Question Set C

CBSE 11th Chemistry Hydrocarbons Model Questions Papers Study Material - QB365

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Questions:

Generic 3

7 x 3 = 21

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

Write structural formulas of the following compounds :
(i) 3, 4, 4, 5–Tetramethylheptane
(ii) 2,5-Dimethyhexane

Write IUPAC names of the following compounds :
(i) (CH₃)₃CCH₂C(CH₃)₃
(ii) (CH₃)₂C(C₂H₅)₂
(iii) tetra - tert-butylmethane

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

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

Write IUPAC names of the following compounds:
(a) CH₃CH = C(CH₃)₂
(b) CH₂= CH-C\(\equiv \)C-CH₃
(c)

(d)

(e)

(f) \(CH_{ 3 }({ CH }_{ 2 })\underset { \overset { | }{ { CH }_{ 2 }-CH(CH_{ 3 } } )_{ 2 } }{ CH } (CH_{ 2 })_{ 3 }{ CH }_{ 3 }\)
(g) \({ CH }_{ 3 }-CH=CH-{ CH }_{ 2 }-CH=CH-\underset { \overset { | }{ { C }_{ 2 }{ H }_{ 5 } } }{ CH } -{ CH }_{ 2 }-CH={ CH }_{ 2 }\)

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

Generic 2

1 x 2 = 2

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

Case Study Questions

1 x 4 = 4

The term ‘hydrocarbon’ is self-explanatory which means compounds of carbon and hydrogen only. Hydrocarbons play a key role in our daily life. You must be familiar with the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the abbreviated form of liquified petroleum gas whereas CNG stands for compressed natural gas. Another term ‘LNG’ (liquified natural gas) is also in news these days. This is also a fuel and is obtained by liquifaction of natural gas. Petrol, diesel and kerosene oil are obtained by the fractional distillation of petroleum found under the earth’s crust. Coal gas is obtained by the destructive distillation of coal. Natural gas is found in upper strata during drilling of oil wells. The gas after compression is known as compressed natural gas. LPG is used as a domestic fuel with the least pollution. Kerosene oil is also used as a domestic fuel but it causes some pollution. Automobiles need fuels like petrol, diesel and CNG. Petrol and CNG operated automobiles cause less pollution. All these fuels contain mixture of hydrocarbons, which are sources of energy. Hydrocarbons are also used for the manufacture of polymers like polythene, polypropene, polystyrene etc. Higher hydrocarbons are used as solvents for paints. They are also used as the starting materials for manufacture of many dyes and drugs. Thus, you can well understand the importance of hydrocarbons in your daily life. In this unit, you will learn more about hydrocarbons.
Classification Hydrocarbons are of different types. Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories – (i) saturated
(ii) unsaturated and
(iii) aromatic hydrocarbons. Saturated hydrocarbons contain carbon-carbon and carbon-hydrogen single bonds. If different carbon atoms are joined together to form open chain of carbon atoms with single bonds, they are termed as alkanes. On the other hand, if carbon atoms form a closed chain or a ring, they are termed as cycloalkanes. Unsaturated hydrocarbons contain carbon-carbon multiple bonds – double bonds, triple bonds or both. Aromatic hydrocarbons are a special type of cyclic compounds. You can construct a large number of models of such molecules of both types (open chain and close chain) keeping in mind that carbon is tetravalent and hydrogen is monovalent. For making models of alkanes, you can use toothpicks for bonds and plasticine balls for atoms. For alkenes, alkynes and aromatic hydrocarbons, spring models can be constructed.
Alkanes As already mentioned, alkanes are saturated open chain hydrocarbons containing carbon – carbon single bonds. Methane (CH₄) is the first member of this family. Methane is a gas found in coal mines and marshy places. If replace one hydrogen atom of methane by carbon and join the required number of hydrogens to satisfy the tetravalence of the other carbon atom,it get C2H6. This hydrocarbon with molecular formula C2H6 is known as ethane. Thus it can consider C2H6 as derived from CH4 by replacing one hydrogen atom by -CH3 group. Go on constructing alkanes by doing this theoretical exercise i.e., replacing hydrogen atom by –CH3 group. The next molecules will be C₃H₈, C₄H₁₀ …

These hydrocarbons are inert under normal conditions as they do not react with acids, bases and other reagents. Hence, they were earlier known as paraffins (latin : parum, little; affinis, affinity). the general formula for alkanes is C_nH_2n+2. It represents any particular homologue when n is given appropriate value. methane has a tetrahedral structure , in which carbon atom lies at the centre and the four hydrogen atoms lie at the four corners of a regular tetrahedron. All H-C-H bond angles are of 109.5°.
In alkanes, tetrahedra are joined together in which C-C and C-H bond lengths are 154 pm and 112 pm respectively. We have already read that C–C and C–H σ bonds are formed by head-on overlapping of sp 3 hybrid orbitals of carbon and 1s orbitals of hydrogen atoms.
Difference in properties is due to difference in their structures, they are known as structural isomers. structural isomers which differ in chain of carbon atoms are known as chain isomers.
Preparation- Petroleum and natural gas are the main sources of alkanes. However, alkanes can be prepared by following methods :

1) From unsaturated hydrocarbons Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen – hydrogen bond. Platinum and palladium catalyse the reaction at room temperature but relatively higher temperature and pressure are required with nickel catalysts.
2) From alkyl halides
i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.

ii) Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon atoms.
1) Which of the following catalyst is used for hydrogenation…
a) platinum
b) palladium
c) nickel
d) All the above
2) LPG stands for ….
a) Liquid Pressure Gas
b) Liquefied Petroleum Gas
c) Liquid Platinum Gas
d) Liquid Processed Gas
3) Difference in properties is due to difference in their structures, they are known as …. isomers.
a) Functional
b) Positional
c) Structural
d) Chain
4) Structural isomers which differ in chain of carbon atoms are known as… isomers.
a) Functional
b) Positional
c) Structural
d) Chain
5) CNG Stands for ..
a) Compressed Natural Gas
b) Condensed Natural Gas
c) Compressed Neutral Gas
d) Compressed Neutral Gallium

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Answers Key:

Generic 3

7 x 3 = 21

(i)

Longest chain is of six carbon atoms and not that of five. Hence, correct name is 3-Methylhexane.

Numbering is to be started from the end which gives lower number to ethyl group. Hence, correct name is 3-ethyl-5- methylheptane.

(i) 2, 2, 4, 4-Tetramethylpentane
(ii) 3, 3-Dimethylpentane
(iii) 3,3-Di-tert-butyl -2, 2, 4, 4 - tetramethylpentane

(i) An aldehyde with molar mass of 44 u is ethanal, CH₃CH = O
(ii) Write two moles of ethanal side by side with their oxygen atoms pointing towards each other.
CH₃CH = O O = CHCH₃
Ethanal Ethanal
(ii) Remove the oxygen atoms and join them by a double bond, the structure of alkene 'A' is

As required, but-2-ene has three C-C, eight C-H \(\sigma \)-bonds and one C-C \(\pi\)-bond.

(a) Isomers of C₄H₈ having one double bond are:

(b) (i) \({ CH }_{ 3 }{ CH }_{ 2 }{ CH }_{ 2 }\overset { 2 }{ C } \equiv \overset { 1 }{ C } H\)
Pent-1-yne
(ii) \({ CH }_{ 3 }{ CH }_{ 2 }-C\equiv \overset { 2 }{ C } -\overset { 1 }{ C } { H }_{ 3 }\)
Pent-2-yne
(iii)

Chlorination of methane is a free radical reaction which occurs by the following mechanism:
.........(i)
Termination \({ \overset .{ CH } }_{ 3 }+{ .CH }_{ 3 }\rightarrow { CH }_{ 3 }-{ CH }_{ 3 }\) .....(ii)
Ethane
\(\overset .{ C } H_{ 3 }+\overset .{ C } I\rightarrow { CH }_{ 3 }-CI\)
\(\overset { . }{ C } I+\overset { . }{ C } I\rightarrow CI-CI\) ..(iii)
From the above mechanism, it is evident that during propagation step, \(\overset { . }{ C } H_{ 3 }\)free radicals are produced which may undergo three reactions, i.e., (i) - (iii). In the chain termination step, the two CH³ free radicals combine together to form ethane (CH₃-CH₃) molecule.

Generic 2

1 x 2 = 2

The structures of cis- and trans-isomer of hex-2-ene are:

The boiling point of a molecule depends upon dipole-dipole interactions. Since cis-isomer has higher dipole moment, therefore, it has higher boiling point.