Organic MCQs with Explanatory Answers

 

1. The acid which reduces Fehling’s solution is

(a) Methanoic acid 

(b) Ethanoic acid 

(c) Butanoic acid 

(d) Propanoic acid

Explanation; (Answer; (a)

Methanoic acid resembles with aldehyde due to its structure. So it reduces Fehling reagent.

Formic acid has -CHO group and therefore it reduces Fehling solution.

 

2.Which one is the strongest acid among following options?

(a) FCH₂COOH     

(b) Cl2CH₂COOH                   

(c) ClCH₂COOH     

(d) F₂CH2COOH

Explanation (Answer; (d)

Difluoroacetic acid is the strongest acid among given options as presence of two F atoms increases its acidic nature due to greater inductive effects.

The strength of the acid increases with the negative inductive effect of the electronegative group attached to it (-I effect) and decreases with the presence of the positive inductive effect of electrons releasing group (+I effect). More the number of electronegative atoms attached, more the acidic character of that substituted acid.

The stronger the electron withdrawing group the greater is the strength of the substituted alpha carboxylic acid. Since F is most electronegative sand strongest electron withdrawing group, therefore F substituted alpha carboxylic acid is stronger ac compared to clhlro, bromo or iodo substituted alpha carboxylic acids.

3. Which of the following forces explain the boiling point of aldehydes and ketones?

(a) van der Waal’s forces
(b) Dipole-dipole attraction
(c) Hydrogen bonding
(d) debye force

Explanation

Both aldehydes and ketones are polar compounds showing dipole-dipole interaction which is responsible for their high boiling and melting points as compared to alkanes.


4. The organic reaction which proceeds through heterolytic bond cleavage are called
(a) Ionic
(b) Polar
(c) Non-polar
(d) both (a) and (b)

Explanation; (d)

The organic reaction which proceed through heterolytic bond cleavage are called ionic or polar or heteropolar type. (It is electrophilic, Nucleophilic and elimination type reaction).

the organic reaction, which proceed by homolytic fission are called free radical or homopolar or non-polar reactions

 

5. The shape of methyl carbanion is similar to that of

(a) BF3

(b) NH3 

(c) Methyl free radical 

(d) methyl carbocation

Explanation; (Answer; b)

The negatively charged carbon atom in Methyl carbanion (CH₃⁻) is sp³ hybridized , with three bond pairs and one lone pair of electrons, giving it to pyramidal (distorted tetrahedral) shape same is that of NH₃.

6. Metamers of ethyl propionate are

(a) C₄H₉COOH and HCOOC₄H₉ 

(b) C₄H₉COOH and CH₃COOC₃H₇

(c) CH₃COOCH₃ and CH₃COOC₃H₇ 

(d) CH₃COOC₃H₇ and C₃H₇COOCH₃  

Explanation; (Answer; d)

Metamers have different type of alkyl group around polyvalent functional group. In option d, ester group (-COO-) is linked with different number of carbons on its either side. 

7. Isomers of propionic acid are

(a)HCOOC₂H₅ and CH₃COOCH₃ 

(b) HCOOCC₂H₅ and C₃H₇COOH 

(c) CH₃COOCH₃ and C₃H₇OH 

(d) C₃H₇OH and CH₃COOCH₃

Explanation; (Answer; a)

Propionic acid is isomeric with 3 C ester which has two metamers. 

8. For which of the following parameters the structural isomers C₂H₅OH and CH₃OCH₃ would be expected to have the same values? (assume ideal behaviour)

(a) Gaseous densities at the same T and P                 

(b) Vapour pressure at the same temperature

(c) Heat of vapourization                       

(d) Boiling points 

Explanation; (Answer; a)

Since the molecular weight of C₂​H₅​OH & CH₃​OCH₃​ are same so in its vapour phase at same temperature & pressure the densities will be same.

Vapour density = (Molecular weight)/2. As both the compounds have same molecular weights, both will have the same vapour density. Hence, gaseous density of both ethanol and dimethyl ether would be same under identical conditions of temperature and pressure. The rest of these three properties; vapour pressure, boiling point and heat of vaporization will differ as ethanol has hydrogen bonding whereas ether does not.

9. The number of primary, secondary and tertiary carbons in 3,4-dimethylheptane are respectively

(a) 4, 3 and 2 

(b) 2,3, and 4 

(c) 4,2 and 3 

(d) 3,4 and 2

Explanation; (Answer; a)
In 3,4-dimethylheptane there are 4 primary, 3 secondary and 2 tertiary carbons. It has no quaternary carbon

10. The number of primary, secondary, tertiary and quaternary carbons in neopentane are respectively

(a) 4, 3, 2 and1 

(b) 5,0, 0 and 1 

(c) 4,0,0 and 1 

(d) 4,0, 1 and 1

Explanation; (Answer; c)

In neopentane, there are 4 primary and one quaternary carbon. It has no secondary and tertiary carbons.

11. In which of the following, functional isomerism is not possible?

(a) Cyanides 

(b) haloalkane 

(c) alcohols 

(d) esters

Explanation; (Answer; b)

12. Which are isomers?

(a) ethyl alcohol and dimethyl ether 

(b) Acetone and propionaldehyde 

(c) Glucose and fructose 

 (d) all of them

Explanation; (Answer; d)

13. The compounds CH₃CH=CHCH₃ and CH₃CH₂CH=CH₂

(a) contain same of sp³-sp³, sp³-sp² and sp²-sp² C-C bonds 

(b) exist together in dynamic equilibrium

(c) are position isomers 

(d) are tautomers  

Explanation; (Answer; c)

Both compounds differ in the position of double bond so they are position isomers. 

14. Which of the following carbocation is least stable?

(a) tert-alkyl 

(b)sec-alkyl  

(c) pri-alkyl 

(d) methyl 

Explanation; (Answer; d)

15. Benzoic acid when treated with PCl₅​, the product is:

(a) Benzoyl chloride 

(b)  Benzoyl amide 

(c) Benzoyl anhydride 

(d) None of them

Explanation; (Answer; a)

16. Which class of compounds shows H-bonding even more than in alcohols?

(a) Phenols 

(b) Carboxylic acid 

(c) Aldehydes 

(d) Ethers

Explanation; (Answer; b)

Forms H-bonding be means of two highly electronegative atoms present in it

 

17. In CH₃COOH and HCOOH, HCOOH will be which of the following

(b)More acidic 

(b)Less acidic 

(b)Equally acidic 

(b)None of them

Explanation; (Answer; a)

Presence of methyl group decreases the acidic character of acetic acid due to positive inductive effect (+I)

18. The total number of isomers for C4H8 is
(a) 5
(b) 6
(c) 7
(d) 8

Explanation; (Answer; b)

Five constitutional isomers are possible with C₄H₈.

The five constitutional isomers include:
1) But-1-ene
2) But-2-ene
3) 2-Methylpropene
4) Cyclobutane
5) Methylcyclopropane

But-2-ene exists as two geometrical isomers namely cis and trans

Hence total isomers with this molecular formula are 6. 

19. Which of the following forces explain the boiling point of aldehydes and ketones?

(a) van der Waal’s forces 

(b) Dipole-dipole attraction 

(c) Hydrogen bonding 

(d) debye force

Explanation (Answer; b)

Both aldehydes and ketones are polar compounds showing dipole-dipole interaction which is responsible for their high boiling and melting points as compared to alkanes.

20.With the increase in carbon number, the solubility of an alcohol:

(a) Increases 

 (b) Remains unaffected 

 (c) Decreases 

 (d) None of these

Explanation; (Answer; c)

Alcohols are soluble in water. This is due to the hydroxyl group in the alcohol which is able to form hydrogen bonds with water molecules. Alcohols with a smaller hydrocarbon chain are very soluble. As the length of the hydrocarbon chain increases, the solubility in water decreases.

The reason why the solubility decreases as the length of hydrocarbon chain increases is because it is requires more energy to overcome the hydrogen bonds between the alcohol molecules as the molecules are more tightly packed together as the size and mass increases.

21. The boiling point of alcohols are higher than the corresponding alkanes. This is because:

(a) Alkanes are dipolar compounds

(b) Alcohols are soluble in water

(c) Alcohols are sweet in taste 

(d) of hydrogen bonding existing between alcohol molecules 

Explanation; (Answer; d)

The boiling point of an alcohol is always much higher than the boiling point of the corresponding alkane with the same hydrocarbon chain. The boiling point of alcohols increase as the length of hydrocarbon chain increases.

The reason why alcohols have a higher boiling point than alkanes is because the intermolecular forces of alcohols are hydrogen bonds, unlike alkanes with van der Waals forces as their intermolecular forces.

22. Phenylmagnesium bromide reacts with solid dry ice followed by acid-hydrolysis to form
(a) Bromobenzene 

(b) Acetophenone 

(c) Benzoic acid 

(d) Phenol

Explanation; (Answer; c)

23. Which acid of the following is not a fatty acid? 

(a) Propanoic acid 

(b) Phthalic acid 

 (c) Butanoic acid 

(d) Acetic acid

Explanation; (Answer; b)

Aliphatic monocarboxylic acids are called fatty acids. Phthalic acid is aromatic dicarboxylic acid, so it is not considered as fatty acid. 

23. In the following reaction, the product R is

(a) Benzene 

(b) Toluene 

(c) Chlorobenzene 

(d) Xylene

Explanation; (Answer; b)

24. Which of the following will undergo meta-substitution on mono-chlorination?

(a) Phenol 

(b) Ethoxyethane 

(c) Ethylbenzoate 

(d) Chlorobenzene

Explanation; (Answer; c)

25. The boiling point of alcohols are higher than the corresponding alkanes. This is because:

(a) of hydrogen bonding existing between alcohol molecules 

(b) Alcohols are soluble in water

(c) Alcohols are sweet in taste 

(d) Alkanes are dipolar compounds

Explanation; (Answer; a)

the boiling point of an alcohol is always much higher than the boiling point of the corresponding alkane with the same hydrocarbon chain. The boiling point of alcohols increase as the length of hydrocarbon chain increases.

The reason why alcohols have a higher boiling point than alkanes is because the intermolecular forces of alcohols are hydrogen bonds, unlike alkanes with van der Waals forces as their intermolecular forces.

26. Which of the following reacts immediately with conc HCl in the presence of Zinc Chloride?

(a) Primary Alcohol
(b) Secondary Alcohol 

(c) Tertiary Alcohol 

(d) Ether

Explanation; (Answer; c)

The reagent, a mixture of conc. HCl and anhydrous ZnCl₂ is called Lucas reagent, which is used to distinguish between 1^o, 2^o and 3^o alcohols. The test is called Lucas test.

Note: Lucas reagent is required for primary and secondary alcohol only.

Primary alcohols react with Lucas reagent very slowly on heating to give alkyl chlorides.
Secondary alcohols react with Lucas reagent much faster.
Tertiary alcohols instantaneously react with HCl at room temperature.

3^o alcohol + Lucas reagent → Immediate turbidity (cloudiness).

2^o alcohol + Lucas reagent → Turbidity (cloudiness) after 5 minutes.

1^o alcohol + Lucas reagent → No reaction or cloudiness appears on heating

27. The percentage of s-character of the hybrid orbitals in ethane, ethene and ethyne are respectively:

(a) 50, 75, 100 

(b) 10,20,40 

(c) 25,33,50 

(d) 25, 50, 75

Explanation; (Answer; c)

28. Which of the following have incorrect molecular formula?

(I)     Icosane – C₁₀H₂₂

(II)   Triacontane – C₃₀H₆₂

(III)  Nonane – C₉H₂₀

(IV)  Dodecane – C₁₂H₂₄

(a) I and IV 

(b) IV only 

(c) II and IV 

(d) II only

Explanation; (Answer; d)

Dodecane – C₁₂H₂₆

(being an alkane follows general formula C_nH_2n+2

29. The compound which has one isopropyl group is

(a) 2,2,3,3-tetramethylpentane 

(b) 2,2,-diemthylpentane 

(c) 2,2,3-trimehtylpentane 

(d) 2-methylpentane

Explanation; (Answer; d)

2-methylalkanes always have isopropyl group at the end of chain. 

30. 

(a) Kolbe’s reaction 

(b) Sandmayer reaction 

(c) Diel-Alder reaction 

(d) Friedel-Craft reaction

Explanation; (Answer; d)

The introduction of alkyl or acyl group in organic compound is called Friedel-Craft reaction

31. Decreasing order of C – C bond length in following compounds is

(I)  C₂H₄  (II) C₂H₂  (III)  C₆H₆   (IV) C₂H₆

(a) IV > III > I > II 

(b)  I >II >IV > III 

(c) II > I > IV > III 

(d) IV > I > III > II

Explanation; (Answer; a)

Decreasing order of C – C bond length follows the order 

Alkane > Benzene > Alkene > Alkyne

32.          

(a) 1-nitrochlorobenzene
(b) 3-nitrochlorobenzene
(c) 4-nitrochlorobenzene
(d) 1,2-dinitrochlorobenzene

Explanation; (Answer; b)

Nitration of benzene gives nitrobenzene which on chlorination forms meta-nitrochlorobenzene as nitro group is meta directing group.

33. Which of the following statements is FALSE regarding the reaction between Cl2 and C2H6?

(a) It is a substitution reaction and the reaction mechanism involves free radicals
(b) The reaction will give a single product of C2H5Cl.
(c). The first step in the mechanism is the cleavage of the Cl-Cl bond to give chlorine atoms
(d) The reaction can be initiated with either sunlight or heat.

Explanation; (Answer; b)

The halogenation of alkanes gives a mixture of halogen derivatives. 

34. Which of the following compounds will not show geometrical isomerism?
(a) 3-Phenyl-2-propenoic acid 

(b) 3-Methyl-2-pentenoic acid 

(c) 3-Methyl-2-butenoic acid 

(d) 2-Butene

Explanation; (Answer; c)

Two same groups (Me) on C-3; hence do not show geometrical isomerism

35. How many moles of sodium hydroxide will react with one mole of the given compound?
(a) 1 

(b) 2 

(c)3 

(d) 4

Explanation; (Answer; a)

NaOH reacts with carboxylic acid to form Na salt of  carboxylate.
The given compound has only one -COOH group. Hence it requires only 1 mole of NaOH.

36. Which one of the following compounds will show geometrical isomerism?

Explanation; (Answer; a)

In 2-butene, cis and trans isomers can exist because a free rotation of the substituents is restricted by C = C.

Two methyl groups on the same side form the cis isomers – cis-2-butene

Two methyl groups on the opposite sides form the trans isomers – trans-2-butene

Hence, 2-butene exhibits geometrical isomerism.

In 2-methylpropene, cis and trans forms do not exist because two H atoms are attached to the same carbon atom. Similarly, two – CH3 groups are attached to another carbon atom. Hence 2-methylpropene does not exhibit geometrical isomerism.





In 1-butene, cis and trans forms do not exist because two H atoms are attached to the same carbon atom. Hence 1-butene does not exhibit geometrical isomerism.

37. How many moles of sodium hydroxide will react with one mole of:








(a) 5

(b) 4

(c) 3

(d) 2

Explanation; (Answer; d)

NaOH only reacts with COOH group of carboxylic acid. Since compound has two acidic COOH groups, so 2 mole of NaOH is needed to neutralize it.