REASONING QUESTIONS IN ORGANIC CHEMISTRY
TEXT EXERCISE
1. Why is sulphuric acid is not used during reaction of alcohols with KI?
Ans: Sulphuric acid converts the KI into HI and then oxidizes into I2
2. Arrange each set of compounds in order of increasing boiling poings.
(i) Bromo Ethane , Bromoform, Chloro methane, Dibromo methane
(ii) 1-Chloro Propane, Isopropyl chloride, 1-chloro butane
Ans: Chloro Methane(CH3Cl) < Bromo Ethane(CH3-CH2-Br) < Dibromo
Mehane(CH2Br2) < Bromoform (CHBr3)
CH3CHCl-CH3 < CH3-CH2-CH2Cl < CH3-CH2-CH2-CH2-Cl
As atomic mass increases boiling point increases. And branching compounds
has less spherical contact as compare with straight chain compounds. Hence bp less
for branching chain alkyl halides.
3. Which of the following undergo SN2 faster?
(i) Cl CH2Cl
(i) I Cl
Ans: CH2Cl is more faster under go SN2 because it is Primary alkyl halide
I is more faster to under go SN2 mechanism because I is better
leaving group due to its large size.
4. Predict the order of reactivity of the following compounds in SN1 & SN2 reactions.
(i) The four isomeric bromo butanes
(ii) C6H5CH2Br, C6H5CH2 (C6H5 )Br, C6H5CH2 (CH3)Br, C6H5CH2 (CH3 )(C6H5 )Br
Ans: CH3-CH2-CH2-CH2-Br< CH3- CHCH3- CH2- Br < CH3-CH2-CHBr –CH3<
C(CH3)3Br order for SN1 mechanism
CH3-CH2-CH2-CH2-Br> CH3- CHCH3- CH2- Br > CH3-CH2-CHBr –CH3>
C(CH3)3Br order for SN2 mechanism
C6H5CH2Br< C6H5CH (CH3)Br< C6H5CH (C6H5 )Br< C6H5C (CH3 )(C6H5 )Br
Order for SN1 mechanism
C6H5CH2Br> C6H5CH (CH3)Br> C6H5CH (C6H5 )Br> C6H5C (CH3 )(C6H5 )Br
Order for SN2 mechanism
5. Although Chlorine is an electron with drawing group yet it is orho para directing in
electrophilic aromatic substitution reactions. Why?
Ans: Cl is electron with drawing by inductive effect, and it electron relesing by resonance
and creates negative charge at ortho and para positions. And here resonance effect
is more as compare with inductive effect.
6. Which alkyl halide from the following pairs would you expect to react more rapidly
by an SN2 mechanism? Explain your answer?
1. CH3-CH2-CH2-CH2-Br & CH3-CH2-CHBr –CH3
2. CH3-CH2-CHBr –CH3 & C(CH3)3Br
3. CH3- CHCH3- CH2- CH2- Br & CH3- CH2- CHCH3- CH2- Br
Ans: 1. CH3-CH2-CH2-CH2-Br primary alkyl halide is more reactive than secondary
alkyl halide
2. CH3-CH2-CHBr –CH3 Secondary alkyl halide is more reactive than Tertiary
alkyl halide
3. CH3- CHCH3- CH2- CH2- Br As methyl group is far from Br it will less stabilize
the carbo cations and facilitate the SN2 mechanism.
7. Which of the following compound go faster SN1 reaction:
1. &
Cl
2. & Cl
Ans: 1. first compound ismore stable because tertiary is more reactive than secondary
, alkyl halide for SN1 mechanism.
2. . first compound is more stable because Secondary is more reactive than
primary for SN1 mechanism.
8. Which one of the following has highest boiling point;
CH2Cl2 CHCl3 CCl4
Ans: net Dipole moment of CH2Cl2 & CCl4 is zero due to cancellation dipole moment of
bonds due to its symmetric structure. Where as net dipole moment of CHCl3 is not
zero. Because its structure is not symmetric.
9. Which compound in each of the following pairs react faster in SN2 reaction with OH?
(i) CH3Br or CH3I (ii) (CH3)3CCl or CH3Cl
Ans: CH3I is faster reactive by SN2 mechanism because C-I bond strength is less than
C-Br bond
CH3Cl is faster reactive than (CH3)3CCl by SN2 mechanism because 10 alkyl halide
more reactive by SN2 mechanism.
10. Halo alkanes react with KCN to form alkyl cyanides as main product while AgCN
forms isocyanides as the chief product. Explain.
Ans: KCN is predominantly ionic and provides cyanide ions in solution. Although both
carbon and nitrogen atoms are in a position to donate electron pairs. The attack
takes place mainly through carbon atom not through nitrogen atom since C—C is
more stable than C—N bond. However, AgCN is mainly covalent innatrue and
nitrogen is free to donate electron pair forming isocyanide as the main product.
11. Why does NO2 group show its effect only at ortho and para positions and not at meta
positions?
Ans: The presence of NO2 group at ortho and para positions with draws the electron
density form the benzene ring and thus facilitates the attact of the nucleophile on
halo arene. The carbanaion thus formed is stabilized through resonance. The
negative charge appeared at ortho and para positions with respect to the halogen
substituent is stabilized by NO2 group while in case of meta nitro benzene, none of
the resonating structures bear the negative charge on carbon atom bearing the NO2
group. There fore the presence of nitro group at meta position does not stabilize the
negative charge and no effect on reactivity isobserved by the presence of NO2 group
at meta position.
12. The dipole moment of chloro benzene is lower than that of cyclohexyl chloride.
Why?
Ans: It is due to electron with drawing effect of phenyl group which decrease dipole
moment of chloro benzene.
13. Alkyl halides, though polar, are immiscible with water. Explain.
Ans: Due to absence of hydrogen bond in alkyl halides with water.
14. Grignard reagent should be prepared under anhydrous conditios. Give reason.
Ans: Grignard reagent highly react with water to alkanes.
RMgX + H2O R—H + MgXOH
15. Vinyl chloride is less reactive towards nucleophilic substitution. Explain.
Ans: Due to resonance between C—Cl bond in vinyl chloride show partial double bond
character.
CH2=CH—::Cl:
16. Chloroform stored in dark colored bottles by completely filling it it. Explain.
Ans: Chloroform react with air in presence of sun light it forms poisonous phosogene.
2 CHCl3 + O2 2 COCl2+ 2 HCl
17. Out of C6H5CH2Cl & C6 H5CHCl C6H5 Which is more easily hydrolyzed by KOH?
Ans; The second comound is more resonance stabilized hence easily forms carbo cation
and react faster by SN1 mechanism.
18. The treatment of alkyl halides with aqueous KOH leads to the formation of alcohols
but in the presence of alcoholic KOH alkenes are major products. Explain
Ans: Substitution of alkyl halide gives alcohols and elimination gives alkenes and there is
a competition between substitution and elimination. In the presence of low polarity
solvent like alcohol favours elimination where as in the presence of high polarity
solvent like water substitution favored.
19. Bond angle of ethers is slightly greater than tetrahydral bond angle. Explain.
Ans: due to repulsion of bulky alkyl groups in ethers bond angles is slightly greater than
tetrahydral bond angle.
20. Arrange the following sets of compounds in order of their increasing
TEXT EXERCISE
1. Why is sulphuric acid is not used during reaction of alcohols with KI?
Ans: Sulphuric acid converts the KI into HI and then oxidizes into I2
2. Arrange each set of compounds in order of increasing boiling poings.
(i) Bromo Ethane , Bromoform, Chloro methane, Dibromo methane
(ii) 1-Chloro Propane, Isopropyl chloride, 1-chloro butane
Ans: Chloro Methane(CH3Cl) < Bromo Ethane(CH3-CH2-Br) < Dibromo
Mehane(CH2Br2) < Bromoform (CHBr3)
CH3CHCl-CH3 < CH3-CH2-CH2Cl < CH3-CH2-CH2-CH2-Cl
As atomic mass increases boiling point increases. And branching compounds
has less spherical contact as compare with straight chain compounds. Hence bp less
for branching chain alkyl halides.
3. Which of the following undergo SN2 faster?
(i) Cl CH2Cl
(i) I Cl
Ans: CH2Cl is more faster under go SN2 because it is Primary alkyl halide
I is more faster to under go SN2 mechanism because I is better
leaving group due to its large size.
4. Predict the order of reactivity of the following compounds in SN1 & SN2 reactions.
(i) The four isomeric bromo butanes
(ii) C6H5CH2Br, C6H5CH2 (C6H5 )Br, C6H5CH2 (CH3)Br, C6H5CH2 (CH3 )(C6H5 )Br
Ans: CH3-CH2-CH2-CH2-Br< CH3- CHCH3- CH2- Br < CH3-CH2-CHBr –CH3<
C(CH3)3Br order for SN1 mechanism
CH3-CH2-CH2-CH2-Br> CH3- CHCH3- CH2- Br > CH3-CH2-CHBr –CH3>
C(CH3)3Br order for SN2 mechanism
C6H5CH2Br< C6H5CH (CH3)Br< C6H5CH (C6H5 )Br< C6H5C (CH3 )(C6H5 )Br
Order for SN1 mechanism
C6H5CH2Br> C6H5CH (CH3)Br> C6H5CH (C6H5 )Br> C6H5C (CH3 )(C6H5 )Br
Order for SN2 mechanism
5. Although Chlorine is an electron with drawing group yet it is orho para directing in
electrophilic aromatic substitution reactions. Why?
Ans: Cl is electron with drawing by inductive effect, and it electron relesing by resonance
and creates negative charge at ortho and para positions. And here resonance effect
is more as compare with inductive effect.
6. Which alkyl halide from the following pairs would you expect to react more rapidly
by an SN2 mechanism? Explain your answer?
1. CH3-CH2-CH2-CH2-Br & CH3-CH2-CHBr –CH3
2. CH3-CH2-CHBr –CH3 & C(CH3)3Br
3. CH3- CHCH3- CH2- CH2- Br & CH3- CH2- CHCH3- CH2- Br
Ans: 1. CH3-CH2-CH2-CH2-Br primary alkyl halide is more reactive than secondary
alkyl halide
2. CH3-CH2-CHBr –CH3 Secondary alkyl halide is more reactive than Tertiary
alkyl halide
3. CH3- CHCH3- CH2- CH2- Br As methyl group is far from Br it will less stabilize
the carbo cations and facilitate the SN2 mechanism.
7. Which of the following compound go faster SN1 reaction:
1. &
Cl
2. & Cl
Ans: 1. first compound ismore stable because tertiary is more reactive than secondary
, alkyl halide for SN1 mechanism.
2. . first compound is more stable because Secondary is more reactive than
primary for SN1 mechanism.
8. Which one of the following has highest boiling point;
CH2Cl2 CHCl3 CCl4
Ans: net Dipole moment of CH2Cl2 & CCl4 is zero due to cancellation dipole moment of
bonds due to its symmetric structure. Where as net dipole moment of CHCl3 is not
zero. Because its structure is not symmetric.
9. Which compound in each of the following pairs react faster in SN2 reaction with OH?
(i) CH3Br or CH3I (ii) (CH3)3CCl or CH3Cl
Ans: CH3I is faster reactive by SN2 mechanism because C-I bond strength is less than
C-Br bond
CH3Cl is faster reactive than (CH3)3CCl by SN2 mechanism because 10 alkyl halide
more reactive by SN2 mechanism.
10. Halo alkanes react with KCN to form alkyl cyanides as main product while AgCN
forms isocyanides as the chief product. Explain.
Ans: KCN is predominantly ionic and provides cyanide ions in solution. Although both
carbon and nitrogen atoms are in a position to donate electron pairs. The attack
takes place mainly through carbon atom not through nitrogen atom since C—C is
more stable than C—N bond. However, AgCN is mainly covalent innatrue and
nitrogen is free to donate electron pair forming isocyanide as the main product.
11. Why does NO2 group show its effect only at ortho and para positions and not at meta
positions?
Ans: The presence of NO2 group at ortho and para positions with draws the electron
density form the benzene ring and thus facilitates the attact of the nucleophile on
halo arene. The carbanaion thus formed is stabilized through resonance. The
negative charge appeared at ortho and para positions with respect to the halogen
substituent is stabilized by NO2 group while in case of meta nitro benzene, none of
the resonating structures bear the negative charge on carbon atom bearing the NO2
group. There fore the presence of nitro group at meta position does not stabilize the
negative charge and no effect on reactivity isobserved by the presence of NO2 group
at meta position.
12. The dipole moment of chloro benzene is lower than that of cyclohexyl chloride.
Why?
Ans: It is due to electron with drawing effect of phenyl group which decrease dipole
moment of chloro benzene.
13. Alkyl halides, though polar, are immiscible with water. Explain.
Ans: Due to absence of hydrogen bond in alkyl halides with water.
14. Grignard reagent should be prepared under anhydrous conditios. Give reason.
Ans: Grignard reagent highly react with water to alkanes.
RMgX + H2O R—H + MgXOH
15. Vinyl chloride is less reactive towards nucleophilic substitution. Explain.
Ans: Due to resonance between C—Cl bond in vinyl chloride show partial double bond
character.
CH2=CH—::Cl:
16. Chloroform stored in dark colored bottles by completely filling it it. Explain.
Ans: Chloroform react with air in presence of sun light it forms poisonous phosogene.
2 CHCl3 + O2 2 COCl2+ 2 HCl
17. Out of C6H5CH2Cl & C6 H5CHCl C6H5 Which is more easily hydrolyzed by KOH?
Ans; The second comound is more resonance stabilized hence easily forms carbo cation
and react faster by SN1 mechanism.
18. The treatment of alkyl halides with aqueous KOH leads to the formation of alcohols
but in the presence of alcoholic KOH alkenes are major products. Explain
Ans: Substitution of alkyl halide gives alcohols and elimination gives alkenes and there is
a competition between substitution and elimination. In the presence of low polarity
solvent like alcohol favours elimination where as in the presence of high polarity
solvent like water substitution favored.
19. Bond angle of ethers is slightly greater than tetrahydral bond angle. Explain.
Ans: due to repulsion of bulky alkyl groups in ethers bond angles is slightly greater than
tetrahydral bond angle.
20. Arrange the following sets of compounds in order of their increasing
