Why does halogenation of alkanes takes place at lower temperature in the presence of tetramethyl lead than when done in its absence?
Answers
The halogenation of alkanes is a free radical reaction.
The initiation step involves the homolytic cleavage of a Cl-Cl or Br-Br bond.
It takes energy to break a covalent bond. This energy can come from one of two sources.
1. Heat
A high temperature makes the molecules move more rapidly and have more energetic collisions.
It also makes the bonds vibrate with greater amplitude, so that the bonded atoms are almost falling away from each other.
So a high temperature promotes the initiation step.
2. Ultraviolet Light
The electrons in the bonding MO of the Cl-Cl bond can absorb a quantum of ultraviolet energy. This can excite them to an antibonding MO.
The weakened bond plus the energy of molecular collisions again leads to homolytic cleavage of the Cl-Cl bond.
So heat and light are the conditions that favour halogenation of alkanes.
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Answer:
Halogenation of alkanes takes place at lower temperature in the presence of tetramethyl lead because the weaker bond combined with the energy of molecular collisions causes homolytic cleavage of the Cl-Cl bond once more.
Explanation:
- Halogenation of alkanes is a process involving free radicals.
- The homolytic cleavage of a Cl-Cl or Br-Br bond is the first step.
- A covalent bond requires energy to break. One of two types of energy can be used to generate this energy.
1) By producing heat :
- High temperatures cause molecules to travel faster and collide more energetically.
- It also causes the bonds to oscillate with increased amplitude, almost causing the connected atoms to come apart.
- As a result, a high temperature encourages the initiation process.
2) Ultraviolet Radiation
- The electrons in the Cl-CI bond's bonding MO can absorb a little amount of UV energy. This may cause them to develop an antibonding MO.
The weaker bond, along with the energy of molecular collisions, causes homolytic cleavage of the CI-Cl bond once more.
As a result, heat and light are favourable circumstances for alkane halogenation.