Curly Arrows Workshop 2017 PDF

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Curly Arrows Workshop 2017 A curly arrow represents 2 electrons; its tail starts at an electron-rich site, its head moves to an electron-poor site. One or more curly arrows are used to “interconvert” delocalized [resonance] structures of molecules or molecular ions, and to visualize the mechanism of chemical reactions [representing the electron flow in such transformations].

Question 1 (i)

Which structure among A–C contains “conjugated”  electrons? Give a definition of “conjugation” [hint: it should include  electrons, n electrons, electron-deficient atoms, and the hybridization states of all involved atoms].

(ii)

Using curly arrows “convert” each of structures D–I to an alternative delocalization [resonance] form. Case J: Draw an isoelectronic structure of H [hint: exchange only 1 atom in H for another atom to get a neutral species]. Using curly arrows “convert” structure J to an alternative delocalization [resonance] form.

(iii)

(iv)

(v) (vi)

Consider the acid–base reactions of alcohols K and L with sodium hydride. Indicate the polarity (+/–) of O–H and Na–H bonds. Using curly arrows draw a mechanism for the first reaction and the structures of conjugate base M and conjugate acid O. Using curly arrows draw 4 plausible delocalization [resonance] forms of conjugate base N. Which conjugate base, M or N, is more stable – and why? Assign the pKa values of “10” and “16” to the appropriate acid, K or L. Explain your reasoning. Which is the stronger acid, K or L – and why?

Question 2 The pharmaceutical company Pfizer made the antidepressant reboxetine by the following sequence of reactions.

For Step A: i. Suggest a nucleophile for the reaction; is this neutral or charged? ii. Draw a curly arrows mechanism for this ring opening; is this an SN1 or SN2 reaction? iii. Comment on the relative stereochemistry of the product, how is this explained by the reaction mechanism? For Step B: i. Suggest a nucleophile for the reaction, is this neutral or charged? ii. Draw a curly arrows mechanism for this ring opening; is this an SN1 or SN2 reaction? iii. Draw a diagram showing the HOMO and LUMO orbitals involved in this reaction and how the new sigma bond is formed between them.

Question 3 Suggest reagents and mechanism for the two transformations in the scheme below. O O

O

O

Me

O

O HO

O

Me O

O

O

Question 4 i.

Draw the mechanism for the reaction below and hence determine the structure of A.

ii.

Assess the electron-withdrawing/donating properties of each of the substituents attached to the aromatic ring in compound A, and hence predict the structure of product that would be obtained from mononitration via an electrophilic aromatic substitution reaction.

iii.

Draw the mechanism for the mononitration of compound A (including formation of the nitronium cation).

Question 5 Draw the addition-elimination mechanism for the nucleophilic aromatic substitution reaction shown below. Your mechanism should show how the nitro substituent helps to stabilise the formation of reactive intermediate during the rate-determining step....