Alkyl Halides and Aryl Halides
8.0 Chemical Properties
8.0 Chemical Properties
Nucleophilic Substitution Reactions
- The nature of $C-X$ bonds in alkyl halides is polar. In this reaction, a nucleophile reacts with a haloalkane having a partial positive charge on the carbon atom. A substitution reaction takes place and the nucleophile is attached to the carbon while the halide ion leaves the substrate. Since the nucleophile initiates the reactions, it is called a nucleophilic substitution reaction.
- $C-F$ bonds are very strong and hence organic fluorides do not undergo nucleophilic substitution under ordinary conditions.
- This type of reaction can take place via two mechanisms: $SN_1$ and $SN_2$.
Second order nucleophilic substitution $SN_2$:
- It is bimolecular.
- It is a single step process; bonds break and new bonds form at the same time.
- The rate of this reaction is dependent on the concentration of the alkyl halide and the nucleophile. Rate = $k[RX][Nu]$
- Usually primary alkyl halides react by $SN_2$ mechanism via the formation of a transition state.
- Transition state is a point of maximum energy. In this state the carbon hypothetically has $5$ bonds, $1$ is partially broken while $1$ is partially formed.
- Transition state is not a discrete molecule and it cannot be isolated.
For example, the reaction between methyl chloride and hydroxide ion is a $SN_2$ reaction.
- Hydroxide ion is a strong nucleophile because the oxygen atom has a negative charge as well as unshared pairs of electrons.
- In $CH_3Cl$, carbon has a partial positive charge since $Cl$ is more electronegative and it pulls the shared pair of electrons towards itself, thus decreasing the electron density around carbon. Hence, carbon in $CH_3Cl$ is electrophilic.
- The hydroxide ion attacks the back side of the carbon atom and donates a pair of electrons to form a new bond.
- However, carbon can form a maximum of four bonds ($8$ electrons in its valence shell), so the $C-Cl$ bond starts to break and the $C-O$ bond begins to form.
- As can be seen from the above example, the position of carbon atom in the substrate undergoes inversion. This is because the leaving group is pushed away by the attacking nucleophile. This process of inversion is known as Walden's inversion of configuration.
- Order of reactivity: Methyl > Primary > Secondary > Tertiary
First order nucleophilic substitution $SN_1$:
- It is unimolecular.
- The process involves two steps.
1. Formation of carbocation as an intermediate. (The slowest (reversible), and hence the rate determining step)
2. Fast attack by the nucleophile on the carbocation.
- The rate of this reaction is dependent on the reactant (substrate) only. Rate = $k[RX]$
- Usually tertiary alkyl halides react by $SN_1$ mechanism via the formation of a carbocation as an intermediate.
- Generally carried out in polar protic solvents (water, alcohol, ethanoic acid etc.)
For example, consider the reaction between $C(CH_3)_3Br$ and hydroxide ion.
Step 1: Formation of carbocation (It involves the breaking of $C-Br$ bond. Energy for this is obtained through the solvation of halide ion with the proton of the protic solvent.)
Step 2: Attack of nucleophile
- Order of reactivity: Tertiary > Secondary > Primary > Methyl ; R-I > R-Br > R-Cl > R-F
- This order depends on the stability of the carbocation formed. Greater the stability of the carbocation, easier it will be to form alcohol and thus, the reaction would be faster and more stable.
- Primary allylic and benzylic halides show greater reactivity through $SN_1$ mechanism than tertiary alkyl halides since their carbocations are more stable (due to resonance; more the number of resonant structures, more is the stability).
- For an optically active alkyl halide: this mechanism will result in partial racemisation. In this case, the nucleophile can attack from either side resulting in the formation of two enantiomers which are not formed in exactly equal amounts (since back side attack is more probable).
