Most Important Hydrocarbons MCQs with Answers | Chemistry

The pi (π) bond in alkenes is weaker and more exposed than sigma bonds, making it susceptible to electrophilic attack. This makes alkenes generally more reactive than alkanes, which primarily undergo substitution reactions.

Isomers are compounds that have the same molecular formula but differ in the arrangement of atoms. Structural isomers (or constitutional isomers) specifically have different connectivity of atoms.

Geometric isomerism (a type of stereoisomerism) arises when there is restricted rotation around a bond (like a carbon-carbon double bond) and different groups are attached to the carbons of the double bond, leading to distinct spatial arrangements (cis or trans).

Dehydration of alcohols involves the removal of a water molecule. In the case of ethanol, under the influence of a strong dehydrating agent like concentrated sulfuric acid, ethene (CH₂=CH₂) is formed.

Dehydrohalogenation is an elimination reaction where a hydrogen atom from one carbon and a halogen atom from an adjacent carbon are removed, typically in the presence of a strong base, to form an alkene.

Hydrogenation is the addition of hydrogen (H₂) across a double bond, typically in the presence of a metal catalyst, converting an alkene into an alkane.

Markovnikov's Rule states that in the electrophilic addition of HX (where X is a halogen) to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that already has a greater number of hydrogen atoms.

Hydration is the addition of water across a double bond, typically catalyzed by an acid, leading to the formation of an alcohol.

This is an example of halohydration. When ethene reacts with bromine water, the bromine adds to one carbon and a hydroxyl group (-OH) from water adds to the other carbon of the double bond, resulting in 2-bromoethanol.

Polymerization is a process where many small monomer units join together to form a large polymer molecule. When ethene (ethylene) monomers polymerize, they form polyethylene.

Structural isomers have the same molecular formula but different connectivity of atoms. But-1-ene (C₄H₈) and 2-methylpropene (C₄H₈) both have the formula C₄H₈ but differ in the arrangement of their carbon atoms and double bond position. But-2-ene is a positional isomer, not a structural isomer of but-1-ene in the context of different carbon skeletons.

Halogenation of alkenes is an addition reaction where halogen atoms add across the double bond. For ethene and chlorine, the product is 1,2-dichloroethane.

This is a hydrohalogenation reaction. The hydrogen atom adds to one carbon of the double bond and the chlorine atom adds to the other, resulting in chloroethane.

The pi (π) bond in alkenes contains loosely held, exposed electrons that act as nucleophiles and are readily attacked by electrophiles, leading to addition across the double bond.

Hydrogenation of alkenes typically requires a metal catalyst such as platinum, palladium, or nickel to facilitate the addition of hydrogen across the double bond.

But-1-ene has a straight four-carbon chain, while 2-methylpropene has a branched three-carbon chain. They have the same molecular formula (C₄H₈) but different carbon skeletons, which defines chain isomerism.

This is the direct addition of water to the alkene across the double bond, resulting in an alcohol. This process is called hydration.

Vicinal dihalides undergo dehalogenation when treated with zinc dust, where the two halogen atoms are removed, forming an alkene. This is a common method for alkene preparation.

Each carbon atom in benzene is sp² hybridized. This leads to a planar hexagonal structure where each carbon forms three sigma bonds (two with adjacent carbons and one with a hydrogen atom).

The extra stability of benzene due to the delocalization of its pi electrons is quantified as resonance energy. This energy makes benzene significantly more stable than a hypothetical cyclohexatriene. While aromaticity is the property, resonance energy is the result of this delocalization contributing to stability.

Despite having double bonds, benzene's aromatic stability is preserved by substitution reactions (specifically electrophilic aromatic substitution) where a hydrogen atom is replaced, rather than addition reactions which would disrupt the delocalized pi system.

While benzene primarily undergoes substitution, it can undergo addition reactions under specific harsh conditions, such as hydrogenation (addition of hydrogen) to form cyclohexane, which breaks the aromaticity.

In electrophilic aromatic substitution, the electrophile attacks the pi electron cloud of the benzene ring, forming a resonance-stabilized carbocation intermediate, also known as a sigma complex or arenium ion.

The electrophile for nitration is the nitronium ion (NO₂⁺), which is generated by the reaction of nitric acid with sulfuric acid (HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻).

Electrophilic halogenation of benzene requires a Lewis acid catalyst (like FeCl₃ for chlorination or FeBr₃ for bromination) to polarize the halogen molecule and generate a more powerful electrophile.

Anhydrous aluminum chloride (AlCl₃) is the most common Lewis acid catalyst used in Friedel-Crafts alkylation to generate the electrophilic carbocation from an alkyl halide.

The active electrophile for sulfonation is sulfur trioxide (SO₃), which is present in fuming sulfuric acid or generated from concentrated sulfuric acid.

This statement is false. Benzene preferentially undergoes substitution reactions to maintain its aromatic stability, unlike typical alkenes which readily undergo addition reactions.

The methyl group (-CH₃) is an activating group and an ortho/para director in electrophilic aromatic substitution due to its electron-donating effect via hyperconjugation.

This is a Friedel-Crafts acylation reaction. An acyl group (RCO-) is introduced onto the benzene ring, forming a ketone where one of the groups attached to the carbonyl is a phenyl group.

The nitro group (-NO₂) is a strong electron-withdrawing group, which deactivates the benzene ring towards electrophilic attack and directs incoming electrophiles to the meta position.

Friedel-Crafts alkylation involves carbocations, which can undergo rearrangements, leading to mixtures of products. Friedel-Crafts acylation, on the other hand, involves stable acylium ions, which do not rearrange, thus providing better control over the product.

Toluene has a methyl group (-CH₃), which is an electron-donating group. Electron-donating groups activate the benzene ring towards electrophilic substitution by increasing electron density, making the reaction faster than with benzene itself.

The addition of hydrogen to benzene (hydrogenation) in the presence of a catalyst (e.g., Ni, Pt) converts it into cyclohexane, breaking the aromatic system and forming an aliphatic cyclic compound.

In the absence of a Lewis acid catalyst, and under UV light, benzene can undergo free radical addition reactions, leading to the addition of six chlorine atoms across the ring, forming hexachlorocyclohexane (e.g., Lindane).

Methoxy group (-OCH₃) is an electron-donating group via resonance, which activates the ring and directs incoming electrophiles to the ortho and para positions.

Sulfonation is reversible. Heating benzenesulfonic acid with steam or dilute aqueous acid can reverse the reaction, removing the sulfonic acid group (removal of -SO₃H group) and regenerating benzene.

Activating groups donate electron density to the benzene ring, making it more nucleophilic and thus more susceptible to attack by an electrophile, increasing the reaction rate.

Friedel-Crafts acylation involves the reaction of an acyl halide (RCOCl) or an acid anhydride with benzene in the presence of a Lewis acid catalyst like anhydrous AlCl₃.

The resonance hybrid concept implies that the pi electrons are delocalized over the entire ring, resulting in all C-C bonds having equal length and character, intermediate between typical single and double bonds.

Reaction with Br₂ in the presence of a Lewis acid catalyst (FeBr₃) is electrophilic aromatic substitution (bromination), where a bromine atom substitutes a hydrogen atom on the benzene ring.

Carboxyl groups (-COOH) are electron-withdrawing by resonance and inductive effects, strongly deactivating the ring and directing incoming electrophiles to the meta position.

The delocalized pi electron system and significant resonance energy of benzene make it very stable and resistant to oxidation under conditions that would typically oxidize alkenes.

Alkenes are generally more reactive than alkynes towards electrophilic addition. While alkynes have a triple bond, the sp hybridized carbons make the pi electrons more tightly held. Also, the carbocation formed from an alkyne is less stable than that from an alkene. Benzene undergoes substitution due to aromaticity, and alkanes are saturated.

Alkynes are typically prepared through double elimination reactions, such as the dehydrohalogenation of dihaloalkanes (vicinal or geminal) using strong bases.

The carbon atom in the C-H bond of a terminal alkyne is sp hybridized, which means it has higher s-character (50%) than sp² (33%) or sp³ (25%). This higher s-character makes the carbon more electronegative, drawing electron density closer to the nucleus, and thus making the hydrogen more acidic and easier to remove as a proton.

Complete hydrogenation of an alkyne (acetylene) involves the addition of two moles of hydrogen across the triple bond, converting it into a fully saturated alkane. Thus, ethyne becomes ethane.

Lindlar's catalyst (Pd/BaOS₄ + quinoline) is a poisoned palladium catalyst that allows for the selective partial hydrogenation of alkynes to cis-alkenes, preventing further reduction to alkanes.

In the hydrohalogenation of an unsymmetrical alkyne like propyne, the hydrogen atom adds to the carbon of the triple bond that has more hydrogen atoms (the terminal carbon), and the bromine adds to the more substituted carbon, resulting in 2-bromopropene.

Hydration of alkynes in the presence of mercuric sulfate and sulfuric acid proceeds via an enol intermediate, which rapidly tautomerizes to a more stable carbonyl compound. Ethyne yields acetaldehyde (an aldehyde), while higher alkynes yield ketones.

Terminal alkynes (those with a C≡C-H bond) have an acidic hydrogen atom that can be replaced by metal ions. 1-Butyne is a terminal alkyne and will react with Tollens' reagent to form a silver acetylide precipitate. Internal alkynes (like 2-Butyne) do not have this acidic hydrogen.

In a substitution reaction, one atom or group is taken out and another is put in its place, while the overall saturation of the molecule often remains the same (e.g., replacement of H in an alkane by a halogen).

An addition reaction involves the combination of two or more molecules to form a single product. This typically occurs across unsaturated bonds (double or triple bonds), where the pi bond breaks and new sigma bonds are formed. Polymerization is a specific type of addition reaction where many monomers add to form a very large molecule.