Chemistry #expch001 Important Questions
Ch. 08. Hydrocarbons #expch_008
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1.Isopropyl iodide is formed after reaction of propene with HI, this is due to
(a) more stable free radical
(b) homolysis
(c) more stable carbanion
(d) more stable carbocation
Answer: (d)
- Which one of the following compound forms salt on reaction with NaNH2?
(a) C2H2
(b) C2H6
(c) C6H6
(d) C2H4
Answer: (a)
Explanation
Acetylene on reaction with NaNH2, forms sodium salt.
The sp hybridized carbon atom in acetylene has higher electronegativity as compared to sp2 hybridized carbon atom in ethylene and sp3 hybridized carbon atom in ethane.
The carbon atom of C−H bond in acetylene attracts the shared electron pair towards itself to a greater extent and the hydrogen atom can be easily liberated as the proton.
Hence, the hydrogen atom attached to the triply bonded carbon atom in alkynes has appreciable acidic character .When acetylene reacts with NaNH2 which is the strong base reacts to form a sodium salt.
HC≡CH+NaNH2→HC≡C−Na+NaNH2→NaC≡CNa
sodium acetylide disodium acetylide - Which among the following reagent can be used to distinguish between CH2CHC≡CH from CH3CH≡CCH3??
(a) Br2/CCl4
(b) cold KMnO4
(c) Ammonical AgNO3
(d) Ag(NH3)2OH - The product obtained on heating 2-Bromopentane with potassium ethoxide in ethanol is
(a) 1-pentene
(b) 2-cis-pentene
(c) trans-2-pentene
(d) 2-ethoxy pentane
Answer: (c) - Which among the following product is formed when ethyne undergoes oxidation??
Formaldehyde
Bromobenzene
(c) Chlorobenzene
(d) Chloroethene
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- Choose the process by which liquid hydrocarbons can be converted to gaseous hydrocarbons
(a) hydrolysis
(b) oxidation
(c) cracking
(d) distillation under reduced pressure
Answer: (c)
Explanation
Lower hydrocarbons exist in gaseous state while higher ones are in liquid state or solid state. On cracking or pyrolysis, the hydrocarbon with higher molecular mass gives a mixture of hydrocarbons having lower molecular mass. Hence, we can say that by cracking a liquid hydrocarbon can be converted into a mixture of gaseous hydrocarbons.
7.The simplest member of organic compounds is?
(a) Methanol
(b) Methane
(c) Formaldehyde
(d) Formic acid
Answer: (b)
- Ethane is obtained by electrolyzing __
(a) Potassium formate
(b) Potassium succinate
(c) Potassium acetate
(d) Potassium fumarate
Answer: b
This is an example of Kolbe’s electrolysis. Ethylene is obtained on electrolysis of potassium succinate.
- Cooking gas is a Mixture of following two gases
(a) methane and ethane
(b) Ethane and Propane
(c)Propane and Butane
(d)Butane and Pentane
Answer: c - which of the following product is used for the construction of roads.
(a) Bitumen
(b) coke
(c) parrafin
(d) all of these
Answer: a
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- Petroleum is a mixture of
(a) salts
(b) elements
(c) polymers
(d) Hydrocarbons
Answer: d - Most of the explosions in the mines in due to
(a) methane and air
(b) oxygen with acetylene
(c) hydrogen with oxygen
(d) none of these
Answer: a
13.Which of following fuel has highest calorific value
(a) Methane
(b) Kerosene
(c) Hydrogen
(d) LPG
Answer: c
- Bromine water reacts with?
(a) Alkenes
(b) Alkanes
(c) Alkynes
(d) both and c
Explanation
Bromine water test (saturation test)
The bromine water test is a qualitative test, used to identify the alkene or alkane functional groups present in the compound. Alkene groups react with bromine water in the dark condition and undergo an addition reaction, to give a decolourized solution.
On the other hand, alkane doesn’t react with bromine water and the colour of the bromine water remains the same. Enols, alkenes, aniline, glucose, phenols and acetyl groups are the most common compounds to undergo bromine water test.
The test also identifies the presence of an aldehyde group in the compound. During the process, the colour of the bromine water changes from yellow to colourless.
- Which statement is not true?
(a) Alkanes undergo only substitution reactions
(b) Alkenes and alkynes undergo both substitution and addition reactions
(c) Alkanes undergo both substitution and addition reactions
(d) none of these
Explanation
Alkenes and alkynes are unsaturated – they have π-bonds, so don’t have the full number of hydrogen that they could have.
This means that they are more unstable than alkanes, since π-bonds aren’t as strong as σ-bonds. The alkenes and alkynes want to form more σ-bonds and have a structure more like an alkane, so they undergo addition reactions.
Addition reactions are where more atoms are added to the molecule, not swapped or taken away. This means that the π-bonds have to be taken away and used as σ-bonds with the new atoms, rather than the σ-bonds already there being reattached – it’s easier to break π than σ.
Alkanes do not undergo this reaction because they already only have single σ-bonds, and so they cannot become more stable or stronger structurally – they are already at the peak, and so can only swap things around in substitution reactions.
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- Select the incorrect statement regarding terminal alkynes.?
(a) Methylacetylene is an example of terminal alkynes
(b) Terminal alkynes are more acidic when compared with alkenes
(c) Terminal alkynes are not as acidic as alkanes
(d) These have a replaceable acidic hydrogen atom
17- Which of the following statements is wrong??
(a) When two orbitals overlap in-phase with each other, a bonding molecular orbital forms.
(b) When two orbitals overlap out-of-phase with each other, an antibonding molecular orbital forms.
(c) When one of two atoms connected by a σ bond rotates about the bond axis, orbital overlap is lost.
(d) When one of two atoms connected by a π bond rotates about the bond axis, orbital overlap is lost.6. Select the incorrect statement regarding terminal alkynes.?
18- Incorrect statement is?
(a) Hydroboration-oxidation of a terminal alkyne gives a ketone as the main product
(b) Hydrogenation of an internal alkyne over the Lindlar catalyst gives a cis alkene
(c) In the epoxidation of an alkene with a peroxy acid, the peroxy acid is electrophilic
(d) All are correct
19- Which of the following reaction is not shown by ketones? (PPSC 2020)?
(a) Reaction with HCN
(b) Reaction with NaHSO3
(c) Reaction with 2 4 dinitrophenyl hydrazine
(d) Reaction with Fehling solution
20- Find the incorrect statement for a nucleophile?
(a) A nucleophile is a Lewis acid
(b) Nucleophiles do not seek electron
(c) Ammonia is a nucleophile
(d) Nucleophiles attack low electron density sites
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21- Which among the following is the most deactivating meta-directing group in aromatic substitution reaction? (PPSC 2020)?
(a) -COOH
(b) -SO3H
(c) -NO2
(d) -CN
22- Which of the following statements is wrong about alkene??
(a) The main orbital interactions in the transition state of an electrophilic addition to an alkene are between the LUMO of the electrophile and the HOMO of the alkene.
(b) The lower the LUMO of the electrophile, the more reactive it is.
(c) The lower the HOMO of the alkene, the more reactive it is.
(d) none of these
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