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5.9: Carbohydrates and Diastereomers

  • Page ID
    191217
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    So far, we have mostly been looking at compounds that contain one chiral center -- that is, one tetrahedral carbon atom with four different groups attached to it. Could there be more than one chiral center? Of course. For example, if carbohydrates have an OH group on every carbon in a chain, we might get a number of chiral centers in one molecule. Thinking in Fischer projections, that means each of the OH groups on the chiral centers could be drawn either on the right or on the left.

    Of course, two compounds with multiple chiral centers could still be enantiomers. Maybe we have two carbohydrates, one with all of the OH groups on the right in the Fischer projection, the other with all of them on the left. Those two compounds would still be the exact opposite of each other. They would still be enantiomers.

    Suppose instead that only some of the chiral centers were opposite, and others were the same. The compounds would no longer be exact mirror images of each other. The mirror would be flawed. These isomers would be called diastereomers.

    We know that enantiomers have the same physical properties. Diastereomers do not. For example, they have different melting points, because the two isomers cannot pack together in exactly the same way, and the intermolecular attractions are different in each case.

    • Diastereomers only occur in compounds containing more than one chiral center.
    • In a pair of diastereomers, some of the chiral centers are the same in the two molecules, but others are different.
    • Diastereomers are stereoisomers that are not mirror images of each other.

    Threose is an example of a biological molecule (a carbohydrate) that contains two chiral centers.

    A wedge-and-dash drawing and fischer projection of D-threose. On the Fischer projection, the bottom hydroxyl group is on the right hand side. On the wedge and dash drawing both internal hydroxy groups are dashed and the hydrogens wedged.
    Figure \(\PageIndex{1}\): Pictures of D-threose.
    A ball and stick model of D-threose. The internal hydroxy groups are on the right hand side of the molecule.
    Figure \(\PageIndex{2}\): A ball-and-stick model of D-threose.

    Fischer projections are sometimes used in showing carbohydrates and other chain compounds with many chiral centers. Fischer projections are Picasso-esque drawings in which the point of view alternates from one carbon to the next. The chiral centers are easy to compare in Fischer projections because you simply have to decide whether groups are on the same side or opposite sides of the vertical line.

    Labelled diagrams of D-threose in wedge-dash form and Fischer projection. On the wedge and dash, labels indicate that the carbonyl can be anywhere in wedge-dash, and dashed lines indicate the lines of sight at the first and second internal carbons. On the Fischer projection, the carbonyl must be at the top. Labels highlight that OH is to the left on C2 and to the right on C3. Underneath the Fischer projections reads "D means OH to right on last chiral C.
    Figure \(\PageIndex{3}\): A little more insight into D-threose.

    D-threose is chiral and it does have an enantiomer. Its enantiomer is called L-threose. L-threose has exactly the same physical properties as D-threose, except for one. A solution of D-threose rotates a beam of plane-polarized light to the right, whereas a solution of L-threose rotates the plane to the left.

    Ball-and-stick model of L-threose, showing both internal hydroxy groups on the left hand side.
    Figure \(\PageIndex{4}\): A ball-and-stick model of L-threose.

    However, D-threose also has other stereoisomers that are NOT its mirror image. These isomers are called diastereomers. These compounds have different physical properties from threose, so they have a completely different common name. That's because common names for compounds were often coined before anyone knew about the structure of the compound.

    Ball-and-stick model of D-erythrose, showing C2 hydroxyl group on the left hand side and C3 hydroxy group on the right hand side.
    Figure \(\PageIndex{5}\): A ball-and-stick model of D-erythrose.
    Ball-and-stick model of L-erythrose, showing C2 hydroxy group on the right hand side and C3 hydroxy group on the left hand side.
    Figure \(\PageIndex{6}\): A ball-and-stick model of L-erythrose.

    The relationships between D-threose and its stereoisomers can be seen in wedge-dash projections. The family of aldobutanoses -- that is, four-carbon dugars containing an aldehyde group -- is presented together below.

    Diagram of relationships between D- and L-treose and D- and L-erythrose. D- and L-threose are enantiomers. D- and L-erythrose are enantiomers as well. Erythrose and threose, whether D or L form, are both diastereomers of each other.
    Figure \(\PageIndex{7}\): The relationship between D-threose and its enantiomer, L-threose; also, the relationship between D-threose and its two diastereomers, D- and L-erythrose.

    Alternatively, these relationships can be viewed in Fischer projections.

    Relationships between D-threose, L-threose, D-erythrose, and L-erythrose as Fischer projections.
    Figure \(\PageIndex{8}\): The relationship between D-threose and its enantiomer, L-threose; also, the relationship between D-threose and its two diastereomers, D- and L-erythrose.

    Alternatively, these relationships can be viewed in Fischer projections.

    Relationships between D-threose, L-threose, D-erythrose, and L-erythrose as Fischer projections.
    Figure \(\PageIndex{9}\): The relationship between D-threose and its enantiomer, L-threose; also, the relationship between D-threose and its two diastereomers, D- and L-erythrose. Once more with Fischer.
    • In a pair of diastereomers, some chiral centers are the same and some are opposite. The molecule is neither identical to nor the mirror image of its diastereomer.
    • D-threose is the enantiomer of L-threose. The two are non-identical, but they are mirror images of each other.

    It may help to look at three-dimensional models of these sugars.

    Go to Animation SC9.1. A three-dimensional model of D-threose.

    Go to Animation SC9.2. A three-dimensional model of L-threosee.

    Go to Animation SC9.3. A three-dimensional model of D-erythrose.

    Go to Animation SC9.4. A three-dimensional model of L-erythrose.

    Exercise \(\PageIndex{1}\)

    Take a look at the three-dimensional model of D-threose using the stick model.

    1. If you turn it so that the carbonyl is at the top and the OH and H of the first chiral carbon are coming towards you, is the first OH on the right or on the left?
    2. If you turn it so that the carbonyl is at the top and the OH and H of the second chiral carbon are coming towards you, is the first OH on the right or on the left?
    Answer a:

    left

    Answer b:

    right

    Exercise \(\PageIndex{2}\)

    Take a look at the three-dimensional model of L-threose using the stick model.

    1. If you turn it so that the carbonyl is at the top and the OH and H of the first chiral carbon are coming towards you, is the first OH on the right or on the left?
    2. If you turn it so that the carbonyl is at the top and the OH and H of the second chiral carbon are coming towards you, is the first OH on the right or on the left?
    Answer a:

    right

    Answer b:

    left

    Exercise \(\PageIndex{3}\)

    Take a look at the three-dimensional model of D-erythrose using the stick model.

    1. If you turn it so that the carbonyl is at the top and the OH and H of the first chiral carbon are coming towards you, is the first OH on the right or on the left?
    2. If you turn it so that the carbonyl is at the top and the OH and H of the second chiral carbon are coming towards you, is the first OH on the right or on the left?
    Answer a:

    right

    Answer b:

    right

    Exercise \(\PageIndex{4}\)

    Take a look at the three-dimensional model of L-erythrose using the stick model.

    1. If you turn it so that the carbonyl is at the top and the OH and H of the first chiral carbon are coming towards you, is the first OH on the right or on the left?
    2. If you turn it so that the carbonyl is at the top and the OH and H of the second chiral carbon are coming towards you, is the first OH on the right or on the left?
    Answer a:

    left

    Answer b:

    left

    • When there is one chiral center present, two stereoisomers result.
    • When there are two chiral centers present, four stereoisomers result.
    • For each additional chiral center, the number of stereoisomers doubles.

    Carbohydrates can be classified according to their stereochemistry, forming a sort of carbohydrate family tree. They are considered to originate from the simplest carbohydrate, glyceraldehyde. Addition of a new carbon to the C=O unit of glyceraldehyde produces two possible isomers, because a new steresocenter is formed. From there, additional carbohydrates can be built up, one carbon at a time. In the carbohydrate family tree, glyceraldehyde is great-grandmother.

    Tree of carbohydrates. At the root is D-glyceraldehyde. From this, a carbon is added to the carbon chain, forming two diastereomers: D-erythrose and D-threose. At the next level up, addition of a carbon to D-erythrose forms D-ribose and D-arabinose; D-threose forms D-xylose and D-lyxose. At the next level: D-ribose forms D-allose and D-altrose; D-arabinose forms D-glucose and D-mannose; D-xylose forms D-gulose and D-idose; D-lyxose forms D-galactose and D-talose.
    Figure \(\PageIndex{10}\): Relationships among a family of D-carbohydrates.

    Exercise \(\PageIndex{5}\)

    What are the absolute configurations of the two chiral centers in D-threose? (You will need to number the carbons on which the chiral centers are found. For example, if an S center is found on the second carbon along the chain and an R center is found on the fourth carbon along the chain, the configuration is 2S,4R. Note that you should number the chain from the highest-priority end, which has a carbon with the most bonds to oxygen.)

    Answer

    D-threose

    2S, 3R

    Exercise \(\PageIndex{6}\)

    What are the absolute configurations in L-threose? What do you notice about the relationship between the configurations in D- and L- threose?

    Answer

    L-threose

    2R, 3S

    D- and L-threose are enantiomers of one another

    Exercise \(\PageIndex{7}\)

    What are the absolute configurations in L-erythrose? What do you notice about the relationships between the configurations in L-erythrose and L- threose?

    Answer

    L-erythrose

    2S, 3S

    L-erythrose and L-threose are diastereomers of one another.

    Exercise \(\PageIndex{8}\)

    Suppose a compound contained three chiral centers.

    1. How many possible stereoisomers would there be?
    2. How many different pairs of enantiomers would there be?
    3. Suppose you selected a pair of diastereomers from this group. How many possible pairs could you choose from?
    Answer a:

    (2)3 = 8 possible stereoisomers

    RRR; SSS; RRS; SSR; RSS; SRR; SRS; RSR

    Answer b:

    4 pairs

    Answer c:

    12 different possible pairs of diastereomers

    Exercise \(\PageIndex{9}\)

    Draw one enantiomer and one diastereomer for each of the following compounds.

    Exercise 5.9.9, a (cis-2-methylcyclopentanol) and b (trans-3-methylcyclohexanol).
     
    Answer

    Answers to Exercise 5.9.9, a and b, showing all diastereomers and enantiomers.

    Exercise \(\PageIndex{10}\)

    Draw one enantiomer and one diastereomer for each of the following compounds.

    Exercise 5.9.10, a (pentose with configuration RRS) and b (tetrose with configuration RS).
     
    Answer

    Answers to Exercise 5.9.10, a through b.


    This page titled 5.9: Carbohydrates and Diastereomers is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Chris Schaller via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.