1Chapter 2 Representative Carbon Compounds Functional Groups And Intermolecular Forces 2In this chapter : 1. The major functional groups 2. The correlation between properties of functional groups and molecules and intermolecular forces 3. Infrared (IR) spectroscopy, which can be used to determine what functional groups are present in a molecule 3Bonding in Organic Molecules Carbon forms strong covalent bonds to other carbons and to other elements such as hydrogen, oxygen, nitrogen and sulfur. Some Examples of Bond Dissociation Energies Bond Type kJ/mol kcal/mol C-H 380-435 91-104 C-F 439-451 105-108 C-Cl 330-349 79-84 C-Br 263-293 63-70 C-I 234 56 C-C 334-368 80-88 C-O 380-385 91-92 Organic compounds are grouped into functional group families. 4A functional group is a specific grouping of atoms (e.g. carbon- carbon double bonds are in the family of alkenes, and carbon-oxygen double bonds are in the family of carbonyls) An instrumental technique called infrared (IR) spectroscopy is used to determine the presence of specific functional groups. 5Hydrocarbons: Alkanes, Alkenes Alkynes, and Aromatic Compounds Hydrocarbons contain only carbon and hydrogen atoms. Subgroups of Hydrocarbons: (1) Alkanes contain only carbon-carbon single bonds (Saturated hydrocarbons) (2) Alkenes contain one or more carbon-carbon double bonds. (3) Alkynes contain one or more carbon-carbon triple bonds. (4) Aromatic hydrocarbons contain benzene-like stable structures (discussed later). Unsaturated hydrocarbons: contain double or triple carbon-carbon bonds (alkene, alkynes, aromatics). Capable of reacting with H2 to become saturated. 6Alkanes The principle sources of alkanes are natural gas and petroleum. The smaller alkanes (C1 to C4) are gases at room temperature. Methane is a (1) component of the atmosphere of many planets and (2) major component of natural gas. 7Alkenes •Ethene (ethylene) is a major industrial feedstock.Used in the production of ethanol, ethylene oxide and the polymer polyethylene. •Propene (propylene, C3H6) is also very important in industry. Propene is used to make the polymer polypropylene and is the starting material for the synthesis of acetone. •Many alkenes occur naturally. 8Alkynes •Ethyne (acetylene) is used in welding torches because it burns at high temperature. •Many alkynes are of biological interest. •Capillin is an antifungal agent found naturally. •Dactylyne is a marine natural product. •Ethinyl estradiol is a synthetic estrogen used in oral contraceptives. 9Aromatic Hydrocarbons: Benzene All bond lengths are the same (1.38 Å) (compare with C–C single bond 1.54 Å, C=C double bond 1.34 Å) Resonance theory explains this by suggesting there are two resonance hybrids that contribute equally to the real structure. Extra stabilization due to resonance  aromatic • Benzene is the prototypical aromatic compound. – The Kekulé structure (named after August Kekulé) – is a six-membered ring with alternating double and single bonds 10Molecular orbital theory explains the equal bond lengths of benzene by suggesting there in a continuous overlap of p orbitals over the entire ring. (1) All carbons in benzene are sp2 hybridized. (2) Each carbon also has a p orbital oriented perpendicular to the ring. Each p orbital contributes one electron to the π system. (3) Each p orbital does not just overlap with one adjacent p but overlaps with p orbitals on either side to give a continuous bonding molecular orbital that encompasses all 6 carbons. (4) All 6 p electrons are therefore delocalized over the entire ring and this results in the equivalence of all of the carbon-carbon bonds. 11C C C Oequal sharing of e⊖ (non-polar bond) unequal sharing of e⊖ (polar bond) 2.5 3.5 d+ d- Polar Covalent Bonds Electronegativity 12Polar Covalent Bonds Polar covalent bonds occur when a covalent bond is formed between two atoms of differing electronegativities (esu) 13(1) The more electronegative atom draws electron density closer to itself. (2) The more electronegative atom develops a partial negative charge (d-) and the less. electronegative atom develops a partial positive charge (d+). (3) A bond which is polarized is a dipole and has a dipole moment. (4) The direction of the dipole can be indicated by a dipole arrow. The arrow head is the negative end of a dipole, the crossed end is the positive end. Dipole Moment (m, D) 14Example: hydrogen chloride The more electronegative chlorine draws electron density away from the hydrogen. Chlorine develops a partial negative charge and hydrogen develops a partial positive charge. The dipole moment of a molecule can be measured experimentally. It is the product of the magnitude of the charges (in electrostatic units: esu) and the distance between the charges (in cm). The actual unit of measurement is a Debye (D) which is equivalent to 1 x 10-18 esu cm. 15The Dipole Moments of Representative Molecules In diatomic molecules a dipole exists if the two atoms are of different electronegativity. In more complicated molecules the molecular dipole moment is the sum of the bond dipoles. A bond dipole or bond moment is a vector; it has both direction and magnitude. Some molecules with very polar bonds will have no net molecular dipole because the bond dipoles cancel out. The center of positive charge and negative charge coincide in these molecules 16Examples: In carbon tetrachloride the bond dipoles cancel and the overall molecular dipole moment is 0 Debye. In chloromethane the C-H bonds have only small bond dipoles but the C-Cl bond has a large dipole; the result is that the molecule is quite polar. An unshared pair of electrons on atoms such as oxygen and nitrogen contribute a great deal to the dipole moment of a molecule. Water and ammonia are examples. 17Water and ammonia have very large net dipoles 18In Class Problems: Write the symbol δ+ and δ- by the appropriate atoms and, where appropriate, draw a resultant dipole moment vector for each of the following molecules. (1) Tetrachloromethane: CCl4 (1) Trichloromethane: CHCl3 19In Class Problem: Which molecule has the largest dipole moment ? (1) (2) 20 Some cis-trans isomers differ markedly in their dipole moment. In trans 1,2-dichloroethene the two carbon-chlorine dipoles are pointing in opposite