Form 5 Chemistry – ORGANIC CHEMISTRY 1

Home Work:

Anti – markovnikov’s rule (organic peroxide HBr). In 1933 the American chemist M. S. Kharasch discovered that the addition of HBr to unsymmetrical alkenes in the presence of organic peroxide (R – O – O – R) takes a course opposite to that suggested by Markovnikovs rule.

 

NOTE:

It is strictly works using HBr with organic peroxide.

Mechanism:

1.     Peroxide dissociates to give alkoxy free radicals.

     Weekly Test:

1.   (b) N_2(g)   + 3H_2(g)          2NH_3(g)

Given ∆H_r = ^– 92KJ

Since H_f you form 1mole

H_f  =

        =

3. Halogenation

Addition of halogens to alkenes

–          This reaction is best carried out by simply mixing halogens in the inert solvents such as carbon tetrachloride (CCl₄)

E.g.

The reaction of Bromine with alkene can be used to test for the presence of double bonds.

Reason:

Addition of bromine (brown) to alkene since it makes the solution colourless. This is how we test for presence of double bonds.

Bromine tests unsaturation of hydrocarbons (alkenes and alkynes).

4. HYDROGENATION

Addition of hydrogen:

–          Alkenes react with hydrogen in the presence of platinum or nickel catalyst to form alkanes.

 

  This cannot take place without catalyst.

5. Hydration of Alkenes:

–          This follows Markrnkov’s rule.

Homework on hydration of alkenes

–          Find the reagent which should be added to alkene to follow Anti markornikovs rule.

Oxidation reaction of alkenes

6. Addition of Bromine water (Br₂/H₂O)

      

Alkenes decolourize Bromine water (Brown) apart from decolorizing Bromine solution.

Reaction with conc H₂ SO₄

Structure of sulphuric acid

7. Oxidation reactions of Alkenes

_–                     Alkenes react with oxidizing agent to form diols. Oxidizing agent can be either KMnO_4, K₂CrO₄.

–          With cold dilute KMnO₄ or cold alkalineKMnO₄ you form diols.

                                                                  

NOTE: Diol means two OH ^–

(b) When hot concentrated acidified KMnO₄ or K₂Cr₂O₄ is used. Alkenes are oxidized to carboxylic acid or oxidized to ketones or both.

Note:

If double bond is branched, you can’t form carboxylic acid.

Terminal alkene

OZONOLYSIS (O₃)Ozono     ozone

Lysis      Breaking

–          This is a cleavage or breaking of carbon, carbon double bond by using ozone.

–          In ozonolysis C = C is completely broken to produce aldehydes or ketones or both depending on the primary structure of alkene.

NOTE:

Ozonolysis is the best method of locating the position of double bond in unknown alkene.

The oxygenated carbon in carbonyl compound obtained by ozonolysis is the one that were joined by double bonds in the original alkenes.

Ozonolysis has 2 major steps:

i)                    1^st step:

NOTE:

Zn dust is added to prevent oxidation of aldehyde to carboxylic acid.

E.g.

                                                         

 Questions

1.     A certain compound A was unsaturated hydrocarbon with molecular formula C₆ H₁₂. During ozonolysis of A, two compounds C and D were formed with molecular formula C₃H₆O. Compound C and compound D was ketone. Identify A, C and D.

Solution: