BRCC CHM 102 Class Notes Chapter 11 Page 1 of 9 Chapter 11 Alkanes and Cycloalkanes hydrocarbons compounds that contain only carbon and hydrogen * 4 families: 1) alkanes only single bonds (includes cycloalkanes) 2) alkenes C=C double bonds 3) alkynes C C triple bonds 4) aromatics alternating double and single bonds such as in benzene * saturated compounds contain only single bonds (cannot add more hydrogens) * unsaturated compounds contain one or more multiple bonds * Alkanes and cycloalkanes are called aliphatic hydrocarbons because they resemble molecules found in fats and oils. * Normal alkanes consist of a continuous chain without branching. Often the letter n precedes the name to indicate that it is normal. methane CH 4 ethane propane CH 2 n-butane CH 2 CH 2 n-pentane CH 2 CH 2 CH 2 n-hexane write out what this compound would be * boiling point increases as the number of carbon atoms increases * melting point increases as the number of carbon atoms increases * Branching of hydrocarbon chains is also possible. ex. isobutane -CH- * Branching makes for 4 types of carbon atoms primary, 1 attached to one other or no other carbon secondary, 2 attached to two other carbon atoms tertiary, 3 attached to three other carbon atoms quaternary, 4 attached to four other carbon atoms
BRCC CHM 102 Class Notes Chapter 11 Page 2 of 9 Determine the different types of carbons in these compounds: isomers of pentane, C 5 H 12 CH 2 CH 2 CH 2 normal pentane CH 2 CH isopentane C neopentane * Constitutional isomers structural isomers, which are compounds with the same molecular formula, but different structural formulas. * As the number of carbon atoms in a molecular formula increases, the number of isomers with that formula increases. * There is a way to calculate the number of isomers possible, but we will learn that in a later chapter. * The general formula for an alkane is C n H 2n+2, where n = number of carbons. isomers of hexane, C 6 H 14 How do you name all these? CH 2 CH 2 CH 2 CH 2 CH CH 2 CH 2 CH 2 CH CH 2 C CH 2 CH CH
BRCC CHM 102 Class Notes Chapter 11 Page 3 of 9 * How can you tell if structural formulas depict different compounds or are just another way of drawing the same compound? * Carbon atoms in alkanes have freedom of rotation around carbon-carbon single bonds. * They can spin around all day...which means that alkanes with carbon-carbon single bonds can exist in an infinite number of 3-D arrangements. * These 3-D arrangements are called conformations. However, these are NOT conformational isomers. These are changes from one conformation to another by rotating one or more carboncarbon single bonds in the same compound. Example: All of these are the same compound: CH 2 CH 2 CH CH CH 2 CH 2 CH 2 CH 2 CH CH 2 CH 2 CH CH CH 2 CH 2 CH CH 2 CH 2 Nomenclature of Alkanes The textbook has helpful practice problems. * Nomenclature is just a system for naming. In this case, it is the naming of organic compounds. * The system for naming organic compounds is called IUPAC. * IUPAC provides systematic names for compounds so that you don t have to memorize a huge list of Latin or Greek archaic names. * The IUPAC nomenclature names compounds in terms of functional groups.
BRCC CHM 102 Class Notes Chapter 11 Page 4 of 9 IUPAC Nomenclature uses the following prefixes to name organic compounds with 1 to 10 carbons. It is important that you memorize these. Prefix # of Carbon atoms Prefix # of Carbon atoms meth- 1 hex- 6 eth- 2 hept- 7 prop- 3 oct- 8 but- 4 non- 9 pent- 5 dec- 10 The Alkyl Group * The functional group associated with alkanes is called the alkyl group. * If these alkyl groups are unbranched, then they are called normal alkyl groups or n-alkyl groups. examples of normal alkyl groups: CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 methyl ethyl propyl or n-propyl butyl or n-butyl pentyl or n-pentyl * The symbol R is commonly used to represent an alkyl group. Recall the R groups on the functional group handout. * In addition to normal alkyl groups, there are important branched alkyl groups as well. * It is important that you know the branched alkyl groups on the next page.
BRCC CHM 102 Class Notes Chapter 11 Page 5 of 9 examples of branched alkyl groups: CH isopropyl CH CH 2 isobutyl CH 2 CH sec-butyl C tert-butyl Rules for Naming Alkanes 1. Find the longest continuous chain of carbon atoms. This is called the parent chain. Assign a name by adding a prefix from the table to the root ane. CH CH 2 CH 2 5 carbons = pentane 2. Identify groups that are not part of the parent chain. Name these as prefixes. CH CH 2 CH 2 methyl methylpentane
BRCC CHM 102 Class Notes Chapter 11 Page 6 of 9 3. Assign numbers to each of the carbons in the chain. Assign numbers to the groups starting at the end of the chain that gives the lowest numbers to the groups. 1 2 3 4 5 CH CH 2 CH 2 2 methylpentane 4. With two or more identical side groups, use the following prefixes: prefix # of groups prefix # of groups di- 2 hexa- 6 tri- 3 hepta- 7 tetra- 4 octa- 8 penta- 5 nona- 9 C CH 2 2,2 dimethylbutane CH CH 2,3 dimethylbutane CH CH CH CH 2 2,3,4 trimethylhexane 5. If there are two or more different groups, list them in alphabetical order. Prefixes and hyphenated prefixes are not included in alphabetizing. CH CH CH CH 2 CH 2 3 ethyl 2,4 dimethylhexane
BRCC CHM 102 Class Notes Chapter 11 Page 7 of 9 6. The four halogens are named by the following prefixes: F fluoro Cl chloro Br bromo I iodo CH CH 2 2 bromobutane Br * Always try to put the halogen on the parent chain rather than a side chain. This may mean that the parent chain has fewer carbons than if the halogen were not included. Common Names * The IUPAC nomenclature is the preferred way of naming organic compounds. However, common names are sometimes used. * Common names are used when the common name is in almost exclusive use by chemists in the real world. CH isobutane is used more often than 2 methylpropane Cycloalkanes * Cycloalkanes are hydrocarbons whose carbons join to form a ring. They are also called cyclic hydrocarbons. * Abbreviated structural drawings called line-angle drawings are commonly used to depict them. cyclopentane cyclohexane Naming Cycloalkanes 1. Add the prefix cyclo to the corresponding open chain alkane. 2. When there are side groups, number the carbons in the ring beginning with the position that gives the lowest possible numbers for the side groups.
BRCC CHM 102 Class Notes Chapter 11 Page 8 of 9 methylcyclopentane (no number is needed with only one side group) H 3 C 1,3 dimethylcyclohexane Br Br Br 1,1,3 tribromocyclooctane Stereoisomerism in Cyclic Compounds * Cyclic alkanes have no free rotation around the carbon-carbon bonds like openchain alkanes. This leads to Stereoisomerism. What are stereoisomers? * Stereoisomers are compounds that have the same structural formulas, but different 3-D shapes. They cannot be superimposed. * A common type of stereoisomerism for cyclic organic compounds is called cis-trans isomerism. cis two groups are on the same side. trans two groups are on opposite sides. cis trans
BRCC CHM 102 Class Notes Chapter 11 Page 9 of 9 Physical Properties A. Physical State * Alkanes are non-polar compounds. * Intermolecular forces are all London Dispersion Forces these are the weakest. * Alkanes of 1 to 4 carbons are gases at room temperature. * Alkanes of 4 to 17 carbons are liquids at room temperature. * Alkanes of greater than 17 carbons are soft, waxy solids. B. Solubility C. Density * Alkanes are soluble in non-polar solvents like acetone. * Alkanes are NOT soluble in polar solvent like water. * Alkanes are less dense than water and therefore float in water. * The density of alkanes is usually in the range 0.6 to 0.8 g/ml. Chemical Properties Chief Chemical Property * Alkanes are inert except that they burn. * Combustion is the most important reaction (chemical property) of alkanes. * Combustion is an oxidation reaction where an alkane reacts with O 2 to yield CO 2 and H 2 O. example C 3 H 8 + 5 O 2 ----------> 3 CO 2 + 4 H 2 O + Energy (heat) * Ignition point the temperature at which the oxidation of an alkane proceeds rapidly and exothermically. * Incomplete combustion burning is not complete so that the reaction yields a mixture of products, such as CO (carbon monoxide) and soot.