Question

Bromination of an alkane compared to chlorination proceeds (1) at a slower rate (2) at a faster rate (3) with equal rate (4) with equal or different rate depending upon the temperature

Short answer

Bromination of an alkane proceeds at a slower rate compared to chlorination.

Step-by-step solution

Understand Bromination and Chlorination

Bromination and chlorination are types of halogenation reactions where a hydrogen atom in an alkane is replaced by a bromine or chlorine atom respectively.

Compare Reactivity of Halogens

Chlorine is more reactive than bromine. Bromination proceeds through a more selective process, whereas chlorination is less selective and more reactive.

Assess Rate of Reaction

Due to the higher reactivity of chlorine, chlorination of alkanes happens at a faster rate compared to bromination.

Conclusion

Bromination of an alkane proceeds at a slower rate compared to chlorination.

Key concepts

halogenation reactions

Halogenation reactions involve the addition of a halogen (like chlorine or bromine) to another compound. In the context of alkanes, this tends to be a substitution reaction where a hydrogen atom in the alkane is replaced by a halogen atom.
For example, when we take methane (CH₄) and react it with chlorine (Cl₂), in the presence of light or heat, it becomes chloromethane (CH₃Cl) and hydrochloric acid (HCl). Here's how the reaction looks chemically:

\[ CH₄ + Cl₂ → CH₃Cl + HCl \]

In bromination, a similar reaction occurs, but a bromine atom replaces the hydrogen. The reaction would look like this:

\[ CH₄ + Br₂ → CH₃Br + HBr \]

In both cases, the bond between the two halogen atoms breaks first, which then allows one of those atoms to bond with the alkane. While the reactions seem similar, the reactivity and mechanisms can vary significantly.

reactivity of halogens

Understanding the reactivity of halogens helps us grasp why different halogens behave differently in reactions. Halogens are a group in the periodic table that include fluorine, chlorine, bromine, iodine, and astatine.
In terms of reactivity, we often see a pattern where the lighter halogens (such as fluorine and chlorine) are more reactive compared to the heavier ones (such as bromine and iodine).
For chemical reactions involving alkanes, chlorine is highly reactive, meaning it can easily replace a hydrogen atom. This is because chlorine atoms have higher energy and lower bond dissociation energy.
Bromine, on the other hand, is less reactive and more selective. This means that while bromination can occur, it requires more specific conditions (like more heat or light) and tends to be a slower process. The selectivity also means bromine might favor substituting hydrogens at certain positions within a molecule more than chlorine would.
Such differences in reactivity can greatly influence the rate and outcome of halogenation reactions, making it crucial to choose the right halogen for the desired chemical process.

reaction rate comparison

When comparing the rates of bromination and chlorination reactions, the differences in reactivity come into play. Chlorination happens at a faster rate compared to bromination. This is due to the lower bond dissociation energy of chlorine, which means it's easier and faster for chlorine to break its bonds and react with alkanes.
To put it simply:

• Chlorination: Fast, less selective, requires less energy (light or heat)
• Bromination: Slow, more selective, requires more energy

Therefore, if we were to react an alkane with both chlorine and bromine under similar conditions, the chlorine would react more quickly. For example, if we have propane reacting with chlorine and bromine separately, the chlorination process would occur faster and yield more chlorinated product more quickly than brominated product.
This concept is crucial in industrial and laboratory settings, where the speed and selectivity of reactions can affect the efficiency and outcome of chemical processes.