Q37
4 marksSection E

More than three million carbon compounds have been discovered in the field of chemistry. The diversity of these compounds is due to the capacity of carbon atoms for bonding with one another as well as with other atoms. Most of the carbon compounds are poor conductors of electricity and have low melting and boiling points. (a) Write the molecular formula of first two members of homologous series having functional group -Br. (b) Given below are the formulae of some functional groups: −CHO-CHO, −CO−-CO-. Write the name of these functional groups. (c) What would be observed on adding a 5%5\% alkaline potassium permanganate drop by drop to some warm ethanol taken in a test tube? State the role of KMnO4KMnO_4 in the reaction and write the chemical equation for the reaction involved. OR (c) Write the name of the compound formed when ethanol is heated at 443 K temperature with excess of conc. H2SO4H_2SO_4. What is the role of conc. H2SO4H_2SO_4 in the reaction? Write the chemical equation for the reaction involved.

Carbon and its Compounds
Versatile Nature of Carbon
Official Answer

(a) The first two members of the homologous series having the functional group −Br-Br (bromoalkanes) are:

  1. Bromomethane: CH3BrCH_3Br
  2. Bromoethane: C2H5BrC_2H_5Br

(b) The names of the given functional groups are:

  • −CHO-CHO: Aldehyde
  • −CO−-CO-: Ketone

(c) Observation: On adding 5%5\% alkaline potassium permanganate drop by drop to warm ethanol, the purple/pink color of KMnO4KMnO_4 initially disappears. However, when added in excess, the purple color persists.

Role of KMnO4KMnO_4: It acts as an oxidizing agent because it provides oxygen to convert ethanol into ethanoic acid.

Chemical Equation:

CH3CH2OH→Alkaline KMnO4+HeatCH3COOHCH_3CH_2OH \xrightarrow{\text{Alkaline } KMnO_4 + \text{Heat}} CH_3COOH


OR


(c) Name of the compound formed: Ethene.

Role of concentrated H2SO4H_2SO_4: It acts as a dehydrating agent because it removes a water molecule from ethanol.

Chemical Equation:

CH3CH2OH→Hot conc. H2SO4,443 KCH2=CH2+H2OCH_3CH_2OH \xrightarrow{\text{Hot conc. } H_2SO_4, 443\text{ K}} CH_2=CH_2 + H_2O

BromomethaneBromoethaneAldehydeKetoneOxidizing agentEthanoic acidDehydrating agentEtheneDehydration

Marking Scheme

  • 1(a) 1 Mark: 0.5 mark for each correct molecular formula (CH3BrCH_3Br and C2H5BrC_2H_5Br).
  • 2(b) 1 Mark: 0.5 mark for identifying 'Aldehyde' and 0.5 mark for 'Ketone'.
  • 3(c) 2 Marks: 0.5 mark for correct observation, 0.5 mark for identifying KMnO4KMnO_4 as an oxidizing agent, and 1 mark for the balanced chemical equation.
  • 4OR (c) 2 Marks: 0.5 mark for naming 'Ethene', 0.5 mark for identifying conc. H2SO4H_2SO_4 as a dehydrating agent, and 1 mark for the balanced chemical equation.

Hint

Recall that alkaline KMnO4KMnO_4 oxidizes alcohols to carboxylic acids, while hot concentrated H2SO4H_2SO_4 dehydrates alcohols to alkenes.

Quick Oral Answer

Alkaline potassium permanganate oxidizes ethanol to ethanoic acid. Concentrated sulfuric acid dehydrates ethanol at 443 K to form ethene gas.

Analysis & Explanation

  • Part (a): The general formula for haloalkanes is CnH2n+1XC_nH_{2n+1}X. For X=BrX = Br, the first member (n=1n=1) is CH3BrCH_3Br (bromomethane) and the second member (n=2n=2) is C2H5BrC_2H_5Br (bromoethane).
  • Part (b): According to Table 4.3 of the NCERT textbook, the carbonyl group with a hydrogen atom attached (−CHO-CHO) is an aldehyde, and the carbonyl group bonded to two carbon atoms (−CO−-CO-) is a ketone.
  • Part (c): Alkaline KMnO4KMnO_4 is a strong oxidizing agent. When added to ethanol, it oxidizes the alcohol group (−OH-OH) to a carboxylic acid group (−COOH-COOH), forming ethanoic acid. The purple color of MnO4−MnO_4^- ions is discharged as it gets reduced. Once all ethanol is oxidized, any further addition of KMnO4KMnO_4 leaves the purple color unchanged.
  • Alternative Part (c): Heating ethanol at 443 K443\text{ K} with excess concentrated sulfuric acid results in intramolecular dehydration (removal of −H-H and −OH-OH from adjacent carbons) to form an alkene (ethene). Concentrated H2SO4H_2SO_4 has a strong affinity for water, making it an excellent dehydrating agent.

Common Mistakes

  1. 1Writing the molecular formula of bromoethane as C2H6BrC_2H_6Br instead of C2H5BrC_2H_5Br (forgetting to substitute one hydrogen atom).
  2. 2Confusing the role of concentrated H2SO4H_2SO_4 as an oxidizing agent instead of a dehydrating agent in this specific reaction.
  3. 3Forgetting to write the reaction conditions (such as heat, 443 K443\text{ K}, or alkaline KMnO4KMnO_4) over the reaction arrow.

Interesting Facts

Friedrich Wöhler's synthesis of urea in 1828 disproved the 'vital force theory', which claimed organic compounds could only be produced by living organisms.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 4 marks in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Versatile Nature of Carbon" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This question appears in Section E in the CBSE Class 10 Science 2024 paper.