Kinetics of First and Second Order Reactions
In organic chemistry, many reactions happen at different speeds. Some reactions finish very fast, while some take more time. The study of the rate of a chemical reaction and the factors that affect this rate is called chemical kinetics.
In this chapter, we learn two important types of reactions based on their reaction rate: First order reactions and Second order reactions.
What is Reaction Order?
Reaction order tells us how the concentration of reactants affects the speed of a reaction. If changing the concentration of a reactant changes the reaction rate, then the reaction depends on that reactant.
1. First Order Reaction
A first-order reaction depends on the concentration of only one reactant. If the concentration of the reactant becomes double, the reaction rate also becomes double.
Examples:
- Radioactive decay
- Hydrolysis of esters in acidic medium
Rate Equation:
Rate = k[A]
Here, k = rate constant [A] = concentration of the reactant
Characteristics of First Order Reaction:
- Rate depends on only one reactant.
- Unit of k is s−1.
- Half-life (t1/2) is independent of concentration.
- Follows exponential decay pattern.
2. Second Order Reaction
A second-order reaction depends on the concentration of either:
- Two molecules of the same reactant, or
- One molecule each of two different reactants
If the concentration of reactant becomes double, the reaction rate becomes four times.
Rate Equation:
Rate = k[A]² or Rate = k[A][B]
Characteristics of Second Order Reaction:
- Rate depends on two reactant molecules.
- Unit of k is L·mol−1·s−1.
- Half-life depends on concentration.
- Rate decreases much faster than first order reactions.
Nucleophilic Substitution at Saturated Carbon
A substitution reaction is a reaction where one group is replaced by another group. In nucleophilic substitution, a nucleophile (an electron-rich species) replaces a leaving group.
Types of Substitution Reactions:
- Radical substitution
- Electrophilic substitution
- Nucleophilic substitution (discussed here)
In aliphatic compounds (like alkanes), nucleophilic substitution happens at a saturated carbon atom. This means the carbon atom is bonded by single bonds only.
There are two mechanisms:
- SN1 mechanism – two-step process involving a carbocation intermediate
- SN2 mechanism – one-step process with a backside attack
Both SN1 and SN2 mechanisms depend heavily on:
- Strength of nucleophile
- Quality of leaving group
- Structure of the substrate
- Solvent conditions
Summary:
- First order reactions depend on one reactant only and have simple rate equations.
- Second order reactions depend on two molecules (same or different).
- Nucleophilic substitution replaces one group with another at a saturated carbon.
- SN1 and SN2 are the two key mechanisms of nucleophilic substitution.
To see diagrams and reaction mechanisms clearly, refer to the PDF provided at the end of the post.
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/KINETICS OF SECOND AND FIRST ORDER REACTION.