Hydrocarbons

Substitution = Replacement

substitution reaction (halogenation) occurs in three steps : 

a. Initiation

b. Propagation

c. Termination

Combustion = On fire/ to ignite

Exercise Questions

Q1 How can you account for the formation of ethane during chlorination of methane?

Q2 Write IUPAC names of the following compounds: 

Q13.3 For the following compounds, write structural formulas and IPUAC names for all possible isomers having the no. of double or triple bond as indicated: 

a. C4H8 (one double bond).....................b. C5H8 (one triple bond)

Q13.4 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

Q13.5 An alkene 'A' on ozonolysis gives a mixture of ethanal and pentan-3-one. Write structure and IUPAC name of 'A'.

13.6 An alkene 'A' contains three C-C, eight C-H sigma bonds and one C-C pie bond. 'A' on ozonolysis gives two moles of an aldehyde of molar mass 44u. Write IUPAC name of 'A'. 

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

Q13.8 Write chemical equations for combustion reaction of the following hydrocarbons: 

(i) Butane (ii) Pentane (iii) Hexyne (iv) Toluene

Q13.9 Draw the cis and trans structures of hex-2-ene. Which isomer will have higher boiling point and why?

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

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

Ans. Necessary conditions for any system to be aromatic: - 

a. planar (sp2 hybridised or double bond)

b. conjugated ring system

c. complete delocalisation of (4n+2)π electrons.

Q13.12 Explain why the following systems are not aromatic?

(iii) this compound carry 8π electrons and not (4n+2)π electrons.

Q13.13  How will you convert benzene into

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

(i) Benzene to p-Nitrobromobenzene

(ii) Benzene to m-Nitrobromobenzene

(iii) Benzene to p-Nitrotoluene

(iv) Benzene to Acetophenone

Q13.14 In the alkene H3C-CH2-C(CH3)2-CH2-CH-(CH3)2, identify 10, 20, 30 carbon atoms and give the number of H atoms bonded to each one of these.

Q13.15 What effect does branching of an alkane chain has on its boiling point? 

Ans. Alkanes with stronger intermolecular van der waals forces have higher boiling point. With increase in the branching, the surface area of the molecule decreases and vander waals forces of attraction decreases which can be overcome at a relatively lower temperature. Hence, the boiling point of an alkane chain decreased with an increase in branching

Q13.16 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. 

Q13.17 Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene? 

All three products cannot be obtained by any one of the Kekule's structures. This shows that benzene is a resonance hybrid of the two resonating structures. 

Q13.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour. 

Q13.19 Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty? 

Ans. Due to the presence of 6π electrons, benzene behaves as a rich source of electrons, thus, being easily attacked by reagents deficient in electrons.

Hence, benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty

Q13.20 How would you convert the following compounds into benzene? (i) Ethyne (ii) Ethene (iii) Hexane 

Q13.21 Write structures of all the alkenes which on hydrogenation give 2-methylbutane. 

Q13.22 Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+ (a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene (b) Toluene, p-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2. 

Ans. Electron donating groups increase the reactivity while electron-withdrawing groups decrease the reactivity.

(a) chlorobenzene > p-nitrochlorobenzene > 2,4-dinitrochlorobenzene 

(b) p-CH3-C6H4-CH3 > toluene > p-H3C-C6H4-NO2 > p-NO2-C6H4-NO2

Q13.23 Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why? 

Ans. The decreasing relative reactivity with an electrophile is:

Toluene >p-CH3-C6H4 -NO2 > p-O2N-C6H4 -NO2

​

When electron withdrawing nitro group is present on the benzene nucleus, the electron density of benzene ring decreases and the reactivity with an electrophile decreases.

When electron releasing methyl group is present on the benzene nucleus, the electron density of benzene ring increases and the reactivity with an electrophile increases.

Q13.24 Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene. 

Ans. Ethylation means the introduction of ethyl group in Benzene ring. this is usually carried out by Friedel craft alkylation reaction of benzene with ethyl halide { chlorine or bromine } , Ethene or ethanol.

The name of Lewis acid other than Anhydrous AlCl3 used in the reaction are : Anhydrous FeCl3 , SnCl4 , BF3 etc.

Q13.25 Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example. 

Ans. Wurtz reaction not preferred for the preparation of alkanes containing an odd number of carbon atoms because if we take two dissimilar alkyl halide as a reactant, the product will be a mixture of alkane but the reaction is by a free radical mechanism it will produce an alkene also.