HALOALKANES AND HALOARENES Haloalkanes are the compounds formed by the replacement of H from an alkane(R-H) with halogen (R-X).Haloarenes are the compounds formed by the replacement of H from benzene(C6H5-H) with halogen (C6H5-X). Halo- compound produced in our body: Thyroxine(Iodine). Chlorine containing antibiotic: Chloramphenicol. Chlorine compound in Malaria: Chloroquine. Blood substitute: Fully fluorinated/perfluorinated compounds. Compound used in Anaesthasia: Halothane(CF3CHClBr) Classification : Follow book Nomenclature rules: Follow longest chain. Follow lowest locant rule for numbering. Follow alphabetical order for naming Halogen compounds are started with prefix Halo- Solve table 10.1,e.g ,Intext questions on pg no. 284,285. Nature of C—X bond: Since halogens are more electro negative than Carbon, C—X bond is polar. Order of polarity: CH3Cl>CH3F>CH3Br>CH3I (Dipole moment=Charge X distance of separation=Q X D) o-DichloroBenzene>m- DichloroBenzene>p- DichloroBenzene(p-DichloroBenzene has zero µ) Methods of Preparation: Follow book. Melting point and boiling point: Boiling point depends upon the inter molecular interactions where as Melting point depends upon the symmetry and ease with which the compound can fit in a lattice. Order of Boiling point: CH3I>CH3Br>CH3Cl>CH3F(More molecular mass ,more vanderwaal’s force) Para-isomer of dihalobenzene Cl-- --Cl, has more m.p than o-isomer Cl-as it fits into the lattice better. Solubility: Slightly soluble in water as water—water bond is stronger than new bonds formed between water and halocompound due to water repelling Alkyl group. Chemical properties of Haloalkanes: Reactions are divided into 3 categories: 1. Nucleophilic substitution reactions. 2. Elimination reactions 3. Reactions with metals. Nucleophilic substitution reactions Second Order SN2: OHNuceophile --C—Cl --- OH--------C--------Cl HO—C--Haloalkane Formation of C—OH and Hydroxide formation Breaking of C—Cl simult. (Inversion) Nucleophile attacks Carbon from the opposite side of C—X bond Pr. Optical activity: Some compounds rotate the plane polarized light when it passes through their solution. 7. For Ist step energy comes to break bond from salvation of halide ion. Example: our hands and this property is known as Chirality. Achiral Molecule: Propane2-ol. 3. The rate depends on the concentration of Alkyl halide(SN1). This mixture is called as Racemic mixture and process is racemisation. Enantiomers: The sterio isomers related to each other as non-superimposable mirror image are called enantiomers(like our hands) example: : : : : *Carbon should have all 4 different groups. The rate of reaction depends upon the Concentration of both the reactants. they show higher reactivity in (SN1).>Pr. Formation of C—OH bond and breaking of C—Cl bond happens simultaneously. Nucleophilic substitution reactions First Order SN1: (CH3)3—Br========(CH3)3+ + Br+OH-----------(CH3)3—OH Haloalkane form carbocation after removing halide ion. 50% d. Inversion of configuration takes place. Racemic Mixture: A mixture of 50% d-isomer & 50% l-isomer having zero optical activity. Bulkier groups cause more steric hinderance to the incoming nucleophile. 2.(reactivity) due to extra stability of tert. When this light is passed through a Nicol prism it vibrates only in one plane.>sec. Achiral Objects: The objects that are superimposable on their mirror image. Example: A Ball. 5. Tert. Nucleophile attacks on trigonal planar carbocation from either side. 8. halide> tert. Carbocation. Plane polarized light: Ordinary light is a combination of many waves of different wavelength and travels oscillating in all the planes. Halide > sec. Steriocentre: Carbon having all 4 different groups.and 50% l. Chiral molecule: Butan2-ol 6. Some basic terms in Optical behavior: 1. No inter mediate is formed. Retention: Preservation of spatial arrangement after nucleophilic substitution. Halide(reactivity) due to steric hinderance.isomers are obtained in mixture and mixture has 0 optical activity. 4. Example: . Due to greater stabilization of allyl and benzyl carbocation intermediate. Chiral Objects: The objects that are Non superimposable on their mirror image. Bulkier nucleophiles behave as sronger base and eliminate a proton rather than substitution. Example SN2.form example SN1 mechanism. Gives immediate turbidity.form or vice-versa. of alkyl groups attached with doubly bonded C-atom. Sandmeyer’s reaction: Finkelstein Halogen exchange: Swartz halogen exchange: Grignard Reagent: CH3CH2Br + Mg-------dry ether------- CH3CH2MgBr Wurtz Reaction: Fittig Reaction: Wurtz Fittig reaction: . it happens according to Saytzeff rule i.sec.form to l.e “The preferred product is that alkene in which more no. But when reacted with alcoholic KOH they undergo elimination.sec. This elimination is also called as β-elimination or β-dehydrohalogenation as Hydrogen is removed from βposition and halogen from α-position. 10. If there is possibility for more than one alkene due to presence of more α-Hydrogens.9. & tert.. H3C—CH2—CH==CH—CH3 Pent2-ene (81%) OH- H3C—CH2—CH2—CH—CH3 OH-H3C—CH2—CH2—CH==CH2 2-Bromopropene Pent1-ene (19%) When haloalkanes are heated with aqueous KOH . Inversion: Change of d. gives after some time and Pr. Name Reactions: Lucas Test(To distinguish between the Pr. they form corresponding alcohols as Halo group is replaced by –OH group.as nucleophile where as water forms OH. Tertiary haloalkane prefer elimination over substitution. Elimination Reactions: It happens when haloalkane is heated with alcoholic KOH. Racemisation : Formation of equal amount of d.as nucleophile. Alcohols): ROH + HX------ZnCl2-----R-X + H2O Tert. The reason is that alcoholic KOH forms OR.and l. Gives on heating. Presence of electron withdrawing group on ortho or para positions requires less drastic conditions. 3.Reasons for less reactivity of Aryl halides towards nucleophilic substitution reaction: 1. 5. Halogenation: 2. Benzene ring is electron rich so repels the incoming nucleophile. if two --NO2 groups present 368K and if all the oand p. Sulphonation 3. Friedel Craft alkylation: 5. Sp2 Carbon of benzene is more electronegative due to more s-character and forms a shorter bond which is more difficult to break.g if one –NO2 group is present temperature 443K. Resonance effect of halogen causes a partial double bond character. Friedel Craft acylation: . Nitration : 4. 4. ELECTROPHILIC SUBSTITUTION REACTIONS: 1. The nucleophilic substitution happens under drastic conditions of temperature 623K.positions are occupied by three –NO2 groups just warming the reaction mixture will cause substitution reaction.300 atm pressure. 2. 6. For e. Phenyl carbocation is highly unstable so SN1 is ruled out.