Unsaturated Hydrocarbons And Their Halogen Derivatives

IUG, Fall 2012Dr Tarek Zaida

Alkenes and Alkynes

1. Alkenes are compounds containing carbon –carbon double bonds

• The simplest alkene, ethene, is a plant hormone.• Alkenes have physical properties similar to those

of alkanes . • They are less dense than water and, being

nonpolar, are not very soluble in water. • As with alkanes, compounds with four or fewer

carbons are gases, whereas higher homologs are volatile liquids.

2. Alkynes,• compounds containing carbon–carbon triple

bonds,• are similar to alkenes in their physical

properties and chemical behavior.

General structural formula of Alkene and Alkynes

• Both of these classes of hydrocarbons are unsaturated, because they contain fewer hydrogens per carbon than alkanes (CnH2n+2).

• Alkanes can be obtained from alkenes or alkynes by adding 1 or 2 moles of hydrogen.

What are all of the structural possibilities for the compound C3H4?

Nomenclature

• The IUPAC rules for naming alkenes and alkynes are similar to those for alkanes but a few rules must be added for naming and locating the multiple bonds.1. The ending -ene is used to designate a carbon–carbon double bond. When more than one double bond is present, the ending is -diene, -triene, and so on. The ending -yne is used for a triple bond (-diyne for two triple bonds and so on). Compounds with a double and a triple bond are -enynes.2. Select the longest chain that includes both carbons of the double or triple bond.

For example,

3. Number the chain from the end nearest the multiple bond so that the carbon atoms in that bond have the lowest possible numbers.

If the multiple bond is equidistant from both ends of the chain, number the chain from the end nearest the first branch point.

4. Indicate the position of the multiple bond using the lower numbered carbon atom of that bond. For example,

5. If more than one multiple bond is present, number the chain from the end nearest the first multiple bond.

If a double and a triple bond are equidistant from the end of the chain, the double

• bond receives the lowest numbers. For example,

• The root of the name (eth- or prop-) tells us the number of carbons, and the ending (-ane, -ene, or -yne) tells us whether the bonds are single, double, or triple. No number is necessary in these cases, because in each instance, only one structure is possible.

• With four carbons, a number is necessary to locate the double or triple bond.

Branches are named in the usual way

Cycloalkenes

Reactions of Alkenes

1. Hydrogenation:

Same with cycloalkenes

19

2. Addition of Halogens to AlkenesX2 = Cl2 and Br2

C C C C

XX

1,2-dihalidealkene

X2

+ Br2

Br

Br

+

Br

Br

not observed

CH3 CH3Br

BrH

Br2

Polymers

Large organic molecules formed from joining similar smaller molecules together.The process is called polymerization.

Types of polymers

1. Naturally occurring:Proteins,Starch,CelluloseRubberIsoprene (present in natural rubber)

2. Synthetic polymers:PlasticNylonRayon (from cellulose)Dacron (polyester)

Medical Uses of Polymers

1. Synthetic heart valves2. Blood vessels3. Surgical mesh4. Disposable syringes5. Drug containers

Because rotation at carbon–carbon double bonds is restricted, cis–trans isomerism (geometric isomerism) is possible in appropriately substituted alkenes. For example, 1,2-dichloroethene exists in two different forms:

Cis–Trans Isomerism

• Cis-trans isomerism is possible for ring structure too:

Draw the structures of the cis-1,2-dimethylcyclopropane & trans-1,2-dimethylcyclopropane

• Cis-trans occurs in fatty acids• A biologically active cis-trans isomer is

Cisplatin, a chemotherapeutic agent used in the treatment of cancer.

Examples of biologically important Alkenes

• Ethene• Linolenic acid• Arachidonic acid• β-carotene• Vitamin A• Isoprene