Organic Compounds - Alkanes Dr. Sean Bonness Chemistry Department El Camino College Organic compounds • All contain carbon. • Organic chemistry is the study of carbon containing compounds. • The essential needs of daily human life are food, fuel, shelter, and clothing - all are organic in nature, except water (H2O). 1 Bonding characteristics and Isomerism • What is the electronic configuration of Carbon • Mixing of the 2s and 2p orbitals for carbon produce sp3 hybrid orbitals • The hybrid orbitals point towards the corners of a regular tetrahedron. For example methane (CH4). Bonding characteristics and Isomerism (Cont.) • Carbon atoms bond covalently to other carbon atoms to form chains or networks. • A third carbon can join the end of this chain. • The process can continue to form chains of almost any length. 2 Bonding characteristics and Isomerism (Cont.) • Carbon can also share more than on pair of electrons to form multiple bonds • Structural Isomers - compounds that have the same molecular formula but in which the atoms bond in different patterns. • For example, what are the isomers for the compound with molecular formula C4H10? Bonding characteristics and Isomerism (Cont.) • What are the structural isomers of C2H6O? • Structural isomers exert a significant influence on properties. Ethyl alcohol is a liquid at room temp., whereas dimethyl ether is a gas. • What is a structural isomer of C3H6O? 3 Functional groups class Functional Expanded Condensed Name group structure structure Alkane Alkene Alkyne Functional groups (cont.) class Functional Expanded Condensed Name group structure structure Aromatic Alcohol Ether 4 Functional groups (cont.) class Functional Expanded Condensed Name group structure structure Amine Aldehyde Ketone Functional groups (cont.) class Functional Expanded Condensed Name group structure structure Carboxylic Acid Ester Amide 5 Functional Groups (cont.) • Write condensed structural formulas for the following compounds. Alkane structures • Hydrocarbons - simplest of all organic compounds. Only contain Carbon and Hydrogen. • Are called Saturated Hydrocarbons. • Unsaturated hydrocarbons have double and triple bonds. Which of the functional groups are unsaturated? • We also have normal alkanes and branched alkanes, i.e. butane. 6 Conformations of alkanes • Conformations are the different arrangements of atoms in space achieved by rotation about a single bond. • Butane • Ethane • Are these pairs structural isomers? Alkane Nomenclature • Assigning IUPAC names. • Step 1 - name longest chain. • Step 2 - number the longest chain • Step 3 - locate and name attached alkyl groups. • Step 4 - combine longest chain and alkyl groups into name 7 Alkane Nomenclature (cont.) • Step 5 - indicate the number and position of attached alkyl groups. If two or more alkyl groups, then use prefixes di, tri, tetra, etc. Common names • Some chain alkanes have a common name. Rarely used for alkanes having more than five carbon atoms. • Some widely used common names are: 8 Cycloalkanes • For alkanes the structural formula is CnH2n+2 • Is C3H6 an alkane? • If form a ring than • And has the structural formula Cycloalkanes Name Structural formula Condensed formula cyclopropane cyclobutane cyclopentane cyclohexane 9 Shapes of cycloalkanes • Stereoisomers are compounds with the same structural formula but different spatial arrangements of atoms. • For example 1,2-dimethylcyclopentane • Geometric isomers are stereoisomers in which the geometry of their groups are maintained by rings and are called cis-trans isomers. Shapes of cycloalkanes (cont.) • Show geometric isomers for 1,2- dimethylcyclobutane • What are the geometric isomers for 1,3- difluorocyclobutane? 10 Shapes of cycloalkanes (cont.) • Cyclohexane ring is not flat. • In standard conformation, lower right hand carbon is designated C-1. • All other carbons are numbered clockwise. • If atom or group bonded at C-1, then it is in the equatorial position. More stable than axial position. Shapes of cycloalkanes (cont.) • For most stable conformation of disubstituted cyclohexane – Place one substituted atome or group at C-1 in the equatorial position. – Place the second atom or group cis or trans to the first atom. • The most stable conformation for trans- 1,2dichlorocyclohexane is: 11 Physical properties of alkanes • Alkanes are composed of nonpolar carbon-carbon and carbon-hydrogen bonds. • Have lower melting points and boiling points than other organic compounds due to weak attractions. However as chain length increases, the dispersion forces increases and which is why very long chain structures are solids (see figure 11.18 in S&S text) Preparation of alkane • Most common method of preparing an alkane is adding hydrogen (hydrogenation) to an alkene. – Example – The product of the rxn is obtained by adding a H atom to each C in the double bond and changing the double bond to a single bond. 12 Combustion of alkane • Combustion means to burn in air. • Hydrocarbons burn in air to produce carbon dioxide and water. – Example Halogenation of alkane • Halogenation is a substitution rxn. • One H atom of the alkane is replaced by one halogen atom. • The mechanism in halogenation of an alkane is called a free radical mechanism. • The mechanism has four steps: 13.