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Hydrocarbons, where one or more hydrogen atoms have been replaced with halogen atoms, are known as Haloalkanes and Haloarenes. The main point of difference between Haloalkanes and Haloarenes can be said that Haloalkanes are derived from open chained hydrocarbons, also known as alkanes and Haloarenes are derived from aromatic hydrocarbons.
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Key Terms: Organic chemistry, atoms, hydrocarbons, reactions, Alkyl halides, Chlorofluorocarbons.
Haloalkanes
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Haloalkanes
- These have hydrocarbons made up with aliphatic alkanes and one or more hydrogen atoms replaced by halogens (elements such as Chlorine, Bromine, Fluorine, Iodine etc.).
- Here, the halogen atom is attached to the sp3 hybridized carbon atom of the alkyl group.
- Example: CH3Cl (Methyl Chloride)
CH3CH2Br (Ethyl Bromide)
- These are saturated organic compounds where all the chemical bonds are attached to the carbon atom with single bonds and a single carbon atom is attached to the Halogen atom.
- Haloalkanes are made by aliphatic alkanes by the process of free radical halogenation.
- They are odourless compounds.
- These precipitate in SN2 substitution reactions.
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Haloarenes
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- Haloarenes are hydrocarbons consisting of aromatic ring/rings and one or more hydrogen atoms replaced by halogens.
- The halogen atom is attached in the sp3 hybridized carbon atom of the alkyl group.
- Example: Chlorobenzene, Bromobenzene.
- They differ from Haloalkanes by their method of preparation and properties.
- This compound is used for several purposes.
- One of the most important Haloarene is aryl chloride.
- Haloarenes are made by direct halogenation of aromatic rings.
- They have a sweet odour.
- These do not precipitate in SN2 substitution reactions.
Classification of Haloalkanes and Haloarenes
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Classification of Haloalkanes and Haloarenes
Haloalkanes and Haloarenes can be classified based on the following points:
- sp2 hybridized carbon halogen bond
- sp3 hybridized carbon halogen bond
- Number of halogen atoms on the molecule
Classification on the basis of sp2 hybridized carbon halogen bond:
- Vinyl Halide- The halogen atom gets attached to the sp2 hybridized carbon C=C.
- Aryl Halide- The halogen atom gets attached to the sp2 hybridized carbon in an aromatic ring.
Classification on the basis of sp3 hybridized carbon halogen bond:
- Alkyl Halide- Halogen gets attached to the alkyl chain
- Allylic Halide- The halogen atom gets attached to the sp3 hybridized carbon which is situated adjacent to the sp2 hybridized carbon C=C.
- Benzylic Halide- The halogen atom gets attached to the sp3 hybridized carbon, attached to a benzene ring.
Classification on the basis of number of halogen atoms:
- Mono Haloalkanes and Monohaloarenes- They contain one halogen atom.
- DIhaloalkanes and Dihaloarenes- They contain two halogen atoms.
- Tri Haloalkanes and Trihaloarenes- They contain three halogen atoms.
Uses of Haloalkanes and Haloarenes
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- Used as solvents for non-polar compounds also known as hydrophobic compounds.
- Used in the treatment of Malaria (Chloroquine).
- Used for organic synthesis.
- Used as insecticide
- Some derivatives such as chloramphenicol are used for the treatment of typhoid.
- Dichloro-Diphenyl-Trichloroethane or DDT is used in insecticides.
Impact on Environment
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Although Haloalkane and Haloarene compounds are used for various commercial purposes, they can still be harming the environment due to the pollutants and the toxins that they release. For example compounds like methyl bromide act as a very harmful fumigant to the environment and also Chlorofluorocarbons (CFC) that are a major cause of the ozone layer depletion.
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Sample Questions
Ques. What is sp2 hybridized carbon halogen bond?(1 mark)
Ans. When one s and two p atomic orbitals mix together, they form sp2 hybridization, where the combination of such atomic orbitals creates 3 new hybrid orbitals all of equal energy. Here one electron in the s orbital is promoted to one of the 2p atomic orbitals.
Ques. What is sp3 hybridized carbon halogen bond?(1 mark)
Ans. When characters of one 2s-orbital and three 2p-orbitals mix together it is known as sp3 hybridization. The mixing creates four hybrid orbitals, which have similar characteristics. For an atom to get sp3 hybridized, it must contain one s orbital and 3 p orbitals.
Ques. What is the main difference between Haloalkanes and Haloarenes?(1 mark)
Ans. The main point of difference between Haloalkanes and Haloarenes is that Haloalkanes are derived from open chain hydrocarbons but Haloarenes are derived from aromatic hydrocarbons.
Ques. Can Haloalkanes and Haloarenes be used in the medical field?(1 mark)
Ans. Yes, some derivations of Haloalkanes and Haloarenes can be used in treatment of ailments like Malaria and Typhoid.
Ques. Can Haloalkanes and Haloarenes be differentiated on the basis of smell?(1 mark)
Ans. Yes, they can be differentiated on the basis of smell. Haloalkanes are odourless compounds whereas Haloarenes have a sweet odour.
Previous Year Questions
Ques: A solution of hydrolyses CH3CHCICH2CH3 and CH3CH2CH2CH2Cl. Whic one among them is easily hydrolysed? (Delhi 2010) (1 mark)
Ans: In aq. KOH SN1 mechanism occurs and the formed carbocation is stabalised for which 2 degree carbocation is more stabalised carbocation than 1 degree therefore,
hydrolyses faster than CH3CH2CH2CH2Cl
Ques: What is the IUPAC name of the following compund, (1 mark)
Ans:
Ques: Write the IUPAC name of the following compound, CH2 = CHCH2Br? (All India 2010) (1 mark)
Ans: 
The IUPAC name if l-Bromo-prop-2-ene.
Ques: What happens when bromine attacks? (All India 2012) (1 mark)
CH2 = CH – CH2 – C = CH
Ans:
Ques: Draw the structure of a monohalogen product in the following reaction: (Comptt. Delhi 2012) (1 mark)
Ans:
Ques: State how are the following conversions carried out?
(i) Benzyl chloride to benzyl alcohol.
(ii) Methyl magnesium bromide to 2-methyl-propan-2-ol. (Delhi 2010) (2 marks)
Ans. (i) Benzyl chloride to benzyl alcohol
(ii) Methyl magnesium bromide to 2-methyl-propan-2-ol
Ques: Haloalkanes undergo nucleophilic substitution while haloarenes undergo electrophilic substitution, explain. (Comptt. Delhi 2012) (2 marks)
Ans: Haloarenes, for example chlorobenzene, is a resonance hybrid of the following five structures:
Resonance causes lowering of energy and therefore greater stability. On the other hand, no such resonance takes place in haloakanes. Halogens directly attached to benzene rings are o, p-directing in electrophilic substitution reactions is due to greater electron density at these positions in resonance.
Ques: Account for the following;
(i) The C – Cl bond length in chlorobenzene is shorther than that in CH3Cl.
(ii) Chloroform is stored in closed dark brown bottles. (Delhi 2013) (3 marks)
Ans: (i) In haloalkanes, the halogen atom is attached
to sp3 hybridized carbon, whereas in haloalrenes, it is attached to sp2 hybridized carbon whose size is smaller than sp3 orbital carbon. Thus, C – Cl bond in chlorobenzene is shorter than alkyl chloride.
(ii) Chloroform is stored in dark coloured bottles to cut off light because CHCl3 is slowly oxidised by air in the presence of light to form an extremely poisonous gas, that is carbonyl chloride. This is popularly known as phosgene.
Ques: (i) Write one use each of DDT and iodoform.
(ii) Which compound in the following couples can react faster in SN2 displacement and why?
(a) 1-bromopentane or 2-bromopentane
(b) l-bromo-2-methylbutane or 2-bromo-2-methylbutane. (Delhi 2010) (3 marks)
Ans: (i) The usage of DDT: As a powerful insecticide, it is widely used in sugarcane and fodder crops to kill mosquitoes, lice which carry pathogens.
Usahe of iodoform: It is used a s antiseptic for dressing wounds. Its antiseptic action is due to the liberation of iodine when iodoform comes in contact with skin but not due to iodoform itself.
(ii) In SN2 reactions, reactivity depends upon steric hindrance.
(a) 1-bromopentane has less steric hindrance and thus more reactivity than 2-bromopentane due to which it udergoes SN2 reaction faster.
(b) l-bromo-2-methylbutane has less steric hindrance and thus more reactivity towards SN2 reaction than 2-bromo-2-methylbutane that has more steric hindrance.
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