Chapter 1: Structure Determines Properties

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.

Draw a mechanism using double headed (ie 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

Draw a sequence, using double headed (ie 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

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

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 stabilized 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

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 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