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Summary Alkyl Halides and Aryl Halides
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Alkyl Halides & Aryl HalidesVictor Grignard
François Auguste Victor Grignard (1871 - 1935) was a Nobel Prize-winning French
chemist.
He is most noted for devising a new method for creating carbon-carbon bonds (i.e. an
addition reaction) in organic synthesis (Original publication: V. Grignard, Compt. Rend.
Vol. 130, p. 1322 (1900). The synthesis occurs in two steps:
1. Synthesis of the Grignard reagent: an organomagnesium compound (the Grignard
reagent) is made reacting an organohalide (R-X, where R stands for some alkyl, acyl,
or aryl radical and X is a halogen such as usually bromine or iodine) with magnesium
metal dissolved in diethyl ether. The resulting compound, named a Grignard reagent,
has the general chemical formula R-Mg-X.
2. Attack on the carbonyl: A ketone or an aldehyde (both contain a carbonyl group) is
added to the solution containing the Grignard reagent. The carbon atom that is bonded
to the Mg atom bonds to the carbonyl carbon atom by nucleophilic addition, with the
formation of a new compound, which is an alcohol.
The Grignard reaction is an important means of making larger organic compounds from
smaller starting materials. By careful selection of the starting materials, a wide variety of
compounds can be made by this reaction. For this work, Grignard was awarded the Nobel
Prize in Chemistry in 1912 jointly with fellow Frenchman Paul Sabatier.
Introduction
Alkyl Halides are compounds in which a halogen atom is attached to carbon. For example,
H H H
H C Cl H C C Br
H H H
Methyl chloride Ethyl bromide
They have the general formula
R X or C X
Where R - alkyl group; X = Cl, Br, I or F. The halogen atom bonded to carbon is the
functional group of alkyl halides.
59
, Alkyl halides are classified as Primary (1°), Secondary (2°), or Tertiary (3°),
depending upon whether the X atom is attached to a primary, secondary, or a tertiary
carbon.
H Primary carbon R Secondary carbon R Secondary carbon
RCX RCX RCX
H H R
1° Alkyl Halide 2° Alkyl Halide 3° Alkyl Halide
Alkyl halides are among the most useful organic compounds. They are frequently used
to introduce alkyl groups into other molecules.
5.1 Structure
Let us consider methyl chloride (CH3Cl) for illustrating the orbital make up of alkyl
halides in methyl chloride, the carbon atom is sp3 hybridized. The chlorine atom has a
half-filled p orbital in valence shell. The C Cl bond is formed by the overlap of an sp3
orbital of carbon and the half-filled p orbtial of chlorine atom shown in figure. Each C H
bond is formed by the overlap of an sp3 orbital of carbon
H
H
sp3-p
s-sp3 C Cl C
H Cl
H H
109°
H
Figure: Orbital structure of Methyl chloride
and the s orbital of hydrogen. All bonds are bonds. The H C H and H C Cl bond
angles are approximately tetrahedral.
5.2 Nomenclature
Alkyl halides are named in two ways
(1) Common system: In this system the alkyl group attached to the halogen atom is
named first. This is then followed by an appropriate word chloride, bromide, or
fluoride. Notice that the common names of alkyl halides are TWO-WORD names.
Br
CH3 Br CH3CH2 Cl CH3 CH CH3
Methyl bromide Ethyl chloride Isopropyl bromide
(2) IUPAC system: The IUPAC names of alkyl halides are obtained by using the
following rules:
60
, (a) Select the longest carbon chain containing the halogen atom and name the alkyl
halides as a derivative of the corresponding hydrocarbon.
(b) Number the chain so as to give the carbon carrying the halogen atom the lowest
possible number.
(c) Indicate the position of the halogen atom by a number and by the fluoro-,
chloro-, bromo- or iodo-.
(d) Name other substituents and indicate their positions by numbers. The examples
given below show how these rules are applied. Notice that the IUPAC names of
alkyl halides are ONE-WORD names.
Br
CH3 Br CH3CH2 Cl CH3 CH CH3
Methyl bromide Ethyl chloride Isopropyl bromide
5.3 Methods of Preparation
Alkyl halides can be prepared by the following methods:
(1) Halogenation of Alkanes: Alkanes react with Cl2 or Br2 in the presence of UV light
or at high temperature (400°C) to give alkyl halides along with polyhalogen
derivatives.
CH4 CH3Cl CH2Cl2 CHCl3 CCl4
2Cl
UV light
This method is not used in the laboratory because of the difficulty of separating the
products.
(2) Addition of Halogen Acids to Alkenes: Halogen acids (HCl, HBr, HI) add to alkenes
to yield alkyl halides. The mode of addition follows Markovnikov rule, except for the
addition of HBr in the presence of organic peroxides (R O O R).
X
R CH = CH R + HX R CH2 CH R
2-Alkene Alkyl halide
CH2 = CH2 + HI CH3 CH2 I
Ethylene Ethyl iodide
Br
R CH = CH2 + HBr R CH CH3
1-Alkene
Br
CH3 CH = CH2 + HBr CH3 CH CH3
Propene 2-Bromopropane
(Markovnikov product)
peroxide
CH3 CH = CH2 + HBr CH3 CH2 CH2Br
Propene 2-Bromopropane
(anti-Markovnikov product)
(3) Action of Halogen Acids on Alcohols. Alcohols react with HBr or HI to produce
alkyl bromides or alkyl iodides. Alkyl chlorides are produced by the action of dry HCl
in the presence of zinc chloride catalyst.
61
, R OH + H X R X + H2O
Alcohol Alkyl halide
ZnCl2
CH3CH2OH + HCl CH3CH2Cl + H2O
Ethyl alcohol Ethyl chloride
CH 3 CH 2 OH HBr CH 3 CH 2 Br H 2 O
propyl
CH3CHn2OH + alcohol
HBr n propyl
CH 3CH2CH2 Br + H2O
bromide
(4) Action of n-Propyl alcohol n-Propyl bromide
Phosphorus Halides on Alcohols. Alcohols react with phosphorus halides
(PX5 or PX3) to form alkyl halides.
R OH + PX5 (or PX3) RX
Alcohol Alkyl halide
2CH3CH2OH + PCl5 2CH3CH2Cl + POCl3 + H2O
Ethyl alcohol Ethyl chloride
3CH3CH2OH + PBr3 3CH3CH2Br + H3PO3
Ethyl bromide
3CH3OH + PI3 3CH3I + H3PO3
Methyl iodide
PBr3 or PI3 are produced in situ by the addition of Br2 and I2 to red phosphorus.
2P 3Br2
2PBr3
2P 3I2
2PI3
(5) Action of Thionyl chloride on alcohols. Alcohols react with thionyl chloride (SOCl2)
in the presence of pyridine to produce alkyl chlorides. Pyridine (C5H5N) absorbs
hydrogen chloride as it is formed.
pyridine
R OH + SOCl2 R Cl + SO2 + HCl
Alcohol Thionyl Alkyl chloride
chloride
pyridine
CH3CH2OH + SOCl2 CH3CH2Cl + SO2 + HCl
Ethyl Alcohol Ethyl chloride
(6) Halogen Exchange reaction: This reaction is particularly suitable for preparing alkyl
iodides. The alkyl bromide or chloride is heated with a concentrated solution of
sodium iodide in acetone.
acetone
CH3CH2 Br + NaI
CH3CH2 I + NaBr
Ethyl bromide Ethyl iodide
Alkyl fluorides are also prepared by treating an alkyl chloride or bromide with
inorganic fluorides.
acetone
2CH3 Cl + Hg2F2
2CH3 F + Hg2Cl2
Methyl chloride Methyl fluoride
5.4 Physical Properties
(1) CH3Cl, CH3Br, CH3F and CH3CH2Cl are gases at room temperature. Other alkyl
halides upto C18 are colourless liquids. Those beyond C18 are colourless solids.
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