When Mr.Baker is baking cookies, his hands become covered in butter. He only uses water to wash off his hands, but the butter stays on his hands.

Question

Explain why the butter doesn't want to come off of his hands.

Answer 1

This is due to the fact that butter is composed of mostly saturated fats, which are composed of lipids (ie the biochemical building blocks of fats). Lipids generally have two regions, a polar, hydrophillic (water loving) head, and a nonpolar, hydrophobic (water hating) head.

There are two contributing factors that make a compound polar or nonpolar.

1. The presence of bond dipoles due to electronegativity differences between atoms
2. The geometry of a molecule

The hydrocarbon tails of lipids are nonpolar. A hydrocarbon is a molecule composed of hydrogen and carbon. Hydrogen and carbon are not very electronegative atoms. An atom's electronegativity describes an atom's affinity for electrons (atoms with higher electronegativity values will have the greatest affinity for electrons. A bond dipole will form when electrons are not shared equally between two atoms. This generally occurs when there is a significant difference in electronegativity between atoms (this difference is usually between 0.5 - 1.5.

Recall that in hydrocarbons, hydrogen and carbon will form covalent bonds (meaning they share their electrons). Carbon has an electronegativity of 2.6, and hydrogen has an electronegativity of 2.2. When carbon and hydrogen bond, their bond has an electronegativity difference of 0.4. Any bond that has an electronegativity difference less than 0.5 is considered a nonpolar covalent bond. This low electronegativity difference shows us that the electrons shared in a C-H covalent bond are shared relatively equally.

Now let us look at a molecule like water. Oxygen has an electronegativity of 3.4. So an O-H bond in water will have an overall electronegativity difference of 1.2. As a result, the O-H bond in water is considered polar covalent (meaning the electrons in the covalent bond are not equally shared between hydrogen and oxygen. Since oxygen has a high electronegativity, the electrons in the O-H covalent bond are more likely to be found in near the oxygen atom, giving oxygen a partial negative charge. Since oxygen will draw electrons away from hydrogen, it will give the hydrogens a partial positive charge. The resulting charges on the oxygen and hydrogen atoms results in a bond dipole.

Molecular geometry matters when determining polarity. There are molecules that have bond dipoles, but are also considered to be nonpolar. The reason for this is because some molecular geometries can cancel out the dipole moments between atoms.

For example, water has a bent shape (because the lone pairs on the oxygen atom repel the valence electrons on hydrogen, producing a bent shape). A bond dipole always points in the direction of the most electronegative atom. Because both the O-H bond dipole moments in water point in the direction of oxygen at an angle, they do not cancel out. However, in carbon tetrachloride (CCl4), the chlorine atom is more electronegative than carbon, so the bond dipoles point in the direction of the chlorine atoms (with carbon being the central atom in the middle of the molecule). Since these bond dipoles point in opposite directions, they cancel each other out, making CCl4 nonpolar despite the C-Cl bond having a high electronegativity difference.

Now back to your question....

1. The reason why some organic compounds are soluble in water is because they have the ability to form hydrogen bonds with water.
2. Hydrogen bonding is a dipole-dipole interaction, meaning that the partial charge created by a dipole from one molecule is attracted to the opposite partial charge created by a dipole on another molecule. For example, the partial positive hydrogen on a water molecule will form a hydrogen bond with the partial negative oxygen atom on another water molecule.
3. Hydrogen bonding only occurs in polar molecules between hydrogen atoms and highly electronegative atoms (typically N, O, and F).
4. Since the electrons in hydrocarbons are shared evenly between carbon and hydrogen atoms, hydrocarbons will not have bond dipoles, and therefore they cannot form hydrogen bonds with water molecules or other polar solvents. As a result, a nonpolar substance such as butter or other oils will not dissolve in water, and cannot be washed away with water.
5. General rule in chemistry: like dissolves like (you can only dissolve polar solvents in polar solvents, and nonpolar solvents in nonpolar solvents).
6. Although butter is not soluble in water, it can dissolve in other nonpolar solvents (such as oils, hexanes, toluene, etc.)