Question

Draw Fischer projections for the product(s) formed by reaction of D-galactose with the following. In addition, state whether each product is optically active or inactive. (a) \(\mathrm{NaBH}_{4}\) in \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{H}_{2} / \mathrm{Pt}\) (c) \(\mathrm{HNO}_{y}\) warm (d) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O} / \mathrm{CaCO}_{3}\) (e) \(\mathrm{H}_{5} \mathrm{IO}_{6}\) (f) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)

Short answer

Based on the given reactions and their corresponding Fischer projections, here's a summary of the products and their optical activity: (a) Reaction with \(\mathrm{NaBH}_{4}\) in \(\mathrm{H}_{2} \mathrm{O}\): Product: Galactitol Optical activity: Optically active (b) Reaction with \(\mathrm{H}_{2} / \mathrm{Pt}\): Product: Inactive hexane Optical activity: Optically inactive (c) Reaction with \(\mathrm{HNO}_{y}\) warm: Product: Galacturonic acid Optical activity: Optically active (d) Reaction with \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O} / \mathrm{CaCO}_{3}\): Product: Bromohydrin of D-galactose Optical activity: Optically active (e) Reaction with \(\mathrm{H}_{5} \mathrm{IO}_{6}\): Product: Galactaric acid (Mucic acid) Optical activity: Optically active (f) Reaction with \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\): Product: Schiff's base of D-galactose Optical activity: Optically active

Step-by-step solution

Reaction (a) with \(\mathrm{NaBH}_{4}\) in \(\mathrm{H}_{2} \mathrm{O}\)

When D-galactose is treated with \(\mathrm{NaBH}_{4}\) in \(\mathrm{H}_{2} \mathrm{O}\), it reduces the aldehyde group to an alcohol. The structure of D-galactose remains unchanged, except for the topmost carbon atom, where the aldehyde becomes an alcohol. Fischer projection: [asy] unitsize(2 cm); draw((-2,1)--(2,1),linewidth(0.7)); draw((-1,-1)--(1,-1),linewidth(0.7)); draw((-1,0)--(1,0),linewidth(0.7)); draw((0,0)--(0,-2),linewidth(0.7)); label("\(\mathrm{HO}\)", (2,1), N); label("\(\mathrm{H}\)", (-2,1), N); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (0,2), N); label("CHOH", (1,0), NE); label("CHOH", (-1,0), NW); label("CHO", (1,-1), NE); label("HO", (-1,-1), NW); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (1,-2), S); [/asy] Product: Galactitol Optical activity: Optically active

Reaction (b) with \(\mathrm{H}_{2} / \mathrm{Pt}\)

When D-galactose is treated with \(\mathrm{H}_{2} / \mathrm{Pt}\), hydrogenation occurs, and the aldehyde group and all the alcoholic groups (except the terminal alcohol group) are reduced to form alkanes. Fischer projection: [asy] unitsize(2 cm); label("\(\mathrm{H}_{3}\mathrm{C}\)",(0,1), N); label("\(\mathrm{H}\)",(-2,1), N); label("\(\mathrm{H}\)",(2,1), N); label("CH\(_{3}\)", (1,0), NE); label("CH\(_{2}\)", (-1,0), NW); label("CH\(_{2}\)", (1,-1), NE); label("H", (-1,-1), NW); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (1,-2), S); [/asy] Product: Inactive hexane Optical activity: Optically inactive

Reaction (c) with \(\mathrm{HNO}_{y}\) warm

When D-galactose is treated with warm \(\mathrm{HNO}_{y}\), oxidation occurs at the aldehyde function to form an acidic carbonyl group. Fischer projection: [asy] unitsize(2 cm); draw((-2,1)--(2,1),linewidth(0.7)); draw((-1,-1)--(1,-1),linewidth(0.7)); draw((-1,0)--(1,0),linewidth(0.7)); draw((0,0)--(0,-2),linewidth(0.7)); label("OH", (2,1), N); label("H", (-2,1), N); label("\(\mathrm{COOH}\)", (0,2), N); label("CHOH", (1,0), NE); label("CHOH", (-1,0), NW); label("CHO", (1,-1), NE); label("HO", (-1,-1), NW); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (1,-2), S); [/asy] Product: Galacturonic acid Optical activity: Optically active

Reaction (d) with \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O} / \mathrm{CaCO}_{3}\)

Reaction with \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O} / \mathrm{CaCO}_{3}\) converts the aldehyde group into a bromohydrin. D-galactose structure remains the same except for the topmost carbon. Fischer projection: [asy] unitsize(2 cm); draw((-2,1)--(2,1),linewidth(0.7)); draw((-1,-1)--(1,-1),linewidth(0.7)); draw((-1,0)--(1,0),linewidth(0.7)); draw((0,0)--(0,-2),linewidth(0.7)); label("OH", (2,1), N); label("H", (-2,1), N); label("\(\mathrm{CBr(OH)}\)", (0,2), N); label("CHOH", (1,0), NE); label("CHOH", (-1,0), NW); label("CHO", (1,-1), NE); label("HO", (-1,-1), NW); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (1,-2), S); [/asy] Product: Bromohydrin of D-galactose Optical activity: Optically active

Reaction (e) with \(\mathrm{H}_{5} \mathrm{IO}_{6}\)

\(\mathrm{H}_{5} \mathrm{IO}_{6}\) is a strong oxidant that will oxidize both the top aldehyde and the bottom primary alcohol groups of D-galactose to carboxylic acids. Fischer projection: [asy] unitsize(2 cm); draw((-2,1)--(2,1),linewidth(0.7)); draw((-1,-1)--(1,-1),linewidth(0.7)); draw((-1,0)--(1,0),linewidth(0.7)); draw((0,0)--(0,-2),linewidth(0.7)); label("OH", (2,1), N); label("H", (-2,1), N); label("\(\mathrm{COOH}\)", (0,2), N); label("CHOH", (1,0), NE); label("CHOH", (-1,0), NW); label("CHO", (1,-1), NE); label("HO", (-1,-1), NW); label("\(\mathrm{COOH}\)", (1,-2), S); [/asy] Product: Galactaric acid (Mucic acid) Optical activity: Optically active

Reaction (f) with \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)

D-galactose reacts with aniline (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)) to form a Schiff's base. The aldehyde group gets converted into an imine while the rest of the structure remains unchanged. Fischer projection: [asy] unitsize(2 cm); draw((-2,1)--(2,1),linewidth(0.7)); draw((-1,-1)--(1,-1),linewidth(0.7)); draw((-1,0)--(1,0),linewidth(0.7)); draw((0,0)--(0,-2),linewidth(0.7)); label("OH", (2,1), N); label("H", (-2,1), N); label("\(\mathrm{C=N-C}_{6}\mathrm{H}_{5}\)", (0,2), N); label("CHOH", (1,0), NE); label("CHOH", (-1,0), NW); label("CHO", (1,-1), NE); label("HO", (-1,-1), NW); label("\(\mathrm{CH}_{2}\mathrm{OH}\)", (1,-2), S); [/asy] Product: Schiff's base of D-galactose Optical activity: Optically active

Key concepts

Fischer Projections

Fischer projections are a two-dimensional drawing convention used to represent the three-dimensional structure of molecules, especially those in the carbohydrate family, like D-galactose. They are particularly useful for distinguishing between different isomers, which are compounds with the same molecular formula but different arrangements of atoms in space.

In a Fischer projection, the carbon chain is oriented vertically, with the most oxidized carbon at the top. Horizontal lines represent bonds that project out from the plane towards the viewer, while vertical lines represent bonds pointing away. Chiral centers, which are carbon atoms attached to four different groups, are shown as the intersections of horizontal and vertical lines. In the exercise provided, Fischer projections help visualize the changes in D-galactose's structure after chemical reactions, such as reduction and oxidation.

Optical Activity

Optical activity is a measure of how a molecule can rotate the plane of polarization of light. Chiral molecules, or molecules with non-superimposable mirror images, can exist in two forms that are mirror images of each other, known as enantiomers. These enantiomers rotate the plane of polarized light in opposite directions; one is termed 'dextrorotatory' (rotating light clockwise) while the other is 'levorotatory' (rotating light counterclockwise).

A substance's optical activity is determined by the presence of chiral centers and the configuration of these centers. In the reactions of D-galactose, products such as galactitol, galacturonic acid, bromohydrin, and Schiff's base are optically active due to the presence of multiple chiral centers. However, when D-galactose is fully reduced to hexane, the product is no longer optically active since the chiral centers are eliminated.

Organic Compound Reactivity

The reactivity of organic compounds like D-galactose is influenced by their functional groups. For example, the aldehyde group can undergo various reactions including reduction to an alcohol, as seen when D-galactose is treated with NaBH4, or oxidation to a carboxylic acid when treated with a potent oxidizing agent such as H5IO6. Other reactions include the formation of imines by reacting with primary amines, creating Schiff's bases.

D-galactose's chemical versatility illustrates organic compound reactivity concept. Understanding these reactions provides insights into the behavior of carbohydrates during metabolic processes or chemical synthesizing. For instance, the reactivity of the aldehyde group at the topmost carbon can lead to the formation of alcohols, acids, or other derivatives which play significant roles in biochemical pathways and industrial applications.