Chemistry 11. Unit 10 Organic Chemistry Part III Unsaturated and aromatic hydrocarbons

Transcription

1 Chemistry 11 Unit 10 Organic Chemistry Part III Unsaturated and aromatic hydrocarbons

2 2 1. Unsaturated hydrocarbons So far, we have studied the hydrocarbons in which atoms are connected exclusively by single bonds. They are called saturated hydrocarbons. There are many hydrocarbons that can be found in nature containing carbon-carbon double or triple bonds. They are called unsaturated hydrocarbons.

3 3 2. Alkenes Alkene is a class of hydrocarbons, which contain at least one carbon-carbon double bond. The double bond may appear either at the end (called terminal alkene) or in the middle (called internal alkene) of the carbon chain. To show a double bond graphically:

4 4 The naming rules for alkenes are similar to the rules for branched alkanes. (1) Identify the longest straight chain that contains the double bond. (2) Number the carbon atoms on the chain so that the double bond gets the smallest number. (3) Label all the branches accordingly. (4) Add the suffix ene to the parent chain.

5 5 Example: Name the following compounds.

6 6 If more than one C=C bond is present in the alkene molecule, the position of each one has to be indicated separately, plus a numeric prefix should be added before the ending ene. For example, the molecule should be named: 1,3-pentadiene Positions of C=C bonds # of C atoms in parent chain # of C=C bonds Ending of alkene

7 7 Example: Name the following compound.

8 8 3. Geometric isomers of alkenes When atoms are connected by a single bond, there is a large flexibility that groups of atoms can rotate freely about the C-C bond.

9 9 However, when the two C atoms are connected by a double bond, the groups can no longer twist freely but instead their geometries are frozen. This constraint gives rise to the geometric isomers or cis-trans isomers of alkenes, in which the molecule with two substituents in the same direction is referred to as cis, while that with the two substituents in opposite directions is referred to as trans.

10 10 There are 2 conditions for cis-trans isomerism: (1) A double bond is involved. (2) Two groups other than hydrogen are present. For example: There are two types of 2-butene. Two methyl groups are on the same side Two methyl groups are on opposite sides

11 11 Therefore, in order to name an alkene molecule correctly, both the location of the C=C bond and its geometry have to be specified. For example: trans-2-heptene cis-3-heptene

12 12 4. Alkynes In addition to forming a double bond, two neighboring carbon atoms could form a triple bond, and the resulting compound is called an alkyne. Its general molecular formula is C n H 2n 2. Similar to alkenes, the triple bond may be either terminal or internal. Alkyne molecules can be describe in either of the following ways. Please note that, other than alkenes, the triple bond unit is linear in alkyne molecules.

13 13 The naming rules for alkynes are similar to alkenes except the following: (1) The continuous longest chain must contain the carbon-carbon triple bond, and the numbering starts from the side that gives the triple bond the smaller number. Side chains are named accordingly. (3) The ending yne is added to the parent name.

14 14 For example: Name the following compounds.

15 15 If there is more than one carbon-carbon triple bond is present in the molecule, then each triple bond has to be labeled with an appropriate label, and the ending yne has to be modified by the numeric prefix. For example: The molecule should be named: 1,4-heptadiyne Positions of carbon-carbon triple bonds Length of carbon chain # triple bonds Ending for alkyne

16 16 Practice: Name the following compounds.

17 17 5. Benzene The double and triple bonds can appear in alternating patterns in a molecule, resulting in a conjugated system. For example: 1,3,5-hexatriene 1,3,5-heptatriyne When such a situation occurs for a cyclic hydrocarbon, the molecule is said to be aromatic.

18 18 A very well known aromatic compound is called benzene which has the formula of C6H6. There are two equivalent structures of benzene, called resonance structures, which can be drawn: Benzene can be considered as having 3 double bonds and 3 single bonds alternating in a sixmembered ring. In reality, however, all these bonds are of equal length and they behave equally.

19 19 Therefore, benzene ring can be drawn in two different ways: Simplified structure Accurate representation Benzene ring and other related aromatic compounds share the feature that they are planar. In other words, they are flat and two-dimensional.

20 20 Technically, aromatic compounds are defined as those cyclic, planar molecules with a ring of resonance bonds. Naphthalene Anthracene Phenanthrene Of course, not all flat ring molecules are aromatic. To decide whether a molecule is aromatic, the Hückel s rule has to be applied. (Out of syllabus!) In CH11, we only consider the aromatic molecules made of one or more benzene rings.

21 21 The naming of substituted benzenes follows the same rules as naming substituted cyclohexanes. For example: bromobenzene 1-bromo-2-chlorobenzene How about this?

22 22 The naming of naphthalene is rather restricted, and the labeling of carbons is fixed: For example: 1,3,6-trimethylnaphthalene 1-bromo-6,7-dichloro-4- fluoronaphthalene

23 23 The general properties of aromatic compounds: (1) They are in general non-polar and not soluble in water. (2) They are usually unreactive; and therefore they can be used as organic solvents in chemical synthesis. (3) Some are liquids at room temperature while others are solids. (4) They are volatile and carcinogenic!