How To Understand The Chemistry Of Alkanes

Transcription

1 Ch 12. Alkanes Introduction to Organic Chemistry: Simple Hydrocarbons Diagram of Methane. Member of Alkane family of hydrocarbons. Alkanes have only single bonds. When carbon is bonded to four atoms, the bonds are oriented toward the four corners of a regular tetrahedron with carbon in the center. This is shown using normal lines for bonds in the plane of the page, dashed lines for bonds receding behind the page, and wedged lines for bonds coming out of the page. 1. Organic bonds are covalent, i.e arise from sharing of electrons between adjacent atoms 2. In Chloromethane, the carbon atom is attached to an Electronegative chlorine, so the bond has polar character (a polar covalent bond). 3. The Cl atom has a partial negative charge as it has a higher Electronegativity. 4. Chloromethane is an alkyl halide (alkane with a halogen atom). 1

2 In Propane, each C must be bonded to at least one other carbon forming the backbone of the compound. The hydrogens are on the periphery. Isomers Of Alkanes Constitutional Isomers of Alkanes: same molecular formula but Different connections between the atoms Isomers will be covered in more detail in lab. 2

3 Constitutional Isomers The number of constitutional isomers increase with number of carbon atoms. Constitutional isomers could be completely different compounds with Different structures and physical and chemical properties. Conformational Isomers Conformational Isomers or Conformers occurs in alkanes. These Structures have the same molecular formula and group connectivity But differ in spatial arrangement by the rotation of a single bond. Conformers are part of isomers generally classed as Stereoisomers (details on this topic covered in lab..) 3

4 Drawing Organic Structures Condensed structure: A shorthand way of drawing structures in which C-C and C-H bonds are understood rather than shown. Alkane Nomenclature by IUPAC standards All organic compounds under IUPAC have a 3 part naming system. For Alkane nomenclature: 1. Prefix: Except a few, they are derived from greek numbers (memorize table 12.3 of text) 2. Suffix: ane 4

5 Alkyl Groups Alkyl groups are parts of a hydrocarbon after removal of one H atom from the parent. The ane suffix is replaced by yl. Higher alkanes will have more branched chains and more branched alkyl groups than simpler alkanes. Ex: propyl and isopropyl ( from propane and isopropane) N-butyl, sec-butyl, tert-butyl etc depending on the branching. 5

6 4 possible substitutions. Primary (1 o ), Secondary or sec- (2 o ), Tertiary or tert- (3 o ) And quaternary (4 o ) Naming of branched chain alkanes 1. Name the Main Chain 2. Number the carbon atoms in the main chain. Begin at the end nearer the first branch point: 6

7 Naming of branched chain alkanes 3. Identify the number of the carbon atom in the main chain where branching occurs 4. In case of two substituents on the same carbon, the same number Is assigned to both substituent 4: Write the name as a single word, using hyphens to separate the numbers from the different prefixes and commas to separate numbers if necessary. If two or more different substituent groups are present, cite them in alphabetical order (using di, tri, tetra etc..) 7

8 Physical Properties of Alkanes Reactions of Alkanes 1. Combustion: An oxidation reaction that produces water and CO 2. Incomplete oxidation produces CO (carbon monoxide). This is a strongly exothermic reaction. Alkane + O 2 CO 2 + H 2 O + heat 2. Halogenation: This reaction is initiated by heat or light. 8

9 Cycloalkanes Cycloalkanes contain a ring of carbon atoms. Cyclopropane and cyclobutane are less stable and more reactive than other cycloalkanes since their bond angles are considerably compressed. Both cyclopentane and cyclohexane rings have nearly ideal bond angles and are therefore stable. Cyclic and acyclic alkanes are similar in many of their properties. 9

10 Naming Cycloalkanes cycloalkanes are represented simply by polygons. In line structures, a C is located at every intersection, and the number of H atoms necessary to give each C four covalent bonds is understood. Methylcyclohexane, for example, looks like this in a line structure: 10

11 Cycloalkanes are named by a straightforward extension of the rules for naming open-chain alkanes. In most cases, only two steps are needed: 1: Use the cycloalkane name as the parent. That is, compounds are named as alkyl-substituted cycloalkanes rather than as cycloalkyl-substituted alkanes. If there is only one substituent on the ring, it is not even necessary to assign a number because all ring positions are identical. 2: Identify and number the substituents. Start numbering at the group that has alphabetical priority, and proceed around the ring in the direction that gives the second substituent the lower possible number. 11

12 Topics covered 1. Introduction to Organic Chemistry: Alkanes 2. Isomerism in Alkanes 3. Nomenclature of Alkanes 4. Trends in b.p of Alkanes 5. Reactions of Alkanes 6. Cycloalkanes 7. Nomenclature of Cycloalkanes 8. Functional groups (memorization) Practice Identifying functional groups 12

13 What is the IUPAC name of this molecule? Is it a straight or a branched chain molecule? 13