1 Chapter 11: Organic Compounds: Alkanes

2 ORGANIC COMPOUNDS: In 1828, Friedrich Wöhler first synthesized an organic compound from an inorganic source. This discredited the “vital force” theory.

3 Organic chemistry is the study of carbon containing compounds except elemental carbon (diamond, graphite, coal), CO 2, CO, carbonates (CO 3 2- group) and cyanides (CN - group) Inorganic chemistry studies the elements and everything else. The principle components of food, fuels, wood construction, and clothing are organic compounds.

4 An estimated 250,000 inorganic compounds have been identified, but more than 6 million organic compounds are known, and thousands of new ones are synthesized or isolated each year.

5 BONDING CHARACTERISTICS In carbon, the 2s and three 2p orbitals can mix to produce four new sp 3 hybrid orbitals.

6 An sp 3 orbital has a two-lobed shape, similar to the shape of a p orbital but with different-sized lobes. Each carbon-hydrogen bond in methane arises from an overlap of a C (sp 3 ) and an H (1s) orbital. The sharing of two electrons in this overlap region creates a sigma (σ) bond.

7 The four hybrid sp 3 orbitals allow carbon to form four bonds. When carbon is joined to four substituents (i.e. CH 4 ), the resultant configuration is tetrahedral in shape.

8 ISOMERISM Isomers: Compounds that have identical molecular formulas, but different arrangement of atoms. Structural isomers: A type of isomerism in which the atoms bond in different patterns. Ball-and-stick models of the isomers of C 2 H 6 O. Ethyl alcohol is a liquid at room temperature and completely soluble in water, whereas dimethyl ether is a gas at room temperature and only partially soluble in water.

9 Functional Groups: Unique reactive combination of atoms that differentiate organic compounds into classes. Examples: Except for alkanes, each functional group contains a multiple bond or at least one oxygen or nitrogen atom. Click here to play Chemistry Interactive Click here to play Coached Problem

10 REPRESENTING ORGANIC COMPOUNDS Expanded structural formulas show all atoms with bonds. Condensed structural formulas list all the atoms in order implying how they are bound together: CH 3 CH 2 CH 2 CH 3 or CH 3 (CH 2 ) 2 CH 3 Click here to play Chemistry Interactive

11 CLASSIFICATION OF HYDROCARBONS Hydrocarbons contain only carbon and hydrogen. A hydrocarbon that contains only single bonds is a saturated hydrocarbon or alkane. Unsaturated hydrocarbons are called alkenes, alkynes, and aromatics and contain double bonds, triple bonds, or ring systems with alternating double bonds. Click here to play Chemistry Interactive

12 Alkanes can be represented by the general formula C n H 2n+2, where the n is the number of carbon atoms in the molecule. The simplest alkane is methane (CH 4 ), which is the primary compound in natural gas. Ethane (C 2 H 6 ) is a minor component of natural gas. Propane (C 3 H 8 ) is used as a fuel for heating homes and cooking.

13 More complex alkanes can be straight chained (normal) or branched. C — C — C — C — C normal alkane C | C — C — C | C branched alkane

14 CONFORMATIONS OF ALKANES There is free rotation around C-C bonds. The different arrangements of atoms in space achieved by rotation about single bonds are called conformations.

15 Which of the following pairs represent structural isomers, and which are simply the same compound? Which are normal alkanes and which are branched alkanes?

16 NAMING ALKANES The IUPAC method consists of: Click here to play Presentation Simulation

17 NAMING ALKANES, CONT. Step 1: Identify and name the longest carbon chain. This gives the root and ending. (The ending –ane signifies the alkane family.) Step 2: Number the longest carbon chain to give the lowest number to any carbon to which a group is attached. Step 3: Locate and name the attached alkyl groups. Step 4: Combine the longest chain and the branches into the name. Example: CH 3 5 CH 2 — CH 2 — CH — CH 3 (pentane) || methylpentane

18

19 Step 5: For multiple branches, show the location of each branch with numbers. For multiple branches of the same type, modify the name with di-, tri-, tetra-, penta-, etc. and separate the position numbers by commas. List multiple branches alphabetically. Ignore the di-, tri-, sec-, and t- prefixes. | 1 CH 3 — 2 CH — 3 CH — 4 CH — 5 CH 2 — 6 CH 2 — 7 CH 3 || CH 3 CH 3 CH–CH 3 CH 3 | 4-isopropyl-2,3-dimethylheptane Example: Click here to play Coached Problem

20 NAMING CYCLOALKANES Cycloalkanes are alkanes containing rings of carbon atoms. The prefix cyclo- is used before the alkane name. When two or more substituents are attached to the cycloalkanes, the ring numbering begins with the first group alphabetically and proceeds to give lowest numbers possible. Example: CH 2 CH CH 3 1-ethyl-3-methylcyclopentane

21 Click here to play Coached Problem THE SHAPE OF CYCLOALKANES Stereoisomers are compounds with the same structural formula but different spatial arrangements of atoms. Geometric isomers are molecules with restricted rotation around C-C bonds that differ in the three-dimensional arrangements of their atoms in space and not in the order of linkage of atoms.

22 Rotation about C-C single bonds occurs in open-chain compounds but not within rings. Geometric isomerism can result in two geometric isomers of 1,2-dimethylcyclopentane. Cis-substituents on the same side. Trans-substituents on the opposite side. Click here to play Chemistry Interactive Click here to play Coached Problem

23 PHYSICAL PROPERTIES OF ALKANES Non-polar molecules with weak intermolecular forces Not soluble in water (hydrophobic) Low density (less dense than water) Melting points increase with molecular size Boiling points increase with molecular size

24 Click here to play Chemistry Interactive Click here to play Coached Problem

25 ALKANE REACTIONS Alkanes are the least reactive of all organic compounds. The most significant reaction of alkanes is combustion (rapid oxidation). Many alkanes are used as fuels. Methane – natural gas Propane – used in gas grills Butane – lighters Gasoline – a mixture of hydrocarbons

26 Complete Combustion (in the presence of adequate oxygen) Incomplete Combustion (not enough oxygen available)