McMurry (Kap 21,5) - cyclic structures of monosacarides: anomers (cyclic hemiacetals - carbohydrates). Flashcards
What reaction occurs when aldehydes and ketones reacts with alcohol?
They undergo a reversible nucleophilic reaction with alcohol to form hemiacetals.
(Page 750).
If the carbonyl and the hydroxyl (OH) are in the same molecule (which is the case in carbihydrates)
the nucleophilic addition reaction can take place leading to the formation of a cyclic hemiacetal.
Are cyclic hemiacetals stable, and in which form do carbohydrates exist?
5 and 6 membered cyclic hemiacetals are relatively strain free and stable.
Many carbohydrates therefore exist in an equilibrium between open chain and cyclic form.
Which form does glucose for example exist in?
In (aq) solution it exist primarily in its 6 membered pyranose form resulting from the nucleophilic addition reaction of the OH group from C5 to the carbonyl group at C1.
What geometry does the cyclic molecule pyranose and the other hemiacetals have?
chair like conformations with axial and equatorial substituents.
The hemiacetal O are placed in the right rear (page 750).
For D sugars = the CH2OH group is on the top of the ring, and for the L sugar it is on the bottom.
When an open chain monnosakkaride cyclizes to the new pyranose form (glukose) - a new chirality center is generated at the former carbonyl Carbon and two diasteomers (called anomers) are produced.
The hemiacetal carbon atom is referred to as the anomeric center
What kind of two anomers exist?
The compound with its newly generated OH group at C1 cis to the OH at the lowest chirality center = a anomer (a-D-glucopyranose)
The compound with its newly generated OH group at C1 trans to the OH at the lowest chirality center = B anomer (B-D-glucopyranose)
NOTE all the substituents on the ring for B-D-glucopyranose is equatorial - it is therefore the least sterically crowded and most stable of the 8 D aldohexoses
(The eight D-aldohexoses are monosaccharides with six carbon atoms (hexoses) and an aldehyde group. They differ in the configuration of the hydroxyl (-OH) groups attached to the four asymmetric carbons).
Some monnosakkarides exit in a 5 membered cyclic hemiacetal form called a furanose. What is this form?
The pyranose form exist from the result of addition of the OH at c6 to the carbonyl group.
The furanose form result from addition of the OH at c5 to the carbonyl group.
The two anomers have different chemical properties, what?
a anomer has a slightly lower melting point then the b anomer.
a anomer has a specifik rotation of +112,2.
b anomer has a specifik rotation of +18.
(a-D-glucopyranose)
(B-D-glucopyranose)
