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Chapter 28: Problem 43

The amino acid ( \(R\) )-alanine is found in insect larvae. Draw the Fischer projection of this amino acid.

Short Answer

Expert verified
The Fischer projection of (R)-alanine places the amino group (NH2) at the top, the carboxyl group (COOH) at the bottom, the methyl group (CH3) on the left, and hydrogen (H) on the right.

Step by step solution

01

Understand what a Fischer projection is

In a Fischer projection, the molecule is viewed in such a way that the horizontal lines represent bonds projecting out of the page (towards the viewer), and the vertical lines represent bonds going into the plane of the page (away from the viewer). It's a way of representing 3D molecules in 2D.
02

Understand the structure of alanine

Alanine is an α-amino acid that consists of a central carbon (the alpha or α carbon) attached to a hydrogen atom, a carboxyl group (-COOH), an amino group (-NH2), and a methyl group (-CH3). Because the α carbon is attached to four different groups, it's chiral (asymmetric).
03

Determine the (R) configuration

The CIP (Cahn-Ingold-Prelog) priority rules help to determine the (R) or (S) designation of a chiral center. Following these rules, rank the four groups attached to the chiral carbon in descending order of priority based on the atomic numbers of the atoms attached directly to the chiral carbon. Trace a circle from highest to lowest priority. If the trace is clockwise, it's (R); if counterclockwise, it's (S). A slight modification is needed when drawing Fischer projections: if the lowest-priority group (usually Hydrogen) is 'vertically downward', then keep the rules as it is, but if the lowest priority group is 'horizontally to right', then switch the configuration.
04

Draw the Fischer projection of (R)-alanine

First, place the α carbon at the intersection of the vertical and horizontal lines. The highest priority group (NH2) should be placed on top, the carboxyl group is usually written at the bottom. The hydrogen atom has the lowest atomic number, so it is placed on the right, with the methyl group on the left. Because the lowest priority group (Hydrogen) is on horizontal line (not downward), we switch the 'R' configuration to 'S', giving us the correct projection of (R)-alanine.

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Most popular questions from this chapter

Briefly describe each of the following ideas, phenomena, or methods: (a) saponification; (b) chiral carbon atom; (c) racemic mixture; (d) denaturation of a protein.

If \(\mathrm{D}-(+)\) -glyceraldehyde is treated with \(\mathrm{HCN}\) in aqueous solution under basic conditions for three days at room temperature, cyanohydrins are formed (see Chapter 27). The cyanohydrins are not isolated, but are hydrolyzed to hydroxyacids in the same reaction mixture using dilute sulfuric acid. In this process, a new stereocenter is formed in the molecule. The products are diastereomers, formed in unequal amounts, and separable from each other by recrystallization because of their different physical properties, including solubilities. The trihydroxybutanoic acids are separated and then oxidized to tartaric acid with dilute nitric acid, which oxidizes only the primary alcohol group. (a) Ignoring stereochemistry, draw the reaction sequence for the transformations described above and hence deduce the structure of tartaric acid. (b) Starting from the Fischer projection of \(\mathrm{D}-(+)-\) glyceraldehyde and using the reaction scheme from part (a), draw Fischer projections of the two trihydroxybutanoic acids formed and designate the chiral centers as \(R\) or \(S\). (c) Starting from the Fischer projection of \(\mathrm{D}-(+)-\) glyceraldehyde and using the reaction scheme from part (a), draw Fischer projections of the two forms of tartaric acid formed and designate the chiral centers as \(R\) or \(S\). (d) One form of tartaric acid obtained is optically active, rotating the plane of polarized light in a negative sense \((-) .\) The other isomer formed, called meso-tartaric acid, is not optically active. Explain why the other isomer is not optically active. Draw the dashed-wedged line structure that corresponds to the Fischer projection of meso-tartaric acid. Can you describe how the two halves of the molecule are related? Using Fischer projections, write equations for the conversion of \(L-(-)-\) glyceraldehyde to tartaric acid. Show clearly the stereochemistry of the tartaric acids that are formed, and indicate whether you expect them to be optically active.

Refer to a typical Escherichia coli bacterium. This is a cylindrical cell about \(2 \mu\) m long and \(1 \mu\)m in diameter, weighing about \(2 \times 10^{-12}\)g and containing about \(80 \%\) water by volume. The intracellular \(\mathrm{pH}\) is 6.4 and \(\left[\mathrm{K}^{+}\right]=1.5 \times 10^{-4} \mathrm{M}\) Determine the number of (a) \(\mathrm{H}_{3} \mathrm{O}^{+}\) ions and (b) \(\mathrm{K}^{+}\) ions in a typical cell.

Draw the dashed-wedged line structure for ( \(R\) )-proline and (S)-valine.

The following terms are all related to optical isomers. Explain the meaning of each: (a) diastereomers; (b) enantiomers; (c) \((-) ;\) (d) D configuration.
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