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Chapter 1: Structure Determines Properties |  |
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Chapter 1: Structure Determines Properties | |
Pushing Electrons
and Curly Arrows Questions
| Study Tip: First draw all on the compounds whose names are given, and then fill in any intermediates that are described paying close attention to the terminology used. You may need to think about the types of reactions involved. Once you have drawn all the structures, you should then be able to draw all the required curly arrows to account for all the bonding changes in the structures. |
Draw a mechanism using
double headed (i.e. electron
pair = )
curly arrows that represents the reaction described by the following
step-by-step
verbal description:
Step 1:
Protonation of 1-phenylethanol
by hydrochloric acid to give an oxonium ion.
Step 2: Loss of
water from this species
to form a fairly stable carbocation.
Step 3:
Abstraction of a proton from the
carbocation by a base to produce phenylethene (styrene)
Draw resonance structures to show the stability of the carbocation.
Draw a sequence, using double headed (i.e. electron
pair = )
curly arrows, that represents the
events in the equilibrium between
the starting material 2,4-cyclohexadienone and the product phenol.
All the processes involved are described by the following step-by-step
verbal description:
Step 1: Draw 2,4-cyclohexadienone
and a resonance structure that
shows that the oxygen is a good Lewis base.
Step 2: Add a proton to the base to give an oxonium ion.
Step 3: Draw two resonance structures that distribute the
positive
charge to two different carbon atoms.
Step 4: Show a generic base, B:, removing a proton from an sp3
carbon atom to give phenol as the product.
Will the equilibrium favour the starting material or the product and why ?
Draw a sequence, using
double headed (i.e. electron
pair = )
curly arrows, that represents the events in the dehydration reaction of
1,2-dimethylcyclohexanol.
All the processes involved
are described by the following
step-by-step verbal description:
Step 1:
Protonation of 1,2-dimethylcyclohexanol
by sulphuric acid to give an oxonium ion.
Step 2: Loss of
water from this species to form
a stable carbocation.
Step 3: Show a
generic base, B:, removing a proton
from an sp3 carbon atom in the carbocation to give
1,2-dimethylcyclohexene.
Suggest two other alkenes that may have formed by the loss of different protons.
Draw a mechanism sequence using double
headed (i.e.
electron pair = )
curly arrows, that represents the reaction sequence described verbally
by the following points in which phenylethene is treated with aq.
sulphuric
acid to give the alcohol, 1-phenylethanol.
Step 1: Protonation of the
alkene in phenylethene (styrene)
by sulphuric acid to give a stabilised carbocation.
Step 2: Attack of water as a nucleophile on this
carbocation
to give an oxonium ion.
Step 3: Abstraction of a proton from the oxonium by a base (e.g.
H2O) to produce 1-phenylethanol.
Draw a carbocation that is isomeric to the one produced in step (1) and that would result in the formation of 2-phenylethanol if steps (2) and (3) then occurred.
Explain using a short paragraph and / or diagrams why the carbocation that leads to 1-phenylethanol is preferred to the one that leads to 2-phenylethanol.
Draw a mechanism sequence using double
headed (i.e.
electron pair = )
curly arrows that represents the reaction sequence described verbally
by
the following points in which phenylethanone is treated with aq.
sulphuric
acid to give the diol, 1-phenyl-1,1-ethandiol.
Step 1: Protonation of
phenylethanone (acetophenone) by
sulphuric acid to give an oxonium ion.
Step 2: A resonance structure of this species which is a
carbocation.
Step 3: Attack of water (as a nucleophile) on this carbocation
to give a new oxonium ion.
Step 4: Abstraction of a proton by water (as a base) to produce
1-phenyl-1,1-ethanediol.
Draw the other important resonance contributors to show the stability of the carbocation in 2.
Draw a mechanistic sequence using double
headed (i.e.
electron pair = )
curly arrows that represents the single reaction sequence described
verbally
by the following points in which a ketone, 2-propanone, undergoes
alkylation
to give a new ketone, 2-pentanone when reacted with a base then an
alkylating
agent.
Step 1: An acid - base reaction in
which a proton is removed
from the 2-propanone using a base, sodium t-butoxide, to create a
resonance
stabilised carbanion and t-butanol.
Step 2: Attack of the carbanion (as
a nucleophile) on the electrophilic
carbon of ethyl bromide leading to the formation of a new CC sigma bond
and causing the simultaneous loss of a bromide ion.
Question 7:
Draw a mechanism sequence using double headed (i.e.
electron pair = ) curly arrows that represents
the single reaction sequence described verbally by the following points in which
an ester, ethyl ethanoate, is treated with aqueous sodium hydroxide to give
the salt of the carboxylic acid, ethanoic acid and an alcohol, ethanol.
Step 1: Draw a resonance structure of the
ester that shows the electrophilic character of the carbonyl carbon.
Step 2: Attack of the hydroxide (as a nucleophile)
on this electrophilic carbon giving a tetrahedral intermediate with a negatively
charged oxygen atom.
Step 3: Reform the carbonyl group and simultaneously
displacing an alkoxide and generating the carboxylic acid.
Step 4: An acid base reaction that produces
the alcohol, ethanol, and the carboxylate ion.
Draw the resonance contributors to the structure of the carboxylate ion and
rank them in order of importance.
Answer
Question 8:
Draw a mechanism sequence using double headed (i.e.
electron pair = ) curly arrows that represents
the single reaction sequence described verbally by the following points in which
an alkyl halide, 2-phenyl-2-propyl bromide, is hydrolysed in water to give an
alcohol, 2-phenyl-2-propanol.
Step 1: Loss of a leaving group from the 2-phenyl-2-propyl
bromide creating a bromide ion and a resonance stabilised carbocation.
Step 2: Attack of a molecule of water (as
a nucleophile) on this electrophilic carbon leading to the formation of a new
CO sigma bond and giving an oxonium ion.
Step 3: An acid - base reaction in which a
water molecule removes a proton from the oxonium ion producing the alcohol,
2-phenyl-2-propanol and a hydronium ion.
Draw the four other major resonance contributors of the structure of the carbocation produced in step 1.
Question 9:
Draw a mechanism sequence using double headed (i.e.
electron pair = ) curly arrows that represents
the single reaction sequence described verbally by the following points in which
1-phenylpropanol is dehydrated to give E-1-phenyl-1-propene.
Step 1: An acid-base reaction where 1-phenylpropanol is protonated by
sulfuric acid to give an oxonium ion.
Step 2: Loss of a water molecule from this species creating a resonance
stabilized carbocation.
Step 3: Using B: as base, remove a proton from an sp3 hybridized carbon
atom in the carbocation to give E-1-phenyl-1-propene as the major product.
Draw the four major resonance contributors to the structure of the carbocation formed in Step 2. You should use curly arrows to illustrate how you move between the resonance structures.
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© Dr. Ian Hunt, Department of Chemistry, University of Calgary | |