Chemical Properties of Carbon Compounds: Substitution MCQs Quiz | Class 10

This quiz covers Class X Science (Code 086), Unit I: Chemical Substances – Nature and Behaviour, focusing on Chemical Properties of Carbon Compounds: Substitution MCQs. It explores typical reactions of saturated hydrocarbons and the basic idea of halogenation. Attempt all 10 multiple-choice questions and then submit to see your results. You can also download a detailed answer PDF for revision.

Understanding Substitution Reactions in Carbon Compounds

Carbon compounds, especially hydrocarbons, exhibit various chemical properties that define their reactivity. Among these, substitution reactions are particularly important for saturated hydrocarbons like alkanes. Let’s delve deeper into this concept.

What are Saturated Hydrocarbons?

Saturated hydrocarbons are organic compounds that contain only carbon-carbon single bonds and are ‘saturated’ with hydrogen atoms. This means each carbon atom is bonded to the maximum possible number of hydrogen atoms. Examples include methane (CH4), ethane (C2H6), and propane (C3H8). Due to the presence of strong C-C and C-H single bonds, these compounds are generally stable and less reactive than unsaturated hydrocarbons (which contain double or triple bonds).

Substitution Reactions

Unlike unsaturated hydrocarbons which readily undergo addition reactions across their double or triple bonds, saturated hydrocarbons primarily undergo substitution reactions. In a substitution reaction, one or more hydrogen atoms of a hydrocarbon molecule are replaced by other atoms or groups of atoms. These reactions usually require specific conditions like high temperature or the presence of light.

Halogenation of Saturated Hydrocarbons (Alkanes)

Halogenation is a classic example of a substitution reaction. It involves the replacement of one or more hydrogen atoms in a hydrocarbon by halogen atoms (Fluorine, Chlorine, Bromine, Iodine). The most common and studied halogenation reactions involve chlorine and bromine.

Key aspects of Halogenation:

  • Reactants: An alkane and a halogen molecule (e.g., Cl2, Br2).
  • Conditions: These reactions typically require the presence of sunlight (ultraviolet light) or high temperatures (around 300-400°C). Sunlight provides the energy needed to break the halogen-halogen bond, initiating a chain reaction mechanism involving free radicals.
  • Mechanism: The reaction proceeds via a free radical mechanism.
  • Products: Halogenated alkanes (haloalkanes) and hydrogen halide (e.g., HCl, HBr).

Example: Chlorination of Methane

Methane (CH4) reacts with chlorine gas (Cl2) in the presence of sunlight to form chloromethane (CH3Cl) and hydrogen chloride (HCl). If chlorine is in excess and conditions permit, further substitution can occur, leading to dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).

CH4 + Cl2 --(UV Light)--> CH3Cl + HCl  (Chloromethane)
CH3Cl + Cl2 --(UV Light)--> CH2Cl2 + HCl (Dichloromethane)
CH2Cl2 + Cl2 --(UV Light)--> CHCl3 + HCl (Trichloromethane)
CHCl3 + Cl2 --(UV Light)--> CCl4 + HCl  (Tetrachloromethane)

Table: Reactivity of Halogens in Substitution

Halogen Reactivity Conditions Remarks
Fluorine (F2) Extremely Reactive Explosive, even in dark Difficult to control
Chlorine (Cl2) Reactive UV light or heat Commonly used, controllable
Bromine (Br2) Less Reactive than Cl2 UV light or heat (higher temp/longer exposure) Similar to chlorine, but slower
Iodine (I2) Least Reactive Reversible, often requires oxidizing agent Difficult to achieve direct substitution

Quick Revision Points

  • Saturated hydrocarbons contain only single C-C and C-H bonds.
  • They primarily undergo substitution reactions.
  • Halogenation is a type of substitution where H atoms are replaced by halogen atoms.
  • UV light or heat is essential for halogenation of alkanes (except for fluorine, which is too reactive).
  • Multiple substitutions can occur if the halogen is in excess.

Practice Questions (with answers)

1. Why are alkanes generally unreactive?
Answer: Alkanes are generally unreactive because their C-C and C-H bonds are very strong, and they lack double or triple bonds that could act as reaction sites for addition reactions. They are saturated, meaning they contain the maximum number of hydrogen atoms possible.

2. What is the main difference between an addition reaction and a substitution reaction in organic chemistry?
Answer: In an addition reaction, atoms are added across a multiple bond (double or triple bond), saturating the molecule, without the loss of any existing atoms. In a substitution reaction, an atom or group of atoms is replaced by another atom or group, meaning atoms are exchanged.

3. If you mix propane (C3H8) with bromine water in the dark, what would you observe?
Answer: You would observe no significant reaction. Propane is a saturated hydrocarbon and requires UV light or high heat for substitution with bromine. Bromine water is used to test for unsaturation (double/triple bonds), and if no unsaturation is present, the bromine color would persist in the dark.

4. Name the products when ethane (C2H6) undergoes complete chlorination in the presence of excess chlorine and UV light.
Answer: The products would be hexachloroethane (C2Cl6) and hydrogen chloride (HCl). All six hydrogen atoms of ethane would be replaced by chlorine atoms.

5. Why is fluorine halogenation of alkanes difficult to control?
Answer: Fluorine is extremely reactive. The F-F bond is weak, and its reaction with C-H bonds is highly exothermic, often leading to explosive and uncontrolled reactions that break down the carbon chain.