Formal Report

Topics: SN2 reaction, Nucleophilic substitution, SN1 reaction Pages: 24 (4632 words) Published: August 26, 2013
Alkyl Halides Alkyl halides are a class of compounds where a halogen atom or atoms are bound to an sp3 orbital of an alkyl group. CHCl3 (Chloroform: organic solvent) CF2Cl2 (Freon-12: refrigerant CFC) CF3CHClBr (Halothane: anesthetic) Halogen atoms are more electronegative than carbon atoms, and so the C-Hal bond is polarized. H H μ

C + C-l δ δ H

The C-X bond is polarized in such a way that there is partial positive charge on the carbon and partial negative charge on the halogen. Dipole moment, μ = 4.8 x δ x d δ = charge d = bond length. Electronegativities decrease in the order of: F > Cl > Br > I Carbon-halogen bond lengths increase in the order of: C-F < C-Cl < C-Br < C-I Bond Dipole Moments decrease in the order of: μ= C-Cl > 1.56D C-F > 1.51D C-Br > 1.48D C-I 1.29D

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Typically the chemistry of alkyl halides is dominated by this effect, and usually results in the C-X bond being broken (either in a substitution or elimination process). This reactivity makes alkyl halides useful chemical reagents. Nomenclature According to IUPAC, alkyl halides are treated as alkanes with a halogen substituent. The halogen prefixes are Fluoro-, Chloro-, Bromo- and Iodo-. Examples: H Cl CH3 H

F CH2CH3 fluoroethane

trans-1-chloro-3-methylcyclopentane

Often compounds of CH2X2 type are called methylene halides. (CH2Cl2 is methylene chloride). CHX3 type compounds are called haloforms. (CHI3 is iodoform). CX4 type compounds are called carbon tetrahalides. (CF4 is carbon tetrafluoride). Alkyl halides can be primary (1°), secondary (2°) or tertiary (3°). H R H 1

o

R X R H 2
o

R X R R 3o X

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Other types: A geminal (gem) dihalide has two halogens on the same carbon. A vicinal dihalide has halogens on adjacent carbon atoms. Br Br R R R gem-dibromide Cl Cl R R R vicinal dichloride

Preparation of Alkyl Halides Numerous ways to make alkyl halides. (1a) Free Radical Halogenation Usually this method gives mixtures of mono-, di-, tri- etc halogenated compounds, which is considered an inefficient method for the synthesis of a desired compound. Consider propane: + Cl2 hν CH3-CH2-CH2Cl + CH3-CHCl-CH3 CH3-CH2-CHCl2 +CH3-CCl2-CH3 and others

Sometimes if there can be control over the selectivity of halogenation this is a useful route. H H + Cl2 hν H Cl

Chlorocyclohexane (50%)

CH3 H3C H CH3

+ Br2



CH3 H3C Br CH3 t-Butylbromide (90%)

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(1b) Allylic Bromination (Allylic means adjacent to a C=C double bond) The bromination of cyclohexene produces a high yield of 3bromocyclohexene. H H + Br2 hν (80%) H Br + H-Br

An allylic hydrogen has been substituted for a bromine. The bromine atom abstracts an allylic hydrogen because the allylic radical is resonance stabilized. H H H Br H -HBr H H H Br Br H Br H H Br H H H

The radical then reacts with a bromine molecule to continue the chain. O N-Br O + H-Br O N-H O + Br-Br

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A common reagent for these allylic brominations is N-bromosuccinamide (NBS) because it continually generates small amounts of Br2 through reaction with HBr. Other methods for Preparation (These will be covered in detail in appropriate later chapters). From alkenes and alkynes: H-X H X C C X X C C H C H X C X X C X X C X

C C

C C

X-X

C C

2 H-X

C C

2 X-X

From alcohols:
R OH H-X R X

From other halides:
R X' + X

R X

+ 'X-

Reactions of Alkyl Halides The alkyl halides are chemically versatile. The halogen atom may leave with its bonding pair of electrons to give a halide ion which is stable – a halide is called a good leaving group. If an atom replaces the halide the overall reaction is a substitution.

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If the halide loss is accompanied by the loss of another atom, the overall reaction is called an elimination. Very often the other atom lost is a hydrogen (as H+). The elimination of H-X is common, and is...
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