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Organic chemistry A Chapter 5 Alkylhalides (Substitution) By Prof. Dr. Adel M. Awadallah

Organic chemistry A Chapter 5 Alkylhalides (Substitution) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza. Alkyl Halides (Substitution) alkyl halides = Halogenoalkanes = haloalkanes contain a halogen atom - fluorine, chlorine, bromine or iodine - attached to an alkyl

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Organic chemistry A Chapter 5 Alkylhalides (Substitution) By Prof. Dr. Adel M. Awadallah

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  1. Organic chemistry A Chapter 5 Alkylhalides (Substitution) By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

  2. Alkyl Halides (Substitution) • 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 CH3Br • Secondary (2°) Tertiary (3°)

  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

  4. Preparation of Alkyl Halides

  5. Reactions of Alkyl HalidesNucleophilic Aliphatic Substitution

  6. Nucleophilic substitution in primary alkyl halides • NucleophilesA 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.

  7. SN2 reaction (Substitution Nucleophilic bimolecular)The reaction is a one step reaction

  8. Energy Diagram for SN2 reaction

  9. Stereochemistry of SN2 ReactionsInversion of Configuration

  10. Kinetics of SN2 Reactions

  11. SN2 Reactions . Steric Hindrance

  12. Differences in rate between two SN2 reactions are duechiefly tosteric factors.Example: all substrates are primary, but there is a difference in steric hindrance

  13. The SN1 Reaction, Kinetics

  14. Energy Diagram for SN1 Reaction

  15. Nucleophilic substitution in tertiary alkylhalides • The reaction occurs in two steps

  16. The SN1 Reaction, Carbocations

  17. The SN1 Reaction, Structure of Carbocation

  18. The SN1 Reaction, Stereochemistry

  19. The SN1 Reaction, stability of carbocations

  20. The SN1 Reaction, Rearrangement

  21. Nucleophilic substitution in secondary alkylhalides The reaction occurs in both mechanisms depending on : • Nucleophile strength (increases SN2) • Solvent polarity (aprotic solvents increases SN2) • Strength of nucleophiles • A) Charged nucleophiles are stronger than neutral ones • OH- > H2O , EtO- > EtOH, NH2- > NH3 • B) Nucleophile strength increases by going to the left in a period, or down in a group of the periodic table • NH3 > H2O, NH2- > OH- (period) • H2S > H2O, SH- > OH- (group)

  22. 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.

  23. Polyhalogenated aliphatic compounds • CH3Br (methyl bromide) • Methyl Bromide is a highly effective fumigant used to. control a number of pests including insects • CH2Cl2 (dicholromethane or methylene chloride) • solvent • CHCl3 (Chloroform) • Solvent and was used as an anesthetic • (CF2-CF2)n Teflon • Chloroflouorocarbons (CFC) • CCl2F2 (Freon 12, CFC – 12) refrigerating gas

  24. Halons • Bromine containing CFCsare called halons, and used to extinguish fire • CBrClF2 (Halon–1211) • CBrF3 (Halon-1301)

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