Summary A Level Chemistry - Further Kinetics (Part 2)

A Level Chemistry – Further Kinetics (Part 2)

• If reaction takes place via a single step, it is called an
elementary reaction.

However, most reactions would take place via a reaction
mechanism, made up of various steps.




• 𝐒𝐍 𝟏 Mechanisms:

For the alkaline hydrolysis of 2-chlormethylpropane (tertiary
halogenoalkane), the following chemical equation and rate
equation apply:

(CH3 )3 CCl + OH− → (CH3 )3 COH + Cl−

𝑅𝑎𝑡𝑒 = 𝑘[(CH3 )3 CCl]

1st order with respect to the halogenoalkane while 0 order with
respect to the hydroxide.

This means the RDS only contains the halogenoalkane.

, The RDS is the slow ionisation of the halogenoalkane. This is
followed by a fast attack by the hydroxide:

Step 1: (CH3 )3 CCl → (CH3 )3 C + + Cl− SLOW
Step 2: (CH3 )3 C + + OH− → (CH3 )3 COH FAST

This type of reaction is called an 𝐒𝐍 𝟏 reaction:

S = Substitution
N = Nucleophilic
1 = Unimolecular (Only one molecule present in the RDS with
the formation of a carbocation intermediate in step 1)


• 𝐒𝐍 𝟐 Mechanisms:

For the alkaline hydrolysis of 1-chlorobutane (primary
halogenoalkane), the following chemical equation and rate
equation apply:

CH3 CH2 CH2 CH2 Cl + OH− → CH3 CH2 CH2 CH2 OH + Cl−

𝑅𝑎𝑡𝑒 = 𝑘[CH3 CH2 CH2 CH2 Cl][OH− ]

It is first order with respect to both the primary halogenoalkane
and the hydroxide, meaning the RDS contains the
halogenoalkane and the nucleophile (OH− ).




This type of reaction is called an 𝐒𝐍 𝟐 reaction: