HydroCarbons MCQ Quiz - Objective Question with Answer for HydroCarbons - Download Free PDF

CONCEPT:

Ozonolysis of Alkenes

• Ozonolysis: Ozonolysis is a chemical reaction in which an alkene is cleaved by ozone (O₃) followed by reduction with a reducing agent such as zinc (Zn) and water (H₂O). The alkene is broken down into carbonyl-containing products like aldehydes and ketones.
• The positions of the aldehyde and ketone groups formed are determined by the original position of the double bond in the alkene.

EXPLANATION:

​
3-Methyl-6-ketoheptanal 

• Option 1: This structure will not undergo ozonolysis to produce 3-methyl-6-oxoheptanal, as the functional group pattern is not correct.
• Option 2: This is the correct structure. Upon ozonolysis, it will give **3-methyl-6-oxoheptanal** by cleaving at the appropriate locations.
• Option 3 and 4: These compounds will not provide the desired product of 3-methyl-6-oxoheptanal upon ozonolysis.

Therefore, the correct answer is the compound that gives 3-methyl-6-oxoheptanal upon ozonolysis.

CONCEPT:

Reaction of Bromine Water with Compounds

• Bromine water is commonly used to test for unsaturation in organic compounds. Unsaturated compounds like alkenes and alkynes decolorize bromine water by reacting with it to form dibromides or addition products.
• However, compounds that are either saturated or have functional groups that do not undergo addition reactions with bromine will not decolorize bromine water.
• In this question, we are looking for a compound that does not react with bromine water and hence does not decolorize it.

EXPLANATION:

• Option 1: Cyclohexane (C6H12): Cyclohexane is a saturated hydrocarbon (alkane). Alkanes do not undergo addition reactions with bromine water because they do not have double or triple bonds. Therefore, it will not decolorize bromine water.
• Option 2: Phenol (C6H5OH): Phenol has a hydroxyl group attached to an aromatic ring. While phenol does not have double bonds like alkenes or alkynes, the hydroxyl group can act as an electron-donating group, but phenol can still decolorize bromine water by reacting with it, especially under certain conditions (e.g., electrophilic substitution).
  
• Option 3: Styrene (C6H5CH=CH2): Styrene contains a C=C double bond (alkene), which can easily react with bromine water, decolorizing it by undergoing an addition reaction to form a dibromide.
  
• Option 4: Aniline (C6H5NH2): Aniline has an amine group (-NH2), which is electron-donating, and can participate in reactions with bromine, leading to the decolorization of bromine water by forming substitution products like brominated aniline.
  

Therefore, the compound that does not decolorize bromine water is Cyclohexane (Option 1), as it is a saturated alkane and does not have any reactive sites like double bonds or groups that could react with bromine.

Thus, the correct answer is Option (1): Cyclohexane.

CONCEPT:

Effect of Hybridization on Bond Dissociation Energy (BDE)

• The bond dissociation energy (BDE) for C-H bonds depends significantly on the hybridization of the carbon atom involved in the bond:
  • sp³ hybridized carbon: The C-H bond in alkyl groups (sp³) has a relatively higher bond dissociation energy. The electron density around the carbon is more spread out due to the tetrahedral arrangement of the bonds.
  • sp² hybridized carbon: The C-H bond in alkenes or aromatic compounds (sp²) has a lower bond dissociation energy. This is because sp² carbon has more s-character, which pulls the electron density closer to the nucleus, weakening the C-H bond.
  • sp hybridized carbon: The C-H bond in alkynes (sp) has an even lower bond dissociation energy due to the high s-character in the sp hybridization, which leads to a stronger bond between the carbon and hydrogen, but lower C-H bond dissociation energy compared to sp² carbon.
• In this problem, the bond dissociation energy varies based on the hybridization of the carbon atoms to which the hydrogen atoms are bonded.

EXPLANATION:

• I: Benzyl group (sp² carbon attached to the aromatic ring) s-character is 33%
• II: sp carbon attached to a CC Triple bond) s-character  is 50%
• III: Alkane (sp³ carbon) s-character  is 25%

Bond Strength \(\propto \) % s-character 

• The bond dissociation energy follows the trend:
  • sp² (Benzyl) < sp  < sp³ (Alkane)

Therefore, the correct order of C-H bond dissociation energy is II > I > III.

CONCEPT:

Free Radical Chlorination and Stereoisomerism

• In free radical halogenation reactions, such as chlorination (Cl₂/hv), chlorine atoms replace a hydrogen atom on a carbon chain, leading to the formation of different products.
• The reaction proceeds via a radical mechanism, where a chlorine molecule (Cl₂) undergoes homolytic cleavage under the influence of light (hv), producing two chlorine radicals.
• The chlorine radical then reacts with a hydrogen atom on the carbon chain, resulting in the substitution of a hydrogen atom with a chlorine atom, forming a product and a new carbon-centered radical.
• For this type of reaction, stereoisomerism can occur if a chiral center is created, meaning two enantiomers (R and S) may form for each substitution site that creates a chiral center.

EXPLANATION:

• In the given compound:

  CH₃-CH₂-CH₂-CH₃ (butane)

  The monochlorination can occur at different positions along the carbon chain, specifically at the 2^nd and 3^rd carbon atoms, where the chlorine atom can replace a hydrogen atom.
• When chlorine atoms are substituted at the 2^nd and 3^rd carbons, they create chiral centers at those positions, leading to the formation of two stereoisomers (R and S forms) for each of these sites.
• 
• The possible products include:
  • Chlorination at the 2^nd carbon creates a chiral center, leading to 2 stereoisomers (R and S forms).
  • Chlorination at the 3^rd carbon also creates a chiral center, leading to 2 stereoisomers (R and S forms).
  • Chlorination at the 1^st and 4^th carbons does not create chiral centers and leads to only one product each.
• Thus, the total number of products formed from monochlorination, including stereoisomers, is 6: 2 from the 2^nd carbon, 2 from the 3^rd carbon, and 1 each from the 1^st and 4^th positions.

Therefore, the correct answer is Option (4) 6.

CONCEPT:

Cis-Trans Isomerism

• Cis-trans isomerism (or geometric isomerism) occurs when a molecule has restricted rotation, typically due to the presence of a double bond or a ring structure, and two distinct groups are attached to the atoms involved in the restricted rotation.
• For cis-trans isomerism to exist:
  • In alkenes: The carbon atoms in the double bond must each have two different groups attached to them.
  • In cyclic compounds: The ring restricts rotation, and two substituents attached to different carbons in the ring can be oriented on the same side (cis) or opposite sides (trans).

EXPLANATION:

• Pent-1-ene:
  • Pent-1-ene has a terminal double bond (C₁=C₂), and one of the carbons in the double bond (C₁) has two identical hydrogen atoms. Therefore, it cannot exhibit cis-trans isomerism.
    
• 2-Methylhex-2-ene:
  • The double bond in 2-methylhex-2-ene (C₂=C₃) has two same groups attached to both C₂. Therefore, it can not exhibit cis-trans isomerism.
    
• 1,1-Dimethylcyclopropane:
  • In 1,1-dimethylcyclopropane, both methyl groups are attached to the same carbon atom in the ring. Since the substituents are not on different carbons, cis-trans isomerism is not possible.
    
• 1,2-Dimethylcyclohexane:
  • In 1,2-dimethylcyclohexane, the methyl groups are attached to different carbons in the ring, and the ring restricts rotation. Therefore, it can exhibit cis-trans isomerism.
  • 

Therefore, the compounds that can exhibit cis-trans isomerism is 1,2-Dimethylcyclohexane

The correct answer is Si.Key Points

• Catenation refers to the ability of an element to form bonds with other atoms of the same element, resulting in the formation of long chains or rings.
• Carbon is well known for its catenation property, which is why it can form a vast number of organic compounds. 
• Silicon (Si), shows a catenation property like carbon.
• This is because silicon has four valence electrons, like carbon, and can form covalent bonds with other silicon atoms.

Additional Information

• Neon (Ne) does not show catenation property as it is a noble gas and does not readily form bonds with other atoms.
• Oxygen (O) can form limited catenation, but not to the same extent as carbon or silicon.
  • This is because oxygen has only two valence electrons and can only form two covalent bonds with other oxygen atoms.
• Potassium (K) is a metal and does not show catenation property as metals typically lose electrons to form positive ions and do not form covalent bonds with other atoms of the same element.

The correct answer is Ethanol

Key Points

• Ethanol is obtained from the hydration of ethene. The process of hydration involves adding water (H₂O) to ethene (C₂H₄) in the presence of a catalyst.
• The chemical equation for the reaction is: C₂H₄ + H₂O → CH₃CH₂OH
• In this reaction, the double bond in ethene is broken, and the carbon atoms form new bonds with the hydrogen and hydroxyl (OH) groups of water, resulting in the formation of ethanol (CH₃CH₂OH).
• This reaction is an example of an addition reaction, where two or more reactants combine to form a single product. The hydration of ethene is an important industrial process for the production of ethanol, which is used as a fuel, solvent, and in the production of various chemicals.

Explanation:

Green fruits can be ripened by providing acetylene artificially.

Usually, fruits are wrapped in paper with calcium carbide, and water is sprinkled over them.

In reaction with water, calcium carbide produces acetylene gas.

Acetylene gas is used for the artificial ripening of green fruits. Additional Information

• Ethylene is a natural plant hormone that helps in the ripening of fruits.
• It is considered the aging hormone of plants and can also cause a plant to die.
• Ethylene, unlike the rest of the plant hormones, is the only gaseous hormone.
• Ethylene is produced in all higher plants and is produced from methionine in essentially all tissues.

Option 4 is NOT correct.

Key Points

• LPG is Liquified Petroleum Gas and CNG is Compressed Natural Gas.
• CNG contains Methane Gas and LPG contain mainly Propane and Butane.
• They both are Alkanes.
  • Alkanes are comprised of a series of compounds that contain carbon and hydrogen atoms with single covalent bonds.
  • This group of compounds consists of carbon and hydrogen atoms with single covalent bonds.
  • Also, it comprises a homologous series having a molecular formula of C_nH_2n+2.
• The calorific value of LPG is 90 to 95MJ/m³ and that of CNG is 35 to 40MJ/m³. Hence, Option 4 is NOT correct.
• LPG finds application in Domestic and Industrial use.
• CNG is used as an alternative fuel in an automobile.

Concept:

Isomers:

• These are compounds that have the same molecular formula but different structures or stereochemistries.
• There is a wide range of classification of organic molecules based on their structures.

Ring -Chain isomers:

• Ring chain isomerism is a type of structural isomerism.
• It is shown by compounds that are capable of forming stable ring compounds. The minimum number of carbons that must be present in order to show ring chain isomerism is three.
• The phenomenon of a compound existing in open-chain as well as ring form is called ring chain isomerism.
• Cyclic compounds formed by rings of 3,4,5,6 carbon atoms are called propane, butane, pentane hexane respectively.

Explanation:

• The formula of n-hexane is C₆H₁₄, it contains 6 carbon atoms.
• Hexane can form open chain as well as closed or cyclic compounds.
• There are 5 possible isomers of n-Hexane in Chain form which are formed by the different arrangement of carbon atoms along the chain.
• They are given below:

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Hence, there are 5 chain isomers of n-Hexane.

• There are six cyclic isomers of n-Hexane. 

Additional Information

The number of isomers and the carbon atoms are given below:

Concept:

⇒ Polysubstitution is a major drawback of Friedel craft’s alkylation. It is so because activating behaviour of the benzene ring increases with an increase in the number of the alkyl group on the benzene ring.

Additional Information

Poly-substitution is a major drawback in Friedel Craft's acylation because, the alkylated product obtained is more activated than the reactant, hence it undergoes poly substitution.

• The introduced alkyl group is activating and gives polyalkylated products.
• The Fridel Craft’s acylation is defined as an electrophilic aromatic substitution reaction in which hydrogen-bonded to an aromatic ring is substituted by an acryl group. Usually, benzene reacts with acid chloride and AlCl₃ to form an aryl ketone.
• Acylation is the process of adding an acyl group to a compound. The compound providing the acyl group is called the acylating agent.
• Acylation can be used to prevent rearrangement reactions that would normally occur in alkylation.
• Friedel-Crafts reactions cannot be performed when the aromatic ring contains an NH₂, NHR, and NR₂ substituent. Lastly, Friedel-Crafts alkylation can undergo polyalkylation.
• This reaction adds an electron-donating alkyl group, which activates the benzene ring to further alkylation.
• Friedel-Crafts Acylation is an important reaction to form several biological compounds, including DNA. Friedel-Crafts Acylation reacts a Lewis Acid, AlCl₃, with an acyl halogen to form an acylium ion.
• This acylium ion is very electrophilic, so the extra electrons from an aromatic compound can stabilize it.

• This reaction has several advantages over the alkylation reaction. Due to the electron-withdrawing effect of the carbonyl group, the ketone product is always less reactive than the original molecule, so multiple acylations do not occur.
• Also, there are no carbonation rearrangements, as the acylium ion is stabilized by a resonance structure in which the positive charge is on the oxygen.

Explanation:

• Amino, hydroxyl and methyl groups are electron releasing groups. They increase the electron density on the benzene ring. 
• Aniline, phenol and toluene are more reactive towards electrophilic aromatic substitution reaction than nitrobenzene.

• Nitro group is an electron-withdrawing group. It removes electron density from the aromatic nucleus.
• The reactivity of nitrobenzene towards electrophilic aromatic substitution reaction is less.

So, Nitrobenzene molecule is less reactive towards electrophilic aromatic substitution.

Concept:

The reactant bromocyclohexane on reaction with Ko^tBu (Potassium tetra-butoxide) to form the intermediate product, then it reacts with O₃ (ozone or trioxygen) and with dimethyl sulfide to produce the final product.

Concept:

Here A is propyne.

The propyne on reaction with H₂SO₄ (sulphuric acid) and HgSO₄ (mercury (II) sulphate) to produce the intermediate product hydroxyl propyl. Then the hydroxyl propyl undergo tautomerise process.

Tautomers are structural isomers of chemical compounds that readily interconvert. This reaction commonly results in the relocation of a proton. 

After tautomerise process, the reactant produces the acetone. Then acetone react with Sodium tetrahydridoborate which gives the final product as acetone.

This reacts in the presence of HCl and ZnCl₂ and with Lucas reagent which produce the 2° alcohol on turbidity within 5 minutes.

Concept:

The carbocation is formed on reaction with Ag⁺ is given below:

(CH₃)₃CCl → (CH₃)₃C⁺