Appendix E - Polarity Worksheet 491yd

Appendix E Polarity worksheet Dr. Justin Shaffer

Biology 100 Spring 2012

This worksheet will help you learn how to distinguish between polar and non-polar molecules, and whether a molecule is hydrophilic (water loving) or hydrophobic (water fearing). Read through the first two pages, then try out the examples on the last page. How do you tell if a bond is polar or non-polar?  Covalent bonds form between atoms when atoms share electrons  The bonds between carbon (C) and hydrogen (H) in methane are covalent, as are the bonds between oxygen (O) and hydrogen (H) in water

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The polarity of a covalent bond depends on the electronegativity of an atom. Electronegativity refers to how strongly an atoms pulls electrons towards itself. Atoms have different electronegativities. For the major atoms that make up biomolecules, the strength of electronegativities is shown below (from most electronegative to least electronegative) Oxygen (O) > Nitrogen (N) > Carbon (C) ~ Hydrogen

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Oxygen is the most electronegative, followed by nitrogen, followed by carbon and hydrogen, which have about the same electronegativity When two atoms with the same electronegativity form a covalent bond, neither atom pulls electrons closer to itself. There is an equal distribution of electrons (everything is balanced), and this kind of bond is referred to as a non-polar covalent bond. o The bonds between carbon and hydrogen in methane are non-polar covalent bonds because carbon and hydrogen have the same electronegativities.

If two atoms with different electronegativities form a covalent bond, then electrons are pulled closer to the atom that is more electronegative. This results

Appendix E

in that atom becoming more negatively charged, and the other atom becoming more positively charged. There is an unequal distribution of electrons, and this kind of bond is referred to as a polar covalent bond. o The bonds between oxygen and hydrogen in water are polar covalent bonds because oxygen is more electronegative than hydrogen, so it pulls the electrons closer to itself, making the oxygen slightly negative and the hydrogen atoms slightly positive.

How do you tell if a molecule is polar or non-polar?  If a molecule (or part of a molecule, like the R group of an amino acid) contains only non-polar covalent bonds, then the molecule is non-polar. o QUICK TIP: If a molecule (or part of the molecule) only contains carbon (C) and hydrogen (H) atoms then the molecule is always non-polar.  If a molecule (or part of a molecule, like the R group of an amino acid) contains one or more polar covalent bonds, then the molecule is most likely polar. o QUICK TIP: If a molecule (or part of the molecule) contains oxygen (O) or nitrogen (N) atoms bound to carbon (C) or hydrogen (H) atoms, then it is most likely polar. o EXCEPTION: Carbon dioxide (CO2) is non-polar because it is a linear molecule (a straight line). This is a special case.  In the examples above, methane is a non-polar molecule because it contains only non-polar covalent bonds between carbon and hydrogen atoms. Water is a polar molecule because it contains polar covalent bonds between the oxygen and hydrogen atoms ( the QUICK TIPS to help you figure this out!) How do you tell if a molecule is hydrophobic or hydrophilic?  Hydrophobic molecules are water fearing. That is, they don’t mix well with water and would rather interact with other hydrophobic molecules. Hydrophobic molecules can be identified if they contain mostly non-polar covalent bonds. Lipids and amino acids with only carbon and hydrogen R groups are hydrophobic molecules.  Hydrophilic molecules are water loving. That is, they mix very well with water. Hydrophilic molecules can be identified if they contain polar covalent bonds. Carbohydrates, amino acids that contain polar R groups (hydroxyl, carboxyl, and amino groups), and positively or negatively charged molecules are hydrophilic molecules.

Appendix E

Practice set: Try applying the information you just learned to the following examples. In each molecule, identify what bonds are polar or non-polar, and then determine if the entire molecule is polar or non-polar.

For this set of macromolecules, identify which are hydrophilic and which are hydrophobic, and say why. For the amino acids, only evaluate the R groups (highlighted in the pink boxes).