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Organic chemistry for medicine and biology students
Chem 2311
Chapter 6Alkylhalides (Substitution and Elimination)
By Prof. Dr.
Adel M. Awadallah
Islamic University of Gaza
1
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°)
2
•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
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
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)9
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
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.
primaryMainly substitution
secondaryBoth substution. and elimination
tertiaryMainly elimination
16
Mechanisms of Elimination1) E2 mechanism
C C
H
Cl
C C
H
OH
carbocation
Nu:
- H+
+ HOH + Cl-
SN2
E2
+ OH-
+ Cl-
17
Elimination from unsymmetric alkylhalides
C C CC
H
Br
HH
CH2 = CH - CH2 - CH3
CH3 - CH = CH - CH3
:OH :OH
1 21
2
more substituted alkene will form betterNote: cis and trans isomers of this compound will form
18
2 (E1 Elimination
C C
H
Cl C C
H
C C
HC C
H
Cl
+slow
+ Cl-
substrate carbocation
+
carbocation
Nu:
- H+
+ H+
SN1
E1
first step: carbocation formation
Second step: Substitution or elimination
19
Elimination from unsymmetrical alkylhalides
C C CC
H
Br
HH
CH2 = CH - CH2 - CH3
CH3 - CH = CH - CH3
:OH :OH
1 21
2
more substituted alkene will form betterNote: cis and trans isomers of this compound will form
20
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 gas21