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Chemistry Notes: Hydrogen Bonds and Functional Groups

Hydrogen Bonds

Definition and Polarity

  • A hydrogen bond is defined as the attraction of a hydrogen atom to an electronegative atom.

  • Electronegative atoms commonly involved in hydrogen bonding are oxygen (O) or nitrogen (N).

  • In a water molecule (H_2O), oxygen is electronegative, creating a partial negative charge on the oxygen atom. The hydrogen atoms consequently acquire partial positive charges.

  • This polarity leads to attractions: the partial negative charge on oxygen of one water molecule is attracted to the partial positive charge on a hydrogen of another water molecule, forming a hydrogen bond.

Strength and Significance

  • Individual Strength: Individually, hydrogen bonds are generally weak and relatively easy to break.

  • Collective Strength: Their strength is significantly bolstered by their sheer number. For instance, each water molecule can form hydrogen bonds with approximately four other water molecules, creating an extensive network.

Location of Hydrogen Bonds

  • Between Different Molecules: Hydrogen bonds commonly occur between distinct molecules, as seen in water-water interactions.

  • Within the Same Molecule: Hydrogen bonds can also form between different parts of a single large molecule. A prime example is DNA.

    • DNA Example: In the ladder structure of DNA, the upright parts are the sugar-phosphate backbones, linked by strong phosphodiester (covalent) bonds. However, the bases in opposite strands (the

Introduction to Functional Groups

  • Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

  • They determine the chemical properties and reactivity of organic compounds.

  • The same functional group will undergo the same or similar chemical reactions regardless of the size of the molecule it is a part of.

Common Functional Groups

1. Hydroxyl Group

  • Structure: -OH

  • Class of compounds: Alcohols

  • Properties: Polar, can form hydrogen bonds, makes molecules more soluble in water.

2. Carbonyl Group

  • Structure: -C(=O)- (a carbon atom double-bonded to an oxygen atom)

  • Classes of compounds:

    • Ketones: A carbonyl group within a carbon chain (e.g., -C-CO-C-)

    • Aldehydes: A carbonyl group at the end of a carbon chain (e.g., -CHO)

  • Properties: Polar, reactive, important in many biological reactions.

3. Carboxyl Group

  • Structure: -COOH or -C(=O)OH (a carbonyl group linked to a hydroxyl group)

  • Class of compounds: Carboxylic acids

  • Properties: Acidic (donates H^+), polar, can form hydrogen bonds.

4. Amino Group

  • Structure: -NH_2

  • Class of compounds: Amines

  • Properties: Basic (accepts H^+), polar, important components of amino acids and proteins.

5. Phosphate Group

  • Structure: -OPO3^{2-} or -OP(=O)(OH)2

  • Class of compounds: Organic phosphates

  • Properties: Negatively charged, highly polar, important in energy transfer (ATP) and nucleic acids (DNA, RNA).

6. Sulfhydryl Group

  • Structure: -SH

  • Class of compounds: Thiols

  • Properties: Can form disulfide bridges (-S-S-), which are important for protein structure, less polar than hydroxyl.