Topics 10 & 20 Organic Chemistry IB CHEMISTRY Topics 10 & 20 Organic Chemistry Jeff Venables Northwestern High School

Some General Characteristics of Organic Molecules The Structures of Organic Molecules Organic molecules exhibit three different types of hybridization at the carbon center: sp3 hybridized carbons for tetrahedral geometries; sp2 hybridized carbons for trigonal planar geometries; and sp hybridized carbons for linear geometries. Homologous Series: Compounds with the same general formula, neighboring members differing by CH2, and similar chemical properties. There is a predictable gradation in physical properties.

The Structures of Organic Molecules

Carbon and hydrogen have very similar electronegativity values, so the C-H bond is essentially non-polar. Therefore, compounds containing C-C and C-H bonds are nonpolar, thermodynamically stable and kinetically inert (only LDF).

Types of Formulas: Empirical – shows the lowest whole-number ratio of atoms in a molecule (C2H5) Molecular – shows the actual number of atoms of each element in a molecule (C4H10) Structural – shows how atoms are connected in a molecule. 4. Condensed – structural formula with bonds omitted: CH3CH2CH2CH3 or CH3(CH2)2CH3 5. Skeletal – all letters are removed: (not to be used in answers on IB exam)

Introduction to Hydrocarbons Hydrocarbons are compounds with only C and H. There are four classes: alkanes (all  bonds and no  bonds); alkenes (a mixture of  and  bonds, but no triple bonds); alkynes (must contain triple bonds); and aromatics (have planar, ring structures with alternating single and double bonds). Saturated compounds have only  bonds. Unsaturated compounds have both  and  bonds.

Alkanes The name of alkanes varies according to the number of C atoms present in the chain. Since the only intermolecular forces available to alkanes are London dispersion forces, the boiling points increase smoothly as the molar mass increases. Methane to butane are gases at normal pressures. Pentane to decane are liquids at normal pressures. Each carbon in an alkane has 4 single bonds.

Alkanes Names based on number of carbon atoms: 1 = meth 5 = pent 2 = eth 6 = hex 3 = prop 7 = hept 4 = but 8 = oct All end with -ane

Alkanes

Examples – Name the following compounds: C5H12 pentane C7H16 heptane

Alkanes In this table each member differs by one CH2 unit. This is called a homologous series. Structures of Alkanes VSEPR theory predicts each C atom is tetrahedral. Therefore, each C atom has sp3 hybridized orbitals. It is easy to rotate about the C-C bond in alkanes.

Skeletal Drawings of Organic Molecules C – (almost) always has 4 bonds H – always has 1 bond H atoms are not drawn (assumed) C atoms are not written (exist at ends of bonds)

Alkanes Structural Isomers Compounds with the same molecular formula but different arrangement of atoms. Structural isomers have different physical properties.

Nomenclature of Alkanes All organic molecule names have three parts: Prefix, which tells the nature of the substituents; Base, which gives the number of carbons; and the Suffix, which gives the family (alkanes, etc.). Rules for naming compounds are given by the International Union for Pure and Applied Chemistry (IUPAC).

To name alkanes: Find the longest continuous carbon chain and use it as the name of the compound. Number the carbon atoms starting with the end closest to the substituent. Name and give the location of each substituent. When two or more substituents are present list them in alphabetical order.

Examples – Name each molecule 1. 4-ethylheptane 2. 4,7-dimethyldecane