ORGANIC CHEMISTRY
HALOARENES AND HALOALKENES
Haloalkenes- A compound in which a halogen atom is
attached to the sp2 hybridized carbon atom of an alkyl
group is called haloalkenes.
Example: C2H5X
c
H
H
H
H
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Halogen compounds is a very well known topic of organic chemistry. This document gives a brief information about preparation and various reactions of halogen compounds. It will help students to prepare various examinations.
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2024/2025
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ORGANIC CHEMISTRY
HALOARENES AND HALOALKENES
Haloalkenes - A compound in which a halogen atom is attached to the sp 2 hybridized carbon atom of an alkyl group is called haloalkenes. Example: C 2 H 5 X
c
H
H
H
H
Haloarenes- A compound in which a halogen atom is attached to sp 2 hybridized carbon atom of an aryl group is called haloarenes. Example:
- On the basis of number of halogen atom: Classification
X
CH 2 X
- Coumpounds containing sp 3 C ❖ Alkyl halides or haloalkanes: CnH2n+1X : General Formula Primary{1 }: R’
CH 2 X
CH 2 X
X Bond [Where x=F,Br, I] CH 2 X
Secondary{2}: R’ Tertiary{ 3}: R’ ❖ Allylic halides: Compounds in which the halogen atom is bonded to sp 2 hybridized carbon atom next to carbon-carbon double bond. Example:
X
R”
C
H
R”
C X
R’’’
X
❖ Vinylic halides: Compounds in which the halogen atom is bonded to sp 3 hybridized carbon atom of a carbon-carbon double bond. [c=c] Example: ❖ Aryl halides: Compounds in which the halogen atom is bonded to the sp 2 hybridized carbon atom of an aromatic ring.
X
X
Example:
- From alcohols: R OH + HCL R CL + H 2 O R OH + NaBr + H 2 SO 4 R Br + NaHSO 4 + H 2 O
X
X
H 3 C
Methods of preparation
CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CL +
CH 3 CH 2 CHCLCH 3
b) By electrophilic substitution:
- X 2 + Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and CL/UV light Or heat CH 3 Fe dark
CH 3
X
CH 3
X
o-Halotoluene p-Halotoluene
bromine respectively in the presence of Lewis acid catalysts like iron or iron[III] chloride. c) Sandmayer’s Reaction: NH 2 NANO 3 + HX 273 - 278K
N 2 X
Benzene diazonium halide N 2 X
Cu 2 X 2
- N 2 Aryl halides [ where X= Cl, Br]
d) From alkenes: A. Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide by reacting with hydrogen chloride, hydrogen bromide or hydrogen iodide. C C + HX C C Propene yields two products, however only one predominates as per Markonikov’s rule.
H H
CH 3 CH CH 2 + HI CH 3 CH 2 CH 2 I + CH 3 CHICH 3
B. Addition of halogens: In the laboratory, addition of bromine in CCl 4 , to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless. minor v major
The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg 2 F 2 ,CoF 2 ,or SbF 3. H 3 C Br + AgF H 3 C F + AgBr
- Reactions of haloalkenes: a. Nucleophilic substitution reaction: Chemical Reaction
In this type of reaction, a nucleophile reacts with haloalkene (the substrate) having a positive charge on the caron atom bonded to halogen. A substitution takes place and halogen atom called leaving group departs as halide ion. Since the substitution reaction nis initiated by a nucleophile, it is called as nucleophilic substitution reaction. Nu + C X C Nu + X
a new carbon-OH bond. These two processes take place simultaneously in a single step and no intermediate is formed. As the reaction progresses and the bond between the nucleophile and the carbon atom starts forming, the bond the carbon atom and leaving group weakens. As this happens, the configuration of the carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind , while the leaving group is pushed away. This process is called as inversion of configuration. In the transition state, the carbon atom is simultaneously bonded to incoming
nucleophile and the outgoing leaving and such structures are unstable and cannot be isolated. This is the because the carbon atom in the transition state is simultaneously bonded to five atoms and therefore is unstable. Order of reactivity in SN2 reaction are as follows: Primary halide > Secondary halide > Tertiary halide ii. Substitution nucleophilic unimolecular: The reaction between tertiary butyl bromide and hydroxide ion yields tertiary butyl alcohol and follows