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Lecture 6

CHEM 233 Lecture Notes - Lecture 6: Stopcock, Sodium Sulfate, Cengage Learning


Department
Chemistry
Course Code
CHEM 233
Professor
Driver Tom
Lecture
6

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Preparation of Alkyl Halides by Nucleophilic Aliphatic Substitution. NaI and AgNO3 Tests
for Alkyl Halides
Kenil Gandhi
Partners: Victor, Zahra
March 29, 2016
Methods and Background
The objective of this lab is to prepare tertiary alkyl halide by SN1 reaction, primary alkyl halide
by SN2 reaction. The other objectives are to isolate a liquid product by simple distillation, to
properly use a separatory funnel and to distinguish between tertiary, secondary and primary alkyl
halides through NaI and AgNO3 tests and finally to calculate the percent yield of an isolated
product through a synthesis reaction.
As shown in Figure 1, nucleophilic aliphatic substitution reaction involves the conversion
between different functional groups in which Nu: represents a nucleophile and L: represents a
leaving group. Nucleophiles are either neutral or negatively charged. The nonbonding pairs of
electrons on the nucleophile are donated to an electrophilic atom which results in the formation
of a new covalent bond during the process of substitution reaction. This reaction is like a Lewis
acid-base reaction in which the electrophilic carbon atom acts as a Lewis acid and nucleophile
acts as a Lewis base. The leaving group may be neutral or negatively charged and must accept
the pair of bonding electrons from the carbon atom when the C-L bond breaks. Thus, the rate of
substitution reaction depends on the leaving ability of a particular group L: If the better the
leaving group is better at leaving the substance, the faster the reaction will occur. Similarly,
conjugate bases of strong acids are also known for good leaving groups whereas those of weak
acids are poor leaving groups.
Figure 1: Nucleophilic Aliphatic Substitution Reaction
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Figure 2: Summary of Reactions in this lab
As shown in Figure 2, nucleophilic substitution reaction is classified into two types based on
their different mechanistic pathways: SN1 and SN2 reaction. SN1 reaction is an endothermic
process in which the first step of SN1 reaction is much slower than the second step and involves
heterolytic cleavage or ionization of the C-L bond to generate an unstable carbocation. The
breakage of the C-L bond requires higher energy. The intermediate carbocation formed during
the first step can undergo rearrangement to form a more stable carbocation, form an alkene
through elimination reaction or react with the nucleophile to form a stable substitution product.
The second step of SN1 reaction is relatively fast because it is an exothermic process which
involves bond formation between the carbocation and the nucleophile. The first step of the SN1
reaction is the rate-determining step (rds) and the rate of the overall reaction depends only on the
concentration of the substrate R-L which is completely independent of the concentration of the
nucleophile. Thus, SN1 reaction is called a unimolecular reaction because its rate determining
step involves only one species which is the substrate R-L. This concept is further explained by a
rate equation Rate = k1[R-L] where k1 is the first order rate constant. Since the transition state in
SN1 reaction involves the formation of a stable carbocation, these types of reactions favors
compounds with a leaving group from tertiary-bonded carbons due to its stability, which is
tertiary > secondary >>primary. According to Le Chatelier’s principle, if equilibrium is
disturbed, then conditions in the reaction will change to keep it at equilibrium. Therefore, if the
concentration of the reactants increases, the reaction will shift to the right resulting in the
formation of the product.
Figure 3: SN1 versus E1
As shown in Figure 3, unimolecular elimination reaction, E1, competes with SN1 substitution
reactions. The reaction depends on the nature of the nucleophile. Substitution can occur in the
weakly basic conditions and also highly polarizable nucleophiles such as I-, Br-, Cl-, H2O, and
Ch3CO2-. For elimination, strongly basic and only slightly polarizable nucleophiles condition
should be met when RO-, H2N-, H-, and HO- are used. Moreover, larger nucleophiles tend to
favor elimination because the hydrogen atom is more sterically available than is the carbon atom
bearing the leaving group. In this experiment, tertiary alkyl halide, 2-chloro-2-methylbutane will
be prepared through SN1 reaction by reacting 2-methyl-2-butanol with hydrochloric acid. In this
reaction,OH is a poor leaving group which is protonated by hydrochloric acid and oxonium ion
is formed. Since H2O is a good leaving group, it results in the formation of a carbocation which
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at the end is attacked by the nucleophilic chloride ion. The below Sn1 reaction mechanism is
shown in Figure 4.
Figure 4: SN1 Reaction Mechanism
In SN2 reaction, primary alcohols reacting with a hydrogen halide H-X (X= Cl, Br, or I) to obtain
primary alkyl halides. In compare to SN1 reaction, SN2 reaction consists of only one step which is
also the rate-determining step and does not go through carbocation formation. In SN2, reaction,
the nucleophile attacks the carbon atom on the opposite side of the leaving group. When the
nucleophile attaches the carbon atom, the leaving group cleaves the carbon atom. The carbon
atom takes bonding pair of electrons. The rate determining step of this reaction is dependent on
the concentrations of both the substrate, R-L, and the nucleophile, since this reaction is
bimolecular reaction. This concept is explained by a rate equation Rate = k2[R-L][Nu:] where k2
is the second order rate constant. In these reactions, the nucleophile attacks the substrate which is
directly behind the leaving group. Therefore, the more sterically hindered a carbon atom is, the
less chances to undergo an SN2 reaction. Therefore, SN2 reactions favor compounds with a
leaving group from primary-bonded carbons due to its stability, which is primary > secondary >>
tertiary. Based on the stability of the transition state, leaving groups from secondary-bonded
carbon, the reaction can proceed either pathway, SN1 or SN2. According to Le Chatelier’s
principle, the reaction will be favored by increasing the concentration of the reactants. By
increasing the amount of nucleophile, the concentration of the compound in the transition state
increases which increases the rate of product formation. The SN2 reaction prefers leaving groups
that are weaker bases, since they have greater leaving ability.
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