alkyl halides

20
Alkyl Halides Chapter Trihalogen Derivatives The general formula of trihalogen derivative is CHX3 These are also called HALOFORMS. Preparation (i) Haloform test is given by compounds having COCH3 group. CH3COOH cannot give haloform test due to resonance and does not contain pure COCH 3 group. C6H5COC6H5 Benzophenone does not give haloform test due to absence of COCH3 group. 2-alkanol but cannot give haloform test because it is inert towards first step oxidation. Acetoacetic ester even though contains COCH3 group but does not give haloform test due to active methylene group in the molecule where first halogenation occurs. The reacting compounds for haloform test are halogen and alkali which are responsible for halogenation, oxidation and hydrolysis. Reactions involved in haloform test are : (a) Oxidation (b) Halogenation (3 equivalent) (c) Hydrolysis in presence of base

Upload: shivam-gupta

Post on 21-Jul-2016

67 views

Category:

Documents


0 download

DESCRIPTION

Alkyl Halides

TRANSCRIPT

Page 1: Alkyl Halides

Alkyl Halides

Chapter

Trihalogen Derivatives

The general formula of trihalogen derivative is CHX3

These are also called HALOFORMS.

Preparation

(i) Haloform test is given by compounds having –COCH3 group.

CH3COOH cannot give haloform test due to resonance and does not contain pure COCH3 group.

C6H5COC6H5 Benzophenone does not give haloform test due to absence of –COCH3 group.

2-alkanol but cannot give haloform test because it is inert towards first

step oxidation.

Acetoacetic ester even though contains –COCH3 group but does not

give haloform test due to active methylene group in the molecule

where first halogenation occurs.

The reacting compounds for haloform test are halogen and alkali which are responsible for halogenation, oxidation and hydrolysis.

Reactions involved in haloform test are :

(a) Oxidation (b) Halogenation (3 equivalent) (c) Hydrolysis in presence of base

Page 2: Alkyl Halides

Halogen and alkali are used as they form OQ Cl or hypohalite ion which is a strong base to replace all 3-‘H’

The haloform reaction

Halogenation of the µ- carbon atom takes place when an enolate ion is generated in the presence of chlorine, bromine, or iodine

(any halogen).

In the acid-catalyzed halogenation of aldehydes and ketones, rate is independant of the concentration of the halogen; chlorination,

bromination, and iodination all at the same rate. Formation of the enolate is rate-determining, and once formed the enolate ion

reacts rapidly with the halogen but remember three equivalents are required for haloforms.

Unlike its acid-catalyzed route, a-halogenation in base catalysed cannot normally be limited at monohalogenation. Methyl ketones,

for example, undergo polyhalogenation and cleavage on treatment with a halogen in aqueous base.

This is called the haloform reaction because the trihalomethane produced is chloroform, bromoform, or iodoform depending, of

course, on the halogen used. Remember this reaction is done by three equivalent of halogen but if there is only one equivalen t of

halogen then mono halo product is formed.

Normally in seperation test of 2-alkanol or 2-alkanone with other alkanol or alkan one we prefer I2 and alkali for test because

formation of yellow ppt of CHI3 takes place.

The mechanism of first phase of the haloform reaction begin with a-halogenation through the enolate. The electron-attracting –I

effect of an a-halogen increases the acidity of the protons on the carbon to which it is bonded, making each H active and undergo

halogenation at that carbon faster than the preceding one.

Page 3: Alkyl Halides

Alkyl Halides

The trihalomethyl ketone (RCOCX3) so formed when undergoes nucleophilic addition reaction of hydroxide ion to its carbonyl

group, and finally its dissociation by cleavage, of the bond to the CX3 group.

The three –I effect halogen group stabilize the negative charge of the trihalomethide ion (C–X3) converting it to weak conjugate

acid and permitting it to act as a good leaving group in the carbon bond-cleavage step or 2-alkanone.

This reaction is also used for the preparation of carboxylic acid from methyl ketones.

(ii) Haloform test from bleaching powder (CaOCl2)

It is also used as reagent for haloform test due to the presence of OQCl which is a strong base.

Page 4: Alkyl Halides

(iii) From chloral hydrate

It shows intramolecular H-bonding and is thus stable. This method is used to prepare fresh and pure chloroform.

(iv) Reduction of CCl4

Properties of chloroform

(i) Oxidation

Storage of CHl3

a. It is stored in amber coloured bottles up to the brim in dark.

b. Small amount of ethyl alcohol is added which converts poisonous phosgene into nontoxic compound, diethyl carbonate.

(ii) Testing of Chloroform

a. Pure chloroform does not react with AgNO3 due to absence of Cl¯ ion

b. Impure chloroform or oxidised chloroform gives white ppt with AgNO3 due to presence of HCl which contains Cl- ion.

(iii) Reaction with HNO3

(iv) Reaction with acetone

Page 5: Alkyl Halides

(v) Reaction with Benzene

Substitution of halogen by phenyl group occurs in the presence of Lewis acids

In case of CCl4 only 3’Cl’ atoms are replaced by phenylgroup, while 4th ‘Cl’ atom donot undergo replacement by phenyl group

due to high stable intermediate triphenyl methyl carbocation.

(vi) Reaction with aq. KOH

Nucleophilic substitution reaction occurs where ‘Cl’ is substituted by –OH group.

Page 6: Alkyl Halides

Hoffmann?s Carbylamine Reaction

(vii) Hoffmann’s Carbylamine Reaction

When all primary amines reaction with chloroform and alc.

KOH theyform isocyanides with bad smell. It is the most

versatile test for all primary amines. In this a-elimination

occurs.

Mechanism

Separation Tests of some Pairs ?

Page 7: Alkyl Halides

(viii) Reimer Tiemann’s Reaction

Process of formylation of phenols with chloroform in alkaline solution is known as Reimer–Tiemann reaction.

A mixture of ortho-and para-isomers is obtained in which the ortho isomer predominates due to more thermodynamical stability. If

one of the ortho positions is occupied the para-isomer is the major product.

The reaction is carried out by refluxing an alkaline solution of phenol and chloroform at 60°C for sometime (1/2 h). Excess

chloroform is distilled off, the mixture acidified with sulphuric acid and steam distilled. Unreacted phenol and the ortho-isomer

distil over leaving behind the para isomer. The two isomers are further purified by sodium bisulphite.

Page 8: Alkyl Halides

Applications

The reaction mainly used for introducing aldehyde or carboxyl group in phenols.

(i) Preparation of vanillin

(ii) Preparation of piperonal

(iii) Formylation of heterocyclic compounds

Page 9: Alkyl Halides

(v) Preparation of acid (salysylic acid)

When carbon tetrachloride in place of chloroform is used, a carboxyl group is introduced.

(vi) Abnormal Reimer–Tiemann reaction (where ring expansion takes place due to electrocyclic rearrangement)

1. Pyrrole undergoes normal and abnormal RTR

Normal reaction (pyrrole is ortho & para activating compound).

Abnormal reaction (ring expansion)

Page 10: Alkyl Halides
Page 11: Alkyl Halides

Pyrene CCl4

Pyrene CCl4

CCl4 does not react with H2O while SiCl4 reacts due to presence of vacant d–orbital in Si atom used by water molecule as

nucleophile. CCl4 at high temperature reacts with H2O where vacant orbital of carbon at high temperature as used by water to form

phosgene.

Freons : Poly chloro fluoro alkanes are known as freons. They are colourless, odourless, non toxic, non inflammable liquids with

very less chemical reactivity & high stability. They are used in refrigerators and air conditioners for cooling purpose and as

propellents in rockets and jets.

Ozone hole

Chloroflorocarbons known as freons, commercially used for refrigeration purpose they are highly volatile and stable in nature (life

time is more than 100 years) due to this they easily move up into the higher zone of atmosphere (stratosphere), and reacts with

ozone causing hole resulting in to as ozone depletion.

Reaction follows free radical mechanism

(i) Chain initiation step

Page 12: Alkyl Halides

(ii) Chain propagation step

The damage of ozone layer in stratosphere clear the path for harmful ultraviolet radiation to come on earth’s surface.

Now a days hydroflorocarbons (HFC’S) are used as substitutes for CFC’S because CFC’S are involved in depletion of ozone layer .

Ozone has a tendency to absorb uv rays and works as natural shield from harmful rays.

DIHALOGEN DERIVATIVES

Preparation

Properties

Ethyledene chloride and Ethylene chloride gives reactions with following reagents in almost same pattern by nucleophilic

substitution reaction, for example

Preparation of Mono halogen derivatives (RX)

(i) From alcohol

It is the (Darzon’s) best method for preparation of halide from alcohol because pyridine forms a complex with HCl. SO2 is

liberated, thus pure RX is obtained. Pyridine behaves as a weak base due to the presence of lone pair of electrons.

Page 13: Alkyl Halides

alcohols do not react directly with HCl but react in presence of ZnCl2 because in its presence they form a complex which is

stronger acid than HCl itself.

Page 14: Alkyl Halides

Alkyl Halides

Order of reactivity of halogen acid HI > HBr > HCl, because acid which give quickly. Proton is maximum reactive while order of

reactivity of alcohol is 3o > 2o >1o > CH3OH due to intermediate carbocation formation where more the stability of intermedia te

higher the reactivity.

(iv) From 1° amines with NOCl or Tildens reagent (nitrosyl chloride)

(v) From alkenes

(vi) Halogenation of alkanes

This reaction involves decarboxylation and Bromination both simultaneously. This reaction is used for bromides only. It is used to

prepare lower homologues (to step down) in alkyl halide series.

(viii) Substitution of one halogen by another (Halogen exchange)

In the reaction described in earlier part of this chapter, we have already taken examples of nucleophilic substitutions involving

halide leaving groups. Halide ions may also works as nucleophiles. In a reaction known as halogen exchange, one halogen

displaces another from an alkyl halide.

Since the halide which undergo displacement is also a nucleophile, an equilibrium is established. Synthetic chemistry application is

how to shift the position of equilibrium so as to make this reaction an effective and spontaneous one for the preparation of alkyl

fluorides and alkyl iodides.

In the preparation of alkyl fluorides, an alkyl chloride, bromide, or iodide is heated with potassium fluoride in a high boil ing

alcohol solvent such as propylene glycol.

Alkyl fluorides have the lowest boiling points (Lowest mass) of all the alkyl halides and are removed from the reaction mixture by

distillation as they are formed. According to Le Chatelier’s principle, the system responds by forming more alkyl fluoride at the

expense of the original alkyl halide. Even if the alkyl fluoride were not removed by distillation, it would predominate at

Page 15: Alkyl Halides

equilibrium because the reaction favours spontaneity of the stronger C — F bond in place of weaker C –– I, C –– Br, or C –– Cl

bonds of low dissociation energy. Since synthetic method for these compounds, this reaction is known as Swart’s reaction.

Alkyl iodides may be prepared from alkyl chlorides and bromides by treatment with sodium iodide in acetone as the solvent,

known as (Finkelstein) connent reaction.

Le Chatelier’s principle under go application in this reaction. Sodium iodide is soluble in acetone due to more covalent nature but

sodium bromide and sodium chloride are not. In these reactions, sodium chloride and sodium bromide precipitate from the react ion

mixture, causing the position of equilibrium to shift in forward direction (activemass is unity for solid) so as to favour formation of

the alkyl iodides.

Physical Properties

(a) Bond strength: In haloalkanes bond strength of carbon–halogen bond decrease with an increase in bond length, as one moves

from fluorine to iodine. This is attributed to the size of p orbital, which increases from fluorine to iodine and thus makes the sp3 -p

overlap less effective.

(b) Solubility and state of matter: Haloalkanes are insoluble in water but soluble in organic solvents. Lower molecular mass

haloalkanes are gases at room temperature while higher molecular mass haloalkanes are liquid at room temperature.

(c) Boiling point: Boiling point of haloalkanes is higher compared to corresponding alkanes due to dipole–dipole interaction. With

an increase in molecular mass, there is an increase in boiling point. For the same alkyl group the boiling point increases from

fluoroalkane (R–F) to iodoalkane (R–I). Iodine has a larger surface area and outer electrons are loosely bound. This makes iodine a

highly polarizable atom. A polarizable atom has increased London forces of attraction (section 1.7), which causes an increase in

boiling point. The branched chain haloalkanes follow a similar increasing order of boiling points.

(d) Density: The densities of haloalkanes increase with atomic mass of the halogen and decrease with increasing size of the alkyl

group. In monohaloalkanes, iodomethane (CH3I) has the maximum density.

Page 16: Alkyl Halides

The general characteristics of monohaloalkanes can be enumerated as follows:

(a) Fluoro and chloroalkanes are less dense than water whereas bromo and iodoalkanes are denser than water.

(b) Monofluroalkanes are unstable and on heating, H–F is eliminated to produce alkene.

(c) Bromo and iodoalkanes are generally photosensitive and are stored in brown opaque bottles. Otherwise, they liberate free

bromine and iodine respectively.

Properties of mono halogen derivatives

Mono halogen derivatives follows nucleophilic substitution reactions which are mainly of S

N

1, S

N

2 type. Reaction follows which type of mechanism is mainly decided by halide quality and medium (solvent). Detail study of

mechanism is given in the next chapter of the book.

Page 17: Alkyl Halides
Page 18: Alkyl Halides

Synthesis of cyanide and Isocyanide

(i) Synthesis of cyanide and Isocyanide

With KCN, halides mainly form cyanides because in KCN molecule ionic bonding occurs which produces CN- with ambident

nature where two sites of attachment are there as negative charge and lone pair of electrons. Attachment prefers at carbon site

because negative charge is more active than pure lone pair. Reaction with AgCN (covalent bonding) produces isoaganides as major

product.

NO2 also works as ambident nucleophile because it has two sites for attachment one is nitrogen lone pair while other is negative

charge on oxygen atom. There is ionic bonding in KNO2 molecule so it produces exclusively alkyl nitrite while with AgNO2

(covalent) attachment prefer from nitrogen site because lone pair of nitrogen is more reactive than lone pair of oxygen.

When primary halides reacts with ammonia to form primary amines as major product but when excess of alkyl halides are taken

than exhaustive alkylation occurs, where synthesis of quaterary ammonium salts takes place. By heating they undergo b-

elimination through Hoffmann’s orientation forms less substituted alkenes as major product.

Page 19: Alkyl Halides

In above cases alkene formation takes place by ethyl group & not by propyl group because b–carbon of propyl is 2° in nature &

abstraction of proton order in case of Hoffmann’s b–eliminations is 1°>2°>3° carbon atom otherwise –CH2NO2 > –CH2Cl > 1°. If

any –I effect group is present on b carbon it increases the reactivity for elimination from that site.

Other chemical reaction mainly follows E-1 or E-2 reaction for elimination (dehydration and dehydrohalogenation) where sytzeff’s

rule for stabilization occurs for example.

(iv) Some other synthetic methods from alkyl halide

Page 20: Alkyl Halides