CHEM 50: WATER: SOLVENT FOR BIOCHEMISTRY

Water

Water is the principal component of most cells and influences the properties of living systems.

Electronegativity

• The tendency of an atom to attract electrons in a chemical bond is called ____.

• Atoms of the same element share electrons equally in bonds, but atoms of different elements may not.

Polarity

occurs when two atoms with different electronegativities form a bond, resulting in unequal sharing of electrons

partial positive charge (δ+) and partial negative charge (δ-), polar

This difference in electronegativity causes a ____ on the hydrogen and a ____ on the oxygen, creating a ____ bond.

nonpolar bond

When the electronegativity difference is small (e.g., C-H in methane), the electrons are shared almost equally, creating a _____.

molecular geometry

plays a key role in determining if a molecule is polar or nonpolar

Carbon Dioxide (CO₂)

The C=O bonds are polar, but the linear geometry cancels out the dipoles, making CO₂ nonpolar.

Water (H2O)

• Water has a bent shape with a bond angle of 104.3°.

• The uneven sharing of electrons in the O-H bonds is not cancelled out, resulting in a polar molecule.

• This creates a dipole where the oxygen end is more negative and the hydrogen end is more positive.

Dipole

Bonds with distinct positive and negative ends, like those in water

Ionic and polar

___ compounds (e.g., potassium chloride, KCl) and ___ compounds (e.g., ethyl alcohol, acetone) dissolve in water due to electrostatic attraction

electrostatic attraction

Ionic compounds (e.g., potassium chloride, KCl) and polar compounds (e.g., ethyl alcohol, acetone) dissolve in water due to ____

electrostatic interactions

• The negative end of water attracts positive ions or the positive end of dipoles.

• The positive end of water attracts negative ions or the negative end of dipoles.

• These interactions lower the energy and increase stability, making the system more likely to exist.

hydrophilic

• Ionic and polar compounds are ____ and dissolve in water

• Examples: Small organic molecules with electronegative atoms (e.g., alcohols, amines, carboxylic acids)

Hydrocarbons

• ____ (compounds of carbon and hydrogen) are nonpolar

nonpolar

• Hydrocarbons (compounds of carbon and hydrogen) are ___

• Ion–dipole and dipole–dipole interactions that aid the solubility of ionic and polar compounds are absent.

• Interactions between nonpolar molecules and water molecules are weaker than dipole–dipole interactions.

Nonpolar compounds do not dissolve in water because:

dipole–induced dipole interactions

• The permanent dipole of water can induce a temporary dipole in nonpolar molecules, creating ____

• These interactions are weaker than those between permanent dipoles (e.g., between water molecules).

hydrophobic

Nonpolar molecules are ____ ("water-hating")

• Hydrocarbons tend to sequester themselves from an aqueous environment.

• A nonpolar solid remains undissolved in water.

• A nonpolar liquid forms a two-layer system with water (e.g., an oil slick)

Behavior of Nonpolar Substances in Water

hydrophobic

The interactions between nonpolar molecules are called ____ interactions or sometimes ____ bonds.

• Amphipathic Molecules

• Some molecules have both polar (hydrophilic) and nonpolar (hydrophobic) portions. These molecules are called ____.

• Example: A long-chain fatty acid has a polar carboxylic acid group (hydrophilic "head") and a nonpolar hydrocarbon tail (hydrophobic "tail")

• micelles

• In water, amphipathic molecules form structures called ____, where:

  • The polar head groups interact with the aqueous environment.

  • The nonpolar tails are sequestered from the water.

• a spherical arrangement of organic molecules in a solution clustered

• van der Waals

• This type of weak interaction is called a _____ interaction

dipole–dipole interaction and electrostatic

Hydrogen bonding is a type of _____, and is of ____ origin

hydrogen bonding

• Occurs when hydrogen is covalently bonded to a highly electronegative atom like oxygen or nitrogen.

• This bond creates a partial positive charge on hydrogen.

• This partial positive hydrogen can interact with the unshared (nonbonding) electrons of another electronegative atom.

Hydrogen-bond donor

• The electronegative atom bonded to hydrogen.

Hydrogen-bond acceptor

• The electronegative atom providing the unshared electron pair for the interaction.