halogens

Chlorine containing antibiotic

chloramphenicol

chloramphenicol produced by
use

chloromycetin produced by streptomyces venezuelae//
bacteriostatic antibiotic- treatment of typhoid

Our body produces iodine
containing hormone,

thyroxine

deficiency of throxine causes

goiter

synthetic halogen compound for malaria treatment

chloroquine

halothane

y. Certain fully fluorinated compounds

are being considered as potential blood substitutes
in surgery.

alkyl halides

the halogen atom is bonded to an alkyl group (R).
They form a homologous series represented by CnH2n+1X.

Allylic halides

These are the compounds in which the halogen atom is bonded to an
sp3
-hybridised carbon atom adjacent to carbon-carbon double bond
(C=C) i.e. to an allylic carbon

Benzylic halides

These are the compounds in which the halogen atom is bonded to an sp3 -hybridised carbon atom attached to an aromatic ring.

Compounds Containing
sp2 C—X
Bond

vinylic and aryl halides

In the IUPAC system
of nomenclature, alkyl halides are named as

halosubstituted hydrocarbons.

alkylidene or alkylene

The dihaloalkanes having the same type of halogen atoms are named as alkylidene (same carbon) or alkylene (adjacent carbons) dihalides.

geminal di halides

when both the halogen atoms are present on the same carbon atom of the chain

vicinal halides or vic-dihalides

when halogen atoms are present on adjacent carbon atoms

Halogen atoms are more electronegative than carbon, therefore,
carbon-halogen bond of alkyl halide is

polarised
the carbon atom bears a partial positive charge whereas the halogen atom bears a partial negative charge

arrange acc to dipole moment
Me-F, Me-Cl, Me-Br, Me-I

cl>f>br>i

arrange acc to bond enthalphy
Me-F, Me-Cl, Me-Br, Me-I

f>cl>br>i

The hydroxyl group of an alcohol is replaced by halogen on reaction with (3)

1. conc acid
2. thionyl chloride
3. phosphorous halides

which method is preferred?
(The hydroxyl group of an alcohol is replaced by halogen on reaction with )

thionyl chloride,
results in release of gases- SO2 and HCl which can escape and give PURE alkyl halides

The reactions of primary and
secondary alcohols with HCl require the presence of a catalyst//
three degree?

ZnCl2 //
With tertiary alcohols, the reaction is conducted by simply shaking the
alcohol with concentrated HCl at room temperature.

alkyl bromide preparation from alcohol
what method?
how much quantity

constant boiling
conc HBr 48%

Good yields of
R—I may be obtained by heating alcohols w

sodium or potassium iodide in 95% orthophosphoric acid

Phosphorus tribromide and triiodide
are usually generated

in situ

Phosphorus tribromide and triiodide
are usually generated in situ (produced in the reaction mixture) by the
reaction of

red phosphorus with bromine and iodine respectively

ROH + Na-Br + H2SO4

R-Br + NaHSO4 + H2O

The preparation of alkyl chloride is carried out (2)

1. alcohol soln+ dry HCl gas passing
2. heating soln of alcohol and conc aq halogen acid

The above methods are not applicable for the preparation of aryl
halides because

the carbon-oxygen bond in phenols has a partial double
bond character and is difficult to break being stronger than a single bond

an important method for the detection of double bond in a molecule

: In the laboratory, addition of bromine in
CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless

Finkelstein
facilitates which reaction?
why

RX + NaI --dry acetone--> RI + NaX
X= Cl or Br//
forward reaction
NaX ppt in acetone

Swarts

H3C-Br + AgF --> H3C-F + AgBr

which metallic fluorides can be used in swarts ?

AgF
Hg2F2
CoF2
SbF3

haloarenes how to separate ortho and para isomers

The ortho and para isomers can be easily separated due to large difference in their melting points. R

Reactions with iodine are ______
in nature and require the presence of __

reversible//
oxidising agent like HIO4 or HNO3 to oxidise HI --> I2 + H2O

Fluoro compounds
are not prepared by this method (w hydrocarbons by electrophilic substitution) _____

due to high reactivity of fluorine.

Many volatile halogen compounds
have _______ smell.

sweet

gases at room temperature.

Me F, Methyl chloride, methyl bromide, ethyl chloride and some
chlorofluoromethanes

how is force of attraction in halogen derivatives?
why?
which forces of attraction

1. Due to greater polarity as well as
2. higher molecular mass as compared to the parent hydrocarbon,
the intermolecular forces of attraction (dipole-dipole and van der Waals)are stronger in the halogen derivatives.

The attractions get _______ as the molecules get bigger in size and
have more electrons.

stronger

For the same alkyl group, the boiling
points of alkyl halides decrease in the order: RI> RBr> RCl> RF
why?

1. increase mass
2. increase size
vander waals force increases

Boiling points of isomeric dihalobenzenes are very nearly the same
which has max
why

However, the para-isomers are high melting as compared to their orthoand meta-isomers. It is due to symmetry of para-isomers that fits in
crystal lattice better as compared to ortho- and meta-isomers.

________ derivatives of hydrocarbons are heavier than water

Bromo, iodo and polychloro

The haloalkanes are very slightly soluble in water

why?

in order to dissolve haloalkane in water, energy is required to

1\. overcome the attractions between the haloalkane molecules and

2\. break the hydrogen bonds between water molecules.

3\. Less energy is released when new attractions are set up between the haloalkane and the water molecules as these are not as strong as the original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. H

However, haloalkanes tend to dissolve in organic solvents because

the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules

The reactions of haloalkanes may be divided into the following categories

1\. nucleophilic

2\. electrophilic

3\. reactions with metals

nucleophilic substitution reaction

\

The reaction in which a nucleophile replaces an already existing nucleophile in a molecule.

nucleophilic substitution reaction

why is it called so?

Since the substitution reaction is **initiated** by a nucleophile, it is called nucleophilic substitution reaction.

ambident nucleophiles.

Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles

example of ambident nucleophiles

CN both names

NO2 both names

Actually

1\. cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways \[VC≡N ↔ :C=NV\], i.e., linking through carbon atom resulting in **alkyl cyanides** and through nitrogen atom leading to **isocyanides**

2\. Similarly nitrite ion also represents an ambident nucleophile with two different points of linkage \[–O— N i i =O\]. The linkage through oxygen results in **alkyl nitrites** while through nitrogen atom, it leads to **nitroalkanes**

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

1\. KCN is ionic, gives K+ and CN-

2\. C and N both can attack, but C-C bond> C-N bond (strength)

3\. AgCN is covalent in nature → N can donate electrons easier

second order kinetics

the rate depends upon the concentration of both the reactants.

proposed a mechanism for an SN 2 reaction.

Edward Davies Hughes and Sir Christopher Ingold.

(David and Chris from tsitp)

Hughes worked under Ingold and earned a D.Sc. degree from the University of London.

transition state

how many bonds does carbon make?

can it be isolated?

5

no it is unstable and cannot be isolated

configuration

Spacial arrangement of functional groups around carbon is called its configuration

presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect. Why?

Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group,

Of the simple alkyl halides, _____ react most rapidly in SN 2 reactions because there are only three small hydrogen atoms.

methyl halides

_______ are the least reactive because bulky groups hinder the approaching nucleophiles

Tertiary halides

relative rate of SN2 reaction

Methyl

1

2

3

30

1

0\.02

0

polar protic solvents example

water

acetic acid

alcohol

first order kinetics

in SN1 which reactant?

the rate of reaction depends upon the concentration of only one reactant, which is substrate

the polarised C—Br bond undergoes _____ cleavage to produce a carbocation and a bromide ion.

slow and reversible

how is energy obtained from C-Br bond breaking?

It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent

why is rate of SN1 dependent only on the substrate and not the Nu?

Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion.

Slowest step is generation of carbocation from substrate

developed the first prism that produced plane polarised light

William Nicol

optically active compounds

Plane of plane polarised light produced by passing ordinary light through Nicol prism is rotated when it is passed through the solutions of certain compounds. Such compounds are called optically active compounds.

how is angle of rotation measured

polarimeter

rotation to right

dextrorotatory, d, +

rotation to left

laevo-rotatory , l, -

Such (+) and (–) isomers of a compound are called-

the phenomenon is called

optical isomers//

optical isomerism

Louis Pasteur (1848) observation (2)

what did he believe?

“that crystals of certain compounds exist in the form of mirror images” laid the foundation of modern stereochemistry//

He demonstrated that aqueous solutions of both types of crystals showed optical rotation, equal in magnitude (for solution of equal concentration) but opposite in direction//

He believed that this difference in optical activity was associated with the three dimensional arrangements of atoms in the molecules (configurations) of 2 types of crystals

who received 1st nobel prize in chemistry?

Jacobus Hendricus Van’t Hoff (1852-1911) received the first Nobel Prize in Chemistry in 1901 for his work on solutions.

J. Van’t Hoff and French scientist, C. Le Bel independantly argued-

the spatial arrangement of four groups (valencies) around a central carbon is tetrahedral and if all the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed (overlapped) on the molecule; such a carbon is called asymmetric carbon or stereocentre.

who proposed stereocentre

Vant Hoff (dutch)

Bel (french)

what is responsible for the optical activity in such organic compounds?

1\. asymmetry

2\. non super-imposability of mirror images

chiral objects//

what is this property called

The objects which are non superimposable on their mirror image (like a pair of hands) are said to be chiral//

chirality

achiral objects

objects, which are, superimposable on their mirror images are called achiral.

enantiomers

The stereoisomers related to each other as non superimposable mirror images are called enantiomers

Enantiomers possess identical -. They only differ -

physical properties namely, melting point, boiling point, refractive index, etc//

with respect to the rotation of plane polarised light.

racemic mixture or racemic modification.

A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known

^^External compensation^^

racemisation

The process of conversion of enantiomer into a racemic mixture is known as racemisation.

Retention

Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.

_______, the product will have the same general configuration of groups around the stereocentre as that of reactant.

if during a reaction, no bond to the stereocentre is broken

e that configuration at a symmetric centre in the reactant and product is same but the sign of optical rotation has changed in the product.

This is so because two different compounds with same configuration at asymmetric centre may have different optical rotation. One may be dextrorotatory (plus sign of optical rotation) while other may be laevorotatory (negative sign of optical rotation)

In case of optically active alkyl halides, the product formed as a result of SN2 mechanism has the _______ configuration as compared to the reactant

why?

inverted //

This is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present.

Back-side attack

In case of optically active alkyl halides, SN1 reactions are accompanied by ________

racemisation.//

Actually the carbocation formed in the slow step being sp 2 hybridised is planar (achiral). The attack of the nucleophile may be accomplished from either side of the plane of carbocation resulting in a mixture of products, one having the same configuration (the –OH attaching on the same position as halide ion) and the other having opposite configuration (the –OH attaching on the side opposite to halide ion).

When a haloalkane with β-hydrogen atom is heated with ___ __solution of potassium hydroxide, there is elimination of atom from β-carbon and a__ _____ atom from the α-carbon atom.

alcoholic//

hydrogen//

halogen

why is it called B elimination

. Since β-hydrogen atom is involved in elimination, it is often called β-elimination.

Alexander Zaitsev (also pronounced as Saytzeff)

“in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.

organo-metallic compounds.

most organic chlorides, bromides and iodides react with certain metals to give compounds containing carbon-metal bonds. Such compounds are known

An important class of organo-metallic compounds discovered by _______ in 1900 is alkyl magnesium halide, RMgX

Victor Grignard

It is therefore necessary to avoid even traces of moisture from a Grignard reagent. why?

Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.

RMgX + H2O -→ RH + Mg(OH)X

Chlorobenzene can be converted into phenol

temp

reagent

pressure

623 K

1\. NaOH

2\. H+

300 atm

p-nitro chlorobenzene -→ p nitro phenol

temp

NaOH 443K

H+

o, p-di nitro chlorobenzene -→ o, p dinitro phenol

temp

NaOH 368K

H+

o, p-tri nitro chlorobenzene -→ o, p trinitro phenol

temp

NaOh warm

H2O

Can you think why does NO2 group show its effect only at ortho- and para- positions and not at meta- position?

As shown, the presence of nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene. The carbanion thus formed is stabilised through resonance. The negative charge appeared at ortho- and para- positions with respect to the halogen substituent is stabilised by –NO2 group while in case of meta-nitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the –NO2 group. Therefore, the presence of nitro group at meta- position does not stabilise the negative charge and no effect on reactivity is observed by the presence of –NO2 group at meta-position.

Halogenation

chlorobenzene + Cl2 + AlCl3-→

major is 1,4 dichlorobenzene

minor is 1,2

Nitration

chlorobenzene + HNO3 + conc H2SO4

p nitro > o nitro

sulphonation

chlorobenzene + conc H2SO4

4- chloro benzeneculphonic acid > 2

FC reaction is ortho or para major?

para