Pushing Curly Arrows

Chapter 5: Pushing Curly Arrows

Pushing Electrons and Curly Arrows Questions

Here are a few problems where you need to draw a sequence including curly arrows that represents the organic chemistry that is being described.

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.

Question 1:

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.

Answer

Question 2:

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 ?

Answer

Question 3:

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.

Answer

Question 4:

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.

Answer

Question 5:

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.

Answer

Question 6:

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.

Answer

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.

Answer

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.

Answer