Hydrocarbon Test 4

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MCQs 1

Which carbon atom among the following is the most electronegative?

III II I

CH3 – CH2 – C ≡ CH





Topic: Electronegativity in Carbon Atoms

Correct Answer: C) I

Detailed Hint:
  • Electronegativity refers to the measure of the ability of an atom's nucleus to attract electrons from a different atom within a covalent bond.
  • In hybridization, carbon atoms of different types (sp, sp2, sp3) have varying electronegativity. The more s character in the carbon atom, the more electronegative it is.
  • In this molecule, carbon atom I is sp hybridized, while carbon atoms II and III are sp2 hybridized. Thus, carbon atom I is the most electronegative.
  • Therefore, the correct answer is C, "I".

MCQs 2

Which of the following is not an electrophile?





Topic: Chemical Reactivity and Electrophiles

Correct Answer: D) NH3

Detailed Hint:
  • Electrophiles are substances that are electron deficient and can accept pairs of electrons to form a new bond.
  • Compounds like BF3, AlCl3, and ZnCl2 are electron deficient molecules, making them electrophiles.
  • On the other hand, NH3 or Ammonia is a nucleophile. It has a lone pair of electrons that can donate to form a new bond, acting opposite to an electrophile.
  • Thus, the correct answer is D, "NH3".

MCQs 3

Which group deactivates the benzene ring for electrophilic substitution?





Topic: Benzene Rings and Electrophilic Substitution

Correct Answer: B) Chlorine

Detailed Hint:
  • Deactivating groups on a benzene ring reduce the ring's reactivity towards electrophilic aromatic substitution because of their electron-withdrawing nature.
  • Chlorine, despite being an ortho/para director due to its lone electron pair, is more electronegative than carbon. This pulls electron density away from the pi system, destabilizing the intermediate cation and thus deactivating the benzene ring.
  • Hence, the correct answer is B, "Chlorine".

MCQs 4

The overlapping of orbitals in benzene is of which type?





Topic: Molecular Orbital Theory and Benzene

Correct Answer: C) sp2 – sp2

Detailed Hint:
  • The overlapping of orbitals determines the type of bonds in a molecule. In benzene, all six carbon atoms undergo sp2 hybridization, forming double bonds.
  • This leads to the overlapping of sp2 hybridized orbitals, which is characteristic of the benzene structure.
  • Therefore, the correct answer is C, "sp2 – sp2".

MCQs 5

What is the structural formula of benzene?





Topic: Structural Chemistry and Benzene

Correct Answer: C) Option C

Detailed Hint:
  • Benzene is a cyclic compound that displays a continuous pi electron cloud above and below the plane of the molecule.
  • Possessing a molecular formula of C6H6, it alternates between single and double bonds in its structure.
  • Upon reviewing the given options, it becomes evident that answer C accurately represents the known structural formula of benzene.

MCQs 6

Which acid serves as a catalyst in Friedel-Craft's reaction?





Topic: Acid Catalysis and Friedel-Craft's Reaction

Correct Answer: A) AlCl3

Detailed Hint:
  • The Friedel-Craft's reaction is a subtype of electrophilic aromatic substitution where an alkyl or acyl group is introduced into an aromatic system.
  • This reaction commonly uses a Lewis acid like AlCl3 as a catalyst to generate the electrophile.
  • The electrophile thus produced attacks the benzene ring, bringing about the electrophilic substitution reaction.
  • Hence, the correct answer is AlCl3 as it efficiently facilitates this reaction.

MCQs 7

Which statement accurately describes ethane, ethene, and ethyne?





Topic: Comparative Analysis of Ethane, Ethene, and Ethyne

Correct Answer: B) Ethyne is the strongest acid and C-H bond length is shortest

Detailed Hint:
  • The nature of chemical bonding and hybridization affects both acidity and bond length in organic compounds.
  • Ethyne contains a triple bond due to sp hybridization, leading to a shorter bond length between carbon atoms.
  • As sp-hybridized carbons have more s character, they are more electronegative, making hydrogen in ethyne more acidic than in ethene and ethane (which contain sp2 and sp3 hybridized carbons respectively).
  • Therefore, the correct statement is "Ethyne is the strongest acid and C-H bond length is shortest".

Question 8:

What is the number of sigma and pi bonds in a benzene molecule?





Topic: Structure of Benzene and Bond Characteristics

Correct Answer: B) 12 sigma and 3 pi

Detailed Hints:
  • Benzene, with the molecular formula C6H6, has a unique structure with alternating double and single bonds between carbon atoms.
  • Each carbon atom forms three sigma (σ) bonds: two with adjacent carbon atoms and one with a hydrogen atom.
  • Additionally, the double bond has a sigma and pi bond, but only each alternate bond is a double bond. Therefore, there are 3 pi bonds in total.
  • With 6 carbon atoms, this leads to a total of 12 sigma and 3 pi bonds.

Question 9:

Which electrophile is generated during the sulfonation of benzene by H2SO4?





Topic: Sulfonation Reaction and Electrophile Generation

Correct Answer: C) SO3

Detailed Hints:
  • Sulfonation of benzene is an important reaction in the field of organic chemistry, where the electrophile plays a pivotal role.
  • In the sulfonation process, concentrated sulfuric acid (H2SO4) is used, which generates the electrophile.
  • The electrophile for this reaction is the sulfur trioxide (SO3) ion.
  • Therefore, the correct electrophile generated during the sulfonation of benzene is SO3.

Question 10:

Given that a saturated hydrocarbon is represented by CnH2O, what is the value of 'n' in this compound?





Topic: Structure of Saturated Hydrocarbons

Correct Answer: C) 9

Detailed Hints:
  • Saturated hydrocarbons, also known as alkanes, adhere to the general formula CnH2n+2.
  • In this type of hydrocarbon, all the carbon-carbon bonds are single bonds - hence, they are 'saturated' with hydrogen atoms.
  • Given the formula CnH2O, if we equate 2n+2 to 20 (to coincide with the hydrogen count in the formula), we find that n equals 9.
  • Therefore, the value of 'n' in this compound is 9.

Question 11:

What acts as a catalyst when benzene reacts with chlorine to form benzene hexachloride?





Topic: Reaction of Benzene with Chlorine

Correct Answer: C) Bright sunlight

Detailed Hints:
  • Reactions involving benzene often need a catalyst to take place effectively.
  • In the formation of benzene hexachloride from benzene, the reaction is facilitated by a catalyst.
  • In this case, benzene reacts with chlorine in the presence of bright sunlight, which aids in the reaction's progress.
  • Therefore, bright sunlight serves as the catalyst in this reaction.

Question 12:

How many types of disubstituted derivatives are possible in benzene?





Topic: Types of Disubstituted Derivatives of Benzene

Correct Answer: C) 3

Detailed Hints:
  • Benzene, as an aromatic compound, can form substituted derivatives.
  • Specifically, disubstituted derivatives of benzene are those in which two substituents have replaced two hydrogen atoms on the benzene ring.
  • There are three different types of disubstituted derivatives possible depending on the position of the substituents: ortho, meta, and para.
  • Therefore, the correct number of types of disubstituted derivatives of benzene is 3.

Question 13:

Which among the following compounds acts as an unsaturated hydrocarbon?





Topic: Characteristics of Unsaturated Hydrocarbons

Correct Answer: D) Ethene

Detailed Hints:
  • Hydrocarbons can be classified as either saturated or unsaturated based on their structure.
  • Saturated hydrocarbons have only single bonds between carbon atoms, while unsaturated hydrocarbons have double or triple bonds.
  • Among the given options, ethene has a double bond between carbon atoms, making it an unsaturated hydrocarbon.
  • Therefore, Ethene is the correct answer as it behaves as an unsaturated hydrocarbon.

Question 14:

Why do aromatic compounds produce a sooty flame when burned?





Topic: Combustion Characteristics of Aromatic Compounds

Correct Answer: C) They consist of a high percentage of carbon

Detailed Hints:
  • The combustion characteristics of a compound are governed by its elemental composition.
  • Aromatic compounds are characterized by ring structures, usually with a significant carbon content.
  • When aromatic compounds burn, the high carbon content can result in incomplete combustion, leading to the creation of soot particles.
  • Therefore, the sooty flame observed during the combustion of aromatic compounds is due to their high carbon content.

Question 15:

What is the order of electronegativity in carbon when it's in sp, sp2, and sp3 hybridized states?





Topic: Electronegativity in Different Hybridized States of Carbon

Correct Answer: A) sp > sp2 > sp3

Detailed Hints:
  • Electronegativity is the ability of an atom to attract shared pairs of electrons.
  • In hybridized states of carbon, the percentage of s character influences the electronegativity.
  • The sp hybridized state has 50% s character, sp2 has 33.33%, and sp3 has 25%.
  • Hence, the order of electronegativity: sp (with highest %s character) > sp2 > sp3.

Question 16:

Of what are aromatic hydrocarbons derivatives?





Topic: Origins of Aromatic Hydrocarbons

Correct Answer: A) Benzene

Detailed Hints:
  • Hydrocarbons can be broadly classified into different categories, including alkanes, paraffins, and aromatic hydrocarbons.
  • Aromatic hydrocarbons are a distinct group, characterized by the presence of a resonance-stabilized ring, such as in benzene.
  • This means that aromatic hydrocarbons are largely derived from benzene or its derivatives.
  • Thus, the correct answer is Benzene.

Question 17:

What is the general formula for alkynes?





Topic: General Formula of Alkynes

Correct Answer: C) CnH2n – 2

Detailed Hints:
  • Alkynes are a category of hydrocarbons characterized by the presence of a carbon-carbon triple bond in their structure.
  • The general formula of a chemical category helps to understand the relation between the number of carbon and hydrogen atoms.
  • For alkynes, the general formula is CnH2n–2, indicating the existence of a triple bond.
  • Therefore, CnH2n – 2 is the correct formula for alkynes.

Question 18:

Which group among the following directs ortho and para positions?





Topic: Ortho-Para Directing Groups

Correct Answer: B) -OH

Detailed Hints:
  • In aromatic compounds, some substituent groups are known as directing groups because they influence the location of subsequent substitutions.
  • Ortho-para directing groups donate electron density into the ring, making ortho and para positions more attractive for electrophilic attack.
  • Among the options, -OH is an ortho-para directing group.
  • Therefore -OH is the correct answer as it directs ortho and para positions.

Question 19:

In which compound are the benzene rings separated?





Topic: Structure of Aromatic Compounds

Correct Answer: A) Diphenyl methane

Detailed Hints:
  • Many aromatic compounds consist of multiple benzene rings.
  • The spatial arrangement and connections between these rings can vary widely, creating unique compounds.
  • In the case of Diphenyl methane, two benzene rings are separated by a -CH2– unit.
  • Therefore, Diphenyl methane is the correct answer as in this compound the benzene rings are isolated.

Question 20:

Among the following alkanes, which has the lowest boiling point?





Topic: Boiling Points of Alkanes

Correct Answer: D) neopentane

Detailed Hints:
  • The boiling point of alkanes is influenced by the molecular structure, specifically branching.
  • A higher degree of branching reduces the surface area of the molecule, which in turn decreases the intermolecular forces.
  • These weaker intermolecular forces result in a lower boiling point.
  • Therefore, among the options, neopentane, which has the most branching, has the lowest boiling point.

Question 21:

Which compound among the following undergoes electrophilic substitution slower than benzene?





Topic: Speed of Electrophilic Substitution Reactions

Correct Answer: B) Nitrobenzene

Detailed Hints:
  • The rate of electrophilic substitution in aromatic compounds is determined by the electron density of the benzene ring.
  • Electron-withdrawing groups, like nitro (-NO2), decrease the electron density, making the benzene ring less reactive.
  • Thus, nitrobenzene, which contains a nitro group, undergoes electrophilic substitution slower than benzene.
  • Therefore, nitrobenzene is the correct answer.

MCQ 22:

Which of the following groups does not participate in addition reactions?





Topic: Addition Reactions of Hydrocarbons

Correct Answer: A) Alkanes

Detailed Hints:
  • Addition reactions characterize unsaturated hydrocarbons, which have double or triple bonds.
  • These reactions involve breaking of a pi bond and addition of two atoms or groups.
  • Among the options, alkanes, composed of single carbon-carbon bonds, do not undergo addition reactions.
  • Therefore, Alkanes is the correct answer as it does not show addition reactions.

MCQ 23:

Identify the aromatic compound that is a fused cyclic structure from the options below:





Topic: Structure of Aromatic Compounds

Correct Answer: C) Naphthalene

Detailed Hints:
  • Aromatic compounds can have different structural configurations, including isolated, fused, or bridged rings.
  • Fused aromatic rings share two or more common atoms.
  • Among the options, Naphthalene is a polycyclic aromatic hydrocarbon composed of two fused benzene rings.
  • Therefore, Naphthalene is the correct answer, being a fused cyclic aromatic compound.

MCQ 24:

Which of the following compounds is classified as an alkane?





Topic: Classification of Hydrocarbons

Correct Answer: A) C10H22

Detailed Hints:
  • Hydrocarbons are organic compounds made up of carbon and hydrogen atoms. They are classified into alkanes, alkenes, and alkynes based on their bonded structures.
  • Alkanes follow the general formula CnH2n+2.
  • Among the options, C10H22 matches the general formula for alkanes, representing Decane.
  • Therefore, C10H22 is the correct answer as it represents an alkane compound.

MCQ 25:

The halogenation of alkanes exemplifies which type of reaction?





Topic: Reactions of Alkanes

Correct Answer: D) Free radical substitution

Detailed Hints:
  • Alkanes can undergo various chemical reactions, including halogenation.
  • Halogenation of alkanes typically involves the substitution of a hydrogen atom with a halogen atom.
  • This process typically takes place through a free radical mechanism, which involves the formation of highly reactive species with unpaired electrons.
  • Therefore, Free radical substitution is the correct answer, being the mechanism through which halogenation of alkanes occurs.

MCQ 26:

What is the appropriate label for the following chemical reaction?





Topic: Chemical Reactions

Correct Answer: B) Hydrogenation

Detailed Hints:
  • Various types of chemical reactions exist, including substitution, hydrogenation, alkenes formation, and dehydrohalogenation.
  • Hydrogenation is a type of reaction where hydrogen atoms are added to a molecule.
  • In this context, Hydrogenation is identified as the correct answer.

MCQ 27:

Which reaction do alkenes normally undergo?





Topic: Reactions of Alkenes

Correct Answer: A) Addition polymerization

Detailed Hints:
  • Alkenes are a class of hydrocarbons that may undergo various types of reactions.
  • One of these reactions is Addition polymerization, where the double bonds of alkenes are broken, and monomer units are added one at a time.
  • Hence, Addition polymerization is the correct answer.

MCQ 28:

Select the compound that possesses a ring-carbon chain structure from the following:





Topic: Carbon Chain Structures

Correct Answer: D) Benzene

Detailed Hint:
  • Compounds may have different types of structures, including linear, branched, or ring structures.
  • A ring-carbon chain structure, such as benzene, consists of carbon atoms connected in a ring.
  • Benzene, composed of six carbon atoms alternately connected by single and double bonds, fits this description.
  • Therefore, Benzene is the correct answer.

Question 29:

What type of reactions is benzene predominantly involved in?





Topic: Reactions of Benzene

Correct Answer: C) Substitution

Detailed Hints:
  • Benzene, a cyclic compound, can participate in a variety of chemical reactions.
  • However, its most common reaction type is substitution, in which one of the hydrogen atoms in the benzene molecule is replaced by another group.
  • Consequently, Substitution is identified as the correct response.

Question 30:

In which order does the percentage increase of s-character in the hybrid orbitals of carbon follow in CH4, C2H4, and C2H2?





Topic: Hybridization of Carbon Atoms

Correct Answer: C. CH4 < C2H4 < C2H2

Detailed Hint:
  • The s-character of a hybrid orbital in a carbon atom represents the orbital's contribution from the s orbital.
  • It increases as the hybridization changes from sp3 (CH4) to sp2 (C2H4) to sp (C2H2).
  • Therefore, the correct order of % increase is CH4 < C2H4 < C2H2.

Question 31:

What is the length of the C-C bond in benzene?





Topic: Bond Length in Benzene

Correct Answer: C. 1.39 Å

Detailed Hint:
  • The bond length between carbon atoms in a molecule is a key physical characteristic.
  • In benzene, the C-C bond length is intermediate between a single bond (around 1.54 Å) and a double bond (around 1.34 Å).
  • Therefore, the C-C bond length in benzene is approximately 1.39 Å.

Question 32:

Which is the correct order of acidity among the following?





Topic: Acidity of Hydrocarbons

Correct Answer: A) 1-Alkyne > Alkene > Alkane

Detailed Hint:
  • Acidity is a chemical property that measures the tendency of a compound to donate a proton (H+).
  • Hydrocarbons like alkanes, alkenes, and alkynes exhibit different levels of acidity.
  • 1-alkynes are more acidic than alkenes and alkanes due to the ability of the alkyne to stabilize the negative charge that results from the loss of a proton.
  • Therefore, the correct order of acidity is 1-Alkyne > Alkene > Alkane.

Question 33:

What is the ratio of sigma to pi bonds in a benzene molecule?





Topic: Bonding in Benzene

Correct Answer: B) 4:1

Detailed Hint:
  • Benzene, C6H6, is a planar molecule that contains both sigma (σ) and pi (π) bonds.
  • Sigma bonds are formed by the direct overlap of atomic orbitals, while pi bonds result from the lateral overlap of p orbitals.
  • In the benzene molecule, there are 12 sigma and 3 pi bonds, giving a ratio of 4:1.
  • Therefore, the correct ratio of sigma to pi bonds in benzene is 4:1.

Question 34:

Compare the chain structures of butane and iso-butane.





Topic: Structure of Hydrocarbons

Correct Answer: A) Butane has a straight chain, and iso-butane has a branched chain.

Detailed Hint:
  • The structure of hydrocarbons varies and can be straight, branched, or cyclic.
  • Butane, C4H10, is a hydrocarbon with a straight chain.
  • Iso-butane, a structural isomer of butane, has a branched chain.
  • Therefore, butane has a straight chain, and iso-butane has a branched chain.

Question 35:

What class of organic compounds does Acetophenone belong to?





Topic: Classification of Organic Compounds

Correct Answer: B) Ketone

Detailed Hint:
  • Organic chemistry categorizes compounds into various classes based on their functional groups, like ether, ketone, aldehyde, and ester.
  • Acetophenone is an organic compound with the formula C6H5C(O)CH3, consisting of a phenyl group connected to a carbonyl group with a methyl substituent.
  • The carbonyl group with a carbon-oxygen double bond is a characteristic functional group of ketones.
  • Therefore, Acetophenone is classified as a ketone.
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