Outline the mechanism

Question

1. for nucleophilic substitution Between 2-bromopropane and potassium cyanide and name the product reaction

Answer 1

Hello Derick

 

The reaction of alkyl halides with cyanide forms nitriles as the main product.  The general structure of a nitrile is R-C≡N (or R-CN in a condensed form).

 

The main reactant in Potassium cyanide (KCN) is the cyanide anion (CN‾).  The potassium ion (K⁺) exists only as a spectator ion in the overall reaction.

 

2-Bromopropane + KCN → 2-Methylpropane nitrile (name of main product) + KBr (by-product)

 

CH₃-CHBr-CH₃   →  ** I am going to rewrite the structure of 2-bromopropane in this form  →   (CH₃)₂-CH-Br

 

(CH₃)₂-CH-Br + CN‾   →   (CH₃)₂-CH-CN +  Br‾

 

The carbon of CN in the final product will be carbon #1 as per IUPAC nomenclature rules.  The longest chain is 3 carbons (propane) and on Carbon-2, a methyl group is present.  When naming nitriles, you write the carbon # and name of any substituent first, then write the name of the longest chain alkane and finally, write the name "nitrile" at the end.  Hence, the final product name is 2-methylpropane nitrile.  I will attempt to draw the structure below, and hopefully, the formatting does not get distorted.

 

CH₃-CH-CN

        ι

       CH₃

 

In the reaction, 2-bromopropane is the alkyl substrate and the cyanide ion (CN‾) is the nucleophile.  The lewis structure of CN‾ is [:C≡N:]‾, and the negative charge is actually present on the carbon atom, and NOT the nitrogen!

 

In the mechanism, the negative carbon atom of CN‾ with its lone pair of electrons starts to form a covalent bond with the carbon atom bonded to Br in 2-bromopropane.  At the same time, Br takes away the electron pair from the carbon it was bonded to to form bromide ion (Br‾) as the by-product.  Br‾ serves as the leaving group in this reaction. At the end of the reaction, the CN group substitutes for the Br group in the original structure.

 

Hope the above was helpful!