Chapter 10 Introduction to Organic Chemistry: Alkanes 10.2 Alkanes Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Structural Formulas Alkanes are written with structural formulas that are expanded to show each bond. condensed to show each carbon atom and its attached hydrogen atoms. Expanded Condensed H H C H CH4 , methane In methane, CH4 the four valence electrons of carbon are shared with the single electrons of four hydrogen (H) atoms. Each pair of electrons is a single bond, which can be drawn as a line. When a structure is drawn to show each bond, it is called a complete structural formula.

Expanded and Condensed Structures

Structural Formulas Condensed formulas are written for expanded structural formula by showing each carbon and the attached hydrogen atoms. Expanded Condensed H H H H │ │ │ │ H─C ─C ─C ─C ─ H CH3─CH2─CH2─CH3

Names of Alkanes The names of alkanes are determined by the IUPAC (International Union of Pure and Applied Chemistry) system. end in –ane. with 1-4 carbons in a chain use prefixes as follows. Name # Carbons Structural Formula Methane 1 CH4 Ethane 2 CH3CH3 Propane 3 CH3CH2CH3 Butane 4 CH3CH2CH2CH3

Names of Alkanes Alkanes with 5-10 carbon atoms in a chain use Greek prefixes. Name # Carbons Structural Formula Pentane 5 CH3CH2CH2CH2CH3 Hexane 6 CH3CH2CH2CH2CH2CH3 Heptane 7 CH3CH2CH2CH2CH2CH2CH3 Octane 8 CH3CH2CH2CH2CH2CH2CH2CH3 Nonane 9 CH3 CH2 CH2CH2CH2CH2CH2CH2CH3 Decane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Learning Check A. Write the condensed formula for: H H H H H H C C C C C H H H H H H B. What is its molecular formula? C. What is its name?

Solution A. CH3─CH2─CH2─CH2─CH3 B. C5H12 C. pentane

Writing Structural Formulas Carbon atoms in a chain maintain tetrahedral shape. are connected in a zig-zag pattern. are drawn as 2-dimensional. Ccan be written in several conformations.

Some Structures for Butane

Hexane has Six Carbon Atoms is an alkane with six carbon atoms in a continuous chain. has a “zig-zag” look because each carbon atom is at the center of a tetrahedron. is represented by a ball-and-stick model as shown below.

Learning Check Write the condensed structural formula for A. ethane. B. heptane.

Solution Write the condensed structural formula for A. ethane CH3─CH3 B. heptane CH3─CH2─CH2─CH2─CH2─CH2─CH3

Cycloalkanes Cycloalkanes are cyclic alkanes. have two hydrogen atoms fewer than the open chain. are named by using the prefix cyclo- before the name of the alkane chain with the same number of carbon atoms.

Cycloalkanes The structural formulas of cycloalkanes are usually represented by geometric figures, Cyclopropane CH2 CH2 CH2 Cyclobutane CH2 CH2 CH2 CH2

More Cycloalkanes Cyclopentane CH2 CH2 CH2 CH2 CH2 Cyclohexane CH2

Learning Check Name the following. A. CH3─CH2─CH2─CH3 B. C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 D.

Solution Name the following: A. CH3─CH2─CH2─CH3 butane B. cyclopropane C. CH3─CH2─CH2─CH2─CH2─CH2─CH2─CH3 octane D. cyclohexane