AROMATIC HYDROCARBONS
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) The term aromatic was derived from Greek word ________ meaning ________. (aroma, fragrant)
(ii) Aromatic hydrocarbons include benzene and all those compounds which are _________ related to benzene. (structurally)
(iii) _________ is recognized as the simplest member of the class of Aromatic Hydrocarbons. (benzene)
(iv) Benzene has _________ structure. (regular hexagonal planar)
(v) The removal of hydrogen atom from aromatic hydrocarbons gives a radical. The radicals are called _________. (aryl)
(vi) Benzene was discovered by Michael Faraday in _______. (1825)
(vii) The unhybridized 2pz orbitals in benzene partially overlap to form a _________ of electron cloud. (sheath)
(viii) The introduction of halogen group in benzene ring is called _______. (halogenation)
(ix) The molecular formula of C6H6 indicates that it is highly ________ compound. (unsaturated)
(x) On oxidation in the presence of V2O5 benzene gives _________. (maleic anhydride)
TRUE/FALSE
Q.02: Indicate True or False:
(i) Benzene is more reactive than alkene and less reactive than alkane. (FALSE)
CORRECT: Benzene is more reactive than alkane and less reactive than alkene.
(ii) Benzene has a pentagonal structure. (FALSE)
CORRECT: Benzene has a hexagonal structure.
(iii) The C—C bond length in benzene molecule is 1.397 Å. (TRUE)
(iv) The state of hybridization of carbon in benzene molecule is sp3. (FALSE)
CORRECT: The state of hybridization of carbon in benzene molecule is sp2.
(v) There are six sigma bonds in benzene molecule. (FALSE)
CORRECT: There are twelve sigma bonds in benzene molecule.
(vi) Halogenonium ion produced in electrophilic substitution reactions is a powerful electrophile. (TRUE)
(vii) In electrophilic substitution reactions, addition products are favourable. (FALSE)
CORRECT: In electrophilic substitution reactions, substitution products are favourable.
(viii) Sulphonation is carried out when benzene is heated with conc. HNO3. (FALSE)
CORRECT: Nitration is carried out when benzene is heated with conc. HNO3.
(ix) In ozonolysis, benzene directly gives glyoxal. (FALSE)
CORRECT: In ozonolysis, benzene directly gives maleic anhydride.
(x) Benzene has five resonance contributing structures. (TRUE)
MCQs
Q.03: Multiple choice questions.
(i) The benzene molecule contains:
(a) Three double bonds
(b) Two double bonds
(c) One double bond
(d) Delocalized π- electron charge
ANSWER: (d) Delocalized π- electron charge
EXPLANATION: The six unhybrid p-orbitals of each carbon in benzene, overlap in a parallel way to form a ring of delocalized pi electron cloud over and below the hexagonal planar sigma framework of benzene molecule.
(ii) Aromatic hydrocarbons are the derivatives of:
(a) Normal series of paraffins
(b) Alkene
(c) Benzene
(d) Cyclohexane
ANSWER: (c) Benzene
EXPLANATION: Aromatic compounds are by definition benzene and its derivatives, or all those compounds that are structurally related to benzene.
(iii) Which of the following acids can be used as a catalyst in Friedel-Crafts reactions?
(a) AlCl3
(b) HNO3
(c) BeCl2
(d) NaCl
ANSWER: (a) AlCl3
EXPLANATION: In Friedel-Crafts reaction, a Lewis acid which is electron deficient and can accept a pair of electrons, such as AlCl3 or FeCl3 etc., is used to generate a powerful electrophile (R+, X+ etc.), which then substitutes a hydrogen atom from benzene ring, resulting in electrophilic substitution product.
(iv) Benzene can’t undergo:
(a) Substitution reactions
(b) Addition reactions
(c) Oxidation reaction
(d) Elimination reactions
ANSWER: (d) Elimination reactions
EXPLANATION: Benzene is highly unsaturated compound with stable delocalized pi electron cloud. So, it cannot undergo elimination of any molecule for further unsaturation.
(v) Amongst the following, the compound that can be most readily sulphonated is:
(a) Toluene
(b) Benzene
(c) Nitrobenzene
(d) Chlorobenzene
ANSWER: (a) Toluene
EXPLANATION: Sulphonation is an electrophilic aromatic substitution reaction. The ease of this reaction depends on the activation of the aromatic ring towards the electrophilic attack. The methyl group (CH₃) has a strong +I effect or electron donating effect, due to hyperconjugation. It activates the ring towards electrophilic attack, and is thus most readily sulphonated.
(vi) During nitration of benzene, the active nitrating agent is:
(a) NO3
(b) NO2+
(c) NO2–
(d) HNO3
ANSWER: (b) NO2+
EXPLANATION: NO2+ is a powerful electrophile, so can penetrate the pi electron ring in benzene. It is, therefore, active nitrating agent during nitration of benzene.
(vii) Which compound is the most reactive one?
(a) Benzene
(b) Ethene
(c) Ethane
(d) Ethyne
ANSWER: (b) Ethene
EXPLANATION: Ethene is the most reactive compound in the given choices because it has pi electrons which can be easily attacked by any electrophile in electrophilic additions. The pi electrons in ethyne are relatively strongly held between the carbons due to shorter bond length of triple bond. The pi electrons in benzene are delocalized, so they also cannot be easily attacked.
(viii) The electrophile in aromatic sulphonation is:
(a) H2SO4
(b) HSO4–
(c) SO3
(d) SO3+
ANSWER: (c) SO3
EXPLANATION: The S atom of SO3 is strongly partial positive due to its bonds with three powerful O-atoms. It, therefore, acts as a powerful electrophile in aromatic Sulphonation
(ix) Aromatic compounds burn with sooty flame because:
(a) They have high percentage of hydrogen
(b) They have a ring structure
(c) They have high percentage of carbon
(d) They resist reaction with air
ANSWER: (c) They have high percentage of carbon
EXPLANATION: Aromatic compounds have high proportion of carbon as compared to aliphatic hydrocarbons. During combustion, some of the carbon particles escape encounter with oxygen molecules, so they remain unburnt and cause sooty fire.
(x) The conversion of n-hexane into benzene by heating in the presence of Pt is called:
(a) Isomerization
(b) Aromatization
(c) Dealkylation
(d) Rearrangement
ANSWER: (b) Aromatization
EXPLANATION: The conversion of straight chain non-cyclic compounds into cyclic ones is called cyclization, while into aromatic compounds like benzene is called aromatization.