Main classes of organic chemistry reactions.pptx

Organic chemistry reactions

1. Main classes of organic reactions Main organic reactions are: ( i ) addition reaction, (ii) elimination reaction and (iii) substitution reaction. All of these reactions may take place in either ( i ) polar mechanism, or through (ii) free radical mechanism. Polar reactions may be electrophilic or nucleophilic.

2. Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymetrical Symmetrical- homolytic Unsymmetrical- heterolytic

3. Not as common as polar reactions Radicals react to complete electron octet of valence shell A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule A radical can add to an alkene to give a new radical, causing an addition reaction Free Radical Reactions

4. Three types of steps Initiation – homolytic formation of two reactive species with unpaired electrons Example – formation of Cl atoms form Cl 2 and light Propagation – reaction with molecule to generate radical Example - reaction of chlorine atom with methane to give HCl and CH 3 . Termination – combination of two radicals to form a stable product : CH 3 . + CH 3 .  CH 3 CH 3 Steps in Radical Substitution

5. Molecules can contain local unsymmetrical electron distributions due to differences in electronegativities This causes a partial negative charge on an atom and a compensating partial positive charge on an adjacent atom The more electronegative atom has the greater electron density Elements such as O, F, N, Cl more electronegative than carbon Polar Reactions

6. Rearrangement reactions – a molecule undergoes changes in the way its atoms are connected Nucleophilic Substitution Reaction Reaction Reaction

7. Nucleophilic addition reactions: Electrophilic addition reactions:

8. Addition reactions – two molecules combine

9. Addition of Cl 2 and Br 2 carried out with either the pure reagents or in an inert solvent such as CH 2 Cl 2 addition of bromine or chlorine to a cycloalkene gives a trans - dihalocycloalkane addition occurs with anti stereoselectivity ; halogen atoms add from the opposite face of the double bond

10. Racemic mixture

11. Addition of HX : Carried out with pure reagents or in a polar solvent such as acetic acid Addition is regioselective Markovnikov’s rule: in the addition of HX, H 2 O, or ROH to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens

12. On similar reduction other hydrocarbons are produced. CH 2 = CH 2 +H 2 Ethane CH 3 CH= CH 2 +H 2 Propane CH 3 CH= CH CH 3 +H 2 Butane

13. Addition of H 2 O addition of water is called hydration acid-catalyzed hydration of an alkene is regioselective ; hydrogen adds preferentially to the less substituted carbon of the double bond HOH adds in accordance with Markovnikov’s rule

14.  - Elimination : removal of atoms or groups of atoms from adjacent carbons to form a carbon-carbon double bond a type of b -elimination called dehydrohalogenation (the elimination of HX)

15. Saytzeff rule: the major product of a elimination is the more stable (the more highly substituted ) alkene

16. Substitution reactions – parts from two molecules exchange Free Radical Substitution Reaction Electrophilic Substitution Reaction

17. Nucleophilic reactions: nucleophilic substitution (S N ) Nucleophilic substitution: -> reagent is nucleophil -> nucleophil replaces leaving group -> competing reaction (elimination + rearrangements) in the following general reaction, substitution takes place on an sp 3 hybridized (tetrahedral) carbon tert -butyl bromide tert -butyl alcohol

18. 21 Nucleophilic Substitution Some nucleophilic substitution reactions

19. S N 1 reaction: unimolecular nucleophilic substitution S N 1 is illustrated by the solvolysis of tert -butyl bromide Step 1: ionization of the C-X bond gives a carbocation intermediate Step 2: reaction of the carbocation (an electrophile) with aq. NaOH (a nucleophile) gives an alcohol

20. Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion Step 3: proton transfer completes the reaction + A racemic mixture Cl C 6 H 5 C 6 H 5 C OCH 3 H CH 3 O C H Cl (R)-Enantiomer (S)-Enantiomer

21. 24 An energy diagram for an S N 1 reaction S N 1

22. S N 2 reaction: bimolecular nucleophilic substitution both reactants are involved in the transition state of the rate-determining step the nucleophile attacks the reactive center from the side opposite the leaving group An energy diagram for an S N 2 reaction there is one transition state and no reactive intermediate