Comparison of the
SN2 preparation of
Nitriles at Primary
• Nitriles have widely been used as intermediates
for the synthesis of fine chemicals such as
agricultural chemicals, dyes and medicines.
• The most general methods for the synthesis of
nitriles is the nucleophilic substitution reaction
of alkyl halides with metal cyanides that
follows the reactivities from an SN2 reaction.
• The SN2 reaction is fastest with unhindered
• As the number of R groups on the carbon with
the leaving group increases, the rate of an SN2
• The steric hindrance caused by bulky R
groups makes nucleophilic attack from the
backside more difficult, thus slowing the
Ø If the primary and secondary haloalkane is
reacted with the metal cyanide, then the rate
of reaction (SN2 reaction) in formation of
cyanoalkane is faster in primary position
than the secondary.
3) CHEMICALS REQUIRED:
Ø 1- Bromobutane
Ø Sodium Cyanide
Ø Dimethyl Sulfoxide (DMSO)
Ø Saturated Aqueous Sodium Chloride
ØCalcium Chloride ( 4 Mesh)
4) REACTIONS INVOLVED:
q1-Bromobutane/ 2-bromobutane, Sodium
cyanide (grounded and then dried), dried DMSO
were heated to about 600 C with continuous
qAt this point the heat is removed; and since the
reaction is exothermic, a rapid rise in temperature
qAt about 700 C, the reaction is cooled in an ice
water bath to hold the temperature.
qThe cooled reaction mixture is then poured
into a seperatory funnel containing water and
saturated aqueous NaCl.
qThe product is extracted with a portion of
dichoromethane, a part of which is used to rinse
the reaction flask.
qThe mixture is well shaken and two layers are
formed in the funnel.
qSince the desired product is denser i.e
cyanobutane, it resides the bottom layer.
qAfter the separation is made, drying is
accomplished by using 4 mesh Calcium chloride
qFinally, the Infrared Spectroscopy(IR) and Gas
chromatography(GC) of the product are taken.
Both the IR and GC showed that there was a
presence of nitrile group in the product.
From Infrared Spectroscopy
§ Nitriles appear just above 2200 cm-1.
§ A sharp peak was seen at about approximately
2300 cm-1 in the IR of the product from both
the reactants that makes the presence of nitrile
group evident in the product.
§ The area under the nitrile peaks was almost
similar in both the IRs so it was hard to
compare the rate of reaction between primary
and secondary bromobutane.
From Gas Chromatography
§ Three peaks were seen in the GC of the product,
of which the first one from above was from the
solvent i.e dichloromethane.
§ The second one was from the reactant i.e 1-
bromobutane and 2- bromobutane respectively.
§ And the third peak was from the nitriles.
§ The ratio of 1-bromobutane to the cyano group
was found to be 3.
§ Similarly, the ratio of 2-bromobutane to the
cyano group was found to be 2.8.
§ The ratios were almost similar so, it was hard
to tell that which one had a faster rate of
§ Theoretically, the ratio of yield should have
been more in primary bromobutane than in
the secondary one, but while doing the
experiment we found it to be almost similar.
§ The rate of reaction was found to be the same
in both the primary and secondary
7) CONCLUSION :
o The IR shows that there was a presence of
cyano group in the product, thus we were
successful in preparing the nitrile.
o SN2 reactions are generally favored in primary
alkyl halides than secondary alkyl halides and
occur at a negligible rate in tertiary alkyl
halides due to steric hindrance.
o There might be a possibility of SN1 reaction
taking place at the side in 2-bromobutane that
probably got collided with the SN2 reaction and
hence made the reaction as fast as in 1-
o Due to limited time, the experiment was not
performed enough number of times to get a
perfect IR or GC from the product nor the
product was pure.
• Cason , J. ;Rapoport , H. Laboratory Text in Organic Chemistry,3rd ed.
;Prentice-Hall, Inc. ,New Jersey,1950; pp 358-365.
• Smith, J. Organic Chemistry, 2nd ed. ;McGraw Hill , Singapore, 2008; pp