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 \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O} / \mathrm{CaCO}_{3}\) (b) \(\mathrm{H}_{2} / \mathrm{Pt}\) (c) \(\mathrm{HNO}_{y}\), warm (f) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\)

Short answer

Question: Determine the products formed and their optical activity when D-galactose reacts with the following reagents: (a) NaBH4 in H2O, (b) H2/Pt, (c) HNOy, warm, (d) Br2/H2O/CaCO3, and (e) C6H5NH2. Draw the Fischer projections for each product. Answer: (a) The product is galactitol, and it's optically active. Fischer projection: HOCH2 | HOCH-C | HOCH-H | HOCH-H | HOCH-H | CH2OH (b) No reaction occurs and the product is the same as the starting material (D-galactose), which is optically active. (c) The product is D-galactonic acid, and it's optically active. Fischer projection: HOCH2 | HOCH-C | HOCH-H | HOCH-H | HOCH-H | COOH (d) The product is brominated D-galactonic acid, and it's optically active. Fischer projection: COOH | HOCH-C | HOCH-H | HOCH-H | HOCH-H | BrCH2 (e) No reaction occurs and the product is the same as the starting material (D-galactose), which is optically active.

Step-by-step solution

Reaction with NaBH4 in H2O (a)

NaBH4 is a reducing agent that reduces the aldehyde group in the open-chain form of D-galactose to an alcohol. The product will be a galactitol, retaining all chiral centers. Here is the Fischer projection of the product: HOCH2 | HOCH-C | HOCH-H | HOCH-H | HOCH-H | CH2OH The product is optically active due to the presence of chiral centers.

Reaction with H2/Pt (b)

H2/Pt is a hydrogenation catalyst that reduces the carbon-carbon double bonds. Since there are no double bonds in D-galactose, there will be no reaction. The product is the same as the starting material, D-galactose, which is optically active.

Reaction with HNOy, warm (c)

HNOy, when warm, is an oxidizing agent. It will oxidize the primary alcohol group of D-galactose to a carboxylic acid, forming D-galactonic acid. The Fischer projection for this product is: HOCH2 | HOCH-C | HOCH-H | HOCH-H | HOCH-H | COOH The product is optically active due to the presence of chiral centers.

Reaction with Br2/H2O/CaCO3 (d)

Br2/H2O/CaCO3 is a halogenation oxidizing agent. It will selectively oxidize the primary alcohol group (CH2OH) in D-galactose to a carboxylic acid (COOH), forming brominated D-galactonic acid. The Fischer projection for this product is: COOH | HOCH-C | HOCH-H | HOCH-H | HOCH-H | BrCH2 The product is optically active due to the presence of chiral centers.

Reaction with C6H5NH2 (e)

C6H5NH2, or phenylamine, is an aromatic amine. Since there are no suitable functional groups on D-galactose for a reaction with phenylamine, no reaction occurs. The product will be the same as the starting material, D-galactose, which is optically active.