Organic chemistry for medicine and biology students

Chem 2311

Chapter 6Alkylhalides (Substitution and Elimination)

By Prof. Dr.

Adel M. Awadallah

Islamic University of Gaza

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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°)

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•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

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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.

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

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Energy Diagram for SN2 reaction

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Nucleophilic substitution in tertiary alkylhalides

• The reaction occurs in two steps

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Energy Diagram for SN1 Reaction

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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

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The reaction with hydroxide ions • Primary alkylhalides

• Tertiary alkylhalides

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The Reaction with water

•Primary alkylhalides

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Tertiary alkylhalides

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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

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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

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Mechanisms of Elimination1) E2 mechanism

C C

H

Cl

C C

H

OH

carbocation

Nu:

- H+

+ HOH + Cl-

SN2

E2

+ OH-

+ Cl-

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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

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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

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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

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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

• Halons

• Bromine containing CFCsare called halons, and used to extinguish fire

• CBrClF2 (Halon–1211)

• CBrF3 (Halon-1301)

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