Chapter 9. Hydrocarbons

Class 11 Chemistry Chapter 9 Hydrocarbons Internal Questions and Answers :

Problem 9.1 :  Write structures of different chain isomers of alkanes corresponding to the molecular formula  . Also write their IUPAC names.

Answer :  (i) N- Hexane :

 

(ii) 2-Methylpentane :

(iii) 3-Methylpentane

 

(iv) 2,3-Dimethylbutane :

(v) 2,2-Dimethylbutane

Problem 9.2 :  Write structures of different isomeric alkyl groups corresponding to the molecular formula  . Write IUPAC names of alcohols obtained by attachment of  groups at different carbons of the chain.

Answer :

Problem 9.3 :  Write IUPAC names of the following compounds :

(i) 

(ii) 

(iii) tetra – tert-butylmethane .

Answer : (i)   

                 2, 2, 4, 4-Tetramethylpentane .

(ii)   

     

3, 3-Dimethylpentane

(iii) tetra – tert-butylmethane 

   

3,3-Di-tert-butyl -2, 2, 4, 4 –tetramethylpentane .

Problem 9.4 :  Write structural formulas of the following compounds :

(i) 3, 4, 4, 5 -Tetramethylheptane

(ii) 2,5-Dimethyhexane

Answer :  (i) 3, 4, 4, 5 -Tetramethylheptane

    

(ii) 2,5-Dimethyhexane

Problem 9.5 :  Write structures for each of the following compounds. Why are the given names incorrect? Write correct IUPAC names.

(i) 2-Ethylpentane

(ii) 5-Ethyl – 3-methylheptane

Answer :  (i) 2-Ethylpentane : Longest chain is of six carbon atoms and not that of five.

      

Hence, correct name is 3-Methylhexane.

(ii) 5-Ethyl – 3-methylheptane : Numbering is to be started from the end which gives lower number to ethyl group.

    

Hence, correct name is 3-ethyl-5-methylheptane.

Problem 9.6 : Sodium salt of which acid will be needed for the preparation of propane ? Write chemical equation for the reaction.

Answer : The chemical reaction is :

Problem 9.7 :  Write IUPAC names of the following compounds:

Answer : The IUPAC name of the compounds are :

(i)  2,8-Dimethyl-3, 6-decadiene

  

(ii) 1,3,5,7 Octatetraene

     

(iii) 2-n-Propylpent-1-ene

(iv) 4-Ethyl-2,6-dimethyl-dec-4-ene

  

Problem 9.8 : Calculate number of sigma (σ) and pi (π) bonds in the given Compounds .

Answer : (i)  2,8-Dimethyl-3, 6-decadiene

  

σ bonds : 33, π bonds : 2

(ii) 1,3,5,7 Octatetraene

     

σ bonds : 17, π bonds : 4

(iii) 2-n-Propylpent-1-ene

σ bonds : 23, π bond : 1

(iv) 4-Ethyl-2,6-dimethyl-dec-4-ene

  

σ bonds : 41, π bond : 1

Problem 9.9 :  Write structures and IUPAC names of different structural isomers of alkenes corresponding to .

Answer : The different structural isomers of alkanes () and IUPAC name :

Problem 9.10 : Draw  and  isomers of the following compounds. Also write their IUPAC names :

(i) 

(ii) 

Answer: (i)

cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

(ii)

  

cis-3,4-Dimethylhex-3-ene

    

trans-3,4-Dimethylhex-3-ene

Problem 9.11 : Which of the following compounds will show -  isomerism?

(i) 

(ii)

(iii)

(iv)

Answer : (iii)

(iv)

In structures (i) and (ii), two identical groups are attached to one of the doubly bonded carbon atom.

(i) 

(ii)

Problem 9.12 :  Write IUPAC names of the products obtained by addition reactions of  to hex-1-ene

(i) in the absence of peroxide and

(ii) in the presence of peroxide.

Answer :

(i) 

 

2-bromohexane

(ii)

 

1-Bromohexane

Problem 9.13 : Write structures of different isomers corresponding to the  member of alkyne series. Also write IUPAC names of all the isomers. What type of isomerism is exhibited by different pairs of isomers?

Answer : The  member of alkyne series has the molecular formula .

The IUPAC name and the isomers of  are : 

The chain and position isomerism shown by different pairs .

Problem 9.14 : How will you convert ethanoic acid into benzene?

Answer :

Class 11 Chemistry Chapter 9 Hydrocarbons Exercises Questions and Answers :

9.1 How do you account for the formation of ethane during chlorination of methane ?

9.2 Write IUPAC names of the following compounds :

9.3 For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated :

(a)  (one double bond) (b)  (one triple bond)

9.4 Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (i) Pent-2-ene (ii) 3,4-Dimethylhept-3-ene (iii) 2-Ethylbut-1-ene (iv) 1-Phenylbut-1-ene

9.5 An alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3-one. Write structure and IUPAC name of ‘A’.

9.6 An alkene ‘A’ contains three  , eight  bonds and one  bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of ‘A’.

9.7 Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene?

9.8 Write chemical equations for combustion reaction of the following hydrocarbons: (i) Butane (ii) Pentene (iii) Hexyne (iv) Toluene

9.9 Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?

9.10 Why is benzene extra ordinarily stable though it contains three double bonds?

9.11 What are the necessary conditions for any system to be aromatic?

9.12 Explain why the following systems are not aromatic?

9.13 How will you convert benzene into

(i) p-nitrobromobenzene (ii) m- nitrochlorobenzene (iii) p - nitrotoluene (iv) acetophenone?

9.14 In the alkane  , identify 1,2,3 carbon atoms and give the number of H atoms bonded to each one of these.

9.15 What effect does branching of an alkane chain has on its boiling point?

9.16 Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.

9.17 Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene?

9.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

9.19 Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?

9.20 How would you convert the following compounds into benzene?

(i) Ethyne (ii) Ethene (iii) Hexane

9.21 Write structures of all the alkenes which on hydrogenation give 2-methylbutane.

9.22 Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile,

(a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene

(b) Toluene, ,

9.23 Out of benzene, m-dinitrobenzene and toluene which will undergo nitration most easily and why?

9.24 Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene.

9.25 Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example.