McMurry (Kap 21.0-21.3) - carbohydrates, fischer projection, D and L sugars. Flashcards
Are carbohydrates rare?
No, they are very apparent and occur in every living organism.
Found in sugars, and starch in food, the cellulose in paper, wood and cotton.
Also they are a part of the nucleic acids that carry our genetic information.
What are the term carbohydrates used for?
To describe the broad class of polyhydrozelated aldehydes and ketones - commonly called sugars.
Glucose is an example.
How are carbohydrates synthesized by plants?
During photosynthesiz - sunlight provides energy to convert CO2 and H2O into glucose + oxygen.
How are carbohydrates classified?
As either simple or complex
Simple sugars (monnosakkkarider) = carbohydrates like glucose or fructose (cant be converted into smaller sugars by hydrolysis).
Complex carbohydrates = made of two or more simple sugars linked together by acetal bonds.
Sucrose (table sugar) for example is made up of glucose linked with fructose.
Cellulose is made of several thousands of glucose linked together.
How can a complex carbohydrate break down into its simple carbohydrates?
enzyme catalyzed hydrolysis can break the complex carbohydrate into its constituent monnosakkarider.
What are monnosakkarider/simple carbohydrates further classified as?
Aldoses or Ketoses.
the -ose suffix (designates a carbohydrate)
the -aldo or -keto prefix identify the kind of carbonyl group in the molecule (whether ketone or aldehyde).
The number of C-atoms on the monnosakkaride is indicated by the prefix tri-tetr-pent-hex and so on.
glucose = aldohexose (six carbon aldehyde sugar)
fructose = ketohexose (six carbon ketone sugar)
ribose = alsopentose (five carbon aldehyde sugar).
Carbohydrates have numerous chirale centers - what is a quick method used to identify their stereochemistry?
The Fischer Projections.
Describe the Fischer Projection?
The vertical lines (lodret) represent bonds going away from the viewer (into the plane of the paper).
The horizontal lines (vandret) represent bonds coming toward the viewer (out of the plane of the paper).
(a tetrahedral C-atom is represented by two crossed lines)
The bonds that go out of page should be placed to the sides, and the bonds that go in plane/page should be placed up/down.
How do you test to see if two fischer projections represent the same or different enantiomer?
Can be rotated 180 degrees, if the same groups go into and out of the plane - same configuration
If yoou rotate 90 degrees, you swap the groups that go in and out - thereby you change the configuration.
If you hold one group steady and rotate the other three in a clockwise/counterclockwise rotation - the configuration does not change.
R and S configuration to a fischer projection chirale C-atoms - how do you assign?
(see page 743-744 for worked example)
Rank the 4 substituents in usual way.
Place the group of lowest ranking (usually H) on the top by holding one of the groups steady and rotating the others until H is on the top.
Determine the direction of rotation for the remaining three groups and assign R or S configuration.
Carbohydrates with more than one chirality centers are shown by stacking the centers on top of one another, with the carbonyl group (either aldehyde or ketone) near the top, and the -OH group near the bottom.
see page 743 for example of the Fischer projection of the glucose.
A simple aldose (glyseraldehyde) has only one chirality center, therefore have 2 enantiomers (dextrorotary and levorotatory) - only the dextrorotory enantiomer occurs naturally.
(R)-(+)-glyceraldehyde is also reffered to as D-glyceraldehyde (D for dextrorotary).
The other enantiomer (S)-(-)-glyceraldehyde is known as L-glyceraldehyde (L for levorotatory).
True.
Because of the way monnosacharides are biosynthesized in nature (glucose, fructose etc.) have the same R steochemistry as D-glyceraldehyde at the chirality center furthest away from then carbonyl group (carbonyl group always on the top).
How is that illustrated in the fischer projection?
The most naturally occuring sugars therefore have their hydroxyl (OH) group at the bottom chirality center pointing to the right. (refered to as D sugars).
D sugars (see illustrations in page 745.
What was it again?
The naturally occuring sugars have their hydroxyl group attached to the bottom chirale center pointing towards the right.
In contrast to D sugars, how is L sugars illustrated in the fischer projection?
They dont have an R configuration but an S configuration at the lowest chirality center (furthest away from the carbonyl group).
The bottom OH is pointed to the left (not right).
