Question Set A

cbse 12th Standard Chemistry Aldehydes , Ketones and Carboxylic Acids Important Questions And Answers Study Material - QB365

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Questions:

Generic 3

3 x 3 = 9

Write the reaction and conditions for the following conversions:
(i) Benzoyl chloride benzaldehyde
(ii) Acetaldehyde to crotonaldehyde

Suggest the reagents to bring about the following conversion:
(i) Hexan -l- ol to hexanal
(ii) Cyclohexanol to cyclohexanone
(iii) 4-Fluorotoluene to 4-fluorobenzaldehyde
(iv) Ethanenitrile to ethanal
(v) Allyl alcohol to propenal
(vi) But-2-ene to ethanal

Two moles of organic compound 'A' one treatment with a strong base gives two compounds B and C. Compound 'B' on dehydrogenation with Cu gives 'A' while acidification of 'C' yields carboxylic acid 'D' having molecular formula of CH₂O₂. Identify the compounds A, B, C and D.

Generic 5

3 x 5 = 15

(i) Write structures of A, B, C and D in the following reaction sequence:

(ii) Arrange the following compounds in the increasing order of their boiling points:
\(\begin{equation} \mathrm{CH}_{3} \mathrm{CHO}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{COOH} \end{equation}\)

Write the products of the following reactions:
(i) \(2C_{ 6 }H_{ 3 }CHO+conc\quad NaOH\longrightarrow \)
(ii) \(CH_{ 3 }COOH\underrightarrow { Cl_{ 2 }/p } \)
(b) Give, simple chemical tests to distinguish between the following pairs of compounds:
(i) Benzaldehyde and benzoic acid
(il) Propanal and propanone.

(a) Describe the following giving linked chemical equations:
(i) Cannizzaro reaction
(ii) Decarboxylation
(b) Complete the following chemical equations:

(iii) \(C_6H_5CONH_2 \xrightarrow [heat] {H_3O^+}\)

Multiple Choice Question

10 x 1 = 10

Which of the following is most reactive in nucleophilic addition reactions?

(a)

HCHO

(b)

CH₃CHO

(c)

CH₃COCH₃

(d)

CH₃COC₂H₅

Compounds A and C in the following reaction are

(a)

identical

(b)

positional isomers

(c)

functional isomers

(d)

optical isomers

Acetophenone when reacted with base C₂H₅ -ONa, yields a stable product:

(a)

(b)

(c)

(d)

10.

\(\mathrm{CH}_{3} \mathrm{COOH} \stackrel{\mathrm{SOCl}_{2}}{\longrightarrow} \ ^{\prime} \mathrm{A}^{\prime} \frac{\text { Benzene }}{\mathrm{Anhy} \mathrm{AlCl}_{3}}> ^{\prime} \mathrm{B}^{\prime} \stackrel{\mathrm{HCN}}{\longrightarrow} \ ^{\prime} \mathrm{C} ^{\prime} \stackrel{\mathrm{HOH}}{\longrightarrow} \ ^{\prime} \mathrm{D} ^{\prime}\)
The product 'D' is:

(a)

(b)

(c)

(d)

11.

Trichloroacetaldehyde was subjected to Cannizaro's reaction by using NaOH. The mixture and another products contains sodium trichloroacetate and other compound. The other compound is

(a)

trichloromethanol

(b)

2,2,2-trichloropropanol

(c)

chloroform

(d)

2,2,2-trichloromethanol

12.

A compound of molecular formula CH 100% (A) reacts with Tollens' reagent to give silver mirror but does not undergo aldol condensation. The compound A is

(a)

3-pentanone

(b)

2,2-dimethylpropanal

(c)

3-hydroxy-2-pentene

(d)

3-methyllbutanone

13.

For making distination between 2-pentanone and 3-pentanone, the reagent employed is

(a)

K₂Cr₂O₇/H₂SO₄

(b)

Zn-Hg/HCI

(c)

SeO₂

(d)

Iodine/NaOH

14.

CH₃CHO and C₆H₅CH₂can be distinguished chemically by

(a)

Benedict's rect

(b)

Iodoform test

(c)

Tollens'reagent test

(d)

Fehling's solution test

15.

C₃H₅O did not give a silver mirror with Tollens' reagent, but give an oxmic with hydroxmine. reagent, but gave an oxmie with hydroxylamine. It can give possitive

(a)

iodoform test

(b)

Fehling's test

(c)

Schiff's test

(d)

Carbylamine test

16.

Acetic acid is treated with Ca(OH)2 and the product so obtained is subject to dry distillation. The final product is

(a)

propanal

(b)

ethanol

(c)

ethanal

(d)

propanone

Generic 2

3 x 2 = 6

17.

Give the simple chemical test to distinguish between ethanal and propanal.

18.

Draw the structure of p-methylbenzaldehyde molecule.

19.

Draw the molecular structure of the compound 4-methylpent-3-en-2-one.

Assertion and reason

4 x 1 = 4

20.

Assertion: Benzaldehyde resists nucleophilic addition in comparison with Ethanal
Reason: The Phenyl group in benzaldehyde is too bulky and therefore prevents the attack of Nucleophile.
Codes:
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but the reason is not the correct explanation of the assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
(e) If the assertion and reason both are false

21.

Assertion: Acetaldehyde on treatment with alkaline gives aldol.
Reason: Acetaldehyde molecules contain a hydrogen atom.
Codes:
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but the reason is not the correct explanation of the assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
(e) If the assertion and reason both are false

22.

Assertion: Ketones can be converted into acids by haloform reaction.
Reason: Addition of Grignard reagents to dry ice followed by hydrolysis gives ketones.
Codes:
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

23.

Assertion: Fluoroacetic acid is stronger than chloroacetic acid.
Reason: Carboxylic acids are weak acids and turn blue litmus red.
Codes:
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

Case Study Questions

2 x 4 = 8

24.

Study the table in whick pka of various is given. Answer the questions based on table and related studied concepts.

Compounds	pka
Acetic acid	4.76
HCOOH	3.75
CH₃CH₂COOH	4.38
CICH₂COOH	2.87
FCH₂COOH	2.59
CH₂ = CH-COOH	4.30
Benzoic acid	4.19
p- Toluic acid	4.38
Salicylic acid	2.98
p-nitro benzoic acid	3.44
p-methoxy benzoic acid	4.88
p-chloro benzoic acid	3.99
p-hydroxy benzoic acid	4.58

(a) Why is HCOOH stronger than CH₃ COOH?
(b) Why is p-hydroxy benzoic acid more acidic than p-methoxy benzoic acid?
(c) Why is salicylic acid stronger than benzoic acid?
(d) What happens when salicylic acid is heated with zinc dust?
(e) Give one use of salicylic acid.

25.

Read the passage given below and answer the following questions:
Aldehydes and ketones undergo nucleophilic addition reactions .

Carbonyl carbon is electron deficient hence acts as an electrophile. Nucleophile attacks on the electrophilic carbon atom of the carbonyl group from a direction perpendicular to the plane of the molecule.

In this process, hybridisation of carbon atom changes from sp² to sp³ and a tetrahedral alkoxide ion is formed as intermediate. This intermediate captures proton from the reaction medium to give the neutral product. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions.
In these questions ( i - iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: Benzaldehyde is more reactive than ethanal towards nucleophilic attack.
Reason: The overall effect of -land +R effect of phenyl group decreases the electron density on the carbon atom of ＞C=O group in benzaldehyde
(ii) Assertion: (CH₃)₃CCOC(CH₃)₃ and acetone can be distinguished by the reaction with NaHSO₃.
Reason: HSO₃ is the nucleophile in bisulphite addition.
(iii) Assertion: Ease of nucleophilic addition of the compounds (I). CH₃CHO(II) and CH₃COCH₃(III) is I > II > III.
Reason : Aldehydes and ketones undergo nucleophilic addition reactions.
(iv) Assertion: is more reactive towards nucleophilic addition reaction than
Reason : Reactivity of carbonyl group is due to electrophilic nature of carbonyl carbon.

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Answers Key:

Generic 3

3 x 3 = 9

\((i)\ { C }_{ 5 }{ H }_{ 5 }{ NH }^{ + }{ CrO }_{ 3 }{ Cl }^{ - }(pcc)\ in\ { CH }_{ 2 }{ Cl }_{ 2 }\)
\( (ii)\ { K }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 }\ in\ acidic\ medium\)
\((iii)\ Cr{ O }_{ 2 }\ in\ the\ presence\ of\ acetic\ anhydride\ (273-283K)\ or\ Cr{ O }_{ 2 }{ Cl }_{ 2 }, { H }_{ 2 }O\)
\((iv)\ DIBALH\:\ (Diisobutyl)\ aluminium\ hydride, 78°C,{ H }_{ 2 }O\)
\((v)\ PCC\ in\ { CH }_{ 2 }{ Cl }_{ 2 }\)
\((vi)\ { O }_{ 3 }/{ H }_{ 2 }O-Zn\ dust\)

Generic 5

3 x 5 = 15

(ii) The correct increasing order of the boiling points is :
\(\begin{equation} \begin{array}{r} \mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{3}<\mathrm{CH}_{3} \mathrm{CHO}<\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} <\mathrm{CH}_{3} \mathrm{COOH} \end{array} \end{equation}\)

(a) Benzaldehyde and benzoic acid NaHCO₃ test Benzoic acid being an acid reacts with NaHCO₃ solution, to produce brisk effervescence due to evolution of CO₂ while benzaldehyde does not give this test.

(b) Propanal and propanone Propanone responds to iodoform test whereas propanal (CH₃CH₂CHO) does not due to the absence of CH₃CO- group.

(a) (i) Cannizzaro's reaction:
When aldehydes which do not have \(\alpha\)-hydrogen gets oxidised as well as reduced in presence of conc. alkali, it is called Cannizzaro's reaction.
\(\underset { Methanal }{ 2HCHO } \overset { 50%NaOH }{ \longrightarrow } \underset { Sodium\quad Methanoate }{ HCOONa } +\underset { Methanol }{ CH_{ 3 }OH } \)
(ii) Decarboxylation : Sodium salts of acids when heated with soda lime, lower alkanes are formed
\(\underset { Sodium\quad acetate }{ CH_{ 3 }COONa } +\underset { Sodalime }{ NaOH } \longrightarrow \underset { Methane }{ CH_{ 4 } } +\underset { sodium\quad carbonate }{ Na_{ 2 }CO_{ 3 } } \)

Multiple Choice Question

10 x 1 = 10

(a)
HCHO

(b)
positional isomers

10.

11.

(d)
2,2,2-trichloromethanol

12.

(b)
2,2-dimethylpropanal

13.

(d)
Iodine/NaOH

14.

(b)
Iodoform test

15.

(a)
iodoform test

16.

(d)
propanone

Generic 2

3 x 2 = 6

17.

Iodoform test Ethanal, because of the presence of CH₃CO-skeleton gives iodoform test whereas propanal due to the absence of such a skeleton does not.

18.

19.

Assertion and reason

4 x 1 = 4

20.

21.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

22.

(c): Addition of Grignard reagents to dry ice followed by hydrolysis gives carboxylic acid not ketone.

23.

(b) : The -I effect of F is greater than inductive effect of Cl and thus fluoroacetate ion is more stabilized to show more tendency to lose proton by its acid.

Case Study Questions

2 x 4 = 8

24.

(a) It is because HCOO^- is more stable than CH₃COO^-.
(b) It is because -OH has more -I effect than -OCH₃ group.
(c) It is because salicylate is more stable than benzoate ion due to intra molecular H-bonding.
(d)

(e) It is used for preparation of Aspirin which is an important Analgesics, Antipyretic and prevents heart attack.

25.

(i) (a)
(ii) (b): \(\mathrm{HSO}_{3}^{-}\) is a bulky nucleophile, hence, cannot attack on sterically hindered ketones.
(iii) (d): Aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues towards nucleophilic addition reactions due to the +R effect of benzene ring. Further, aldehydes are more reactive than ketones due to +I effect and steric effect of alkyl group. Therefore, the ease of nucleophilic addition will follow the order:

(iv) (b): Electron withdrawing group (-NO₂) increases the reactivity towards nucleophilic addition reactions, whereas electron donating group (-CH₃) decreases the reactivity towards nucleophilic addition reactions.