Chapter 24Organic Chemistry• Most of you taking CHM 116 probably plan to continue on to organic chemistry next year.• Organic chemistry deals with chemicals composed largely of carbon, hydrogen and oxygen that are the basis for all life on earth.• Most of the chemicals we interact with each day, from the paper you write on to the food you eat, are organic compounds.Definition of Organic Chemicals• The original definition of organic chemicals was chemicals that were produced by organisms.– However, synthesis processes were developed to make “organic” chemicals out of inorganic chemicals.– Pharmaceuticals are an obvious example.• The more modern, and rather nebulous, definition of organic chemicals that contain carbon and hydrogen. Additional elements can also be present (e.g. O, N, S, P, halogens)– This definition is not absolutely perfect either, CF4is considered “organic” even though it has no hydrogen, but this is the best working definition.Quick Electronegativity Review• A lot of the reactivity of organic molecules is due to differences in electron density in the molecule, so we will have a quick refresher on electronegativity.• Electronegativity is the ability for an atom in a molecule to attract electrons to itself.• In general, electronegativity increases as you move from left to right in a row of the periodic table and it increases as you go up a column in the periodic table.Electronegativity(Pauling Scale)Notice that the first row of the “p” block elements are about 0.5 units apartNotice that the second row of the “p” block elements are about 1.0 units lower than the corresponding first row element.Electronegativity• The difference in electronegativity of the two atoms in a bond determines whether the bond is a nonpolar covalent bond, a polar covalent bond or an ionic bond.– Difference in electronegativity < 0.5 units results in a nonpolar covalent bond. (equal electron sharing)– Difference in electronegativity between 0.5 and 2.0 units results in a polar covalent bond. (unequal electron sharing)– Difference in electronegativity > 2.0 units results in an ionic bond.• Different people sometimes use different cut points. If a metal is involved the cutoff point is usually considered to be closer to 1.7.• Note: The largest difference of those elements that I expect you to know was H and F (difference of ~1.9). The only pairings that are < 0.5 are C with H and N with Cl. So all of the pairs (except C-H and N-Cl) are polar covalent. Which means you only really have to remember which element is more electronegative.ExamplesHC2.5 2.1Difference = 0.4Non-polar covalent bondHO3.5 2.1Difference = 1.4Polar covalent bondClNa0.9 3.0Difference = 2.1Ionic bondExamples:• Are the following bonds in an organic molecule polar or non-polar? Quantify your answer.C=OC−NN−HC−BrC−FC−S−H (both bonds) C−O−H (both bonds)C = 2.5 and O = 3.5; difference = 1.0, so it is a polar bondC = 2.5 and N = 3.0; difference = 0.5, so it is barely a polar bondN = 3.0 and H = 2.1; difference = 0.9, so it is a polar bondC = 2.5 and Br = 2.8; difference = 0.3, so it is a nonpolar bondC = 2.5 and F = 4.0; difference = 1.5, so it is a polar bondC = 2.5, S = 2.5 and H = 2.1; differences = 0 and 0.4, so they are nonpolar bondsC= 2.5, O = 3.5 and H = 2.1; differences = 1.0 and 1.4, so they are polar bondsWhy Do We Care About Polarity of Organic Compounds?• Molecules with many polar bonds can undergo dipole-dipole intermolecular interactions.– This means that they tend to have higher melting points and boiling points than similar-sized nonpolar molecules.– Polar molecules tend to be soluble in water because they can interact with the dipole of water molecules.• In contrast, nonpolar molecules are NOT water soluble (e.g. oils).• Molecules with many dipoles tend to be more reactive than nonpolar molecules.– The differences in electron density tends to create sites for reaction.The Chemistry of Carbon• The field of organic chemistry is dominated by the chemistry of carbon, hydrogen and oxygen.• The chemistry of these all-important elements is derived from their electronic structure, which dictates their geometries.• So, let’s briefly review the electronic configuration of these elements.• Note that this is just a quick refresher. You will likely need to review the molecular geometry chapter (Chapter 9) covered in CHM 113.The Chemistry of Carbon• Carbon has an atomic number of 6 and a mass of about 12. Therefore, carbon has 6 electrons.• Its electronic configuration is 1s22s22p2, which means it has four valence electrons.• To fill its octet, it needs to obtain 4 more electrons through bonding. Therefore, carbon forms 4 bonds.– There are some exceptions where carbon has three bonds and an ionic charge, but you will not see them in this class.– Carbon CANNOT form stable compounds with 5 or more bonds under terrestrial conditions because it cannot expand its valence shell.The Chemistry of Carbon• The physical geometry of the carbon atom depends on the number of electron domains around it.• If all four bonds are single bonds, then the carbon undergoes sp3orbital hybridization and it assumes a tetrahedral shape.TetrahedralFour electron domains all bond angles = 109.5°109.5°CHHHHThe Chemistry of Carbon• If there are only three electron domains, then the carbon undergoes sp2and assumes a trigonal planar geometry. Three electron domains means that one of the electron domains is a double bond. Trigonal planarThree electron domainsall atoms in one plane, this forms an equilateral triangle, bond angles 120°CHHOThe Chemistry of Carbon• If there are only two electron domains, then the carbon undergoes sp hybridization and assumes a linear geometry. Two electron domains means that one of the electron domains is a triple bond or both bonds are double bonds (very rare). LinearTwo electron domainsBond angle = 180°180°CNHThe Chemistry of Carbon• Carbon can form single, double and triple bonds. These bonds have different degrees of reactivity.– Triple bonds are the strongest and shortest. The bond enthalpy of the C  C bond is 832 kJ/mol.– Double bonds are longer and weaker than triple bonds. The bond enthalpy of the C=C bond is 614 kJ/mol.– Single bonds are the longest and the weakest. The bond enthalpy for the CC bond is 348 kJ/mol.Rotation Around Bonds• Remember that a single bond consists of a sigma