Precursor to Organic Chem CHEMISTRY I BASICS

THE CHEMISTRY OF CARBON Carbon is the star in organic chemistry, so we must ensure that we understand all things carbon before we begin… Carbon Atomic Number 6 Atomic Mass Two stable isotopes Carbon-12 and Carbon-13, Carbon-14 is radioactive Electron configuration 1s 2 2s 2 2p 2 Oxidation numbers range from +4 to -4 Usually forms 4 covalent bonds, may multiple bond May form polar or nonpolar bonds, dictated by electronegativity differences EN= 2.5

SATURATED VS UNSATURATED When there are only single bonds between carbon atoms, we call those compounds saturated. This is because they contain as many hydrogen atoms as possible. They are “saturated’ with hydrogens. When double or triple bonds are present between carbons, those molecules are unsaturated. This is because they would have more hydrogens attached if there were single bonds.

The valence shell electron pair repulsion theory states that since electron pairs repel each other, they will try to spread out as much as possible around an atom. Lone pairs of electrons occupy more space than shared electron pairs. Thus, lone pairs on atoms will orient themselves as far apart from each other as possible causing the atoms to be spread out. Singly bonded carbons have a tetrahedral shape, with bond angles of 109.5°. Double and triple bonds have a significant effect on the shape of molecules. See below… VSEPR THEORY

Another important aspect to consider when determining shapes and bonding patterns in organic compounds is called hybridization. Hybridization refers to the combination of orbitals to create orbitals that are unique… a combination of atomic orbitals. To determine hybridization, count the regions around the atom of interest: A lone pair counts as a region Single, double or triple bonds only count as one region each Add up s and p exponents to get a total equal to the number of regions: 4 regions= sp 3 3 regions= sp 2 2 regions= sp HYBRIDIZATION

Models are of extreme importance to organic chemists. There are many ways to represent molecules and each has its advantages and disadvantages. Ball and stick - shows atoms as different colored balls and bonds as sticks Advantages- can see the bonds clearly and the geometry is more obvious Disadvantages- does not accurately represent how lone pairs take up space around each atom Too hard to draw Space-filling model - Advantages- more accurate representation of what it actually looks like in 3-D Disadvantages- cannot see bonds Too hard to draw MOLECULAR MODELS

Lewis structures are the way that chemists draw organic molecules, but it is important to realize that actual 3-D bond angles are much different than the way they are represented on paper. How to draw Lewis structures: 1) Add up all the valence electrons for each atom in the molecule. 2) Bond all atoms together using sngle bonds. 3) Add lone pairs so that each atom has an octet. (Except: H, B) 4) Add up all electron in the structure. If it is the same, you are most likely done. If it is greater than your total in #1, you have multiple bonds. If it is less than your total in #1, place extra lone pairs on the central atom. 5) CONSIDER FORMAL CHARGE!!!! Sulfate Ion and Formal Charge LEWIS STRUCTURES

These are the way that organic chemists write formulas that show the number of atoms in the molecule AND the arrangement of the atoms. Each carbon is grouped with the hydrogens bonded to it and then written as a formula. Example: CH 3 CH 2 CH 2 CH 3 CONDENSED STRUCTURAL FORMULAS

Functional groups are characteristic atoms or groups of atoms that help us categorize the molecule. Organic compounds from different classes can have the same chemical formula, but have different chemical and physical properties. CLASSIFYING ORGANIC COMPOUNDS