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Chapter 23: Q50P (page 1255)

(a) Give the products expected when (-)-erythrose reacts with HCN.

(b) What is the relationship between the products? How might they be separated?

(c) Are the products optically active? Explain.

Short Answer

Expert verified

(a) (-)-erythrose:

(b) The relationship between the products is that they are diastereomers of each other. They could be separated by crystallization, distillation process.

(c) Yes, both the products are optically active.

Step by step solution

01

(a) Give the products expected when (-)-erythrose reacts with HCN.

(-)-erythrose on reaction with HCN generates two products which are diastereomers of each other.

Cyanide ion acts as a nucleophile in this reaction and attacks at the carbonyl carbon of the aldehyde which is planar in nature, thereby generating chiral center, hence the position of hydroxyl group can be left or at right in the fischer projection.

02

The relationship between the products:

The products formed are diastereomers which possess different physical properties. Separation of diastereomers can be done by various methods such as crystallization, distillation, or chromatography. HPLC is a method of chromatography which is employed to separate mixture of diastereomers as diastereomers have different degrees of attraction to a stationary phase.

03

(c) Explain the products are optically active or not.

Both the products obtained are optically active. Each product possess two chiral centers and no plane of symmetry. Optical activity of the molecule depends whether the molecule has plane of symmetry or axis of symmetry, if it possess plane of symmetry or axis of symmetry, then the molecule will be optically inactive.

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Most popular questions from this chapter

Question: Exposure to nitrous acid (see Section 19-16), sometimes found in cells, can convert cytosine to uracil.

  1. Propose a mechanism for this conversion.
  2. Explain how this conversion would be mutagenic upon replication.
  3. DNA generally includes thymine, rather than uracil(found in RNA). Based on this fact, explain why the nitrous acid-induced mutation of cytosine to uracil is more easily repaired in DNA than it is in RNA.

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of α-D-galactopyranoseis +150.70 , and that of the β anomer is +52.80 . When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.20. Determine the percentages of the two anomers present at equilibrium.

The relative configurations of the stereoisomers of tartaric acid were established by the following synthesis:

(1) D-(+)-glyceraldehydediastereomers A and B (separated)

(2) Hydrolysis of A and B using aqueous Ba(OH)2 gave C and D, respectively.

(3) HNO3 oxidation of C and D gave (-)-tartaric acid and meso-tartaric acid, respectively.

(a) You know the absolute configuration of D-(+)-glyceraldehyde, Use Fischer projections to show the absolute configurations of products A, B, C, and D.

(b) Show the absolute configurations of the three stereoisomers of tartaric acid: (+)-tartaric acid, (-)-tartaric acid, and meso-tartaric acid.

All of the rings of the four heterocyclic bases are aromatic. This is more apparent when the polar resonance forms of the amide groups are drawn, as is done for thymine at left. Redraw the hydrogen-bonded guanine-cytosine and adenine-thymine pairs shown in figure 23-24, using the polar resonance forms of the amides. Show how these forms help to explain why the hydrogen bonds involved in these pairings are particularly strong. Remember that a hydrogen bond arises between an electron-deficient hydrogen atom and electron-rich pair of nonbonding electrons.

Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:

G = C and A = T

(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?
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