Chapter 21: Alkyl Halides
A: MULTIPLE CHOICE QUESTIONS
1. Which one of the following structures is 2,3-dibromo-4-ethylheptane?
CH3
Br
CH3
Br
Br
H3C
H3C
CH3
CH3 H3C
Br
Br
CH3
H3C
Br
Br
H3C
a)
H3C
b)
ANS: b
CH3
Br
c)
d)
REF: Section 21.3
2. Which one of the following could be effectively chlorinated using gaseous hydrochloric acid in ether?
OH
OH H3C
OH
OH
H3C
CH3
H3C
H3C
CH3
H3C
CH3
a)
b)
ANS: b
CH3
CH3
c)
d)
REF: Page 816
3. Which one of those listed is the product of the following reaction?
CH3
CH3
+
NaOH
+
Na Cl
Cl
CH3
CH3
CH3
CH3
CH3
CH3
CH3
HO
CH3
HO
HO
a)
ANS: a
Cl
b)
HO
Cl
+
Cl
c)
d)
REF: Page 818, 819
4. The following compound can be synthesized from ethyl chloride, according to the reaction shown,
using which one of the listed nucleophiles?
CH3
+
Cl
?
CH3
O
Na Cl
+
CH3
a)
b)
c)
d)
NaNH2, sodium amide
CH3OH, methanol
NaOH, sodium hydroxide
NaOCH3, sodium methoxide
ANS: d
REF: Pages 819, 820
5. Choose from the options given, the correct product of the following reaction.
H3C
H
H3C
CH3
Cl
CH3
H2N
H
R
Cl
R
-
NH2
H
R
H
CH3
NH2
NH2
H3C
Cl
CH3
S
H
R
CH3
a)
CH3
b)
ANS: b
c)
d)
REF: Section 21.6
6. Of the following compounds which one would be most reactive to substitution using sodium cyanide?
Cl
Cl
CH3
CH3
H3C
CH3
H3C
Cl
a)
ANS: a
Cl
b)
REF:
c)
CH3
d)
section 21.6
7. Which one of the following is true regarding SN1 reactions?
a) They involve a carbanion intermediate and the intermediate is planar
b) Kinetics are 1st order and formation of the carbocation is the fastest step
c) Kinetics are 2nd order and the rate is higher with tertiary than secondary
d) A racemic mixture is formed from chiral substrates
ANS: d
REF: Pages 825, 826
8. To favour an E1 reaction over an SN2 or E2 reaction when using a tertiary alkyl-halide you can do
which one of the following?
a) Use a strong base in a basic solution
b) Use a weak base in a basic solution
c) Use a good nucleophile in a neutral solution
d) Use a weak acid and it’s conjugate base as nucleophile
ANS: c
REF: Pages 832, 833
9. 1-chloropropane is reacted with magnesium metal shavings in ether at 35 0C. Water is then added to
the reaction flask resulting in the formation of which one of the following compounds?
a) CH3CH2CH3
b) CH3CH2CH2MgCl
c) CH3CH2CH2OH
d) CH3CH2CH2Cl
ANS: a
REF: Page 834
10. Assuming that the following compound reacts via an E2 mechanism, which of the compounds given
is the predominant product?
H3C
Cl
H
H3C
H
H
H
H3C
H3C
Cl
H
H
H3C
H3C
H
H3C
H
H3C
H
H
H
ANS: c
H
H3C
H
H
H
H
H
H
a)
Cl
H3C
b)
H
c)
d)
REF: Page 829
B: SHORT ANSWER QUESTIONS
1. Starting from an alkene, synthesize 2-chloro-2-methylpentane.
ANS:
From an alkene, alkyl halides are synthesized through addition reactions. Since the product is a
monoalkene we need to use the hydrohalic acid. Addition must follow Markovnikov’s rule so the
chlorine must be added to the most substituted carbon in the final product. This fits the
requirement.
H3C
H3C
CH3
+
HCl
H3C
Cl
H
CH3
H
H3C
REF: Page 816
2. Show the mechanism for an SN1 reaction of hydrochloric acid with 1-methylcyclohexanol.
ANS:
H
+
HO H
H3C
H
OH
CH3
+
OH
H3C
+
C
Cl
CH3
Cl
-
REF: Section 21.7
3. Show the mechanism for SN2 substitution of chlorocyclopentane with hydroxide ion.
ANS:
Cl
H
H
OH
+
HO
Cl
-
-
REF: Section 21.6
4. What two products can you expect from treatment of 1-chloro-1-phenylethane with sodium
methoxide?
ANS:
H H
H
H
H
H
Cl
+
O
-
H
+
CH3
H3C
OH
H H
H H
H
H
+
Cl
H
H
O
+
Cl
-
CH3
O
-
CH3
REF: Section 21.5
5. The rates of SN2 substitution using 1-chlorobutane are 500 times higher than for 2-chlorobutane.
Explain why.
ANS:
This is a steric effect. Comparing the two molecules we can see that 2-chlorobutane is a
secondary alkyl halide and 1-chlorobutane is a primary alkyl halide. In 2-chlorobutane, the
halide group is flanked by a methyl and an ethyl group. In the 10 alkyl halide, it is surrounded by
hydrogens on the common carbon and a propyl group. Clearly, the carbon of the 10 alkyl halide
is less sterically hindered for attack by a small nucleophile.
In the above diagram the chlorine atom is the large green atom. The hydrogens are small and
white and the carbons are blue and median in size.
REF: Pages 823, 824
6. Hydroxide ion is a poor leaving group. This makes sense because hydroxide is a strong base and
strong bases are very reactive. High reactivity equates with instability. Good leaving groups are
relatively non-reactive and stable. However, a hydroxyl group will leave if you protonate it. Once
protonated, it becomes water as a leaving group. Water is stable.
Thus, by protonating an alcohol we can create a leaving group. Unfortunately, alcohols are weak
bases and only protonated significantly by strong acids. Strong acids have weak conjugate bases that
tend to make relatively poor nucleophiles. In an aqueous solution the reaction can transiently occur
but then it simply reverses since water acts as a better nucleophile than most of the halide ions. The
halogenated product often can’t be isolated. The exception to this is when strong acids of high
concentration are used, such as sulfuric or phosphoric. Suggest a way to promote the acid dependent
substitution reaction in a secondary alcohol.
ANS:
If we can use a solvent that promotes ionization of the acid but does not compete as a
nucleophile, we could facilitate the process. This is why the reaction conditions on Page 21.4 for
halogenation of a 20 alcohol use gas phase hydrochloric acid and ether. The ether is able, at these
high temperatures, to facilitate ionization of the acid yet the ether doesn’t compete as a
nucleophile with the chloride ion.
Other solvents may be suitable, especially if they have some polarity but are relatively nonreactive. Trichloromethane, dichloroethane, etc., may be effective. Temperature may be a factor,
however and since these boil at low temperatures, reaction might have to be carried out at high
pressures.
Lastly, dehydration of alcohols is routinely performed using sulfuric acid. Only concentrated
solutions of the acid work (low water concentration). Even primary alcohols will dehydrate to
alkenes but they do so by an E2 mechanism. 20 and 30 eliminate by an E1 mechanism where
water departs from the alcohol.
REF: All prior chapters and concepts
7. Show the mechanism for an E2 reaction of tert-butylchloride with sodium acetate.
ANS:
CH3
CH3
H3C
H3C
Cl
CH3
+
C
CH3
CH3
CH3
+
O
H3C
Na Cl
CH3
O
O
-
CH3
+
Na
O
REF: Pages 831, 832
8. Grignard Reagents are prepared in diethyl ether at its boiling point or in tetrahydrofuran (THF) at
higher temperatures for recalcitrant substrates. Prepare a Grignard reagent from 2-chloro-2,3dimethylbutane and react it with the appropriate substrate to produce the alkoxide shown.
H3C
Cl
CH3
O
H3C
H
H3C
CH3
-
CH3
ANS:
Dissolve the 2-chloro-2,3-dimethylbutane in diethyl ether and add magnesium shavings. Incubate
at 35 0C (boiling point of the ether) for 1 hour or more. This produces the Grignard reagent
which has the structure;
Cl
H3C
H3C
Mg
H
CH3
CH3
The alcohol can now be added to the reaction flask. The Grignard will deprotonated the alcohol to
form the alkoxide.
H 3C
OH
H 3C
CH 3
Cl
MgCl
Mg
-
O
CH 3
+
H 3C
H 3C
H
+
H 3C
CH 3
H
+
CH 3
H 3C
H
CH 3
CH 3
REF: Section 21.12
9. Given the known behavior of terminal alkynes, what reaction would be expected in the reaction of
propyne with a Grignard?
ANS:
Propyne is a terminal alkyne so its terminal hydrogen is acidic. Given the strength of Grignards
as bases, they will abstract a proton from this extremely weak acid. It is more acidic than an
alcohol and alcohols lose their protons to Grignards.
The predicted reaction is;
R-MgX + HC≡CH  R-H + HC≡C-MgX
The product is the magnesium chloride salt of the acetylide ion.
REF: Section 21.12 and Page 760
10. Grignard reagents are made from alkyl halides through reaction of the alkyl halide in ether with
magnesium turnings. Given that Grignard reagents are strong nucleophiles, what is a potential
complication of this synthesis?
ANS:
They can attack the unreacted alkyl halides in solution, in an SN2 reaction, as the Grignard molecules
form.
Ie,
R-Cl + Mg(s) in ether  R-MgX
then: R-MgX + R’-Cl  R-R’ + MgX2
Fortunately this secondary reaction is slow and only occurs at a minimal level except with especially
reactive alkyl halides such as allylic alkyl halides.
REF: Logic
C: LONG-ANSWER QUESTIONS
1. Dichloromethane is synthesized by high temperature incubation of methane with chlorine gas.
Chloromethane, trichloromethane, and carbon tetrachloride are produced, as well. What are some
common uses of dichloromethane, including those listed in your text.
ANS:
 It is also used as a plastic adhesive. It is one of the primary ingredients in some of the rapid
setting glues and almost all plastic-cements.
 It is a common solvent and has been used in the cleaning of semi-conductor chips,
decaffeination of tea and coffee and industrial cleaning
 It is used in materials science to dissolve the binding agents in substances such as macadam
and asphalt
 It is used as a propellant for polyurea foams and for canned methanol
 In organic chemistry, it is often used as a polar but non-reactive solvent for various types of
synthesis that must exclude water
REF: http://en.wikipedia.org/wiki/Dichloromethane and Page 813 of your text
2. For the following compounds indicate whether they would react preferentially through an SN2, SN1,
E2, or E1 mechanism in the presence of a polar solvent (hydroxylic) and medium to strong
nucleophile.
Cl
Cl
H3C
CH3
Cl
H3C
CH3
Cl
Cl
H3C
Cl
ANS:
Cl
Cl
H3 C
No SN1, no E1, SN2
favoured and E2 competes
Benzylic cations are resonance
stabilized. SN1 and SN2
CH3
Cl
H3 C
CH3
Cl
20 alkyl halide, therefore
subject to all four
This is the allylic position, the cation will conjugate
with the double bond and be stabilized. Therefore it is
subject to E1 and SN1 as well as E2, SN2.
Cl
Cl
H3 C
20 alkyl halide, therefore all four
will occur.
Completely non reactive.
REF: Pages 817 to 833
3. As mentioned in the prior question, an allylic alkyl halide produces an especially stable carbocation
due to conjugation with the pi-electrons in the alkene double bond. Predict both the substitution and
the elimination products from the following compound when reacting with an alcohol as nucleophile.
Use ethanol as your alcohol. Show the mechanisms.
Br
ANS:
H 3C
H
H 3C
O
O
Br
HC
+
CH 3
H
O
+
H
Br
+
O
+
CH3
+
Br
-
H
H
O
CH3
H
+
Br
Br
HC
-
+
+
O
+
CH3
H
H
H
H
CH3
O
H
REF: Sections 21.5 to 21.10
4. Zaitsev’s rule says that elimination of the hydrogen during the elimination reaction occurs mostly from
the carbon next to the carbon bound to the halogen (the beta carbon), that has the fewest hydrogens.
This results in the double bond forming between that carbon and the alpha carbon.
this one is the most
substituted -carbon and the
proton is removed from here
and not here
H3C
H
H H
H


H
H
Cl
Ie;
Considering Zaitsev’s rule, predict the elimination products of the following reactions.
a) The substrate shown above
b)
Cl
H3C
CH3
CH3
c)
CH3
Cl
H3C
CH3
d)
H3C
Cl
ANS:
a)
H
H3C

H

H
H
H
b)
CH3
H3C
CH3
c)
CH3
H3C
CH3
d)
H3C
REF: Page 829
5. Illustrate the mechanism of an SN2 reaction using bromo-chloro-fluoromethane as the substrate for
attack by the hydroxide ion; showing the transition intermediate and inversion of configuration with
the best leaving group departing. Also, give the R/S configurations for reactants and products.
ANS:
Cl
HO
-
Cl
Cl
F
F
Br
HO
-
Br
-
HO
F
R
H
H
+
H
Bromine is the best leaving group of the three halogens.
The product is S configuration since chlorine is #1, fluorine is #2 and oxygen is #3. With
hydrogen pointing away (ranked #4) the progression is counterclockwise. Therefore, S
Br
-
configuration.
REF: Pages 822 and 824