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

6. Energy Diagram for SN2 reaction

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

8. Energy Diagram for SN1 Reaction

9. 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)

12. The reaction with hydroxide ions
Primary alkylhalides
Tertiary alkylhalides

13. The Reaction with water
Primary alkylhalides

14. Tertiary alkylhalides

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

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 substitution
primary
Both substution. and elimination
secondary
Mainly elimination
tertiary

17. Mechanisms of Elimination 1) E2 mechanism

18. Elimination from unsymmetric alkylhalides

19. 2) E1 Elimination

20. Elimination from unsymmetrical alkylhalides

21. 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

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