Properties and Reactions of Haloalkanes | Organic Chemistry 1

Properties and reactivities of haloalkanes are 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):

Chemical compounds consisting of an alkane with one or more hydrogens replaced by a halogen atom X
Halogens are the elements occupying group VIIA (17) of the periodic table: fluorine, chlorine, bromine, iodine, and astatine
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Properties:

Halogens are more electronegative than the carbon atom. Therefore, the electron density along the C–X bond is shifted in the direction of X ⇒ The C–X bond is polarized

The bond strength of C–X decreases as the size of X increases ⇒ Iodine is a better leaving group than fluorine

Polar Reactions

 

Polar reactions occur between an electron poor or electrophile atom and an electron rich or nucleophile atom
Some regions of a molecule can be poor or rich in electrons: they are created by the polarity of the bonds due to the difference in electronegativity of the bonded atoms

 

In blue: electrophile (electron poor atom)
In green: nucleophile (electron rich  atom)

 

Polar reactions involve species that have an even number of valence electrons and thus have only electron pairs in their orbitals (≠ radical reactions) ⇒ heterolytic cleavages occurs during this process

 

Nucleophilic Substitution Reactions

SN Reactions:

Class of reactions in which a nucleophile attacks an electrophile to replace a good leaving group
 

the nucleophile attacks the haloalkane and replaces the halogen atom


 

Examples of nucleophiles and leaving groups:

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

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. The kinetic rate of a SN2 reaction involves 2 components: the nucleophile and the electrophile reagents


Mechanism:

Because of its unique step, SN2 reaction results in an inversion of configuration at the reaction centre. 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: