Chapter 13 Hydrocarbons

Question 1. Arrange the following in decreasing order of their boiling points.

(A) n–butane

(B) 2–methylbutane

(C) n-pentane

(D) 2,2–dimethylpropane

(i) A > B > C > D

(ii) B > C > D > A

(iii) D > C > B > A

(iv) C > B > D > A

Question 2. Arrange the halogens $F_2, Cl_2, Br_2, I_2$, in order of their increasing reactivity with alkanes.

(i) $I_2 < Br_2 < Cl_2 < F_2$

(ii) $Br_2 < Cl_2 < F_2 < I_2$

(iii) $F_2 < Cl_2 < Br_2 < I_2$

(iv) $Br_2 < I_2 < Cl_2 < F_2$

Question 3. The increasing order of reduction of alkyl halides with zinc and dilute HCl is

(i) R–Cl < R–I < R–Br

(ii) R–Cl < R–Br < R–I

(iii) R–I < R–Br < R–Cl

(iv) R–Br < R–I < R–Cl

Question 4. The correct IUPAC name of the following alkane is

(i) 3,6 – Diethyl – 2 – methyloctane

(ii) 5 – Isopropyl – 3 – ethyloctane

(iii) 3 – Ethyl – 5 – isopropyloctane

(iv) 3 – Isopropyl – 6 – ethyloctane

Question 5. The addition of HBr to 1-butene gives a mixture of products A, B and C

(A)

(B)

(C)

The mixture consists of

(i) A and B as major and C as minor products

(ii) B as major, A and C as minor products

(iii) B as minor, A and C as major products

(iv) A and B as minor and C as major products

Question 6. Which of the following will not show geometrical isomerism?

(i)

(ii)

(iii)

(iv)

Question 7. Arrange the following hydrogen halides in order of their decreasing reactivity with propene.

(i) HCl > HBr > HI

(ii) HBr > HI > HCl

(iii) HI > HBr > HCl

(iv) HCl > HI > HBr

Question 8. Arrange the following carbanions in order of their decreasing stability.

(A) $H_3C – C \equiv C^–$

(B) $H – C \equiv C^–$

(C) $CH_3-CH_2^-$

(i) A > B > C

(ii) B > A > C

(iii) C > B > A

(iv) C > A > B

Question 9. Arrange the following alkyl halides in decreasing order of the rate of $\beta$–elimination reaction with alcoholic KOH.

(A)

(B) $CH_3—CH_2—Br$

(C) $CH_3—CH_2—CH_2—Br$

(i) A > B > C

(ii) C > B > A

(iii) B > C > A

(iv) A > C > B

Question 10. Which of the following reactions of methane is incomplete combustion:

(i) $2CH_4 + O_2 \xrightarrow{Cu/523K/100atm} 2CH_3OH$

(ii) $CH_4 + O_2 \xrightarrow{Mo_2O_3} HCHO + H_2O$

(iii) $CH_4 + O_2 \rightarrow C(s) + 2H_2O(l)$

(iv) $CH_4 + 2O_2 \rightarrow CO_2(g) + 2H_2O(l)$

Question 11. Some oxidation reactions of methane are given below. Which of them is/are controlled oxidation reactions?

(i) $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

(ii) $CH_4(g) + O_2(g) \rightarrow C(s) + 2H_2O(l)$

(iii) $CH_4(g) + O_2(g) \xrightarrow{Mo_2O_3} HCHO + H_2O$

(iv) $2CH_4(g) + O_2(g) \xrightarrow{Cu/523K/100atm} 2CH_3OH$

Question 12. Which of the following alkenes on ozonolysis give a mixture of ketones only?

(i) $CH_3—CH = CH—CH_3$

(ii)

(iii)

(iv)

Question 13. Which are the correct IUPAC names of the following compound?

(i) 5–Butyl – 4– isopropyldecane

(ii) 5–Ethyl – 4– propyldecane

(iii) 5–sec-Butyl – 4– iso-propyldecane

(iv) 4–(1-methylethyl)– 5 – (1-methylpropyl)-decane

Question 14. Which are the correct IUPAC names of the following compound?

(i) 5–(2,2–Dimethylpropyl)-decane

(ii) 4–Butyl – 2,2– dimethylnonane

(iii) 2,2–Dimethyl – 4– pentyloctane

(iv) 5–neo-Pentyldecane

Question 15. For an electrophilic substitution reaction, the presence of a halogen atom in the benzene ring _______.

(i) deactivates the ring by inductive effect

(ii) deactivates the ring by resonance

(iii) increases the charge density at ortho and para position relative to meta position by resonance

(iv) directs the incoming electrophile to meta position by increasing the charge density relative to ortho and para position.

Question 16. In an electrophilic substitution reaction of nitrobenzene, the presence of nitro group ________.

(i) deactivates the ring by inductive effect.

(ii) activates the ring by inductive effect.

(iii) decreases the charge density at ortho and para position of the ring relative to meta position by resonance.

(iv) increases the charge density at meta position relative to the ortho and para positions of the ring by resonance.

Question 17. Which of the following are correct?

(i) $CH_3—O—\overset{+}{C}H_2$ is more stable than $CH_3—\overset{+}{C}H_2$

(ii) $(CH_3)_2\overset{+}{C}H$ is less stable than $CH_3—CH_2—\overset{+}{C}H_2$

(iii) $CH_2=CH—\overset{+}{C}H_2$ is more stable than $CH_3—CH_2—\overset{+}{C}H_2$

(iv) $CH_2= \overset{+}{C}H$ is more stable than $CH_3—\overset{+}{C}H_2$

Question 18. Four structures are given in options (i) to (iv). Examine them and select the aromatic structures.

(i)

(ii)

(iii)

(iv)

Question 19. The molecules having dipole moment are __________.

(i) 2,2-Dimethylpropane

(ii) trans-Pent-2-ene

(iii) cis-Hex-3-ene

(iv) 2, 2, 3, 3 - Tetramethylbutane.

Question 20. Why do alkenes prefer to undergo electrophilic addition reaction while arenes prefer electrophilic substitution reactions? Explain.

Question 21. Alkynes on reduction with sodium in liquid ammonia form trans alkenes. Will the butene thus formed on reduction of 2-butyne show the geometrical isomerism?

Question 22. Rotation around carbon-carbon single bond of ethane is not completely free. Justify the statement.

Question 23. Draw Newman and Sawhorse projections for the eclipsed and staggered conformations of ethane. Which of these conformations is more stable and why?

Question 24. The intermediate carbocation formed in the reactions of HI, HBr and HCl with propene is the same and the bond energy of HCl, HBr and HI is $430.5 \text{ kJ mol}^{–1}, 363.7 \text{ kJ mol}^{–1}$ and $296.8 \text{ kJ mol}^{–1}$ respectively. What will be the order of reactivity of these halogen acids?

Question 25. What will be the product obtained as a result of the following reaction and why?

Question 26. How will you convert benzene into

(i) p–nitrobromobenzene

(ii) m–nitrobromobenzene

Question 27. Arrange the following set of compounds in the order of their decreasing relative reactivity with an electrophile.Give reason.

Question 28. Despite their -I effect, halogens are o- and p-directing in haloarenes. Explain.

Question 29. Why does presence of a nitro group make the benzene ring less reactive in comparison to the unsubstituted benzene ring. Explain.

Question 30. Suggest a route for the preparation of nitrobenzene starting from acetylene?

Question 31. Predict the major product(s) of the following reactions and explain their formation.

$H_3C—CH=CH_2 \xrightarrow{HBr, (Ph-CO-O)_2}$

$H_3C—CH=CH_2 \xrightarrow{HBr}$

Question 32. Nucleophiles and electrophiles are reaction intermediates having electron rich and electron deficient centres respectively. Hence, they tend to attack electron deficient and electron rich centres respectively. Classify the following species as electrophiles and nucleophiles.

(i) $H_3CO^–$

(ii)

(iii)

(iv)

(v) $(H_3C)_3C^+$

(vi) $Br^–$

(vii) $H_3COH$

(viii) R–NH–R

Question 33. The relative reactivity of 1°, 2°, 3° hydrogen’s towards chlorination is 1 : 3.8 : 5. Calculate the percentages of all monochlorinated products obtained from 2-methylbutane.

Question 34. Write the structures and names of products obtained in the reactions of sodium with a mixture of 1-iodo-2-methylpropane and 2-iodopropane.

Question 35. Write hydrocarbon radicals that can be formed as intermediates during monochlorination of 2-methylpropane? Which of them is more stable? Give reasons.

Question 36. An alkane $C_8H_{18}$ is obtained as the only product on subjecting a primary alkyl halide to Wurtz reaction. On monobromination this alkane yields a single isomer of a tertiary bromide. Write the structure of alkane and the tertiary bromide.

Question 37. The ring systems having following characteristics are aromatic.

(i) Planar ring containing conjugated $\pi$ bonds.

(ii) Complete delocalisation of the $\pi$−electrons in ring system i.e. each atom in the ring has unhybridised p-orbital, and

(iii) Presence of $(4n+2)\pi$−electrons in the ring where n is an integer (n = 0, 1, 2,...........) [Huckel rule].

Using this information classify the following compounds as aromatic/nonaromatic.

Question 38. Which of the following compounds are aromatic according to Huckel’s rule?

(A)

(B)

(C)

(D)

(E)

(F)

Question 39. Suggest a route to prepare ethyl hydrogensulphate ($CH_3–CH_2–OSO_2—OH$) starting from ethanol ($C_2H_5OH$).

Question 40. Match the reagent from Column I which on reaction with $CH_3—CH=CH_2$ gives some product given in Column II as per the codes given below :

Column I

(i) $O_3/Zn + H_2O$

(ii) $KMnO_4/H^+$

(iii) $KMnO_4/OH^–$

(iv) $H_2O/H^+$

(v) $B_2H_6/NaOH$ and $H_2O_2$

Column II

(a) Acetic acid and $CO_2$

(b) Propan-1-ol

(c) Propan-2-ol

(d) Acetaldehyde and formaldehyde

(e) Propane-1,2-diol

Question 41. Match the hydrocarbons in Column I with the boiling points given in Column II.

Column I

(i) n–Pentane

(ii) iso-Pentane

(iii) neo-Pentane

Column II

(a) 282.5 K

(b) 309 K

(c) 301 K

Question 42. Match the following reactants in Column I with the corresponding reaction products in Column II.

Column I

(i) Benzene + $Cl_2 \xrightarrow{AlCl_3}$

(ii) Benzene + $CH_3Cl \xrightarrow{AlCl_3}$

(iii) Benzene + $CH_3COCl \xrightarrow{AlCl_3}$

(iv) Toluene $\xrightarrow{KMnO_4/NaOH}$

Column II

(a) Benzoic acid

(b) Methyl phenyl ketone

(c) Toluene

(d) Chlorobenzene

(e) Benzene hexachloride

Question 43. Match the reactions given in Column I with the reaction types in Column II.

Column I

(i) $CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$

(ii) $CH_2=CH_2 + H_2 \xrightarrow{Pd} CH_3—CH_3$

(iii) $CH_2=CH_2 + Cl_2 \rightarrow Cl—CH_2—CH_2—Cl$

(iv) $3 CH \equiv CH \xrightarrow[\text{Heat}]{\text{Cu tube}} C_6H_6$

Column II

(a) Hydrogenation

(b) Halogenation

(c) Polymerisation

(d) Hydration

(e) Condensation

Question 44. Assertion (A) : The compound cyclooctane has the following structural formula :

It is cyclic and has conjugated 8 $\pi$-electron system but it is not an aromatic compound.

Reason (R) : (4n + 2)$\pi$ electrons rule does not hold good and ring is not planar.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

Question 45. Assertion (A) : Toluene on Friedal Crafts methylation gives o– and p–xylene.

Reason (R) : $CH_3$-group bonded to benzene ring increases electron density at o– and p– position.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

Question 46. Assertion (A) : Nitration of benzene with nitric acid requires the use of concentrated sulphuric acid.

Reason (R) : The mixture of concentrated sulphuric acid and concentrated nitric acid produces the electrophile, $NO_2^+$.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

Question 47. Assertion (A) : Among isomeric pentanes, 2, 2-dimethylpentane has highest boiling point.

Reason (R) : Branching does not affect the boiling point.

(i) Both A and R are correct and R is the correct explanation of A.

(ii) Both A and R are correct but R is not the correct explanation of A.

(iii) Both A and R are not correct.

(iv) A is not correct but R is correct.

Question 48. An alkyl halide $C_5H_{11}Br$ (A) reacts with ethanolic KOH to give an alkene ‘B’, which reacts with $Br_2$ to give a compound ‘C’, which on dehydrobromination gives an alkyne ‘D’. On treatment with sodium metal in liquid ammonia one mole of ‘D’ gives one mole of the sodium salt of ‘D’ and half a mole of hydrogen gas. Complete hydrogenation of ‘D’ yields a straight chain alkane. Identify A,B, C and D. Give the reactions invovled.

Question 49. 896 mL vapour of a hydrocarbon ‘A’ having carbon 87.80% and hydrogen 12.19% weighs 3.28g at STP. Hydrogenation of ‘A’ gives 2-methylpentane. Also ‘A’ on hydration in the presence of $H_2SO_4$ and $HgSO_4$ gives a ketone ‘B’ having molecular formula $C_6H_{12}O$. The ketone ‘B’ gives a positive iodoform test. Find the structure of ‘A’ and give the reactions involved.

Question 50. An unsaturated hydrocarbon ‘A’ adds two molecules of H2 and on reductive ozonolysis gives butane-1,4-dial, ethanal and propanone. Give the structure of ‘A’, write its IUPAC name and explain the reactions involved.

Question 51. In the presence of peroxide addition of HBr to propene takes place according to anti Markovnikov’s rule but peroxide effect is not seen in the case of HCl and HI. Explain.