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Chemistry Case study (4 Marks)

Hydrocarbons · Rajasthan Board (RBSE) STD 11 Science (Rajasthan - English Medium). 4 questions.

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Case study (4 Marks)

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Question 14 Marks
Read the passage given below and answer the following questions from 1 to 5.
Alkynes– Like alkenes, alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms. The number of hydrogen atoms is still less in alkynes as compared to alkenes or alkanes. Their general formula is $C_nH_{2n–2}.$ The first stable member of alkyne series is ethyne which is popularly known as acetylene. Acetylene is used for arc welding purposes in the form of oxyacetylene flame obtained by mixing acetylene with oxygen gas. Alkynes are starting materials for a large number of organic compounds. Hence, it is interesting to study this class of organic compounds.
Structure of Triple Bond Ethyne is the simplest molecule of alkyne series. Structure of ethyne is shown in Figure. Each carbon atom of ethyne has two sp hybridised orbitals. Carbon-carbon sigma $(\sigma)$ bond is obtained by the head-on overlapping of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridised orbital of each carbon atom undergoes overlapping along the internuclear axis with the 1s orbital of each of the two hydrogen atoms forming two C-H sigma bonds. H-C-C bond angle is of $180^\circ .$

Each carbon has two unhybridised p orbitals which are perpendicular to each other as well as to the plane of the C C sigma bond. The $2 p$ orbitals of one carbon atom are parallel to the $2 p$ orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi ( $\pi$ ) bonds between two carbon atoms. Thus ethyne molecule consists of one $C - C \sigma$ bond, two $C - H \sigma$ bonds and two $C - C \pi$ bonds. The strength of $C \equiv C$ bond (bond enthalpy $823 kJ mol ^{-1}$ ) is more than those of $C = C$ bond (bond enthalpy $681 kJ mol ^{-1}$ ) and $C - C$ bond (bond enthalpy 348 kJ $mol ^{-1}$ ). The $C \equiv C$ bond length is shorter $(120 pm )$ than those of $C = C (133 pm )$ and $C - C (154 pm )$. Electron cloud between two carbon atoms is cylindrically symmetrical about the internuclear axis. Thus, ethyne is a linear molecule. Preparation - From calcium carbide: On industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions:

$\text{C}\text{aC}_2+2\text{H}_2\text{O}\rightarrow\text{Ca}(\text{OH})_2+\text{C}_2\text{H}_2$
From vicinal dihalides: Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne.


Aromatic hydrocarbon- These hydrocarbons are also known as ‘arenes’. Since most of them possess pleasant odour, the class of compounds was named as ‘aromatic compounds’. Most of such compounds were found to contain benzene ring. Benzene ring is highly unsaturated but in a majority of reactions of aromatic compounds, the unsaturation of benzene ring is retained. However, there are examples of aromatic hydrocarbons which do not contain a benzene ring but instead contain other highly unsaturated ring. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids. Some examples of arenes are given below:

Friedel-Crafts alkylation reaction: When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.
Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids ($AlCl_3$) yields acyl benzene.

If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous $AlCl_3$​​​​​​​ can be chlorinated to hexachlorobenzene $(C_6Cl_6)​​​​​​​$​​​​​​​
  1. The general formula of Alkynes is …?
  1. $C_nH_{2n–2}$​​​​​​​
  2. $C_nH_{2n–2}$
  3. $C_nH_{2n–2}$
  4. $C_nH_{2n–2}$
  1. Calcium carbide is prepared by heating quick lime with …?
  1. Backing soda
  2. Coke
  3. Carbide
  4. Salt
  1. Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo …?
  1. Dehydrogenation.
  2. Hydrohalogenation.
  3. Dehydrohalogenation
  4. Dehalogenation.
  1. The bond enthalpy of C ≡ C is …?
  1. 523 kJ mol
  2. 623 kJ mol
  3. 723 kJ mol
  4. 823 kJ mol
  1. The C ≡ C bond length is …?
  1. 1200 pm
  2. 620 pm
  3. 240 pm
  4. 120 pm
Answer
  1. (a) $C_nH_{2n–2}$
  2. (b)Coke
  3. (c) Dehydrohalogenation.
  4. (d) 823 kJ mol
  5. (d) 120 pm
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Question 24 Marks
Read the passage given below and answer the following questions from 1 to 5.
Alkenes are unsaturated hydrocarbons containing at least one double bond. What should be the general formula of alkenes? If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes. Hence, general formula for alkenes is $C_nH_{2n}$. Alkenes are also known as olefins (oil forming) since the first member, ethylene or ethene $(C_2H_4)$ was found to form an oily liquid on reaction with chlorine.
Structure of Double Bond Carbon-carbon double bond in alkenes consists of one strong sigma $(\sigma)$ bond (bond enthalpy about $397\ kJ\ mol^{–1)}$ due to head-on overlapping of $sp^2$ hybridised orbitals and one weak pi $\pi$ bond (bond enthalpy about $284\ kJ\ mol^{–1})$ obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms. The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm). You have already read that the pi $(\pi)$ bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals. Thus, the presence of the pi $(\pi)$bond makes alkenes behave as sources of loosely held mobile electrons. Therefore, alkenes are easily attacked by reagents or compounds which are in search of electrons. Such reagents are called electrophilic reagents. The presence of weaker$(\pi)$-bond makes alkenes unstable molecules in comparison to alkanes and thus, alkenes can be changed into single bond compounds by combining with the electrophilic reagents. Strength of the double bond (bond enthalpy, $681\ kJ\ mol^{–1}$) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, $348\ kJ\ mol^{–1}$). Orbital diagrams of ethene molecule are shown in Figure.

Geometrical isomerism: Doubly bonded Carbon atoms have to satisfy the remaining two Valences by joining with two atoms or groups. If the two atoms or groups attached to each Carbon atom are different, they can be Represented by YX C = C XY like structure. YX C = C XY can be represented in space in the Following two ways:

In (a), the two identical atoms i.e., both the X or both the Y lie on the same side of the Double bond but in (b) the two X or two Y lie Across the double bond or on the opposite Sides of the double bond. This results in Different geometry of (a) and (b) i.e. disposition Of atoms or groups in space in the two Arrangements is different. Therefore, they are Stereoisomers. They would have the same Geometry if atoms or groups around C = C bond Can be rotated but rotation around C = C bond Is not free. It is restricted. For understanding This concept, take two pieces of strong Cardboards and join them with the help of two Nails. Hold one cardboard in your one hand And try to rotate the other. Can you really rotate The other cardboard ? The answer is no. The Rotation is restricted. This illustrates that the Restricted rotation of atoms or groups around The doubly bonded carbon atoms gives rise to Different geometries of such compounds. The Stereoisomers of this type are called Geometrical isomers. The isomer of the type (a), in which two identical atoms or groups lie On the same side of the double bond is called Cis isomer and the other isomer of the type (b), in which identical atoms or groups lie on The opposite sides of the double bond is called Trans isomer. Thus cis and trans isomers Have the same structure but have different Configuration (arrangement of atoms or groups In space). Due to different arrangement of Atoms or groups in space, these isomers differ In their properties like melting point, boiling Point, dipole moment, solubility etc. Geometrical or cis-trans isomers of but-2-ene Are represented below:

Cis form of alkene is found to be more polar Than the trans form. For example, dipole Moment of cis - but - 2-ene is 0.33 Debye, Whereas, dipole moment of the trans form Is almost zero or it can be said that trans - but - 2 -ene is non-polar. This can be understood by drawing geometries of the two forms as given below from which it is clear that in the trans - but - 2 -ene, the two methyl groups are in opposite directions, Therefore, dipole moments of $C - CH_3$ bonds cancel, thus making the trans form non-polar.

In the case of solids, it is observed that the trans isomer has higher melting point than the cis form. Geometrical or cis-trans isomerism is also shown by alkenes of the types XYC = CXZ and XYC = CZW
Preparation – From alkynes: Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur
compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.

From alkyl halides: Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol, say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of $\beta-$elimination reaction, since hydrogen atom is eliminated from the $\beta$ carbon atom (carbon atom next to the carbon to which halogen is attached).

Nature of halogen atom and the alkyl group determine rate of the reaction. It is observed that for halogens, the rate is: iodine > bromine > chlorine, while for alkyl groups it is: tert > secondary > primary.
Physical properties Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non- polar solvents like benzene, petroleum ether. They show a regular increase in boiling point with increase in size i.e., every $–CH_2$ group added increases boiling point by 20–30 K. Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds.
  1. The first three members of alkenes are …?
  1. Gases
  2. Liquids
  3. Solids
  4. None of above
  1. General formula for alkenes is ….?
  1. $C_nH_{2n+1}$
  2. $C_nH_{2n}$
  3. $C_nH_{2n-1}$
  4. $C_nH_{2n+2}$
  1. The colour of ethene gas is …?
  1. Red
  2. White
  3. Pale Green
  4. None of above
  1. The bond length of carbon carbon double bond is … pm ?
  1. 154
  2. 143
  3. 134
  4. 120
  1. Alkenes are also knows as …?
  1. Olefines
  2. Paraffines
  3. Oleofines
  4. Paracetofines
Answer
  1. (a) Gases
  2. (b) $C_nH_{2n}$
  3. (d) None of above
  4. (c) 134
  5. (a) Olefines
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Question 34 Marks
Read the passage given below and answer the following questions from 1 to 5.

Alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions.
1) Substitution reactions One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below: Halogenation
$\text{CH}_3-\text{CH}_3+\text{CL}_2\xrightarrow{\text{hv}}\text{CH}_3-\text{CH}_2\text{Cl}+\text{HCl}$

It is found that the rate of reaction of alkanes with halogens is $F_2 > C_{l2} > Br_2 > I_2$. Rate of replacement of hydrogens of alkanes is: $3^\circ > 2^\circ > 1^\circ$ ​​​​​​​. Fluorination is too violent to be controlled. Iodination is very slow and a reversible reaction. It can be carried out in the presence of oxidizing agents like $HIO_3$ or $HNO_3$.
$\text{CH}_4+\text{I}_2\rightleftharpoons\text{CH}_3\text{I}+\text{HI}$
$\text{HIO}_3+5\text{HI}\rightarrow3\text{I}_2+3\text{H}_2\text{O}$
Halogenation is supposed to proceed via free radical chain mechanism involving three steps namely initiation, propagation and termination.

The General combustion equation for any alkane is:
$\text{C}_\text{n}\text{H}_{2\text{n}+2}+\Bigg(\frac{3\text{n}+1}{2}\Bigg)\text{O}_2\rightarrow\text{nCO}_2+(\text{n}+1)\text{H}_2\text{O}$
Combustion
Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
Due to the evolution of large amount of heat during combustion, alkanes are used as fuels. During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.

Controlled oxidation Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
Ordinarily alkanes resist oxidation but alkanes having tertiary H atom can be oxidized to corresponding alcohols by potassium permanganate .
Pyrolysis Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.

Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to 973K in the presence of platinum, palladium or nickel gives a mixture of heptane and pentene.

Conformations- Alkanes contain carbon-carbon sigma $(\sigma)$bonds. Electron distribution of the sigma molecular orbital is symmetrical around the internuclear axis of the C–C bond which is not disturbed due to rotation about its axis. This permits free rotation about C–C single bond. This rotation results into different spatial arrangements of atoms in space which can change into one another. Such spatial arrangements of atoms which can be converted into one another by rotation around a C-C single bond are called conformations or conformers or rotamers. Alkanes can thus have infinite number of conformations by rotation around C-C single bonds. However, it may be remembered that rotation around a C-C single bond is not completely free. It is hindered by a small energy barrier of $1^{-20}kJ\ mol–1$ due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain. Conformations of ethane : Ethane molecule $(C_2H_6)$ contains a carbon – carbon single bond with each carbon atom attached to three hydrogen atoms. Considering the ball and stick model of ethane, keep one carbon atom stationary and rotate the other carbon atom around the C-C axis. This rotation results into infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom. These are called conformational isomers (conformers). Thus there are infinite number of conformations of ethane. However, there are two extreme cases. One such conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation and the other in which hydrogens are as far apart as possible is known as the staggered conformation. Any other intermediate conformation is called a skew conformation. It may be remembered that in all the conformations, the bond angles and the bond lengths remain the same. Eclipsed and the staggered conformations can be represented by Sawhorse and Newman projections.
  1. Alkanes contain carbon-carbon … bonds.
  1. sigma $\sigma$
  2. pi bond$\pi$
  3. delta$\delta$
  4. eta $\eta$
  1. C-C single bond is hindered by a small energy barrier of…. $kJ ~mol^{–1}$​​​​​​​
  1. 10 - 200
  2. 1 - 20
  3. 100 - 427
  4. 342 - 786
  1. A decomposition reaction into smaller fragments by the application of heat is called as ….
  1. pyrolysis
  2. cracking
  3. both (a) & (b)
  4. combustion
  1. Which of the following steps are involving in free radical chain mechanism
  1. initiation
  2. propagation
  3. termination
  4. All the above
  1. The … reaction in which alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
  1. pyrolysis
  2. cracking
  3. both (a) & (b)
  4. combustion
Answer
  1. (a) Sigma $\sigma$
  2. (b) 1 - 20
  3. (c) Both (a) and (b)
  4. (d) All the above
  5. (d) Combustion
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Question 44 Marks
Read the passage given below and answer the following questions from $1$ to $5$.
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 $(CH_4)$ 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 $C_2H_6$. This hydrocarbon with molecular formula $C_2H_6$ is known as ethane. Thus it can consider $C_2H_6$ as derived from $CH_4$ by replacing one hydrogen atom by $-CH_3$ group. Go on constructing alkanes by doing this theoretical exercise i.e., replacing hydrogen atom by $–CH_3$ group. The next molecules will be $C_3H_8, C_4H_{10} …$

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 $109.5^\circ$ .
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 $\sigma$ 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…
  1. platinum
  2. palladium
  3. nickel
  4. All the above
  1. LPG stands for ….
  1. Liquid Pressure Gas
  2. Liquefied Petroleum Gas
  3. Liquid Platinum Gas
  4. Liquid Processed Gas
  1. Difference in properties is due to difference in their structures, they are known as …. isomers.
  1. Functional
  2. Positional
  3. Structural
  4. Chain
  1. Structural isomers which differ in chain of carbon atoms are known as… isomers.
  1. Functional
  2. Positional
  3. Structural
  4. Chain
  1. CNG Stands for ..
  1. Compressed Natural Gas
  2. Condensed Natural Gas
  3. Compressed Neutral Gas
  4. Compressed Neutral Gallium
Answer
  1. (d) All the above
  2. (b) Liquefied Petroleum Gas
  3. (c) Structural
  4. (d) chain
  5. (a) compressed natural gas
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