Organic chemistry - Alkanes (3)
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1.
A student carried out a reaction between methane (CH4) and chlorine (Cl2) under direct sunlight. They observed the formation of chloromethane (CH3Cl) and hydrogen chloride (HCl). State what type of reaction this is and explain why direct sunlight is required.
This is a substitution reaction. Specifically, it's a free radical substitution reaction.
Why is direct sunlight required?
Direct sunlight provides the energy (photons) needed to break the Cl-Cl bond in chlorine (Cl2). This homolytic cleavage generates two chlorine radicals (Cl•). These radicals are highly reactive and initiate a chain reaction, where a chlorine radical abstracts a hydrogen atom from methane, forming a methyl radical (CH3•). The methyl radical then reacts with another chlorine molecule to form chloromethane and regenerate a chlorine radical, continuing the chain. Without the UV light, the Cl2 molecule would not be broken down into radicals, and the reaction would not proceed effectively. The UV light acts as an initiator for the reaction.
2.
Describe the type of bonding present in alkanes. Explain why alkanes are classified as hydrocarbons. State whether alkanes are saturated or unsaturated and explain your answer.
Alkanes contain single covalent bonds between carbon atoms and single covalent bonds between carbon and hydrogen atoms. Covalent bonds are formed by the sharing of electrons between atoms.
Alkanes are classified as hydrocarbons because they are composed entirely of hydrogen and carbon atoms. This is the defining characteristic of hydrocarbons.
Alkanes are classified as saturated hydrocarbons. This is because each carbon atom is bonded to the maximum number of hydrogen atoms possible (four). Since they are saturated with hydrogen, they cannot accommodate any more hydrogen atoms without breaking existing single bonds. The presence of only single bonds between carbon atoms further supports their classification as saturated.
3.
State, with examples, what is meant by a substitution reaction in organic chemistry.
A substitution reaction is a type of chemical reaction where one atom or group of atoms in a molecule is replaced by another atom or group of atoms.
Example 1: Halogenation of Alkanes: In the reaction of an alkane (like methane, CH4) with a halogen (like chlorine, Cl2) in the presence of UV light, one hydrogen atom is replaced by a halogen atom.
CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)
Example 2: Hydrohalogenation of Alkenes: An alkene (like ethene, C2H4) reacts with a hydrogen halide (like hydrochloric acid, HCl) to replace a hydrogen atom on the alkene with a hydrogen halide group.
C2H4(g) + HCl(g) → CH3Cl(g)
In both examples, a specific atom or group of atoms is directly replaced by another.