Chapter 16

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Chapter 16

There are several definitions you should know from this chapter.

Carbohydrates: aldehydes and ketones that contain several hydroxyl groups. The simple carbohydrates are called sugars.
Aldose: a carbohydrate with the carboxyl group on the end.
Ketose: a carbohydrate with the carboxyl group in the middle somewhere.
Asymmetric Carbon: A carbon with 4 different things attached to it. Any compound that contains an asymmetric carbon will have two molecular forms that are nonsuperimposable mirror images. These two images are referred to as stereoisomers.
Fischer projections: represent straight chain structures of sugars.
Haworth projections: represent sugars in cyclic form.
D and L forms of a sugar are determined by the position of the hydroxyl on the asymmetric carbon furthest from the carbonyl group. If the hydroxyl groups points to the right, the sugar is a D form. If it points left sugar is an L form. Most sugars in nature are D forms. D and L forms are identical in chemical and physical porperties, except that they rotate plane-polarized light in opposite directions. However, you cannot tell what direction it rotates the light just by the D and L designations
Optical Isomers (means the same things as stereoisomers) rotate plane-polarized light. A (+) indicates rotation to the right, and a (-) indicates rotation to the left.

Draw out the Fischer projections for D-glyceraldehyde, D-ribose, D-deoxyribose, D-glucose, and D-fructose. Memorize these structures. :o)

Cyclic Structures of Sugars

A five membered sugar ring is called furanose and a six member ring is called pyranose. Alpha designates a hydroxyl group that points down, and beta designates a hydroxyl group that points up.
Here are the cyclic structures for the sugars that you've been instructed to memorize. Be aware that each of these could be in either alpha or beta form, although only one form is shown. (There are only four out of the five sugars that you have memorized. Why is that?) Name each one, and number the carbons. Where are the asymmetric carbons and how many are there? What is the difference in the way that an aldose will form a cyclic structure verses how a ketose will form one?

Starches: polymers that consist entirely of D-glucose units, most commonly with alpha one to four linkages. What that means is that the number one carbond on one alpha form ring attaches to another alpha form ring at it's fourth carbon.
Reducing Sugar: gives positive tests with Benedict's and Tollen's reagents.
Non-reducing Sugar: gives negative tests with Benedict's and Tollen's reagents. Any sugar that is an acetal or ketal is nonreducing. Large polysaccharides such as starch and cellulose are usually not reducing sugars.