SSS 3: ORGANIC CHEMISTRY (VI)

CARBOHYDRATES are naturally occurring organic compounds containing carbon, hydrogen and oxygen, with the hydrogen and oxygen present in the ratio of 2:1, as in water. The general molecular formula of carbohydrate is Cₓ(H₂O)ᵧ or CₓH₂ᵧOᵧ.

Carbohydrates can be classified into simple sugars and complex sugars. Simple sugars are crystalline, soluble in water and have a sweet taste. Structurally, they can be further divided into monosaccharides, e.g. glucose, and disaccharides, e.g. sucrose.

Classification of Carbohydrates

Monosaccharides

Monosaccharides are simple sugars with three to six carbon atoms per molecule. However, the most common and important of these are the ones containing six carbon atoms per molecule, the hexoses, e.g. glucose, fructose, galactose and mannose. All hexoses have the same molecular formula, C₆H₁₂O₆, but possess different spatial arrangements.

Hexoses can exist both in the open chain and cyclic forms in solution. In solid crystals, they exist in the ring form.

Straight-chain forms

Glucose

     H

      |

H — C — OH

      |

HO — C — H

      |

H — C — OH

      |

H — C — OH

      |

    CH₂OH

Fructose

CH₂OH

      |

     C=O

      |

HO — C — H

      |

H — C — OH

      |

H — C — OH

      |

    CH₂OH

Cyclic forms

Disaccharides

Disaccharides are derived from two molecules of monosaccharides.

Complex Sugars

Complex sugars are also known as polysaccharides. They are non-crystalline, insoluble and tasteless substances, e.g. starch and cellulose. They have very high relative molecular masses and are polymers derived from monosaccharides