The fermentation of starch with malt produces ethyl alcohol. During this process, the hydrolysis of — Organic Chemistry Chemistry Question
Stereochemistry (Organic synthesis – 1)
The fermentation of starch with malt produces ethyl alcohol. During this process, the hydrolysis of starch is catalyzed by the enzyme diastase present in malt to produce maltose, a disaccharide.
Maltose (C12H22O11) reduces Tollens’ and Fehling’s reagents and it is oxidized by bromine in water to maltobionic acid ((C11H21O10)COOH), a mono carboxylic acid. In order to deduce its structure, maltose was subjected to series of reactions:
[VISUAL]
Draw structures B, D – N in a Fisher projection.
Model Answer
Structures B, D – N in Fischer projection:
B (D-glucose):
CHO
H-|-OH
HO-|-H
H-|-OH
H-|-OH
CH2OH
D (2,3,4,6-tetra-O-methyl-D-glucose):
CHO
H-|-OMe
MeO-|-H
H-|-OMe
H-|-OMe
CH2OMe
E (2,3,5,6-tetra-O-methyl-D-gluconic acid):
COOH
H-|-OMe
MeO-|-H
H-|-OH
H-|-OMe
CH2OMe
F (2-methoxymalonic acid / methoxybutanedioic acid derivative):
COOH
H-|-OMe
COOH
F' (2,3-di-O-methylglyceric acid):
COOH
H-|-OMe
CH2OMe
G (2,3-di-O-methyl-D-threuric/erythruric acid):
CHO
H-|-OMe
MeO-|-H
COOH
H (methoxyacetic acid):
COOH
|
CH2OMe
I (2,3,5,6-tetra-O-methyl-4-keto-D-gluconic acid):
COOH
H-|-OMe
MeO-|-H
|=O
H-|-OMe
CH2OMe
J (2,3,5,6-tetra-O-methyl-D-glucitol):
CH2OH
H-|-OMe
MeO-|-H
H-|-OH
H-|-OMe
CH2OMe
K (2,3,5,6-tetra-O-methyl-D-galactitol precursor):
CH2OH
H-|-OMe
MeO-|-H
HO-|-H
H-|-OMe
CH2OMe
L (1,2,3,5,6-penta-O-methyl-D-glucitol):
CH2OMe
H-|-OMe
MeO-|-H
H-|-OMe
H-|-OMe
CH2OMe
M (1,2,3,5,6-penta-O-methyl-D-galactitol - meso / optically inactive):
CH2OMe
H-|-OMe
MeO-|-H
MeO-|-H
H-|-OMe
CH2OMe
N (D-glucaric acid):
COOH
H-|-OH
HO-|-H
H-|-OH
H-|-OH
COOH
Draw structures for maltose, maltobionic acid and C in a Haworth projection.
Model Answer
Structures in Haworth projection:
Maltose:
- Consists of two D-glucopyranose units linked by an alpha-(1->4) glycosidic bond.
- Left ring (non-reducing): alpha-D-glucopyranose with OH groups down at C2 and C4, up at C3, and CH2OH up at C5.
- Right ring (reducing): D-glucopyranose linked through its O4 oxygen, with OH at C1 shown as either alpha (down) or beta (up).
Maltobionic acid:
- Left ring: alpha-D-glucopyranose (same as maltose).
- Right ring: Open-chain form representing D-gluconic acid. C1 is a carboxylic acid (-COOH). C2 (OH down), C3 (OH up), C4 is linked to O-1 of the glucopyranosyl unit, C5 (OH down), C6 is -CH2OH.
Compound C (Octa-O-methylmaltobionic acid):
- Left ring: 2,3,4,6-tetra-O-methyl-alpha-D-glucopyranose (pyranose ring with -OMe instead of -OH at C2, C3, C4, C6).
- Right ring: Open-chain 2,3,5,6-tetra-O-methyl-D-gluconic acid. C1 is -COOH, C2 (-OMe), C3 (-OMe), C4 is connected to O-1 of the glucopyranosyl unit, C5 (-OMe), C6 is -CH2OMe.