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SN1 and SN2 Reactions - Kinetics, Order of Reactivity of Alkyl Halides

When 2-Bromo-2-methylpropane reacts with water, the rate depends only on the concentration of the alkyl halide, and not concentration of nucleophile.

SN1 reaction – kinetics

When 2-Bromo-2-methylpropane reacts with water, the rate depends only on the concentration of the alkyl halide, and not the concentration of the nucleophile. When the concentration of alkyl halides is doubled, the reaction rate doubles. When the nucleophile concentration is doubled, there is no change in reaction rate. As a result, the reaction rate is determined only by the alkyl halide concentration.

Following are the components of the rate law:

Reaction rate = k [2-bromo-2-methylpropane] mol/(l s)

Note - In chemical reactions, the speed of every reaction is determined by its slowest step.

In addition to being the slowest step, it is also the one with the greatest activation energy. During the transition between two states, energy consumption for a transition is determined by the conversion factor. Below you will find reactions energy diagrams.

The first figure shows:
Activation energy determines a large part of each step.

The second figure shows:
Step 2 is the step that determines the rate with the most activation energy.





SN2 reaction – kinetics

Nucleophile concentration and substrate concentration affect the rate of SN2 reactions (e.g., alkyl halide). As both reactants are present in the rate-determining step, the reaction follows second-order kinetics.


Stereochemistry and Mechanism of SN2 Reaction:
SN2 reactions involve only a single step. As a result, halogen – carbon (C – X) bonds are broken and carbon – nucleophile bonds are made simultaneously.

Nucleophiles attack carbon attached to halogen atoms from the opposite side (i.e. backside attack). The halogen atom forms a partially-bonded bond to both the nucleophilic group and leaving group (carbon atom), which results in a transition state (activated complex).

An activated complex is an unstable species that contains a large amount of energy. Upon leaving the molecule as a bromide ion, the hydroxide ion forms a covalent bond with carbon on the product, which is an alcohol having an inverted configuration.



Order of reactivity of alkyl halides

The compounds that are also called alkyl halides (haloalkanes) occur when halogen atoms (fluorine, chlorine, bromine, or iodine) are substituted for hydrogen atoms.

Haloalkanes (also known as haloalkenes) are organic molecules that contain one or more atoms of halogen (fluorine, chlorine, bromine, or iodine). The carbon atom bonded to halogen and the halogen atom has polarized electronegativity differences; the halogen atom is somewhat negative while the carbon atom is slightly positive. The two atoms are therefore linked by polarized covalent bonds. Therefore, there is a difference in electronegativity.

Halogen atoms become smaller and their electronegativity decreases as they move down a periodic table. Hence, the carbon and halogen bonds lengthen as the halogen atoms change from fluorine to iodine. (Iodine and fluorine are the largest and smallest atoms of group 1717 of the periodic table, respectively). It is faster to remove Iodine atoms than Bromine and Chlorine atoms since the Iodine atoms are weaker during the Elimination or Substitution reaction. RI > RBr > RCl is the decreasing order in which alkyl halides react.>
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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