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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

Draw the structures (using chair conformations of pyranoses) of the following disaccharides.

(a) 4-O-(伪 -D-glucopyranosyl)-D-galactopyranose

(b) 伪 -D-fructofuranosyl-尾 -D-mannopyranoside

(c) 6-O-(尾 -D-galactopyranosyl)-D-glucopyranose

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.

Except for the Tollens test, basic aqueous conditions are generally avoided with sugars because they lead to fast isomerizations.

  1. Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonyl group is reversibly removed, and the resulting enolate ion is no longer asymmetric. Reprotonation can occur on either face of the enolate, giving either the original structure or its epimer. Because a mixture of epimer results, this process is called epimerization. Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of glucose and its C2 epimer, mannose.
  2. Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol intermediate, shown immediately above. Note that the enediol has twoprotons and removing one or the other gives two different enolate ions.

Draw the Haworth projection for the cyclic structure of D-mannose by laying down the Fischer projection.

When the gum of the shrub Sterculia setigera is subjected to acidic hydrolysis, one of the water-soluble components of the hydrolysate is found to be tagatose. The following information is known about tagatose:

(1) Molecular formula C6H12O6

(2) Undergoes mutarotation.

(3) Does not react with bromine water.

(4) Reduces Tollens reagent to give d-galactonic acid and d-talonic acid.

(5) Methylation of tagatose (using excess CH3 I and Ag2O) followed by acidic hydrolysis gives 1,3,4,5-tetra-O-methyltagatose.

(a) Draw a Fischer projection structure for the open-chain form of tagatose.

(b) Draw the most stable conformation of the most stable cyclic hemiacetal form of tagatose.

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