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

The answer is 1,2- dibromo cyclopropane

Concept:-

Optical isomerism: Optical isomerism is the type of stereoisomerism that have the same molecular formula and connectivity of atoms but different spatial arrangement.

Geometrical isomerism: Geometrical isomerism is also the type of stereoisomerism in which the same substituent is attached differently to the carbon-carbon double bond.

Explanation:-

maleic acid can only show geometrical isomerism

Tartaric acid is a compound with two chiral centers, making it optically active. but not geometrical isomerism.

Propylene dibromide has the potential for optica isomerism due to a double bond, but it lacks chiral centers and cannot exhibit optical isomerism.
Tartaric acid lacks a plane of symmetry, making it chiral and enabling the existence of optical isomers.

1,2-Dibromo cyclopropane can exhibit geometrical isomerism,in the form of cis trans.

Also exhibiting optical isomers

Conclusion:-

So, The compound which can show both geometrical isomerism and optical isomerism is 1,2-Dibromo cyclopropan

CONCEPT:

Calculation of Stereoisomers

\(\text{Number of stereoisomers} = 2^n\)

• Stereoisomers are molecules that have the same molecular formula but differ in the spatial arrangement of atoms.
• The number of possible stereoisomers is determined using the formula:
• where n is the number of stereogenic centers (chiral centers and/or geometrical isomers).

EXPLANATION:

n (stereogenic unit) = 2, 22 = 4 stereoisomers are possible. 

• For 5-phenylpent-4-en-2-ol, we analyze the stereogenic units:
  • One chiral center at C2 (which has -OH, alkyl, and two other different groups).
  • One C=C double bond at C4=C5, which gives rise to cis-trans (E/Z) isomerism.
• Thus, the total number of stereogenic units is n = 2.
• Using the formula: 2² = 4 stereoisomers are possible.

Therefore, the correct answer is: 4 stereoisomers.

CONCEPT:

Monobromo Derivatives and π Electrons in Hydrocarbons

• The given molecular formula is C6H6 , which corresponds to benzene or a similar compound with conjugated bonds.
• The compound forms only one monobromo derivative and takes up four moles of hydrogen per mole for complete hydrogenation. This indicates that the compound is unsaturated and likely contains a benzene ring (or similar conjugated system).
• The number of π electrons ( π  electrons) in such a system determines its reactivity and characteristics. Benzene, for example, has 6 π electrons in its conjugated system, which is important for reactions such as bromination.

EXPLANATION:

• Upon complete hydrogenation, the compound takes up four moles of hydrogen per mole. This suggests a structure with two degrees of unsaturation (which is characteristic of a conjugated system such as benzene).
• The monobromo derivative indicates that only one position in the conjugated system can undergo bromination, pointing to a symmetric molecule like benzene or its derivatives.
• The π electron count for such a structure is crucial in understanding its reactivity, with benzene having 6 π electrons in total.

Correct Option: The correct answer is 8 & 6 both.

CONCEPT:

Formation of Benzene

• Benzene (C₆H₆) can be formed through several reactions, primarily involving aromatic compounds or derivatives.
• Common methods to synthesize benzene include decarboxylation, reduction of phenol, and other specific organic reactions.

EXPLANATION:

• 1) C₆H₅COONa + Sodalime → C₆H₆
  • This is a decarboxylation reaction where sodium benzoate reacts with soda lime to produce benzene.
  • 
  • 2)C₆H₅N₂⁺Cl^- + H₃PO₂ + H₂O → C₆H₆
    • This is a reduction of a diazonium salt, which produces benzene.
    • 
  • 3) C₆H₅OH + Zn → C₆H₆
    • This reaction involves the reduction of phenol using zinc dust to produce benzene.
    • 
  • 4) C₆H₅OH + H₂CrO₄ → [O]
    • This reaction is an oxidation of phenol, typically leading to the formation of quinones or other oxidized products, not benzene.
    • 

Therefore, the reaction that does not produce benzene is option 4: oxidation of phenol with chromic acid (H₂CrO₄).

CONCEPT:

Aldol Condensation Reaction

• Aldol condensation is a reaction between an enol or enolate and a carbonyl compound to form a β-hydroxy aldehyde or β-hydroxy ketone, followed by dehydration to yield a conjugated enone.
• The reaction usually involves the formation of a new carbon-carbon bond.

EXPLANATION:

• In the given problem, we need to find the alkene that, after undergoing ozonolysis, gives a product that can undergo aldol condensation.
• Ozonolysis of alkenes cleaves the double bond to form carbonyl compounds (aldehydes or ketones).
• The given compound is an aldol condensation product, which suggests it is formed by the reaction of two carbonyl compounds.
• We need to identify the alkene that, when cleaved by ozonolysis, gives these carbonyl compounds.
• Option 1 is the correct answer because it is the alkene that, after ozonolysis, produces the carbonyl compounds needed for the aldol condensation to form the given product.

Therefore, the correct answer is option 1.

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.

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:

​

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:

• In a 6-membered heterocyclic system, the electronegative group joined with the adjacent carbon of hetero-atom prefers a more sterically hindered axial configuration over a less steric equatorial position.
• The reason behind this is anomeric effect as shown by the picture given below:

• As we can see in axial conformation the antibonding orbital of the adjacent group is parallel to the non-bonding orbitals of oxygen. The lone pairs can be shared to the antibonding orbital, providing them extra stability. This is called Anomeric effect.
• No such effect can be seen in equatorial configuration.

Explanation:

The given structure is: 

As we can see two of the chlorine groups on the either left or right side of O are trans to each other. Therefore, option (2) and (3) are incorrect.

Now, the possible configurations are :

and

considering the arrangement of Cl and anomeric  effect the correct and more stable configuration will be the with axial C-Cl bonds, that is:

Conclusion:

Hence, the correct option is (4).

Explanation:

• The longest chain has six carbon atoms.
• So the parent name of the compound is hex.
• There are three functional groups present in the compound: Aldehyde, Ketone, and alkene.
• The main priority of the functional group is aldehyde and ketone and alkene are substituents.
• According to IUPAC nomenclature, the suffix name of the aldehyde is -al, the name of substituent ketone is -keto, and the name of alkene is -en.

• There is one methyl group attached to carbon number-2.
• Hence, the IUPAC name of the compound is 3-keto-2-methylhex-4-enal.

Concept:

Markovnikov’s rule states that with the addition of a protic acid HX to an asymmetric alkene, the acid hydrogen (H) gets attached to the carbon with more hydrogen substituents, and the halide (X) group gets attached to the carbon with more alkyl substituents. Alternatively, the rule can be stated that the hydrogen atom is added to the carbon with the greatest number of hydrogen atoms while the X component is added to the carbon with the least number of hydrogen atoms.

Both additions follow Markovnikov’s rule.

Here, in DCl, the chlorine gets attached to the less hydrogen containing substituents whereas the acid hydrogen (D) gets attached to the carbon with more hydrogen substituents. In this way, DI also gets attached to the compound.

The Correct answer is option 1 that is C2H6.

Concept:

Salts:

• Salts are generally strong electrolytes and thus are almost completely ionized in water.
• They are mainly formed by neutralisation reaction.

On the basis of their nature, salts can be classified as:

• Acidic salts:
  • These salts are formed by the combination of a strong acid and a weak base.
  • These are the salts that may H+ ions from acids that can be still replaced.
  • Acid salts are prepared from acids that have more than one replaceable H+ ions for example H2SO4.
  • Examples of such types of salts are NaHCO3, NaHSO4, NH4Cl, etc.
• Basic salts:
  • These salts are formed by the combination of a strong base and a weak acid.
  • These salts may have hydroxyl ions OH- that could still be replaced.
  • examples of such types of salts are CH3COONa, Zn(OH)Cl, etc.
• Neutral salts:
  • These types of salts are formed by the combination of either weak acid and weak base or by the combination of strong acid and a strong base.
  • Examples are CH3COONH4, NaCl, etc.

Explanation:

• An aqueous solution of Potassium acetate gives Ethane on the electrolysis. Ethane is an organic compound. The molecular of Ethane is C2H6 Ethane is obtained when potassium acetate is electrolyzed. This process is an example of a Kolbe electrolysis.

Correct answer: 4) 

Concept:

• Elimination reactions are endothermic reactions that occur at high temperatures (>50℃).
• Saturated alkyl halides and alcohols give unsaturated alkenes by elimination reactions.
• In elimination reactions, competition occurs between nucleophilic substitution and elimination.
• Elimination reactions are used in the preparation of alkenes by alkyl halides and alcohols. 
• In the dehydration method, there is the elimination of a water molecule mostly from compounds such as alcohol.
• Sometimes, this method is also called a Beta elimination reaction where the leaving group and H are placed at neighbouring carbon atoms.
• On the other hand, in dehydrohalogenation, there is a removal of a hydrogen atom and a halogen atom.

Explanation:

• Alkyl halides on heating with alcoholic potash eliminates one molecule of halogen acid to form alkene.
• Hydrogen is eliminated from  β -carbon atom." id="MathJax-Element-1-Frame" role="presentation" style="position: relative;" tabindex="0">-carbon atom.-carbon atom. -carbon atom.
• Nature of alkyl group determine the rate of reaction i.e., 3^o> 2^o>1^o.
• More the number of β- substituents (alkyl groups), more stable alkene it will form on β- elimination and more will be the reactivity.

• Hence, the decreasing order of the rate of β-elimination reaction with alcoholic KOH is →A > C > B

Conclusion:

Hence, the decreasing order of the rate of β-elimination reaction with alcoholic KOH is →A > C > B

Additional Information

Correct answer: 1) 

Concept:

• Markovnikoff's Rule:
  • First Rule: When molecules of an HX add up on unsymmetrical unsaturated hydrocarbon, the halogen atom goes to the unsaturated carbon atom bearing the lesser number of hydrogen atoms. 
  • Markownikoff's rule can also be stated as: The electrophilic addition to unsymmetrical alkenes always occurs through the formation of a more stable carbocation intermediate.
  • Second Rule: In the addition of HX to vinyl halide and analogous compounds, the halogen attaches itself to the carbon atom, on which the halogen atom is already present. 

Explanation:

• HBr selectively adds to the alkenes through Markownikoff's addition.
• According to Markovnikov’s rule, the major product is 2-Bromobutane, and the minor product is I-Bromobutane. Butane-1 is unsymmetrical.
• 2-Bromobutane has chiral carbon and hence exists in two enantiomers.
• Thus, it is clear that the major products are A & B while the minor product is C.

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Conclusion:

Thus, the mixture consists of A and B as major and C as minor products. 

Additional Information

Concept:

• Kolbe's electrolysis is the reaction in which decarboxylation takes place along with the formation of an alkane by passing electricity through sodium/potassium carboxylate salts.

Explanation:

The given reactant molecule is sodium acetate.

• Through this salt solution when electricity is passed then at the cathode hydrogen gas is released.
• At the anode, ethane gas is formed.

The overall reaction is as shown below:

• 2CH₃COONa + 2H₂0   \(\overset{electrolysis}{\rightarrow}\)   CH₃CH₃ + 2CO₂ + H₂ + 2NaOH

          (sodium acetate)                              ethane

This reaction proceeds through the free radical mechanism.

The chemical reaction at the cathode is shown below:

• 2H₂0 \(\overset{+2e-}{\rightarrow}\)  2OH^-  + 2 H⁺

The chemical reaction at the anode is shown below:

There fore we can conclude that ethane is produced on the electrolysis of an aqueous solution of sodium acetate.