Properties and Reactions of Haloalkanes | Organic Chemistry 1

Properties and reactivities of Haloalkanes is studied in this chapter: physical properties of haloalkanes, polar reactions, introduction to one of the most important types of reactions in organic chemistry: nucleophilic substitution reactions (SN reactions).

Physical Properties of Haloalkanes

Haloalkanes (or alkyl halides) = group of chemical compounds comprised of an alkane with one or more hydrogens replaced by a halogen atom X (X = F, Cl, Br or I).
 


 

The C–X bond is polarized: this is the main characteristic of the haloalkanes.
Halogens are more electronegative than carbon atom.
Thus, the electron density along the C–X bond is displaced in the direction of X.

The bond strength of C–X decreases as the size of X increases.

Polar Reactions

Polar reactions involve species that have an even number of valence electrons and thus have only electron pairs in their orbitals (≠ radical reactions).


 

Polar reactions occur between the positive (electron poor or electrophile atom) and negative (electron rich or nucleophile atom) regions of molecules. These regions are created by the polarity of the bonds due to the difference in electronegativity of the bonded atoms.

 

In green: electron rich / nucleophile atom
In blue: electron poor / electrophile atom

 

 

Nucleophilic Substitution (SN Reactions)

In this process, a nucleophile attacks an electrophile and replaces a good leaving group.

Good nucleophile (Nu-): NaCN (NC-), NaSR (RS-), NaOR (RO-)
Good leaving group: I-, Br-, TsO-

 

 In this chapter: a nucleophile attacks the haloalkane and replaces the halides (X).

SN2 Reactions

SN2 reactions = nucleophilic substitutions which do not proceed via an intermediate: SN2 reactions are bimolecular with simultaneous bond-making and bond-breaking steps.
Because of this unique step, SN2 reactions give inversion of stereochemistry at the reaction centre.
The kinetic rate involves 2 components: the nucleophile and the electrophile reagents.

 

 

The nucleophile attacks the electrophile center on the opposite side of the good leaving group.
The other substituents flip from one side to the other: this is the inversion of configuration.

Steric effects are particularly important in SN2 reactions: