1. Organic chemistry for medicine and biology students Chem 2311 Chapter 6 Alkylhalides (Substitution and Elimination) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza 1

2. Alkyl Halides (Substitution and Elimination) alkyl halides = Halogenoalkanes = haloalkanes contain a halogen atom - fluorine, chlorine, bromine or iodine - attached to an alkyl The different kinds of halogenoalkanes Primary (1°) includes CH 3 Br Secondary (2°) Tertiary (3°) 2

3. The polarity of the carbon-halogen bonds Electro negativity values (Pauling scale) C = 2.5, F = 4.0, Cl = 3.0, Br = 2.8, I = 2.5 Strengths of various bonds (all values in kJ mol -1 ). C-H = 413 C-F = 467 (very polar, but very strong and difficult to break) C-Cl = 346 C-Br = 290 C-I = 228 (non polar, but it is polarized in the reaction by any nucleophile) Rates of reaction: RCl < RBr < RI 3

4. Nucleophilic substitution in primary alkyl halides Nucleophiles A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else. Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia. Notice that each of these contains at least one lone pair of electrons, either on an atom carrying a full negative charge, or on a very electronegative atom carrying a substantial - charge. 4

5. S N 2 reaction (Substitution Nucleophilic bimolecular) The reaction is a one step reaction 5

6. Energy Diagram for S N 2 reaction 6

7. Nucleophilic substitution in tertiary alkylhalides The reaction occurs in two steps 7

8. Energy Diagram for S N 1 Reaction 8

9. Nucleophilic substitution in secondary alkylhalides The reaction occurs in both mechanisms depending on : Nucleophile strength (increases S N 2) Solvent polarity (aprotic solvents increases S N 2) Strength of nucleophiles A) Charged nucleophiles are stronger than neutral ones OH - > H 2 O, EtO - > EtOH, NH 2 - > NH 3 B) Nucleophile strength increases by going to the left in a period, or down in a group of the periodic table NH 3 > H 2 O, NH2 - > OH - (period) H 2 S > H 2 O, SH - > OH - (group) 9

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12. The reaction with hydroxide ions Primary alkylhalides Tertiary alkylhalides 12

13. The Reaction with water Primary alkylhalides 13

14. Tertiary alkylhalides 14

15. Elimination Reactions Elimination (dehydrohalogenation) reactions of alkyl halides A hydrogen atom and a halogen atom from adjacent carbons are eliminated and a carbon –carbon double bond is formed Competition between substitution and elimination 15

16. Factors deciding substitution or elimination 1) Type of alkylhalide 2) The solvent Water encourages substitution, Ethanol encourages elimination. 3) The temperature Higher temperatures encourage elimination. 4) Concentration of the sodium or potassium hydroxide solution Higher concentrations favor elimination. Mainly substitutionprimary Both substution. and eliminationsecondary Mainly eliminationtertiary 16

17. Mechanisms of Elimination 1) E2 mechanism 17

18. Elimination from unsymmetric alkylhalides 18

19. 2) E1 Elimination 19

20. Elimination from unsymmetrical alkylhalides 20

21. Polyhalogenated aliphatic compounds CH 3 Br (methyl bromide) Methyl Bromide is a highly effective fumigant used to control a number of pests including insects CH 2 Cl 2 (dicholromethane or methylene chloride) solvent CHCl 3 (Chloroform) Solvent and was used as an anesthetic (CF 2 -CF 2 ) n Teflon Chloroflouorocarbons (CFC) CCl2F2 (Freon 12, CFC – 12) refrigerating gas 21

22. Halons Bromine containing CFCsare called halons, and used to extinguish fire CBrClF 2 (Halon–1211) CBrF 3 (Halon-1301) 22