13

Q1. What is decarboxylation? How is methane prepared from sodium ethanoate?

Solution

Sodium salts of carboxylic acids on heating with soda lime (mixture of sodiumhydroxide and calcium oxide) gives methane with one carbon atom less than the carboxylic acid. CH₃COO^-Na⁺   + NaOH     CH₄ + Na₂CO₃  

Q2. What do the terms ortho-, meta or para- stand for?

Solution

The aromatic compounds containing the two groups at the adjacent positions, i.e. the 1,2-positions are called ortho, denoted as 'o-'.The aromatic compounds containing the two groups at the alternate positions, i.e. the 1,3-positions are called meta, denoted as 'm-'.The aromatic compounds containing the two groups at the diagonal positions, i.e. the 1,4-positions are called para-, denoted as 'p-'.

Q3. Why alkanes are also known as paraffin?

Solution

Alkanes are relatively unreactive under ordinary conditions and so they are also known as paraffins.

Q4. What types of isomerism is shown by alkenes?

Solution

Alkenes generally show following three types of isomerism:a) Position Isomerismb) Chain Isomerismc) Geometrical Isomerism

Q5. How do the activating and deactivating effects account for the orientation of entering E⁺referred?

Solution

Activating and deactivating effects are strongest at the ortho and para positions. Activating G's stabilize all positions, but o-and p- more than m. Deactivating G's destabilize all positions, but o and p more than m.

Q6. How is dihydrogen gas prepared industrially?

Solution

Industrially, methane is allowed to react with steam at 1273 K in the presence of nickel catalyst to form carbon monoxide and dihydrogen. CH₄ + H₂O          CO   +   3H₂O

Q7. Is it possible to isolate pure staggered ethane or pure eclipsed ethane at room temperature?

Solution

It is not possible to isolate pure staggered form of ethane or pure eclipsed form of ethane at room temperature because of less difference in their energy they can get converted into each other very fast.

Q8. What effect does the branching of an alkane has on its melting point?

Solution

Branching of an alkane decreases the melting point.

Q9. What are alkanes?

Solution

Alkanes are saturated hydrocarbons with general formula C_nH_2n+2. Alkanes form a homologous series in which successive members differ by unit of -CH₂ group.

Q10. How can a t-butyl substituent be used as a blocking group in aromatic electrophilic substitutions?

Solution

The Friedel-Crafts alkylation can be reversed, especially when a 3° alkyl group such as Me₃C is present. Dealkylation is affected with AlCl₃ by transfer of Me₃C⁺ to another arene, used as the solvent, or at high temperatures by protonation with H⁺ and loss of Me₃C⁺as Me₂C=CH₂. Me₃C- , introduced into a ring, is used to block an active position and/or to direct another group into the ring , and then removed.

Q11. Does the eclipsed conformation of propane have same energy or different as the eclipsed conformation of ethane?

Solution

Different energy. Eclipsed conformation of ethane is more stable than that of propane.

Q12. Give the product of acylation of naphthalene with CH₃COCl and AlCl₃ in the solvent (i) CS₂ and (ii) PhNO₂.

Solution

(i)1-Acetylnaphthalene(ii) 2-Acetylnaphthalene

Q13. What is E/Z isomerism?

Solution

When two groups of highest priority are on the same side of the double bond, the isomer is designated as Z or Zusammen and when such groups are on the opposite sides, the isomer is known as E or Entagegen.

Q14. Why is Friedel-Crafts acylation but not alkylation of naphthalene practical?

Solution

Acylation introduces a deactivating group (- COR) that prevents further acylation from occurring. Introduction of an activating -R group induces polyalkylations which is not possible.

Q15. What are the harmful facts related to polynuclear hydrocarbons?

Solution

• The polynuclear aromatic hydrocarbons penetrate the body rapidly and reach to the fat containing tissues.
• Short-term low exposure to naphthalene may cause eye and skin irritation. If naphthalene is ingested it has the potential to cause hemolytic anaemia, a condition that involves the breakdown of red blood cells. Naphthalene is a suspected human carcinogen, and has been proven to cause damage to the kidneys and to the liver. Higher incidences of lung and skin tumors have been reported for people who have been occupationally exposed to naphthalene and other Polynuclear aromatic hydrocarbons.
• An exposure to Anthracene can cause skin and eye irritation. Repeated exposure may cause alteration of skin pigments as well as cancerous growth. Inhaled anthracene can cause bronchitis like symptoms. There is limited information on human reproductive implications. Anthracene may cause genetic mutations in cells.
• Phenanthrene is also a skin and eye irritant, with increasing effects in sunlight due to photosensitization. There is currently no data available for human oral and inhalation exposure. It is however, a suspected carcinogen, although there is no data for humans.

Q16. Why is PhNO₂ a suitable solvent for the Friedel-Crafts alkylation of PhBr while benzene is not?

Solution

Benzene is more reactive than bromobenzene in the Friedel-Crafts reaction and would alkylate more rapidly. Nitrobenzene may be used because it does not undergo the Friedel-Crafts reaction owing to the deactivating effect of the NO₂ group (CS₂ can also be used).

Q17. What are alkynes?

Solution

Alkynes are unsaturated hydrocarbons with the general formula C_nH_2n-2. These are the most reactive compounds when compared to alkenes and alkanes.

Q18. Which salt on treatment with soda lime gives ethane?

Solution

Sodium propanoate.

Q19. What are alkenes?

Solution

The class of unsaturated hydrocarbons which possess C=C bond and have general formula C_nH_2n are known as alkenes.

Q20. Arrange the different type of conformation of butane in the decreasing order of stability.

Solution

Anti > Skew > Eclipsed > Fully Eclipsed

Q21. Discuss the physical properties of alkenes.

Solution

a) Physical State: At room temperature, alkenes with four or less than four carbon atoms exist in gaseous state. Alkenes with 5 to 17 carbon atoms generally exist in liquid state in room temperature. Alkenes with 18 carbon atoms and more are generally solids.b) Solubility: Alkenes are insoluble in water but soluble in alcohol, ether and other organic solvents.c) Alkenes are less dense than water and so they float on water.d) Alkenes show geometrical isomerism due to the presence of  carbon-carbon double bond.e) Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.

Q22. Account for the greater reactivity and the o, p-orientation in electrophilic substitution of biphenyl despite the electron-attracting inductive effect of the phenyl group.

Solution

In attack at the o-,p- positions the unreacted ring can stabilize the + charge on the attached C of the σ complex by electron-releasing delocalization that results in delocalizing the + charge to both rings. Such delocalization is impossible with the intermediate from meta substitution which does not have the + charge on the C bonded to the Ph substituent. The + charge cannot be incorporated into the second ring, and delocalization is confined to one ring.

Q23. How will you define aromatic hydrocarbons?

Solution

Aromatic hydrocarbons that contain one or more benzene rings are called Arenes. A benzene ring is a six-membered ring containing carbon atoms linked to each other with alternate single and double bonds. Arenes have general formula C_nH_2n-6 (where n = 6 or more).

Q24. Give the IUPAC name of the following compounds: (i) CH₂ = CH – CH = CH₂ (ii)CH₂ = CH – CH – CH₃                                                                |                        CH₃

Solution

(i)  Buta – 1, 3 - diene (ii) 3-Methylbut-1-ene

Q25. What happens when?

Isobutylene is added to water

HBr is added to an unsymmetrical alkene in presence of peroxides.

Solution

1.                                                                                                                                                                                                                         Tert-butyl alcohol 2.                                                                                                                      Tert-butyl alcohol

Q26. Give the IUPAC name of the following compound:   (i)CH₃ – CH₂ – CH₂ – CH₂– CH₂ – OH  (ii) CH₃ – CH₂ – CH – CH₂ – CH – CH₂ – CH₃                     l                 l                     CH₃            C₂H₅

Solution

(i) Pentan-1-ol (ii) 3-ethyl-5-methylheptane

Q27. Compare the rate of nitration under similar conditions of Ph-O-Me and Ph-S-Me. Explain

Solution

PhOMe>PhSMe . The ∏ bond from O (which uses a 2p orbital) to the ring Carbon is shorter and stronger than the comparable bond from S (which uses a 3p orbital) to the ring Carbon and -OCH₃ being an activating group has higher rate of nitration.

Q28. Name the two extreme type of conformation of ethane.

Solution

Staggered, eclipsed

Q29. What happens when:

n-hexane is heated with AlCl₃ and HCl gas.

Hexane on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium.

Solution

1. CH₃-(CH₂)₄ CH₃    CH₃ CH – (CH₂)₂ – CH₃ + CH₃CH₂-CH-CH₂-CH₃                                     l                                             l                                  CH₃                                          CH₃                                  2-Methylpentane                     3-Methylpentane 2.     n-hexane                                         Benzene

Q30. What type of isomerism do alkynes show?

Solution

Alkynes show three types of isomerism, which are described below. viz.,(a) Chain isomerism(b) Position isomerism(c) Functional isomerism.

Q31. What is hydrogenation? How is propyne prepared by the process of hydrogenation?

Solution

The addition of dihydrogen to alkenes and alkynes in the presence of platinum, palladium or nickel to form alkanes is called hydrogenation. Hydrogen adds to propyne in the presence of platinum, palladium or nickel to form propane.

Q32. Name the products which may be obtained when a mixture of methyl bromide and ethyl bromide is treated with sodium metal in ether.

Solution

Ethane, Propane and butane.

Q33. What is cis/tras isomerism?

Solution

When the groups of similar nature are on the same side of the double bond, the isomer is known as cis-isomer. On the other hand if the groups of similar nature are on the opposite sides, the isomer is known as trans-isomer.

Q34. How do substituent groups (G) on an aromatic ring influence the course of electrophilic aromatic substitution?

Solution

Such substituent groups affect the electrophilic aromatic substitution in two ways : a) Reactivity or rate of reaction: The ones more reactive than benzene are activating, and those less reactive are deactivating. b)Orientation : Whether Electrophilic group enters ortho , meta or para position.

Q35. Define the following: (a)Structural isomers (b)Chain isomers

Solution

(a)Structural isomers: The difference in properties due to difference in their structures are known as structural isomers. (b) Chain isomers: The structural isomers which differ in chain of carbon atoms are known as chain isomers.

Q36. Out of boat and chair form of cyclohexane, which is more stable?

Solution

Chair form is more stable.

Q37. Why do alkenes show geometrical isomerism? Explain with examples.

Solution

• Due to C=C bond, the rotation around this bond is restricted.
• Geometrical isomerism is due to the presence of like groups on the same side(cis-isomer) or on the opposite side (trans-isomer).
• The presence of two different groups at the doubly bonded carbon is the necessary and sufficient condition for geometrical isomerism.

             C(a,b) = C(a,b) , C(a,b) = C(e,f) or               C(a,b) = C(a,d) all show geometrical isomerism.

Q38. How many chain isomers are possible for C₄H₁₀.Draw their structure.

Solution

2 isomers are possible for C₄H_10. CH₃ CH₂ CH₂ CH₃   and  CH₃CH(CH₃)₂

Q39. Explain Wurtz reaction with an example.

Solution

The reaction in which alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon atoms. Example: Methyl bromide on treating with sodium metal in dry ethereal solution gives ethane.    

Q40. Explain that halogens are o-, p- directors, but are deactivating.

Solution

The electronegative halogens inductively withdraw e^-(s)from all the carbon atoms of the ring of the σ complex, and the o- , p- and m- benzonium ions each have a higher ∆H^‡than that of the cation from benzene .The halogens have unshared electrons and can delocalize electron density to the o- and p-carbon atoms. Halogens are said to be Π-donating but σ - withdrawing.