Lactose (milk sugar) is produced on a fairly large scale in The Netherlands starting from whey (a by — Analytical Chemistry Chemistry Question
Lactose Chemistry
Lactose (milk sugar) is produced on a fairly large scale in The Netherlands starting from whey (a by- product of cheese manufacture). Lactose is applied in baby food and in pharmaceutical tablets. It is a disaccharide composed of the monosaccharides D-galactose and D-glucose. The structure is shown below (Haworth projection). The left hand monosaccharide unit is D-galactose.
Draw the Fischer projection of D-galactose and D-glucose.
The acid-catalysed hydrolysis of lactose gives D-galactose and D-glucose.
Indicate with an arrow in the formula of lactose:
(a) To which oxygen atom a proton will be attached in order to effect hydrolysis.
(b) Which carbon-oxygen bond will be cleaved during the hydrolysis reaction.
(c) Which carbon atom will be involved in the reaction with Fehling’s reagent (this reagent is used to detect reducing sugars).
The hydrolysis of lactose can be coupled with hydrogenation using a metal catalyst which leads to the polyalcohols sorbitol and galactitol, also known as glucitol and dulcitol, respectively.
Draw Fischer projections of sorbitol and galactitol.
Indicate whether these compounds are optically active or inactive.
Sorbitol: Optically active – Yes / No,
Galactitol. Optically active – Yes / No,
In industrial processes lactose is isomerised to lactulose, which is an intestinal pharmaceutical. Hydrogenation of lactose leads to lactitol, a C12-poIyol which is a low-calorific sweetener. Both processes are executed in The Netherlands.
(a) Draw the Haworth structure of lactulose.
(Hint: The glucose part of lactose has been isomerised to be keto-sugar fructose.)
(b) Draw the Haworth structure of lactitol.
Model Answer
Lactulose is a mixture of the furanose and pyranose form in the ration 4 : 6. When a student gives either the furanose or the pyranose form, he/she will receive full marks.