HALOALKANES
Alkyl halides : General formula is RX, where R = alkyl group.
Classification :
General methods of preparation :
≫ From alcohols : Alkyl halides can be prepared from alcohols in a number of ways :
– Action of hydrogen halides :
• Order of reactivity of halogen acids towards the above reaction is :
HI > HBr > HCl
• Order of reactivity of different alcohols towards the above reaction is :
allyl benzyl > tertiary > secondary > primary
– Action of phosphorus halides :
– Action of thionyl chloride (Darzen’s process) :
≫ Halogenation of alkanes :
– Abstraction of hydrogen for a particularhalogen follows the orderallylic > 3° > 2° > 1° > CH4– Reactivity of halogens for the above reactionfollows the order :F2 > Cl2 > Br2 > I2
≫ Halide exchange method :
This is known as Finkelstein reaction.
– Fluoroalkanes can also be prepared by halide exchange method by reacting alkyl chloride with inorganic fluorides. This is known as Swart’s reaction.
≫ Action of hydrogen halides on ethers :
≫ Addition of hydrogen halides to alkene :
Alkenes add on a molecule of hydrogen halide to form alkyl halide.
– Reaction follows electrophilic addition mechanism and takes place as per Markownikoff’s rule. However, in presence of peroxide addition of HBr takes place as per anti-Markownikoff’s rule.
Anti-Markownikoff’s rule is also known as Peroxide effect or Kharasch effect. HCl and HI do not show peroxide effect.
Physical properties :
≫ Alkyl halides being polar in nature are insoluble in water as they cannot break H-bonding already existing in water.
≫ They have higher melting and boiling points. For the same alkyl group boiling point follows the order as : R I > R Br > R Cl > R F
≫ Decreasing order of boiling points among the isomeric alkyl halides follows the order : 1° > 2° > 3° alkyl halides
≫ Decreasing order of density among the alkyl halides is RI > RBr > RCl > RF.
For alkyl iodide decreasing order of density is as follows :
CH3I > CH3CH2I > CH3CH2CH2I.
≫ Bond strength of C X bond follows the order
CH3F > CH3Cl > CH3Br > CH3I
i.e., bond strength of C X bond decreases as the size of halogen atoms increases.
≫ Correct stability order of RX is as follows :
R F > R Cl > R Br > R I
Chemical properties :
≫ Dehydrohalogenation :
≫ Friedel Crafts reacton :
≫Reaction with Mg metal :
≫Wurtz reaction :
≫Wurtz Fittig reaction :
≫Fittig reaction :
≫Reduction of alkyl halides :
HALOARENES
≫Aryl halides : General formula is ArX, where Ar = aryl group.
General methods of preparation :
≫By Raschig process :
From benzenediazonium salt :
By direct halogenation of benzene :
Physical properties :
≫Aryl halides are colourless stable liquids with pleasant odour.
≫These are insoluble in water but readily miscible with organic solvents.
≫Most of them are steam volatile, heavier than water.
≫Their boiling points are higher than corresponding alkyl halides. The boiling points rise gradually from fluoro to iodo compounds.
Chemical properties :
≫Nucleophilic substitution reactions :
The presence of an electron withdrawing group ( NO2) at ortho- and para-positions increases the reactivity of haloarenes.
Electrophilic substitution reactions :
– Halogenation :
– Nitration :
– Sulphonation :
– Friedal-Crafts reaction :
Reaction with metals :
Fitting reaction :
SN1 and SN2 mechanisms : e nucleophilic substitution can proceed via SN1 mechanism or SN2 mechanism.
| Unimolecular (SN1) | Bimolecular (SN2) |
| It is first order reaction. | It is second order reaction. |
| Generally carried out in polar protic solvents like water, alcohol and acetic acid. | Carried out in polar aprotic solvents like acetone, DMSO, acetonitrile, or DMF. |
| Takes place in two steps through carbocation as the intermediate. | Takes place in one step through transition state. |
| Rate of reaction : 3° > 2° > 1° > CH3 (fastest) (slowest) Greater the stability of carbocation, faster will be the reaction. | Rate of reaction : CH3 > 1° > 2° > 3° halides (fastest) (slowest) Less the steric hindrance in T.S., faster will be the reaction. |
| Tends to proceed with weak nucleophiles, e.g.; CH3OH, H2O, CH3CH2OH, etc. | Tends to proceed with strong nucleophiles, e.g.; CH3O–, CN–, OH–, etc. |
| Configuration is retained but in front attack inversion takes place (racemisation and inversion). | Inversion of configuration takes place (Walden inversion). |
≫Primary allylic and primary benzylic halides show higher reactivity in SN1 reactions than other primary alkyl halides due to the greater stabilisation of allylic and benzylic carbocation intermediates by resonance.
≫Vinylic and aryl halides are unreactive in nucleophilic substitution reactions. This is because of double bond character of C-X bond due to resonance.
Optical isomerism/Enantiomerism :
≫Optical isomer is known as dextrorotatory isomer (latin : dexter means right) (d-form or +ve) if it rotates the plane polarised light to the right (clockwise) and laevorotatory isomer (latin : laevo means left) (l-form or –ve) if it rotates the plane polarised light to the left (anticlockwise).
≫An equimolar mixture of the d- form and l-form will be optically inactive and is called racemic mixture (or dl- form or (±)-mixture).
≫The process of conversion of an enantiomer into racemic mixture is known as racemisation.
≫Chirality : The compound is said to have chirality if the central carbon atom is attached to four different groups and this centre is called chiral (asymmetric) centre or stereogenic centre or stereocentre.
≫Achirality : The compound is said to have achirality if the central carbon atom have atleast two identical groups and this centre is called achiral (symmetric).
≫If molecule has a plane of symmetry it is achiral (not chiral) and if molecule has no plane of symmetry it is chiral.
≫Enantiomers/d and l isomers : They are the optical isomers which are non-superimposable mirror images (or dissymmetric).
≫Diastereomers : They are the optical isomers which are not mirror images of each other. They have different physical properties and magnitude of specific rotation.
≫Meso compounds : These compounds which have two or more even number of chiral carbon atoms and have an internal plane of symmetry.
They are optically inactive due to internal compensation.
Uses and environmental effects of some important compounds :
Compounds
(1) Chloroform (CHCl3)
Uses
– Its major use is in the production of Freon refrigerant, R-22.
– It is used as a solvent for resins, rubbers, oils and fats, alkaloids, iodine and many other substances.
– In the past, it was extensively used as anaesthetic for surgery but now it is rarely used as it causes liver damage.
– It is used in preparation of chloretone (drug) and chloropicrin (insecticide).
– It is used to preserve anatomical species.
Effects
– It is oxidised to poisonous gas, carbonylchloride, known as phosgene.
Phosgene gas causes liver and kidneydamage.– Inhaling chloroform vapours depressesthe CNS, causes dizziness, fatigue andheadache.
(2) Iodoform (CHI3)
Uses
– It is used as an antiseptic in dressing of wounds due to liberation of iodine.
– It is used as methylating agent in organic synthesis.
Effects
– It has strong smell.
Freons
– They are used as refrigerants, blowing agents, propellants in medical applications and degreasing solvent.
– Freons cause disruption of ozone layer by initiating radical chain reactions in stratosphere.
– This anthropogenic compound is a greenhouse gas and its effect is more than CO2.
(3) DDT
Uses
– In 1940, it was used as a pesticide.
Effects
– It is a persistent organic pollutant, strongly absorbed by soil.
– It is lipophilic so has a high potential to bioaccumulate.
– It may be directly genotoxic but may also induce enzymes to produce other genotoxic intermediates and DNA adducts.
