NEET Chemistry – Haloalkanes and Haloarenes- Study Notes

HALOALKANES & HALOARENES

PREPARATION

• From alcohols with  (X = I, Br, Cl) or SOCl2

• • Order of reactivity among HX : HI > HBr > HCl >> HF

• • Order of reactivity among ROH : 3° > 2° > 1° > CH3OH
  • tert-Alcohols react with HX by SN1 pathway, while primary alcohols react via SN2, secondary may follow either or both of the paths (SN1 and SN2 are discussed later).
  • Mixture of conc. HCl and anhydrous ZnCl2 is used for differentiating three types of alcohols (3° > 2° > 1°) under the name of Lucas reagent.
  • Primary and secondary alcohols can best be converted to the corresponding chlorides (by SOCl2), bromides (by PBr3) and iodides, by PI3 (P + I2).
  • SOBr2 is less stable and SOI2 does not exist, PBr5 and PI5 are highly unstable hence not used.
• Alkyl halides are generally not prepared by direct halogenation of alkanes because the reaction generally gives a mixture of several compounds. However, compounds containing only one type of hydrogen atom can be converted into mono-halogeno products in good yield by taking excess of the concerned hydrocarbon; examples of such compounds are CH4, CH3CH3, (CH3)4C, C6H5CH3 etc.
  • Since allylic hydrogen atoms are much more reactive than the vinylic hydrogen, former are easily replaced by halogen.

      

• • Benzylic hydrogens (hydrogen present on C attached directly to benzene) are more reactive, hence easily replaced than 1°, 2° or 3° hydrogen.

• • •          
  • Halogenation of hydrocarbons in presence of light, heat and absence of halogen carrier takes place through free-radical mechanism.
  • More reactivity of benzylic and allylic hydrogens is due to stability of the corresponding free radical due to resonance.
  • Bromination at allylic and benzylic positions may best be carried by N-bromosuccinimide, NBS (Wohl-Ziegler Reaction).
• Addition of hydrogen halides to alkenes is an example of electrophilic addition involving carbocations as intermediates (ionic mechanism).

• Halide exchange method (anti-Markownikoff’s addition) is considered to be best for alkyl fluorides and alkyl iodides. For alkyl fluorides

CH3–Br + AgF CH3F + AgBr

      

• Hunsdiecker method (suitable for bromides)

• • Reaction involves free radical mechanism.
  • Reaction gives an alkyl halide having one carbon atom less.
  • Yield of alkyl bromides follows the order : 1° > 2° > 3°.
  • Yield is very low in case of chlorides, while iodine in such cases react differently.

PHYSICAL PROPERTIES

• Alkyl halides, although polar, are insoluble in water due to inability to form hydrogen bonds with water molecules.
• Density

• Boiling points

• Dipole moment
  Except fluoride, dipole moment decreases with the decrease in electronegativity from Cl to I. Fluorides, although having highest electronegativity have lower dipole moment than chloride due to the very small size of F and hence very small C–F bond length which outweighs the effect of electronegativity (recall that m = d × e). Thus the order for dipole moment is CH3Cl > CH3F > CH3Br > CH3I.
• Stability order : R – F > R – Cl > R – Br > R – I    (Similar to C–X bond length)

CHEMICAL PROPERTIES

NUCLEOPHILIC SUBSTITUTION

• Alkyl halides undergo nucleophilic substitution very easily.

• SN1 (Unimolecular nucleophilic substitution) : Although it is a two step process, the rate of reaction depends only upon the first (slow) step which involves ionization of the alkyl halide to form carbocation. Hence rate of reaction depends only upon the concentration of the alkyl halides, r = k[RX] and is independent of the concentration of the nucleophile which adds on the carbocation in the second (fast) step.

• In SN1 mechanism, carbocations are formed as intermediate, hence more the stability of the intermediate carbocation, greater are chances for their formation and hence more reactive will be the parent alkyl halide for SN1 reaction. Hence the order of reactivity of alkyl halides toward SN1 reaction follows the order : 3° > 2° > 1°
• When the intermediate carbocation is capable of undergoing rearrangement, lesser stable carbocation (1° < 2° < 3°) rearranges to the more stable carbocation and hence under such conditions unexpected product is formed.

• If the alkyl halide is optically active, the product formed in  reaction is always a racemic mixture. This is due to the formation of carbocations as intermediates which, being planar (sp2 hybridised) can be equally attacked by the nucleophile on either side of the face forming two enantiomers.

• SN2 (Bimolecular Nucleophilic Substitution) : The rate of SN2 reactions depends on the concentration of alkyl halide as well as nucleophile, i.e. r = k[RX][Nu]. This implies that both the reactants are involved in the rate-determining step, i.e. the reaction occurs in one step only or it is a concerted reaction. Concerted reactions occur through a transition state (an imaginary state in which both the reactant molecules are partially linked to each other).

• Polar solvents favour SN1 reactions while non-polar solvents favour SN2 reactions.
• Low concentration of nucleophile favours SN1 reactions, while high concentration favours SN2.
• Rate of reaction in SN1 mechanism is independent of the nature of the attacking nucleophile because it is not involved in rate-determining step; while rate of SN2 reactions depends upon the strength of the nucleophile. Strength of some common nucleophiles is

• Primary alkyl halides undergo SN2  reactions, 3° halides SN1, while 2° halides may undergo SN2 and/or SN1 mechanism.

ELIMINATION REACTIONS

• Alkyl halides lose a molecule of hydrogen halide (dehydrohalogenation) when heated with alcoholic potash. Dehydrohalogenation is a b-elimination reaction in which halogen and hydrogen atoms are lost from the two adjacent carbon atoms.

• Dehydrohalogenation is governed by Saytzeff rule according to which more highly substituted alkene is the major product, i.e. hydrogen atom is lost from the carbon atom carrying minimum number of hydrogen atoms (poor becomes poorer).

• Ease of dehydrohalogenation among halides is : 3° > 2° > 1°
• Elimination reactions dominate over substitution when strong Bronsted base
  e.g.  etc.) is used and alkyl halide is 3° or 2°.

• 1° Halides undergo SN2 reactions except when a hindered strong base like  is used.
• 2° Halides undergo SN2 reactions with weak base like I–, CN–, RCOO– etc., and elimination reaction with strong base like RO–.
• 3° Halides undergo SN1 reaction in absence of a strong base and only solvent acts as a base/nucleophile (solvolysis), however in presence of strong base (–OR) elimination reaction predominates.

REACTION WITH METALS

(Grignard reagent)

(Wurtz reaction)

• Solvent used must be perfectly anhydrous because even a trace of water or alcohol reacts with metals to form insoluble hydroxide or alkoxide that coat the surface of the metal. Moreover, water or alcohol may react with the product (organometallic compound) to form hydrocarbon.
• Alkyl lithiums react with copper halides to form higher alkanes (Corey-House synthesis)

• Among alkyl halides, order of reactivity is I > Br > Cl > F
• The reactivity of a metal toward an alkyl halide depends upon its reduction potential; more easily a metal is reduced, more reactive it is, e.g. Mg > Zn

REDUCTION

• 
• In this reaction, zinc atoms transfer electrons to the carbon atom of the alkyl halide. Zinc is a good reducing agent because it has two electrons in an orbital far from the nucleus, which are readily donated to an electron acceptor.
• 
• 
• 
• 

HALOGENATION

ISOMERISATION

ARYL SUBSTITUTED ALKYL HALIDES

• Aryl halides are the compounds having halogen atom directly attached to the aromatic ring. They are represented by the general formula Ar–X where Ar may be phenyl, substituted phenyl or other aromatic system, e.g. naphthyl.
  All compounds containing aromatic ring and a halogen atom should not be considered as aryl halides, e.g. benzyl chloride, C6H5CH2Cl because chlorine is not directly attached to the ring. Such compounds resemble alkyl halides in structure as well as in properties, hence grouped as aryl substituted alkyl halides.
• The carbon-halogen bonds of aryl halides are both shorter and stronger (due to possibility of resonance) than the carbon-halogen bonds of R–X and in this respect as well as in their chemical behaviour, they resemble vinyl halides (CH2 = CHX) more than alkyl halides.

• The strength of the C–X bonds causes aryl halides to react very slowly in reactions in which cleavage of C–X bond is rate determining, i.e. nucleophilic substitution. 

PREPARATION

• By direct halogenation of aromatic hydrocarbons

• By the decomposition of diazonium salts

• • Fluorides and iodides can be easily prepared.
  • Halogenation gives a mixture of o- and p- isomers which are difficult to separate.
• Hunsdiecker method :

• Industrial method for chlorobenzene (Raschig process)

PHYSICAL PROPERTIES

• Like alkyl halides, aryl halides are insoluble in water due to their incapability of forming H-bonds.
• Aryl halides are less polar than alkyl halides because in aryl halides, halogen is present on sp2 hybridised carbon which is more electronegative than the sp3 hybridised carbon of alkyl halides or halo cyclohexanes. Consequently, the electronegativity difference between C and Cl is low in aryl halides than in alkyl halides.
• Possibility of resonance in chlorobenzene makes the C–Cl bond shorter and hence, its dipole moment is less than that of cyclohexyl chloride.

• Although, the three isomeric dihalobenzenes have nearly same boiling points, the p-isomer has higher melting point than the o- and m-isomers. This is due to symmetrical nature of the para isomer due to which it is better packed in the crystal lattice. This explains why only the para isomer crystallises on cooling a solution containing ortho and para isomers.
• Due to strong intercrystalline forces, the higher melting point of para isomer is less soluble in a given solvent than the ortho isomer.

CHEMICAL PROPERTIES

LOW REACTIVITY TOWARDS NUCLEOPHILIC SUBSTITUTION

• Aryl halides (like vinyl halides) are less reactive towards nucleophilic substitutions under ordinary conditions (difference from alkyl halides). This low reactivity is due to
  (a) resonance effect
  (b) sp2 hybridisation of carbon atom holding the halogen atom
  (c) less polarity of the C–X bond.

• • Stabilisation of the molecule by delocalisation of electrons.
  • The C–Cl bond acquires some double bond character (structures III, IV and V) and thus becomes shorter and stronger than the C–Cl bond in alkyl halides.

• Conversely, benzyl halides do not show delocalisation of electrons of the halogen atom, hence its halogen is quite reactive towards nucleophilic substitutions (similarity with alkyl halides). Moreover, benzyl halides are even more reactive than alkyl halides, because the carbocation (benzyl) formed after the removal of halide ion is stabilised by resonance (remember higher is the stability of a species, easier will be its formation).

       

• Thus halogen derivatives can be categorised into two main groups on the basis of reactivity of the halogen atom.
  • Those in which halogen is present on sp3-hybridised carbon atom, such halogens and corresponding halides are highly reactive. For example,

• • Those in which halogen is present on sp2-hybridised carbon atom, such halogens are relatively inert. For example,

• However, aryl halides having electron-withdrawing groups (like –NO2, –CN, –COOH, –SO3H etc.) in ortho and para positions undergo nucleophilic substitution very easily. Further, greater the number of such groups in o- and p- positions, more rapid is the reaction and hence less vigorous conditions are required. Thus, reactivity of chlorine in the following compounds towards nucleophilic substitution follows the order.

• Since electron-withdrawing groups activate halogen toward nucleophilic substitution, electron-releasing groups (like –OH, –NH2, –OCH3, –R etc.) deactivate toward nucleophilic substitution.
• Aromatic nucleophilic substitution proceeds through mechanisms different from the SN2 and SN1 mechanism of alkyl halides.
  • Bimolecular mechanism is applicable to reactions of aryl halides having electron-withdrawing groups with the usual nucleophiles like NaOH, CH3ONa etc. Here carbanion is formed as an intermediate.
  • Benzyne mechanism is applicable when aryl halides are treated with a strong base like  or with the usual nucleophiles like NaOH, NH3, CH3ONa etc. under drastic conditions.
• Relative reactivity of halogens towards nucleophilic substitution. The order of reactivity of the halogen atom in aryl halides is opposite to that observed in alkyl halides (R – I > R – Br > R – Cl > R – F). In aromatic nucleophilic substitution, fluoride is the most reactive and iodide the least. Ar – F > Ar – Cl > Ar – Br > Ar – I

IMPORTANT NUCLEOPHILIC SUBSTITUTION REACTIONS

  

(where –Y = –NH2, –OH, –CN)

• However, simple aryl halides undergo nucleophilic substitution very easily by strong basic nucleophiles, like amide ion, via benzyne mechanism.

ELECTROPHILIC SUBSTITUTION

OTHER REACTIONS

• Formation of Grignard reagent : Since chlorobenzene is less reactive than methyl chloride, THF (a higher boiling solvent) is used.

• Fittig reaction :

• Wurtz-Fittig reaction :

• Ullmann reaction :

• Reduction :
  

Benzyl chloride undergoes all the above reactions formingand  respectively.

CONDENSATION WITH CHLORAL TO FORM DDT

OXIDATION OF THE SIDE CHAIN

POLYHALOGEN  DERIVATIVES

          

PREPARATION

• (For vic-dihalides)
• (For vic-dihalides)
• (For gem-dihalides)
• (For gem-dihalides)

PROPERTIES

• Dehydrohalogenation : Both give alkynes.

 

• Dehalogenation : Both give alkenes.

          

• Hydrolysis : Two give different products.

   

• Reaction with aq. KCN followed by hydrolysis give different products.

 

CHLOROFORM (TRICHLOROMETHANE)

PREPARATION

• 
• By distilling ethanol or acetone with a paste of bleaching powder (laboratory and commercial method).

• From ethanol :
  • (Oxidation)
  • (Chlorination)
  • (Hydrolysis)
• From acetone :
  • (Chlorination)
  • (Hydrolysis)
• (Commercial method)
• Pure chloroform, required in anaesthesia, is obtained by chloral hydrate with NaOH.                      

PROPERTIES

• Oxidation
  On exposure to air and sunlight, chloroform, a colourless heavy liquid, oxidises to carbonyl chloride (phosgene), a highly poisonous gas used in warfare.

• Reduction

• Chlorination

• Nitration

• Hydrolysis

• Carbylamine reaction

• Reimer-Tiemann reaction

• Condensation with Ketones

• Dehalogenation

IODOFORM

PREPARATION

PROPERTIES

• iodoform when comes in contact with organic matter, decomposes easily to free iodine, an antiseptic.
• iodoform gives yellow precipitate of AgI, on heating with silver nitrate, while chloroform does not give any precipitate with AgNO3 solution.

 

CARBON TETRACHLORIDE (TETRACHLORO METHANE OR PERCHLOROMETHANE)

PREPARATION

PROPERTIES

• It is a colourless, non-inflammable, poisonous liquid, soluble in alcohol and ether.
• On heating with steam at about 773K, it undergoes oxidation forming carbonyl chloride.

• It reacts with HF in presence of SbCl5 forming dichlorodifluoromethane; commonly known as freon-12.

• Reimer-Tiemann reaction

• Reduction

• Hydrolysis

USES

• as a solvent for oils, fats, resins,
• in dry cleaning,
• as a laboratory reagent,
• as anthelmintic (removal of worms) for hookworms
• as a fire extinguisher under the name of pyrene
  Carbon tetrachloride gives dense, and incombustible vapours which cover the burning surface and thus prevents oxygen to reach the fire. After the use of pyrene, the room should be well ventilated to remove poisonous phosgene vapours formed by the oxidation of carbon tetrachloride.

UNSATURATED  HALOGEN DERIVATIVES

• Those in which halogen atom is attached on unsaturated carbon atoms. Since in such compounds lone pair of electrons on halogen are conjugated with the p-bond, these undergo resonance, the C-Cl bond acquires double bond character, hence it is strong and chlorine is non-reactive.

• Those in which halogen is attached on the saturated carbon atom; hence resonance not possible; halogen is reactive.

VINYL HALIDES

• Halogen atom of vinyl halides is inert to nucleophilic substitution. However, like alkyl halides they undergo elimination reaction, and form Grignard reagents.

     

• Like alkenes, it undergoes the usual addition reactions such as addition of H2, X2, HX and polymerisation.

ALLYL CHLORIDE

ALLYL BROMIDE

ALLYL IODIDE

• (Insoluble in acetone)
• 

ISOMERIC DICHLOROETHENES

1,2-Dichloroethene exists in two geometrical isomeric forms, i.e. cis- and trans-.

TYPES OF HALOGENS

• Ionic halogens as in benzene diazonium halides  and quaternary ammonium halides . Aqueous solution of such halides when treated with AgNO3 solution in presence of dil. HNO3 give white or yellow precipitate in cold.
• Labile halogens as in alkyl halides (R – X), allyl halides  and benzyl halides (C6H5CH2X). These halogens are also precipitated as silver halides but under strong conditions. Organic halide is boiled with aqueous KOH solution, cooled, acidified with dil. HNO3 and then treated with AgNO3 solution; appearance of a white or yellow precipitate indicates labile halogen.
• Inert halogen as in aryl halides (ArCl) and vinyl halides (CH2=CHX). These compounds give precipitate with AgNO3 solution neither in cold nor on heating.