Haloalkanes and Haloarenes

How are haloalkanes formed?

Haloalkanes formed by replacing hydrogen atom in hydrocarbon by halogen atom, which is attached to sp3 hybridised C.

How are haloarenes formed?

Replacement of Hydrogen atom attached to sp2 hybridised C of aryl grp by halogen

Applications of Haloalkanes

Solvents fr non-polar compounds

Starting material fr synthesis of Org. compounds.

Fully fluorinated compounds are potential blood substitutes in surgery

Use of chloramphenicol

Cl containing antibiotic

Produced by soil microbes

Treatment of typhoid fever

Use of thyroxine

Iodine containing hormone

Deficiency causes goiter

Use of chloroquine

Synthetic halogen compound

Used for treatment of malaria

Use of halothane

Used as an anaesthetic during surgery.

Classification on Basis of No. of Halogen atoms

Mono (1 halogen)

Di (2 halogens)

Tri ( 3halogens)

Classification of Compounds with sp3 C-X bond

Haloalkanes- Primary, Secondary, Tertiary

Allylic Halide- X-sp3-sp2

Benzylic. X-sp3-Ar

Compounds with sp2 C-X bond

Vinylic Halides- X-sp2

Aryl Halides- X-Ar

Common Nomenclature

Alkyl grp+Halide

dihalogen derivatives- o, m, p

IUPAC nomenclature

Halosubstituted hydrocarbons

Haloarenes

Dihalogen derivatives- 1,2 1,3 1,4 dihaloalkanes

Geminal halides

Halogen atom present on same carbon atom

Alkylidiene halides

Vicinal Halides

Halogen atoms present on adjacent carbon atoms

Alkylene dihalides

Y is the C-X bond polarised?

Halogen more electronegative than carbon

C-partial +ve charge

H-partial -ve charge

Y does C-X bond increase frm F to I?

Size of halogen increases down grp and thus, F is smallest n I largest.

Y does bond enthalpy decrease down grp?

IP and EA decrease down grp

Y does dipole moment largely decrease down grp?

Electronegativity decreases down grp

Y does Cl hv higher dipole moment than F?

Small size, Steric electron repulsion in F

Y are alkyl halides best prepared from alcohols?

Easily accessible

Y is thionyl chloride preferred reaction?

Other 2 products are escapable gases.

Pure alkyl halides obtained

How is phosphorus tribromide/triiodide generated?

Insitu

Reaction of red P with Br and I

Y are alcohol methods of preparation nt suitable for aryl halides?

C-O bond in phenols has partial double bond character, stronger, difficult to brk

Comment on HCl reaction with 1°,2° and 3° alc.

1° and 2° require presence of ZnCl2 catalyst

3° shaken with conc. HCl at room temp.

Y is it difficult to separate products of free radical halogenation?

Similar B. P and yield of any 1 compound is low.

What is the catalyst used in Electrophilic substitution reaction of arenes?

Lewis acid catalysts like Fe or FeCl3

How can Electrophilic substitution be carried out with iodine?

Reversible reaction, oxidizing agent needed to oxidize HI formed during iodination

HNO3, HIO4

How can Electrophilic substitution be carried out with F?

Nt possible due to high reactivity of F

Y can Ortho and Para isomers be easily separated?

Large diff. in M. P

Explain Sandmeyer Reaction.

Primary aromatic amine+ Cold aq. mineral acid+NaNO2 at 273-278K gives Benzene diazonium halide

What is the diazotisation reaction?

Diazonium salt+ CuCl2/CuBr2 gvs aryl halide and nitrogen

Gv diazotisation reaction fr aryl iodide and phenol.

Aryl iodide-KI

phenol- warm water

Wht does discharge of reddish brown colour of Br indicate?

Double bond

Unsaturation

Mention a reaction tht involves halogen exchange.

Finkelstein reaction

Alkyl Cl/Br with NaI in dry acetone

Explain Swarts Reaction

Alkyl Cl/Br+ metallic flouride gives alkyl flouride.

Are alkyl halides coloured??

Colourless when pure, Br and I develop colour in light.

Y do halo compounds have higher B. P than hydrocarbons of the same molecular mass??

Gr8er polarity+High molecular mass leads to strong intermolecular dipole-dipole and Van der Waals forces.

Comment on states of matter of halo compounds.

Lower members are gases at room temp. and higher members r liquids or solids.

As molecular mass increases, B. P increases for halides. GR

Bigger molecules, more electrons, more attraction.

For same alkyl grp, BP of alkyl halides increase down grp. GR

Size n mass of halogen increases, Van der waals forces increase

As branching increases BP of halides decreases. GR

Branching increases, spherical shape, small area of contact, weak forces which r overcome at low temp.

Y do para-isomers of halides hv high MP?

Symmetry of para isomers that fits in crystal lattice betteras compared to ortho n meta isomers.

How can we increase the density of a halide?

Increase no. of C atoms

Increase no. of halogen atoms

Increase atomic mass of halogen atoms.

Haloalkanes are barely soluble in water. GR

Less energy released when new attractions set up b/w haloalkane and water molecules as these are not as strong as the original H bonds in water.

Haloalkanes are highly soluble in organic solvents. GR

New intermolecular attraction strength=strength of bonds brkn in separate haloalkane and solvent molecules.

What is a nucleophilic reaction?

Nucleophile+ partially positive haloalkane leads to substitution and departure of halide ion

Y is nucleophilic reaction called so?

Initiated by nucleophile on positive C attached to halogen.

What are ambidient nucleophiles? Gv 2 examples.

Grps of nucleophiles tht possess 2 nucleophilic centres.

CN(cyanide) and NC(Isocyanide)

ONO( alkyl nitrite) and NO2(nitroalkane)

Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. GR.

KCN-Ionic and provides cyanide ions

C donates as C-C bond is stronger than C-N bond.

AgCN- Covalent, N free to donate electron pair forming isocyanide.

Explain SN2 reaction

Nucleophile+ alkyl halide

C-X bond weakens as C-Nu bond strengthens.

Configuration inverts n C-X bond brks.

Y is reaction called SN2 reaction?

Rate depends upon conc. of both reactants.

Bimolecular nucleophilic displacement reaction.

Wht is Walden's Inversion?

Nu is repelled by X and so attacks frm opposite side.

As bond forms, C-X bond brks n X pushed away.

Y is carbon unstable in transition state?

Simultaneously bonded to 5 atoms at a tym and cannot be isolated.

Pri>Sec>Tert. favoured.Y?

Bulky grps hv inhibiting effect on approach of nucleophiles.

Methyl halides hv small H atoms

Explain the SN1 reaction

Step1- C-Br bond slowly cleaves to gv carbocation n Br-ion

Step 2- Carbocation attacked rapidly by nucleophile.

Wht. are polar protic solvents?

Solvents tht contain free H+ ions and can donate protons readily to reagents

Y is the reaction called SN1?

Rate of reaction depends on conc. of substrate only as it is the slowest step.

Where is the energy fr SN1 reaction obtained frm?

Solvation of halide ion with the proton of protic solvent provides energy fr C-X bond cleavage.

3°>2°>1°.GR

3° carbocation hv gr8er stabilty

Allylic n Benzylic halides undergo SN1 reaction. GR

Carbocations stabilised thru resonance.

In SN1 n SN2, R-X reactivity increases down grp. GR

Large size of I, better leaving grp

What are optically active compounds?

Compounds tht can rotate plane polarised light when it is passed thru their solutions.

What are optical isomers?

Dextrorotatory- rotation of light to right, +ve isomers

Laevorotatory- rotation of light to left, -ve isomers

Both +ve and -ve isomers are called optical isomers.

What is optical isomerism?

Phenomenon of rotation exhibited by optical isomers.

What is a stereocentre or asymmetrical carbon?

4 diff. substituents present around the carbon.

Responsible for optical activity.

What are chiral and achiral objects?

Objects which are non-superimposible on mirror images are chiral and show chirality whilr objects which are superimposable on mirror images are achiral and show achirality.

What are enantiomers?

Stereoisomers related to each other as non-superimposible morror images

What are the properties of enantiomers?

Identical physical properties except opposite rotation of plane polarised light.

What is a racemic mixture?

Mixture containing 2 enantiomers in equal proportions and thus hv no optical rotation.

Define racemisation

The process of conversion of an enantiomer into a racemic mixture.

What is retention?

Same relative configuration after reaction.

Occurs if no bond to the stereocentre is brkn.

What is resolution?

Process of conversion of racemic mixture to its d and l components.

Y do SN1 reactions undergo racemisation?

sp2 hybridised carbocation is achiral

Nucleophile can attack frm either side.

Mix of products with same and opposite configuration.

What is a β-elimination reaction?

Haloalkane+ alc. soln. of KOH gvs alkene after elimination of β-hydrogen and α-halogen atoms.

State Zaitsev's rule.

In dehydrohalogenation reacrions, preferred product is tht alkene which has gr8er no. of alkyl grps attached to doubly bonded carbon atoms.

What factors does a chemical reaction depend upon?

Nature of alkyl halide

strength and size of nucleophile

Reaction conditions.

What are organometallic compounds?

Mg + R-Br with dry ether gives RMgBr, Carbon metal bond.

Also called Grignard's reagent.

Y is Grignard's reagent extremely reactive?

C-Mg bond is polar covalent but Mg-X bond is ionic.

Y shld we avoid traces of moisture frm Grignard's reagent?

Highly reactive

RMgX + H2O gvs RH + Mg(OH)X

Wht is Wurtz reaction?

2RX + Na in dry ether gvs RR + NaX.

Involves increase in C atoms

Y are aryl halides extremely less reactive towards nucleophilic substitution reactions?

Resonance effect- Partial double bond character of C-Cl bond due to resonance.

Sp2 hybridised C has smaller n stronger bond to brk.

No SN1 mechanism as phenyl cation formed by self-ionisation nt stabilised by resonance.

Repulsion due to bulky arene grp.

Y does NO2 show effect at ortho and para positions n nt meta?

Withdraw electron density frm benzene ring

Facilitate attack of nucleophile

Carbanion stabilised by resonance.

No -ve charge at meta position.

Y does substitution wrt X tk place at o and p positions?

X is deactivating+ o, p directing.

Y does Electrophilic substitution in haloarenes occur slowly and under srastic conditions?

Resonance, X takes away e- frm benzene ring due to -I effect, ring gets deactivated.

Though Cl is electron withdrawing, it is ortho para directing in Electrophilic aromatic substitution reactions. GR

Inductive effect- Destabilises carbocation formed during Electrophilic substitution

Resonance- pronounced effect on o and p positions stabilises carbocation.

I>R, net e- withdrawal, net deactivation lessened by R effect.

Explain Wurtz Fittig reaction.

Haloarene + Haloalkane + Na with ether gv Ar-R and NaX

Explain Fittig reaction.

2Ar-X + Na in ether gvs Ar-Ar and 2NaX.

What are polyhalogen compounds?

Compounds tht contain more than 1 halogen atom.

Uses of dichloro methane

CH2Cl2

Solvent in paint remover

Propellant in aerosols

Harmful symptoms of CH2Cl2

Lower- slightly impaired hearing n vision

Higher- Dizziness, nausea, numbness in fingers n toes.

Contact - mild redness, burning of skin n cornea.

Uses of Chloroform (CHCl3)

Producrion of freon refrigerant R-22

Solvent fr fats, alkaloids, iodine and other substances.

General anaesthetic in surgery

Effects of Chloroform

Depresses CNS.

Short term- Dizziness, fatigue, headache

Chronic- Damage liver and kidneys, skin sores.

Y is chloroform stored in closed dark coloured bottles?

Air tight to prevent conversion to carbonyl chloride or phosgene gas.

Use of iodoform

Antiseptic due to free iodine

Replaced due to objectionable smell.

Use of carbon tetrachloride (CCl4)

Manufacture of refrigerants

Propellant fr aerosol cans.

Cleaning fluid

Effect of CCl4

Mild- Dizziness, nausea, vomiting, eye irritation.

Severe- Liver cancer, stupor, coma, nerve cell damage, death.

Atmosphere - Ozone depletion which can cause skin diseases.

Characteristics of freons

Stable

unreactive

non-toxic

non-corrosive

easily liquefiable gases.

Uses of freons

Aerosol propellants

refrigerant

AC purposes

Can upset ozone balance.

Most common freon in industrial use

Freon 12(CCl2F2)

Gv full form of DDT. What is it?

Dichlorodiphenyltrichloroethane

First chlorinated organic insecticide.