Aliphatic Hydrocarbon | Question Bank for Class 12 Chemistry

Ans. Coal and petroleum are two natural sources of hydrocarbons.

Ans. Petroleum is a complex mixture of solid, liquid and gaseous hydrocarbons.

Ans. LPG mainly contains butane and Isobutane.

Ans. Octane number.

Ans. Diesel < kerosene oil < gasoline.

Ans. The compound added to LPG to detect its leakage is ethyl mercaptan $\left(C_{2} H_{5} S H\right)$

Ans. Fuel oil is used as furnace fuel in metallurgical operations.

Ans. A sharp metallic rattling sound produced in an internal combustion engine, is called knocking.

Ans. Octane number is 95.

Ans. - Those compounds which reduces knocking and help in smooth combustion of gasoline are called antiknocking compounds, e.g. tetra ethyl lead $\left(C_{2} H_{5}\right)_{4} P b$

Ans. It is because individual dipoles are equal and opposite such that net dipole moment is zero

Ans. They are called paraffins because under normal conditions alkanes are inert towards common reagents [In Latin, Parum = little, affinis =affinity]

Ans. It is because the difference in their energy is less.

Ans. Lindlar's catalyst is $P d / B a S O_{4} .$ It is used to convert Alkynes to alkenes with the help of $H_{2}$

Ans. $\mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \frac{\mathrm{Ni}}{573 \mathrm{K}} \mathrm{CH}_{3}-\mathrm{CH}_{3}$

Ans. Due to their straight linear geometry.

Ans. X is glyoxal, i.e., ethane 1, 2-dial.

Ans. In the presence of $H g^{+},$ ethyne adds acetic acid to give vinyl ester.

Ans. - Arenes are aromatic hy drocarbons. $C_{n} H_{2 n-6}$ is general formula for monocyclic arenes.

Ans. Gammaxene.

Ans. $\mathrm{SO}_{3} \mathrm{H}^{+}$

Ans. It acts as a base.

Ans. With ozone $o$ -xylene forms triozonide that supports the structure of benzene as cyclohexa- $1,3,5$ triene.

Ans. The crude oil is washed with acidic or basic solution depending upon whether the impurities are basic or acidic in nature. The washed oil is then subjected to fractional distillation by heating it to about $723 K$ and allowing the vapours to pass through a fractionating column. The crude oil is separated into various fractions.

Ans. CNG is compressed natural gas. It mainly consists of methane, $(85$ to $95 \%$ ). Ethane and propane are also present in small quantities. Natural gas is formed in the earth crust by the decomposition of vegetable-matter lying under water by the action of anaerobic bacteria so mostly it is found along with petroleum deposits.

Ans. The staggered conformer of ethane is more stable than eclipsed conformer, because in staggered conformer, the H-atoms are far apart. So, the repulsion is lesser than in eclipsed conformation. The difference in the energy content of staggered and eclipsed conformation is $12.5 \mathrm{kJ} \mathrm{mol}^{-1}$ The order of stability of conformers is in the following order: Staggered $>$ gauche (skew) $>$ eclipsed.

Ans. (i) E (ii) E (iii) E (iv) Z

Ans. Cracking : "The process of breaking down of hydrocarbons of high molecular mass and higher boiling points into smaller hydrocarbons of low molecular mass and lower boiling points, is termed as cracking. For Example: Types of cracking : The process of cracking is of two types: (i) Thermal cracking. (ii) Catalytic cracking. (i) Thermal cracking: When cracking is carried at high temperature, it is called thermal cracking. (ii) Catalytic Cracking: When cracking is carried at low temperature in the presence of a catalyst such as alumina or silica, is called catalytic cracking.

Ans. Each $1^{\circ}$ Carbon is attached to 3 hydrogen, Each $2^{\circ}$ carbon is attached to 2 hydrogen atoms. Each tertiary 'C' atom is bonded to one and quaternary $^{\epsilon} \mathrm{C}^{\prime}$ is not attached to any hydrogen atom. All five Carbon atoms are attached to total 15 hydrogen atoms. TwoCarbon atoms are attached to total 4 hydrogen atoms. OneCarbon atom is attached to total one hydrogen atom OneCarbon atom is not attached to any hydrogen atom.

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Ans. Chair form is more stable than boat form.

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Ans. Propane has only one eclipsed conformer Butane has three eclipsed forms due to changes in dihedral angles. (i), (ii) and (iii) Newman eclipsed projection of butane.

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Ans. (i) 5 -methyl - hept $1,3,6$ -triene. (ii) 3 methyl pent $-1$ -ene- 4 -yne.

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Ans. Conjugated diene undergo 1, 4 addition due to more resonance stabilization of intermediate carbocation.

Ans. (iii) Buta-1, 3-diene undergo polymerisation reaction readily because it will form resonance stabilized intermediate.

Ans. A compound is said to aromatic if there is delocalization of $\pi-$ electrons and it has planar structure. It should follow Huckel's Rule, i.e., in a conjugated, planar, cyclic system if number of delocalized electrons is $(4 n+2) \pi,^{c} n^{\prime}$ being integer, $i . e ., 1,2,3,$ etc. it will be aromatic, otherwise not, e.g. Benzene, naphthalene, anthracene have $6,10,14$ electrons in conjugated, planar, cyclic system.

Ans. These products prove the structure I and II (Kekule structures of benzene).

Ans. $-H-C \equiv C-H$ has $s p$ hybridised carbon which is most electronegative due to $50 \%$ s-character therefore it is most acidic. In Benzene, each carbon is $s p^{2}$ hybridised carbon $(33 \%$ of character), which is less electronegative than hybridised carbon, therefore, it is less acidic. In hexane, each $^{c} C^{\prime}$ is hybridised carbon $(25 \%$ s-character), which is least electronegative, therefore, it is least acidic.

Ans. Benzene has $\pi-$ electrons, therefore, it can undergo electrophilic substitution reactions easily. Nucleophilic substitution are difficult because nucleophile will be repelled by $\pi$ -electrons.

Ans. since electron density is maximum at $o$ and $p$ -positions $S O_{3}$ will attack at $o$ and $p$ -positions. $\mathrm{NO}_{2}$ will attack at $o$ and $p$ -positions because electron density is maximum at ortho and $p$ -positions.

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Ans. (i) When sodium acetate is heated with soda lime, methane gas will be formed (ii) When acetylene is passed through ammonical silver nitrate solution, white precipitate of silver acetylide will be formed. (iii) When chlorobenzene is condensed with chloral, in presence of conc. $H_{2} S O_{4},$ DDT is formed.

Ans. (i) Friedel Crafts Reaction

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Ans. Petroleum gas : It is a mixture of butane, propane, propene, isobutane and ethane. The main constituent of petroleum gas is butane. At ordinary pressure butane $\left(C_{4} H_{10}\right),$ Propane $\left(C_{3} H_{8}\right)$ and ethane $\left(C_{2} H_{6}\right)$ are gases. Under high pressure, they are easily liquefied. The petroleum gas which has been liquefied under pressure is called liquefied petroleum gas (LPG). The domestic gas cylinder contains LPG. LPG a better domestic fuel : Components of LPG burn easily and produce a lot of heat, so LPG is used as a good domestic fuel. Due to the following qualities, LPG is considered as a better fuel. (i) $\mathrm{LPG}$ is an efficient fuel as it has high calorific value, burning of 1 gram of LPG in air produces about 50 kilo joule of heat energy. (ii) LPG burns with a smokeless flame, so causes no pollution. (iii) It leaves no ash on burning so it is a clean fuel. (iv) On burning no poisonous gases are formed. (v) It is easy to handle and convenient in storing.

Ans. Petroleum : It is a naturally occurring fossil fuel that is dark coloured, viscous and foul smelling oil. It is a mixture of several hydrocarbons and other elements such as sulphur, oxygen and nitrogen. Mostly it occurs deep down the earth between two nonporous rocks, that's why it is called as petroleum. (petra =rock, oleum $=$ oil ) or crude - oil. Principle of refining of petroleum :”The process of separating different components of petroleum, is known as refining. It is carried out by the process of fractional distillation which is based on the fact that different components of petroleum have different boiling point ranges. The crude-oil is heated in a tank to a temperature of about and all the vapours formed are fed into a tall fractionating column near the bottom. Except asphalt, all the other components are in vapour state. As the mixture of hot vapours rises up in the column, it begins to cool The vapours of components having higher boiling point-range condense first in the lower part and vapours of components with lower boiling point-range condense later at the upper part of column. Therefore, the components with the highest boiling point condenses first and is collected in the lowest part of the column and the component having lowest boiling point condenses last and is collected in the top most part of column. The vapours that do not liquify, are taken out from the top of the column. This component is called petroleum-gas. The residual oil or liquid residue which does not vapourise in the column is called residual oil. It is collected in the base of column and subjected to further distillation. Products of petroleum : The various components obtained at different heights of fractional column, (in order of top to bottom of column) are as follows :

Ans. (i) Reaction with sulphuric acid: Cold concentrated sulphuric -acid reacts with alkenes to gives addition products (ii) Reaction with halogen : Halogens (chlorine and bromine only react at ordinary temperature and without exposure to $U V$ light to form vicinal halide. (iii) Oxidation (a) Oxidation with dilute aqueous potassium permanganate solution : At low temperature alkenes form vicinal glycols. [This reaction is called hydroxylation as hydroxyl groups are attached across the double bond. $]$ (b) With alkaline potassium permanganate at higher temperature : In such conditions cleavage of $C=C$ bond occurs forming carboxylic acids. (iv) Ozone: With ozone alkenes form ozonides which undergo cleavage to produce carbonyl compounds. This method was extensively used to determine the position of double bond in alkenes in past. Procedure : Ozone is passed through a solution of an alkene in inert solvent such as $\mathrm{CH}_{2} \mathrm{Cl}_{2}, \mathrm{CHCl}_{3}$ or $\mathrm{CCl}_{4}$ at temperature $196-200 K .$ Alkenes are oxidised to ozonised which are not usually isolated but reduced in situ, with zinc dust and water or $H_{2} / P d$ to give carbonyl compound. The location of double bond is assigned on the basis of products. (a) In case of symmetrical alkene, only one aldehyde is formed on reduction with zinc and water: $\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}+\mathrm{O}_{3} \longrightarrow$ (b) If double bond is terminal, methanal is one product and a higher aldehyde is other product. $\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{O}_{3} \longrightarrow$ (c) If ketone is formed, branching is there at C atom having double bond.

Ans. Let us first try to write the structure of alkene. The product of ozonolysis are :

Ans. (i) Octane number is defined as percentage of iso-octane in the mixture of iso-octane and $n$ -heptane which gives same knocking as sample fuel. Higher the octane number, lesser will be knocking. (ii) Functional isomerism.

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