Wood Chemistry Wood Chemistry PSE 406/Chem E 470 Lecture 2: Monosaccharides PSE 406 - Lecture 2 1 Wood Chemistry Why Study Monosaccharides? As we discussed in lecture 1, wood carbohydrates (cellulose and hemicelluloses) are made of various sugars such as glucose, mannose, xylose, etc. In order to understand the properties and reactions of the carbohydrates, we need to understand monosaccharides. D-glucose PSE 406 - Lecture 2 2 Wood Chemistry What is a
Monosaccharide? H Monosaccharides are polyhydroxy aldehydes or ketones (aldoses or ketoses) of 3-9 carbons. Polyhydroxy means that there is more than 1 hydroxyl group (OH) on the carbon backbone. Aldehydes and ketones both contain carbonyl groups. Typically monosaccharides contain either 5 or 6 carbons. PSE 406 - Lecture 2 C O R Aldehyde R C O R Ketone
3 Wood Chemistry Glyceraldehyde This 3 carbon sugar is the simplest monosaccharide. It is an aldehyde. As is the case with all monosaccharides, it contains an asymmetric or chiral carbon. This means there are 4 different groups attached to this carbon Because of this, there are 2 different stereoisomers of this structure known as enantiomers. PSE 406 - Lecture 2 CHO H C OH CH2OH 4 Glyceraldehyde
Representations Wood Chemistry 1. Wedge and Dash CHO CHO H C OH HO C H CH2OH CH2OH D-glyceraldehyde In this representation, the dotted lines indicate bonds going into the paper the wedges indicate bonds coming out of the paper. L-glyceraldehyde 2. Fischer Formula CHO
H C OH CH2OH D-glyceraldehyde Notes CHO HO C H CH2OH In the Fischer representation, vertical lines indicate bonds going into the paper while horizontal lines indicate bonds coming out of the paper. L-glyceraldehyde PSE 406 - Lecture 2 5 Wood Chemistry H C
3 D Views of the Enantiomers of Glyceraldehyde = Carbon CHO OH = Oxygen CH2OH = Hydrogen HO C CHO H C CHO H CH2OH OH CH2OH CHO HO C
H CH2OH D Glyceraldehyde L Glyceraldehyde PSE 406 - Lecture 2 6 Four Carbon Sugars Wood Chemistry (Not Found in Hemicelluloses or Cellulose) CHO H C OH H C OH CHO HO C H Enantiomers HO C H
CH2OH CH2OH D-erythrose L-erythrose Diasteriomers CHO HO H C H C OH CH2OH D-threose CHO Enantiomers H C OH HO C H CH2OH With 2 chiral centers in the molecule, there are 4 sterioisomers; two pairs of
enatiomers and two pairs of diasteriomers. L-threose PSE 406 - Lecture 2 7 Wood Chemistry Stereochemistry Enantiomers are identical in physical properties except that they rotate polarized light in opposite directions. From a biochemical standpoint, they are different. Diastereoisomers possess different physical properties For n asymetric carbons there are: 2n maximum possible number of sterioisomers 2n-1 maximum possible number of enantiometric pairs That means there are 2n-1 possible diastereoisomer pairs PSE 406 - Lecture 2 8 Enatiomers of Erythrose
Wood Chemistry CHO CHO HO H H OH HO H H OH CH2OH CH2OH D Erythrose L Erythrose PSE 406 - Lecture 2 9 Wood Chemistry
Are you a 3D type of Student? How do chemical representations correlate with the actual spatial arrangement of the molecule? The representations are a simplified (but tricky way) of showing the molecule. We dont care about steriochemistry CHO HO H HO H CH2OH PSE 406 - Lecture 2 10
Wood Chemistry Simple Bob Method for 3D Remember, in the Fischer formula, the vertical lines always go into the paper. You dont want your molecule to be a circle so in 3D the carbon backbone alternates in and out of the page. PSE 406 - Lecture 2 11 Wood Chemistry Simple Bob Method for 3D (II) Check out the second carbon down, in the 3d version the carbons attached to this atom both are going into the page. The OH group and the H group attached to this carbon are coming out of the page and pointing to the left and right respectively. Therefore, in the Fischer formula these are
drawn to the left and right. PSE 406 - Lecture 2 CHO HO H HO H CH2OH 12 Wood Chemistry Simple Bob Method for 3D (III) Now if you go to the 3rd carbon from the top, you will notice that the carbon above and below it are coming out of the page; this is unacceptable. To fix this, we rotate the molecule 180. PSE 406 - Lecture 2 13 Wood Chemistry
Simple Bob Method for 3D (IV) If you look at the molecule now you will see that the OH and H groups on the 3rd carbon now stick out of the page and are on the left and right side respectively. Draw them into the Fischer formula and you get your final molecule. It may seem a little tricky but it is a fun thing to do at parties. PSE 406 - Lecture 2 CHO HO H HO H CH2OH 14
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