Question

Using dash-wedge notation to designate stereochemistry, draw (S)-pentane-1,3-diol. Be sure to draw all bonds to the stereocenter (including bonds to hydrogen).

Answer 1

Drawing (S)-pentane-1,3-diol using dash-wedge notation involves a series of steps to ensure the correct stereochemistry:

Step 1: Draw the Carbon Chain

- Draw a straight line representing the five-carbon backbone of pentane.

Step 2: Identify the Chiral Centers

- Mark the first and third carbon atoms, which will be the chiral centers because they will each be bonded to four different groups.

Step 3: Add the Hydroxyl Groups

- Draw a wedged bond (a solid triangle) to represent the hydroxyl group (OH) coming out of the plane of the paper on the first and third carbons.

- Assign priority to the substituents according to the Cahn-Ingold-Prelog rules. For the first carbon, the OH group has the highest priority, followed by the rest of the carbon chain, then the hydrogen, and lastly the methyl group on the other side. The same priority assignment applies to the third carbon.

Step 4: Assign the Correct Stereochemistry

- For (S)-configuration, the substituents should be arranged in counterclockwise order of priority. When looking from the hydrogen (which should be the lowest priority and drawn with a dashed line going back into the plane), the OH group should be positioned to the left, making the configuration S (sinister, or left).

Step 5: Draw the Remaining Hydrogen Atoms

- Draw the hydrogens on the second and fourth carbons as regular lines since they are in the plane of the paper.

- Draw the hydrogen on the first and third carbons with a dashed line to indicate that they are going into the plane of the paper.

Step 6: Complete the Molecule

- Add the rest of the hydrogen atoms to fill the valency of each carbon atom.

- Ensure all carbon atoms have four bonds in total, satisfying the tetravalency of carbon.

Answer 2

The structure of (S)-pentane-1,3-diol is shown in the image attached

The stereochemistry of (S)-pentane-1,3-diol

Using the Cahn-Ingold-Prelog priority rules, the stereochemistry of the chiral center at carbon 3 in (S)-pentane-1,3-diol is ascertained. The substituents are ranked according to atomic number, with the lowest priority group (H) pointing away.

The order is as follows:
`CH_2CH_3`> -OH > -
`CH_3`> H. A counterclockwise direction, following the path from the highest to the lowest priority group, indicates a S configuration. Thus, in (S)-pentane-1,3-diol, the stereochemistry at carbon 3 is (S). One would conduct a comparable analysis on the chiral center at carbon 1.