We are about to explore 4 commonly studied reaction mechanisms, SN1, SN2, E1, & E2. These are all competing
reactions. By this, the reaction conditions determine the mechanism, which ultimately determines the product.
Before we study these, we must first review the idea of nucleophiles & electrophiles.
For each of the following, determine if the species is an electrophile or nucleophile.
RECALL:
Nucleophiles – Nucleus-loving – reacts with positive charged or partially positive charged species.
Neutral atoms can be nucleophiles if they have lone pairs of electrons or pi bonds (double or triple bonds) that can be
used to react with electrophiles.
RECALL:
Electrophiles – electron-loving – reacts with negative charged or partially negative charged species.
Neutral atoms can be electrophiles if they have empty orbitals or can be easily polarizable.
1) For the following, write N for nucleophile and E for electrophile.
NO2+
__E____
C2H4 (H2C=CH2)
OH-
__N____
AlH3
__E___
H2O
__N____
NH3
__N___
Cl-
__N____
any carbonyl (C=O)
CN-
___N___
BH3
_N____
___N___
___E___
Good – now let’s look closer at the STRENGTH of nucleophiles as these help determine if a substitution reaction
occurs via SN1 or SN2.
RECALL: The more negative the species, the stronger the “desire” to obtain + charge, the stronger the nucleophile.
2) Explain why Br- is a stronger nucleophile that HBr. See the Lewis structures below to help answer this question.
Bromide has a greater negative charge than hydrogen bromide.
3) Explain why water is a stronger nucleophile than ammonia. See the Lewis structures below to help answer this
question.
Water has more bare pairs of electrons than ammonia & the electronegativity difference between bonded atoms in
water is greater than that of ammonia.
Another way of measuring the strength of a nucleophile would be to look at the basicity of the species. For most
reactions, there is a direct relationship between the basicity and nucleophilicity of the substance. Stated differently,
the stronger the base, the stronger the nucleophile.
4) Use the table below to answer the following questions:
Which is a stronger nucleophile, NH2- or OH-?
Which is a stronger nucleophile, Cl- or I-?
Both of these have higher pKa values
Yet another way of measuring the strength of a nucleophile is to look at the steric hindrance of that substance. The
more “bulky” the species, the less likely it is to react, the weaker the nucleophile. See below:
NEW VOCABULARY: A Leaving group is molecular fragment that departs with a pair of electrons in heterolytic bond
cleavage.
NOTE: The stronger the nucleophile, the weaker the leaving group. This should be common sense – leaving groups
are stable by themselves & therefore are not as reactive & therefore strong leaving groups are weak nucleophiles.
5) Which is a better leaving group, H2O or OH-? Water is more capable of remaining in an un-bonded state so it is a
stronger leaving group.
Larger halides are more stable than smaller halides. The larger electron cloud allows the added negative charge to be
spread out, making the larger halides more stable.
6) Rank the following in order of increasing nucleophilic strength:
F-, Cl-, Br-, IStronger  weaker
7) Rank the following in order of increasing leaving group strength:
F-, Cl-, Br-, IWeaker stronger
NOW to apply to SN1 & SN2 reactions:
8) SN1 & SN2 reaction mechanisms are mechanisms that explain substitution reactions (something replaces something
else). The end products of both of these are mostly the same. The difference is HOW the product is formed. Use
arrows to show if the description below applies to either SN1 or SN2 mechanisms.
SN1
vs.
SN2
Needs a strong nucleophile
Has a bulky substrate (T-butyl)
Results in carbocation formation
Needs a strong Leaving Group
Fast for methyl substrates
Occurs in 2-steps
Occurs in 1-step
Rate depends on 1 substance
Rate depends on 2 substances
9) Predict the products for each of the following reactions, while also identifying the nucleophile and the leaving group.
CH3 – Br + OH- 
CH3-OH  Br-
Substrate-LG + Nuc:- 
CH3 – Br + CN- 
CH3-CN  Br-
10) Is the above reaction an SN1 or SN2 reaction? ________ Explain how you know the answer to this question.
A methyl substrate favors SN2 (encourages backside attack) & the nucleophiles used are strong.
11) (True / False) Less bulky alkyl groups are favored in the SN1 mechanism.
Due to formation of carbcation, more bulky alkyl groups help stabilizes carbocation formation.
12) (True / False) The SN2 mechanism has a faster rate with a tertiary alkyl halide than with a primary alkyl halide.
SN2 requires backside attack by nucleophile, which is favored with a less bulky, primary alkyl halide.
13) Identify the compound in each of the following pairs that reacts at the faster rate in an S N1 reaction:
a. isopropyl bromide or isobutyl bromide
b. cyclopentyl iodide or 1-methylcyclopentyl iodide
14) Halogenoalkanes undergo nucleophilic substitution reactions. The rates and mechanisms of these reactions depend
on whether the halogenoalkane is primary, secondary or tertiary. Explain the term nucleophilic substitution.
Nucleophilic substitution is an elimination of a substituent followed by replacement by a nucleophile.
15) The formula C4H9Br represents more than one compound. Using this formula, draw a structure (showing all bonds
between carbon atoms) to represent a halogenoalkane that is:
CH 3
CH 2
CH 2
CH 2 Br / CH 2
CH
CH 2 Br
CH 3
primary.
secondary. CH3–––CH2–––CHBr–––CH3
CH 3
CH 3
tertiary.
C
Br
CH 3
16) The reaction of aqueous KOH with 1-bromobutane occurs by a SN2 mechanism.
(a) Draw the mechanism for this reaction, using “curly arrows” to show the movement of electron pairs,
including the structural formulas of 1-bromobutane, the transition state and the organic product.
(b) State and explain how the above reaction is affected when the concentration of the KOH is doubled.
the rate of the reaction doubles;
the rate is proportional to [OH–] / OH− appears in the rate-determining
step / first order with respect to OH−;
(c) State and explain how the rate of reaction of 1-chorobutane in the above SN2 reaction compares with that of
1 bromobutane. Make certain to draw the mechanism for this reaction, including the structural formulas of 1chlorobutane, the transition state and the organic product.
Bromoethane reacts with ammonia as follows.
CH3CH2Br + NH3 → CH3CH2NH3 + Br−
CH3CH2NH3 + NH3 → CH3CH2NH2 + NH4+
The mechanism for this reaction is described as SN2.
See in class
17) Show the mechanism for the reaction between 2-methyl-2-iodopentane and Cl- . Make certain to include
the rate determining step and the name of the product formed.
18) Predict products of the following reactions (while determining the likely mechanism and using curly arrows to
show products being formed):