Chemistry $^{3}$ : Introducing inorganic, organic and physical chemistry

Andrew Burrows, John Holman, Andrew Parsons

Chapter 23 Aldehydes and ketones: nucleophilic addition and $\alpha$-substitution reactions - all with Video Answers

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Chapter Questions

Problem 1

The following questions are based on the reactions of ethanal (1) shown here. (Section 23.2)
(a) Give appropriate reagents for converting 1 into 2 . Is this an example of an oxidation or a reduction reaction?
(b) Draw a reaction mechanism to show how 1 is converted into 3 .
(c) Suggest a method for preparing PhMgBr.
(d) Give structures for organic compounds 4 and 5 .

Nicholas S.

Numerade Educator

07:32

Problem 2

The following questions are based on the reactions of benzaldehyde (6) shown here. (Sections $23.2$ and 23.4)
(a) Draw the structures of organic compounds $7-9 .$
(b) Suggest reagents that will convert 6 into 10 , and 10 into 11 .
(c) Give a mechanism that explains how 6 is converted into 12 .
12

Rajesh S.

Numerade Educator

08:41

Problem 3

In nature, pyridoxal phosphate reacts with an enzyme (abbreviated as $\mathrm{H}_{2} \mathrm{~N}$-Enzyme) to form a coenzyme that catalyses the conversion of $\alpha$-amino acids into $\alpha$-keto acids. (Section 23.2)
<smiles></smiles> $+\mathrm{H}_{2} \mathrm{~N}-$ Enzyme $\rightleftharpoons \mathrm{Coenzyme}+\mathrm{H}_{2} \mathrm{O}$
pyridoxal phosphate
(a) Draw the structure of the coenzyme and name the functional group that is formed.
(b) Is the coenzyme formed by an addition reaction, a substitution reaction, or an addition-elimination reaction? Explain your reasoning.
(c) An acid within the active site of the enzyme plays an important part in the mechanism of the reaction to produce the coenzyme. Explain the role of an acid in this type of transformation.
(d) Give the general structure of an $\alpha$-amino acid, with one $\alpha$-hydrogen atom.
(e) Suggest a method for converting aldehydes and ketones into $\alpha$-amino acids in the laboratory.

Rajesh S.

Numerade Educator

Problem 4

A synthetic route to the naturally occurring terpene geraniol (13) is shown here. (Section 23.2)
<smiles>CC=O</smiles>
<smiles>CC(C)=CCCO</smiles>
15
$\frac{1 . \mathrm{PBr}_{2}}{2 . \mathrm{Mg} \cdot \mathrm{dry} \mathrm{Et}_{2} \mathrm{O}}$
16
<smiles>C=CCCC(C)=CCO</smiles>
13
17
(a) Suggest reagents that will convert 14 into 15 .
(b) Give the structure of organic compound 16 . Why is it important to use dry $\mathrm{Et}_{2} \mathrm{O}$ ?
(c) Suggest reagents for a two-step synthesis of ketone 17 . from 16 .
(d) Propose a mechanism for the reaction of 17 with $\mathrm{Br} \mathrm{Z} \mathrm{nCH}_{2} \mathrm{CO}_{2} \mathrm{Et}$ (Reformatsky reagent), then $\mathrm{H}^{+}$, to give hydroxy-ester $18 .$
(e) Would you expect 14 or 17 to react more rapidly with $\mathrm{Br} \mathrm{ZnCH}_{2} \mathrm{CO}_{2} \mathrm{Et}$ ? Explain your reasoning.

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Problem 5

The following questions are based on the reactions of acetophenone (19) shown here. (Sections $23.3$ and 23.4)
<smiles>CC(=O)c1ccccc1</smiles>
$\mathrm{T}_{2}, \mathrm{HO}^{\mathrm{a}}$ (both in excess)
$20+21$ (Yellow solid)
<smiles>O=C(CBr)c1ccccc1</smiles>
(a) Draw the structures of organic compounds 20 and 21 .
(b) Give a mechanism that explains how 19 is converted into 22 .
(c) Suggest reagents that will convert 19 into $23 .$

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Problem 6

$6^{\circ}$ Isophorone is prepared on a large scale in industry for use as a solvent in, for example, some printing inks. A synthetic route to isophorone is shown below. (Section 23.5)
<smiles>CC(=O)C=C(C)CC(C)=O</smiles>
24
25 Isophorone
(a) Give a reaction scheme to show how 24 could be prepared from propanone.
(b) Draw compound 25 and indicate all of the $\alpha$-hydrogen atoms.
(c) Given that isophorone is formed from 25 in an intramolecular aldol condensation reaction, propose a structure for isophorone.
(d) Give a reaction mechanism to explain the formation of isophorone from 25 .

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