By: Dr. Siham Lahsasni 1 Unsaturated Hydrocarbons 1 Alkenes

 Unsaturated hydrocarbons that contain one or more double carbon-carbon bonds.hydrocarbons  Alkenes with just one double C-C bond form an homologous series with the general formula C n H 2n.  The first few in this series are ethene, propene, butene, and pentene.ethene 2

HYBRIDISATION OF ORBITALS - ALKENES 2s 2 2p 2 2s 1 2p 3 3 x sp 2 2p The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s 2 2s 1 2p 3 Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.

Bonding in Ethylene : Bond length =1.34 Å with the atoms separated by bond angles of 120°. 4 Hybridization Sp2

Nomenclature of alkenes 5 1. The ene suffix indicates an alkenes or cycloalkenes. 2. The longest chain chosen for the root name must include both carbon atoms of the double bond. 3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts. 4. In cycloalkenes the double bond carbons are assigned ring locations C1 and C2. Which of the two is C1 may be determined by the nearest substituent rule. 5. Substituent groups containing double bonds are: H 2 C=CH– Vinyl group H 2 C=CH–CH 2 – Allyl group

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Geometric (cis-trans) isomerism 7 The prefix cis- is used when the two arms of the longest chain are on the same side of the double bond. The prefix trans- is used when they are on opposite sides of the double bond.

E-Z NOTATION FOR GEOMETRIC ISOMERISM 8 If the two groups with the higher molecular weight are on the same side of the double bond, that is described as the (Z)- isomer. So you would write it is (Z)-name of compound. If the two groups with the higher molecular weight are on opposite sides of the double bond, then this is the (E)- isomer. E Z

Physical Properties of Alkenes 9 Alkenes are non polar compounds. Insoluble in water. Soluble in non polar organic solvents. They are less dense than water. Range of physical states: ≤ 4 C's are gases C's are liquids ≥ 18 C's are solids The alkenes has a boiling point which is a small number of degrees lower than the corresponding alkanes.

Preparation of alkenes 1- Dehydration of alcohols: 10

11 Saytzeff s Rule: In every instance in which more than one alkene can be formed, the major product is the alkene with the most alkyl substituents attached on the double bonded carbon.

2- Dehydrohalogenation of Alkyl Halides 12

Reaction of alkenes Additions to the Carbon-Carbon Double Bond 1-1- Addition of hydrogen: Hydrogenation (reduction)

Addition of Halogens: Halogenation

1-3- Addition of Hydrogen Halides: Hydrohalogenation 15

Markonikov’s rule: In addition of HX to unsymmetrical alkenes the hydrogen halide adds to the double bonded carbon that bears the greater number of hydrogen atoms and the negative halide ion adds to the other double bonded carbon. 16

1-4- Addition of HOX: Halohydrin formation 17

Addition of H 2 O: Hydration 18

Addition of HCN

2- OZONOLYSIS 20

21 3- Oxidation 4- Epoxidation