1. 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

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 CH3Br
Secondary (2°) Tertiary (3°)

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 =
Strengths of various bonds (all values in kJ mol-1).
C-H =
C-F = (very polar, but very strong and difficult to break)
C-Cl =
C-Br =
C-I = (non polar, but it is polarized in the reaction by any nucleophile)
Rates of reaction: RCl < RBr < RI

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.

5. 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)

8. 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

9. Elimination from unsymmetrical alkylhalides

10. 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

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