Hydrocarbon Important Questions

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Very Short Answer Questions [1 mark questions]

Ques: What is the structure of the alkene (C₄H₈) which is added on HBr with or without the presence of peroxide to produce the same product named C₄H₉Br.

Ans: 2-Butene that has the structure of CH₃ – CH = CH — CH₃ that is being symmetrical gives the same product, i.e., 2-bromobutane CH₃ CH (Br) CH₂CH₃.

Ques: How to separate propene from propyne?

Ans: We can separate propene from propyne when the mixture is passed through ammoniacal AgNO₃ solution where propyne reacts; on the other hand propene passes over.

Ques: Name the hydrocarbons in respect to the carbon-carbon bond.

Ans: with respect to the carbon–carbon bond that takes place between them, hydrocarbons are classified into three types: 

(a) Saturated hydrocarbon 

(b) Unsaturated hydrocarbon 

(c) Aromatic hydrocarbon.

Ques: What are cycloalkanes? 

Ans: Cycloalkanes are a type of saturated hydrocarbons that are formed when carbon atoms combine to form a closed chain or ring. It has a structural formula C_nH_2n, where n is a number of carbon atoms attached to the organic compound.

Ques: What do you understand by Lindlars’ catalyst?

Ans: The Lindlars’ catalyst is defined as partly deactivated palletized charcoal. It consists of palladium deposited on calcium carbonate or barium sulphate, which is poisoned with several forms of lead or sulphur, such as quinoline or lead acetate.

Ques: Write down the I.U.P.A.C. name of the alkane that has the lowest molecular weight and also contains a quaternary carbon?

Ans: The I.U.P.A.C. name of the alkane that has the lowest molecular weight and also contains a quaternary carbon is 2, 2-Dimethylpropane.

Ques: Name three reagents that help to distinguish between ethyne and ethane?

Ans: Tollen’s reagent, amm. CuCl solution and Ammoniacal AgNO₃ are three reagents that help to distinguish between ethyne and ethane.

Ques: What is Kolbe’s Electrolytic Method? 

Ans: Kolbe’s electrolytic method involves heating of sodium or potassium salt of carboxylic acid to produce the required alkane. 

2CH₃COO^-Na + 2H₂O ------→ CH₃-CH₃ + 2NaOH + 2CO₂ + H₂

Ques: What is Hydrogenation?

Ans: Hydrogenation refers to the preparation of alkane hydrocarbon using nickel as a catalyst. It is also popular with the name Sabatier and Sanderson’s Reaction.

CH₂=CH₂ → (+Hz/Ni) CH₃-CH₃

Ques: What is aromatic hydrocarbons? 

Ans: Aromatic hydrocarbons are a type of hydrocarbon that is popular with the name arenes and consist of only hydrogen and carbon atoms. It is used in the fields of medicine, fashion, and hygiene. 

Ques: Why does methane not react with chlorine in the dark?

Ans: Chlorination of methane is a type of substitution reaction that is based on free radicals. Methane does not react with chlorine in the dark because chlorine cannot be converted into free radicals in the dark. As a result, the reaction cannot be initiated.

Ques: Why does benzene ring become less reactive in presence of a nitro group than the unsubstituted benzene ring?

Ans: It is determined that nitro is an electron-withdrawing group that exhibits -R and -I effects. The nitro group will deactivate the benzene ring, which in turn decreases nucleophilicity for further substitution.

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Short Answer Questions [2 marks questions]

Ques: Cyclobutane is not as reactive as cyclopropane. Explain why?

Ans: Cyclobutane molecule has 90º C-C-C bond angles and in cyclopropane it has 60º. It explains that in cyclobutane, the bond angle difference between the tetrahedral and cyclopropane is smaller. On the other hand, when it is compared to cyclobutane, cyclopropane is majorly into severe strain and therefore they are more reactive in nature.

Ques: According to anti-markovnikov’s rule, the peroxide effect is not visible in the case of HCI and HI that takes place in the presence of peroxide in addition to HBr to propene. Explain.

Ans: H—Br bonds are less stable than the H—Cl bond. Thus the H-Cl bonds are not divided by the free radical, whereas the H—I bond is weak in nature and iodine consists of free radicals that combine to produce iodine molecules in place of adding it to the double bond. Therefore no peroxide effect is seen while addition to HCl and HI.

Ques: During monochlorination of 2-methylpropane what is the new hydrocarbon radicals that are formed as intermediates? Which is the more stable product of them? Give reasons.

Ans: 2-methylpropane provides radicals of two types:

• CH₃−CH(CH₃)−∗CH₂
• CH₃−∗C(CH₃)−CH₃

Tertiary 3° free radical is a more stable compared to primary 1°, since it has 9α hydrogen, and the hyper-conjugation structure is also stabilised in nature. Primary 1° free radical is not a stable radical since it comprises only 1α hydrogen as well as only one hyper conjugative structure.

That is why (2) is more stable than (1)

Ques: How is alkyne produced from vinyl dihalides?

Ans: The preparation of alkynes from dihalides can be explained in two steps:

• The process of dehydrohalogenation involves removing a halogen and a hydrogen atom which leads to the formation of vinylic halides.
• The second step involves the halides that are made to react with a strong base and produce alkynes in the process. Large alkynes are made from the Small ones in the presence of Metal acetylides.

Ques: How is cis – alkene formed by alkyne?

Ans: In the presence of palladised charcoal, alkyne are partially reduced with a distinct amount of di-hydrogen to form Cis-alkene and then they are partially deactivated using poisons like compounds such as sulphur compounds or quinoline.

Ques: Why alkenes undergo electrophilic addition reaction while arenes undergo electrophilic substitution reactions?

Ans: Alkenes consist of loosely held pi electrons, which will undergo electrophilic addition reactions and be more favourable than electrophilic substitution reactions. When arenes undergo electrophilic addition reactions the aromatic character of benzene is damaged while it remain intact during electrophilic substitution reaction.

Ques: What is Le chatelier’s principle?

Ans: Le Chatelier’s principle, also known as Chatelier’s principle, states that when there occurs any change in one variable that describes the system in equilibrium, then it will bring in the direction of equilibrium that counteracts the effect of this change. 

Ques: What is the method to produce alkene from vicinal dihalide?

Ans: Vicinal dihalides is a type of dihalides that consists of two adjacent carbon atoms attached to two halogens. When the compound react with zinc then they will lose halogen molecules which result in the formation of alkenes. This reaction is also known as dehalogenation.

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Long Answer Questions [3 marks questions]

Ques: How will you explain the structure of benzene?

Ans: In benzene, all the six carbon atoms are sp² hybridized. Each carbon atom has two hybrid orbitals that overlap with the sp2 hybrid orbital of adjoining carbon atoms that create six C-C sigma bonds in the hexagonal plane structure.

• Now, each carbon atom stays back in the sp² hybrid orbital that eventually intersects with the s-orbital of the hydrogen atom to give six C-H sigma bonds.
• For each carbon atom, only one hybridized p-orbital in a perpendicular position to the ring plane remains. 
• Carbon atoms that have unhybridized p-orbital will form the pi-bond.

Structure of benzene

Ques: Explain in brief the term polymerization with suitable examples.

Ans: When two or more molecules of unsaturated compounds combine to produce a larger complex under the given best conditions. The product that is produced aftermath is known as a polymer, and the process involved in it is called polymerization. Examples are:

(a) Additional polymerization: Nothing is wasted in this process as the larger molecule (polymer) is an exact multiple of the product produced from the smaller molecule.

(b) Condensation polymerization: During this process, molecules such as hydrochloric acid, water, and others are lost mostly. It does not even give an exact multiple of polymer produced from smaller molecules. 

Ques: How will you differentiate between the following?
a) Butyne-l & Butyne-2

Ans: (a) Butyne-1 has an acetylene hydrogen atom which gives white ppt. when ammoniacal silver nitrate & red ppt. are combined with ammoniacal cuprous chloride. On the other hand, butyne-2- that have no acetylenic hydrogen atom that does not respond to any of the two reagents mentioned above.

(b) Butene-1 & Butene-2
To distinguish between Butene-1 & butene-2, two processes can be operated i.e. Ozonolysis or

Butene-1 & Butene-2

by oxidation with acidic KMnO₂ solution with which they produce various carbonyl compounds.

Ques: What are the uses of acetylene?

Ans: Uses of acetylene are-

• Welding and cutting is done by using Oxyacetylene flame.
• It is also used as an illumining agent.
• Artificial ripening of fruits is done with the help of Acetylene.
• It is also used as a general anesthetic.

Ques: Why do addition reactions occur more often with alkenes & alkynes than with aromatic hydrocarbons?

Ans: When addition reaction occurs to an alkene or alkyne, the energy gained by forming two more sigma bonds makes up for the loss of the number of n bonds. However, in the case of aromatic hydrocarbons, the aromatic ring is firmly stabilized due to the delocalization of n electrons around the ring.

• Hence it requires strong activation energy or force to claim the loss of its aromatic character.
• The most common reaction in arenes is the substitution reaction rather than addition reaction, as substitution does not lead to the loss of aromatic characteristics of the compound.

Ques: What are different types of hydrocarbon? 

Ans: The four types of hydrocarbon are as follows:

• Saturated hydrocarbons: Compounds with a single bond between carbon atoms are known as saturated hydrocarbons. They play a significant part in petroleum fuel.
• Unsaturated hydrocarbons: Hydrocarbons that are unsaturated have at least one double or triple bond between their carbon atoms. Alkynes are organic compounds with triple bonds, while alkenes are compounds with double bonds.
• Aliphatic hydrocarbons: The chemical breakdown of fats produces alpha-hydroxycarbons. These are typically chemical compounds with one or more hydrogen atoms swapped out for halogen atoms.
• Aromatic hydrocarbons: These are identified by the presence of benzene rings. They release a variety of scents. The sole atoms in these hydrocarbons are carbon and hydrogen.

Ques: How does hydrocarbon affects the nature?

Ans: As everyone knows, hydrocarbons may be found all over the surface of the earth in the form of coal, petroleum, and crude oil. Burning these materials as fuel releases toxic compounds like carbon dioxide, which is thought to be the primary cause of global warming.

• In addition, chemical explosions, oil spills, and gas leaks in the ocean all release them into the atmosphere.
• It erodes the soil and eventually turns it into a hazardous substance that cannot be consumed by humans.
• A significant risk to human health and agriculture arises from this kind of environmental hydrocarbon exposure.

Ques: What happens when hydrogen halide is added to alkyne?

Ans:When hydrogen halide is added to the alkyne, an anti-addition reaction takes place, resulting in the formation of geminal dihaloalkane. According to Markovnikov's rule, the order of reactivity for different hydrogen halides includes HI > HBr> HCl > HF. 

• In the preparation of alkynemore, a number of hydrogen atoms is added to the carbon atoms.
• On the other hand, halogen atoms will be added to carbon atoms with fewer hydrogen atoms.
• The most stable carbocation is formed when protonation takes place. 

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Very Long Answer Questions [5 marks questions] 

Ques: Explain in brief three processes involved in the process of preparation of alkynes?

Ans: The two processes involved in the preparation of alkynes are

Dehydrohalogenation: Alkanes formed after the reaction that takes place between alkenes and halogen thereby are passed through alcoholic KOH to form substituted alkenes. It then reacts with sodium amide which results in the formation of alkynes. This whole process is also called dehydrohalogenation as a hydrogen atom is replaced along with a halogen to get an alkyne in the result. 

Dehydrohalogenation

Preparation of Alkynes from Vicinal Dihalides: Vicinal dihalides prepare Alkynes with the help of the process of dehydrohalogenation. Dihalides are obtained from respective alkenes by adding halogen (group 17 elements) in the periodic table. 

• The laboratory preparation of alkynes can be explained in two steps; the first step comprises the preparation of unsaturated halides which are also known as vinylic halides.
• They are not very reactive in nature.
• Then this halide reacts to a strong base leading to the formation of alkynes which is denoted as the second step as well.
• Metal acetylides convert small alkynes into large ones in the process.

Preparation of Alkynes from Vicinal Dihalides

Preparation of Alkynes from Calcium Carbide: Calcium carbide is a major component that facilitates the process of alkynes synthesis on a larger scale. It is prepared by heating quicklime (CaO) in the presence of coke (C).

• The calcium carbide is later reacted with water, which leads to the evolution of calcium hydroxide and acetylene.
• The reaction is as follows:

CaCO₃ \(\to\) CaO + CO₂

CaO + 3C \(\to\) CaC₂ + CO

CaC₂ + 2H₂O \(\to\) Ca (OH)₂ + C₂H₂

Ques: Write the formula involved in the conversion of the following compounds to benzene?

(i). Acetylene (ii). Cyclohexane (iii). Benzene diazonium chloride.

Answer:  (i) Acetylene 

When ethylene is heated to a higher temperature it polymerizes into benzene. The reaction is given below:

3C₂H₂  \(\to\) C₆H₆

(ii) Cyclohexane 

Ans: When cyclohexane is exhibited to iron or quartz in a red hot tube, it starts to oxidise that leads to produce benzene. The reaction is given below:

Cyclohexane 

(iii) Benzene diazonium chloride

Ans: Benzene diazonium chloride is converted to benzene in the presence of hypophosphorous acid. The reaction is given below:

Benzene diazonium chloride

Ques: An alkyl halide C₅H₁₁Br (A) reacts to ethanolic KOH and produce an alkene ‘B’, which then reacts with Br₂ to produce a compound ‘C’, which goes through dehydrobromination and gives an alkyne with ‘D’. On treating with sodium metal in the presence of liquid ammonia, one mole of ‘D’ produces one mole of the sodium salt of ‘D’ and half a mole of hydrogen gas. Complete process of hydrogenation of ‘D’ produces a straight-chain alkane. Identify A, B, C and D from the given and also mention about the reactions involved.

Ans: Here the reaction implies that (D) is a terminal alkyne which means a triple bond is present at the end of the chain. It could be either (I) or (II).

Since alkyne ‘D’ when going through the process of hydrogenation gives out straight-chain alkane, the structure I is alkyne (D).

Hence, the structure of A, B and C are as follows:

• CH₃-CH₂-CH₂-CH₂-CH₂Br
• CH₃-CH₂-CH₂-CH=CH₂
• CH₃-CH₂-CH₂-CH(Br)-CH₂Br

Ques: Why ethyne is acidic in comparison to ethene and ethane?

Ans: In ethyne, hydrogen atoms are attached to sp hybridised carbon atoms. On the other hand, they are bonded with sp2 hybridised and sp3 hybridised carbon atoms in ethene and ethane, respectively.

• Ethyne molecules contain the largest percentage of s-characters, due to which sp hybridised orbitals have the highest electronegativity of any orbital.
• The sp2 hybridised and sp3 hybridised carbon atoms in case of ethene and ethane will attracts the shared pair of the C-H bond of ethyne more strongly.
• And lastly, hydrogen atoms can easily be realised as protons in the ethyne in comparison to ethene and ethane.

Ques: Why is the Wurtz reaction not used for the preparation of alkane hydrocarbons having an odd number of carbon atoms?

Ans: The Wurtz reaction, which employs two identical alkyl halides as reactants to produce an alkane with twice as many carbon atoms, is the sole method available for synthesizing symmetrical alkanes, or alkanes with an even number of carbon atoms.

• For instance unsymmetrical alkanes cannot be produced by the Ethane Wurtz reaction because it yields a mixture of alkanes when two distinct alkyl halides are utilized as the reactants.
• The reaction involves species of free radicals.
• As a result, there is also a side reaction that produces an alkene.
• For example, the reaction between bromoethane with iodoethane yields a variety of alkanes.

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