Chapter 6 Adel M. Awadallah Islamic University of Gaza 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

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 CH3Br Secondary (2°) Tertiary (3°)

Rates of reaction: RCl < RBr < RI 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

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.

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)

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

Elimination from unsymmetrical alkylhalides

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

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