Haloalkanes and Haloarenes

Classification: On the basis of

(i) number of halogen atoms present - as mono, di, tri, tetra, etc., halogen derivatives, e.g.,
monohaloalkane C2H5X, dihaloalkane X- CH2- CH2-X, monohaloarene C6H5X.
(ii) nature of the carbon to which halogen atom is attached - 1°, 2°, 3°, allylic, benzylic (sp3
hybridised carbon), vinylic and aryl derivatives (sp2 hybridised carbon), e.g.,

Nature of C-X Bond - The C-X bond is polarized due to higher electronegativity of halogen atom. There is a
partial positive charge on carbon and a partial negative charge on the halogen atom.
C-X bond length increases from C—F to C—I.

General Methods of Preparation:

1. From Alcohols
HCl+ Anhy. ZnCl2
R-Cl + H2O (Groove’s Process- ZnCl2 is used to weaken the C-OH
bond. With 3° alcohols, ZnCl2 is not required.
HCl+ Anhy. ZnCl2 is called Lucas Reagent.)
NaBr + H2SO4
R-Br + H2O + NaHSO4 (NaI/KI + H2SO4 cannot be used to prepare R-I as
H2SO4 is oxidizing in nature & HI is oxidized to I2.
Non oxidizing acids like phosphoric acid are used
instead.)
PX3
R-OH 3R-X + H3PO3 (PX3 is generated in situ with Red phosphorous and X2;
X= Cl, Br)
PCl5
R-Cl + POCl3 +HCl
SOCl2
R-Cl + HCl + SO2  (best method for preparing RX from R-OH since
both the by products are gaseous and escape easily.)

Red P/X2
R-X (X2= Br2 , I2)

This method is not applicable for aryl halides as carbon –oxygen bond in phenols has a partial double bond
character.
2. From Hydrocarbons:
a) Free Radical Halogenation of Alkanes: gives a complex mixture of isomeric mono- and polyhaloalkanes,
which is difficult to separate as pure compounds. The yield of any one compound is low
UV light or heat
CH3- CH2- CH2- CH3 + Cl2 CH3- CH2- CH2- CH2Cl + CH3- CH2- CH (Cl)-CH3

b) I) Halogenation of Aromatic Hydrocarbons (electrophilic substitution): Ortho and para isomers can be
separated due to large difference in their melting points.
Reactions with I2 are reversible & are carried out in presence of an
oxidising agent (HNO3, HIO4) to oxidise the HI formed to iodine.
Fluoro compounds are not prepared by this method due to high
reactivity of fluorine.

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 II) Side Chain Halogenation involves free radical mechanism:

c) Sandmeyer’s Reaction:

d) From alkenes:
i) Addition of Hydrogen Halides on Alkenes: Br
CH3- CH2- CH = CH2 + HBr CH3- CH2- CH- CH3 (Markonikov addition)
CH3-CH2-CH =CH2 + HBr Peroxide CH3-CH2 CH2-CH2Br (Anti Markonikov addition / Peroxide effect)
ii) Addition of Halogens on Alkenes:
CCl4
CH3- CH2- CH = CH2 + Br2 CH3- CH2- CH- CH2
Br Br
e) Halogen Exchange:
i) Finkelstein Reaction: R-X +NaI Acetone R-I +NaX (X= Cl,Br) NaX is precipitated in dry acetone.
ii) Swarts Reaction: H3C-Br +AgF → H3C-F +AgBr (Hg2F2, CoF2 & SbF3 can also be used.)

Physical Properties
1. Boiling points-higher than those of the hydrocarbons of comparable molecular mass.
a) b.p. increases as polarity & molecular mass increase:
R – I > R – Br > R – CI > R – F and CH3CH2CH2X > CH3CH2X > CH3X
b) b.p. decreases as branching increases: CH3 – (CH2)2 – CH2Br > (CH3)2 CHCH2Br > (CH3)3CBr
2. Bond strength: decreases as the size of the halogen atom increases CH3F > CH3Cl > CH3Br > CH3I
3. Dipole moment decreases as the electronegativity of the halogen decreases but the order of dipole
moments of CH3 –X is CH3 – Cl> CH3 –F> CH3 –Br> CH3 –I due to larger bond length of Chlorine.
Dipole moment of chlorobenzene is less than that of cyclohexylchloride as C-Cl bond is shorter due
to resonance.
4. Haloalkanes though polar but are insoluble in water as the new intermolecular attractions between
haloalkanes and water molecules have less strength as the ones being broken in the separate haloalkane
and water molecules.
5. Density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen
atoms. Thus, RI > RBr > RCl > RF
6. The para-isomers of isomeric dihalobenzenes are high melting as compared to their ortho and meta-
isomers due to symmetry of para-isomers Boiling points of isomeric dihalobenzenes are nearly the same.

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Chemical Reactions:
1. Nucleophilic Substitution Reactions (SN reactions): RX + Nu- →RNu + X-

Groups like cyanides and nitrites

possess two nucleophilic centres and
are called ambident nucleophiles.
[C≡N ↔ :C=N], [–O— N=O].
KCN provides cyanide ions in
solution. The attack takes place
mainly through carbon atom since
C—C bond is more stable than C—
N bond. AgCN is mainly covalent
in nature and only nitrogen is free
to donate electron pair forming
isocyanide as the main product.
Similarly, KNO2 forms RONO
while AgNO2 produces R-NO2
as product.

Vinyl chloride is less reactive towards nucleophilic substitution reactions due to resonance.

Nucleophilic substitution reactions are of two types:

(a) SN1 type (Unimolecular nucleophilic reactions proceed in two steps:
Rate, r = k [RX). It is a first
order reaction.
Reactivity order of alkyl halide
towards SN1 mechanism
3° > 2° > 1°

In SN1 reactions, racemization occurs due to the

possibility of frontal as well as backside attack on
planar carbocation.
Polar solvents, low concentration of nucleophiles
and weak nucleophiles favour SN1 mechanism.
Strength of some common nucleophiles is:
:CN- > :I- > :OR- > :OH- > CH3COO-> H2O > F-

(b) SN2 type (Bimolecular nucleophilic substitution reactions proceed in one step. Rate, r = k[RX] [Nu].
It is a second order reaction
Reactivity of halides towards SN2
mechanism is 1° > 2° > 3°
During SN2 reaction, complete
stereochemical inversion i.e., starting
with dextrorotatory halide a laevo
product is obtained and vice-versa, e.g.,

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.
For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the
mechanisms R–I> R–Br>R–Cl>>R–F.
Non-polar solvents, strong nucleophiles and high concentration of nucleophiles favour SN2 mechanism.

Allylic and benzylic halides show high reactivity towards the

SN1 reaction. The carbocation thus formed gets stabilised
through resonance.

Elimination Reactions (Dehydrohalogenation - a β – elimination reaction) halogen from α-carbon atom

and the hydrogen from the α-carbon is eliminated according to Saytzeff rule, e.g. (“in
dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl
groups attached to the doubly bonded carbon atoms.”)
CH3- CH2- CH- CH3 Alc. KOH CH3- CH= CH- CH3 + CH3- CH2- CH= CH2
-KBr, -H2O
Br Major Minor
Ease of dehydrohalogenation among halides 3° > 2° > 1°

2. Reaction with Metals:

i) Wurtz Reaction: 2RX+ 2Na Dry Ether R-R +2NaX
ii) Wurtz-Fittig Reaction: C6H5Cl + 2Na +CH3Cl Dry Ether C6H5 CH3 +2NaCl
iii) Reaction with Mg: C2H5Br + Mg Dry Ether
C2H5 Mg Br (Grignard’s Reagent)
Grignard reagent - an organometallic compound in which the carbon-magnesium bond is covalent but
highly polar; magnesium halogen bond is essentially ionic. Rδ-Mgδ+Xδ-
Grignard reagents are highly reactive and react with any source of proton (even water, alcohols, amines) to
give hydrocarbons. It is, therefore, necessary to avoid even traces of moisture from a Grignard reagent.
RMgX + H2O → R-H + Mg(OH)X

Chemical Properties of Aryl Halides:

a) Nucleophilic Substitution Reaction: Aryl halides are less reactive towards nucleophilic substitution
reaction due to:
a) resonance, C-X bond has partial double bond character.
b) The carbon atom attached to halogen being sp2-hybridised
has a greater s-character and is more electronegative. It can
hold the electron pair of C—X bond more tightly than sp3-
hybridised carbon in haloalkane.
b) Instability of phenyl carbocation.
c) Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach
electron rich arenes.
Replacement by –OH group:
Aryl halides having electron withdrawing groups (like –
NO2, - SO3H, etc.) at ortho and para positions undergo
nucleophilic substitution reaction easily.

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 At ortho- and para-positions, nitro group, withdraws the
electron density from the benzene ring and thus enables the
attack of nucleophile on haloarene. The carbanion formed is
stabilised by resonance. The negative charge 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.

1. Electrophilic Substitution Reactions: Halogens are deactivating but o, p-directing. The halogen atom
because of its –I effect has some tendency to withdraw electrons from the benzene. Due to resonance, the
electron density increases more at ortho- and para-positions than at meta-positions.
(i) Halogenation (ii) Nitration

(iii) Sulphonation

(iv) Friedel-Crafts reaction

(v) Reaction with Metals: (a) Wurtz Fittig reaction (b) Fitting reaction:

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POLYHALOGEN COMPOUNDS
a) Dichloromethane (CH2Cl2) uses: a) a solvent & propellant in aerosols.
Harms the human central nervous system. Direct contact a) with skin causes intense burning and mild
redness of the skin b) with the eyes can burn the cornea.
b) Chloroform [Trichloromethane, CHCl3] used as (i) a solvent for fats, alkaloids, iodine etc. (ii) in
production of the freon refrigerant R-22.
Breathing in air for a short time can cause dizziness, fatigue, and headache.
Chronic exposure may damage the liver (where it is metabolised to phosgene) and the kidneys.
It is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride,
(phosgene). It is, therefore, stored in closed dark-coloured bottles completely filled so that air is kept
out. CHCl3 + O2 Light COCl2 + 2HCl
c) Iodoform (tri-iodornethane, CHl3) shows antiseptic properties as it liberates free iodine. Due to its
objectionable smell, it has been replaced by other formulations containing iodine.
d) Tetrachloromethane (Carbon Tetrachloride, CCl4) is a colourless, non-inflammable, poisonous
liquid. It is used in the manufacture of refrigerants and propellants for aerosol cans and
chlorofluorocarbons, and general solvent use. Exposure to carbon tetrachloride causes dizziness, light
headedness, nausea and vomiting. It can cause permanent damage to nerve cells, make the heart beat
irregularly or stop. It easily rises to the atmosphere and depletes the ozone layer.
e) Freons are the chlorofluorocarbon compounds of methane and ethane. They are extremely stable,
unreactive, non-toxic, non-corrosive and easily liquefiable gases. Freon-12 (CCl2F2) - most common
freons in industrial use, is manufactured from tetrachloromethane by Swarts reaction. These are used
for aerosol propellants, refrigeration and air conditioning purposes. Most freons, make their way into
the stratosphere where it initiates free radical chain reactions that can upset the natural ozone balance.
f) DDT (p, p’-Dichlorodiphenyltrichloroethane) is the first chlorinated organic insecticide and is
particularly effective against the mosquito that spreads malaria and lice that carry typhus. Its stability
and fat solubility is a great problem. DDT is not metabolised very rapidly by animals; instead, it is
deposited and stored in the fatty tissues. If ingestion continues at a steady rate, DDT builds up within
the animal over time.

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Compounds that rotate the plane polarised light (produced by passing ordinary light through Nicol prism
and has its vibrations contained in a single plane) when it is passed through their solutions are called
optically active compounds.

The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter. If
the compound rotates the plane polarised light to the right, i.e., clockwise direction, it is called
dextrorotatory or the d-form and is indicated by placing a positive (+) sign before the degree of rotation. If
the light is rotated towards left (anticlockwise direction), the compound is said to be laevorotatory or the l-
form and a negative (–) sign is placed before the degree of rotation. Such (+) and (–) isomers of a
compound are called optical isomers and the phenomenon is termed as optical isomerism.

The spatial arrangement of four groups around a central carbon is tetrahedral and if all the substituents
attached to that carbon are different; such a carbon is called asymmetric carbon or stereocentre. The
resulting molecule lacks symmetry and is referred to as asymmetric molecule. Asymmetric molecules are
optically active and are nonsuperimposable on their mirror image (like a pair of hands). They are said to
be chiral. This property is known as chirality. E.g., Butan-2-ol, 2-chlorobutane, 2, 3-dihyroxypropanal,
(OHC–CHOH–CH2OH), bromochloro-iodomethane (BrClCHI), 2-bromopropanoic acid (H3C–CHBr–
COOH), etc. While the molecules, which are, superimposable on their mirror images are called achiral.
The stereoisomers related to each other as nonsuperimposable mirror images are called enantiomers.

Enantiomers possess identical physical properties. They only differ with respect to the rotation of plane
polarised light. If one of the enantiomers is dextro rotatory, the other will be laevo rotatory. 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 as racemic
mixture or racemic modification. A racemic mixture is represented by prefixing dl or (±) before the name,
for example (±) butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known as
racemisation.

Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an

asymmetric centre during a chemical reaction or transformation. Inversion of configuration takes place
when the incoming group attaches itself on the side opposite to the one where the outgoing group is present,
and optical activity of product is opposite to that of the asymmetric reactant molecule. There are three
outcomes for a reaction at an asymmetric carbon atom-retention of configuration, inversion of
configuration or racemisation (If a 50:50 mixture of the above two is obtained and the product is optically
inactive).

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 NOMENCATURE SENIORITY OF FUNCTIONAL GROUPS

Carboxylic acid Carol Acids

Sulphonic acid Sullivan
Acid anhydride Actively answers
Ester Esther derivatives
Acid halide Acosta’s half
Amide Amusing
Cyanide Cyclic Cyanides(poisons)
Isocyanide Issues;
Aldehyde Although carbonyl
Ketone Ken
Alcohol Allows alcohols
Thiol Thrilling
Amine Ambushes
Phenyl Phils
Alkene twenty
Alkyne three
Alkyl albums
Alkoxy/Ether alter
Halide Happy
Nitro news

Substances containing double and triple bonds are called alken ynes. (the name ends in yne).
Chain numbering starts from the end closest to either group, unless they’re both equidistant from the chain
ends, in which case the double bond takes priority and is given the lower number.

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