the carbonyl oxygen of acetic acid, which further activates the carbonyl groups towards nucleophilic addition of ethanol, which ultimately results in the formation of ethyl acetate, which is an ester.
5. Which of the following is used to shift the esterification reaction of carboxylic acids towards the right?
a) Using excess of carboxylic acid
b) Using excess of acid catalyst
c) Removal of water by distillation
d) Removal of ester by distillation
Answer: c
Clarification: Since the esterification of carboxylic acids is a reversible reaction, it can be shifted towards the right by using excess of alcohol or by the removal the water formed by distillation.
6. Which of the following carboxylic acids is most reactive towards esterification with given alcohol?
a) HCOOH
b) CH3COOH
c) (CH3)2CHCOOH
d) (CH3)3CCOOH
Answer: a
Clarification: The rate at which carboxylic acids are esterified depends upon the stearic hinderance and to some extent the acidic strength of the acid, as it is a nucleophilic substitution reaction. Since formic acid has the lowest molecular mass and smallest group, it is the most reactive.
7. Which is the most preferred reagent in the product of acetyl chloride from acetic acid?
a) Cl2
b) PCl3
c) PCl5
d) SOCl2
Answer: d
Clarification: Thionyl chloride reacts with acetic acid to form acetyl chloride along with two gaseous compounds (SO2 and HCl) that escape the reaction mixture, making the purification of the product easier.
8. Benzoic acid reacts with phosphorus pentachloride to give ________
a) chlorobenzene
b) benzyl chloride
c) benzoyl chloride
d) chlorobenzoic acid
Answer: c
Clarification: The hydroxyl group (OH) of benzoic acid id easily replaced by chlorine atom on treatment with PCl5. This results in the formation of C6H5COCl, or benzoyl chloride.
9. Other than ethanoyl chloride, what are the by-products formed when ethanoic acid reacts with PCl3?
a) H3PO3
b) H3PO3 and HCl
c) H3PO2 and HCl
d) POCl3 and HCl
Answer: a
Clarification: Three molecules of ethanoic acid react with one molecule of PCl3 to give three molecules of ethanoyl chloride and one molecule of phosphorous acid (H3PO3).
10. Acetic acid behaves exactly like ethanol on reaction with thionyl chloride.
a) True
b) False
Answer: a
Clarification: Acetic acid as well as ethanol consist of an OH group and reaction with SOCl2 results in the cleavage of C-OH bond with the hydroxyl group to be replaced by Cl atom. The only difference is that acetic acid forms acetyl chloride, whereas ethanal forms ethyl chloride.
Chemistry Objective Questions and Answers for Class 12,