SARASWATI
CHEMISTRY POINT
Haloalkanes and Halorenes
Introduction
The replacement of hydrogen atom(s) from a hydrocarbon (aliphatic or aromatic) by halogen atom(s)
result in the formation of alkyl halide and aryl halide. These may be classified as mono, di or tri halo
compounds depending upon the number of halogen atoms.
(i) Alkyl halides are the compounds whose molecules contain one halo group directly attached to alkyl
chain. For example,
H
R
H
C
X
R
C
R
X
R
C
H
R
R
1° (mono)
2° (dil)
3° (tri)
X
(ii) Aryl halides are the compounds whose molecules contain halo group directly attached to benzene
ring. For example,
X
X
CH3
(X = Cl, Br, I)
 Preparation of Haloalkanes
From Alcohols
Haloalkanes can be obtained from alcohols by the reaction with halogen acids or phosphorus halides
or thionyl chlorides. For example,
 R – OH + HCl ZnCl2 RCl + H2O
 ROH + HBr H2SO4 (aq) RBr + H2O
 3R – OH + PX3
 ROH + PCl5
3RX + H3PO3
RCl + POCl3 + HC
 R – OH P(red)/X2
RX; (X2 = Br2, I2)
 R – OH + SOCl2 Pyridine RCl +SO2 ↑ + HCl ↑
From Alkanes
Alkane in the presence of sunlight shows free radical halogenation. For example,
CH3 – CH2 – CH2 – CH3
Cl₂/UV light
or heat
CH3 – CH2 – CH2 – CH2 – Cl + CH3 – CH2 – CH – CH3
Cl
Halide exchange Method
This includes Finkelstein reaction and swarts reaction.
(i) Finkelstein reaction It is used for the conversion of chloroalkane or bromoalkane to iodoalkane.
RX + NaI Acetone RI + NaX (X = Br, Cl)
(ii) Preparation of alkyl fluorides Alkyl chloride/bromide when heated with metallic fluoride such as
AgF, Hg2F2, CoF2 or SbF3 gives alkyl fluoride. The reaction is termed as Swarts reaction.
CH3 – Br + AgF
PITAM PURA & ROHINI
CH3F + AgBr
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
1
SARASWATI
CHEMISTRY POINT
 Preparation of Aryl Halides (Haloarenes)
By Electrophilic Substitution of Arenes
It is carried out in the presence of a halogen carrier like FeCl3, FeBr3 in dark and at low temperature.
CH3
CH3
Fe,dark Or
+ X2
CH3
+
FeCl3 (anhyd)
X
X
o-halo toluene
(X–Cl, Br, I)
p-halo toluene
Sandmeyer’s Reaction
 In this reaction, benzene diazonium chloride is warmed with cuprous chloride/bromide in HCl to obtain
chloro/bromo arenes. For example,
+
N ≡ NX ¯
NH2
NaNO2 + H𝑋
273−278K
Benzene diazonium halide
+
N2X ¯
X¯
+ N2
CU₂X₂
(X – Cl, Br)
 Iodobenzene is obtained by warming diazonium salt with KI.
+
N2X ¯
I
KI
Gattermann
reaction
+ N2 + KX
 Fluorobenzene is obtained by using fluoroboric acid, HBF4.
F
+ N2 + BF3
The reaction is known as Balz Schiemann reaction.
Hunsdiecker Reaction
COOAg
+ Br₂
Br
CCl₄
Reflux
Silver benzoate
+ CO2 + AgBr
Bromobezene
 Physical Properties o Haloalkanes and Haloarenes
Boiling & Melting Point
 Alkyl halides and haloarenes are polar compounds and have higher b.p. due to dipole-dipole interaction.
Alkyl/aryl halides have higher b.p. than hydrocarbons of comparable molecular mass.
 Decreasing order of b.p. of alkyl halides is as follows
RI > RBr > RCl > RF
 Straight chain alkyl halides have higher b.p. as comparable with branched chain alkyl halide of similar
molecular weight., hence boiling points of isomeric Haloalkanes decrease with increase of isomeric
Haloalkanes decrease with increase in branching due to decrease in surface area.eg,
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
2
SARASWATI
B.p./K
CHEMISTRY POINT
CH3 – CH2 – CH2 – CH2 – Br
375
CH3
CH3 – CH2 – CH – CH3
B.p./K
Cl
H3C – C – CH3
Br
Br
364
346
Cl
Cl
Cl
Cl
B.p./K
B.p./K
364
364
446
446
Cl
448
448
Density and Solubility
Bromo and iodo derivatives of hydrocarbons are heavier than water.
 Lower halo alkanes are very slightly soluble in water but others are insoluble in water because
Haloalkanes/ haloarenes do not form hydrogen bond with water.
 Chemical Properties of Haloalkanes
Haloalkanes are highly reactive compounds due to presence of polar R – X bond. The reactions of
Haloalkanes
may be of two types. These are nucleophilic substitution reactions and elimination reactions.
Nucleophilic substitution Reaction
In Haloalkanes, these reactions proceed either by SN1 or by SN2 type of mechanism.
(a) Substitution nucleophilic unimolecular reaction (SN1)
Tertiary alkyl group and polar solvent favour SN1.
It follows first order kinetics. Rate = K (subtract)
Carbocation is formed and gets rearranged, if possible.
Reaction is completed in two steps.
Nucleophile can be attack on front and back side. Therefore, racemic mixture can be formed.
CH3
CH3
Step I
+
Eg, (CH3)3CBr
+ Br⁻
+ OH⁻ Step II (CH3)3C – OH
H3C
CH
H3C
CH
(i)
(ii)
(iii)
(iv)
(v)
(b) Substitution nucleophilic bimolecular reaction (SN2).
(i) Primary alkyl group and non-polar solvent favour SN2.
(ii) If follows second order kinetics.
(iii) Carbocation does not form.
(iv) Reaction is completed in one step.
(v) It involves complete inversion in configuration.
H
eg,
H
H
Cl + OH⊖
HO
H
H
Cl
H
HO
H + Cl⊖
H
Transition state
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
3
SARASWATI
CHEMISTRY POINT
(Solid bond) – Towards the viewers
(Wedge dash bond) – Away from the viewers
 Reactivity of halides towards SN2 mechanism is 1° > 2° > 3° > neo-pentyl halides.
 In SN2 mechanism, rate of reactions does not depend upon the strength of attacking nucleophile. The
strength of different nucleophiles is
CN⁻ > I⁻ > OR⁻ > OH⁻ > CH3COO⁻ > H2O > F⁻.
Stereochemical Aspects of Nucleophilic
Substitution Reactions
For SN1 and SN2, we need to learn some basic Stereochemical principles and notations:
a
Ordinary light
nicol prism
plane polarized light
Substance
Optically active
If plane polarized light rotate clockwise, when passed through a substance, the substance is known as
dextro rotator or (+) or (d). If rotate anticlockwise, it is known as laevo rotatory or I or (-).
Chiral Carbon. It has four different group s or atoms attached to it.
Chiral Molecule. Optically active molecule is known as chiral molecule.
Racemic mixture Equimolar. Equimolar mixture of enantiomers is known as racemic mixture.
Racemic
Mixture is optically inactive due to external compensation. This phenomenon is known as racemisation.
Inversion of configuration. The back side attack by a nucleophile in SN2 reaction gives rise to a
product whose configuration is opposite to the reactant. Therefore, during SN2 reaction inversion of
configuration occurs.
This 100% inversion of configuration is known as Walden inversion.
Elimination Reactions
 When Haloalkanes having -hydrogen atom is heated with alc KOH. -elimination takes place. Eg,
H
H
H
C
C
H
X
H
H
Alc.KOH
− K𝑋
− H₂O
H
C=C
H
H
 If there is possibility of formation of more than one alkene due to availability of more than one -H
atoms, the elimination of HX takes place according to Saytzeff rule. It states, “In
dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of
alkyl groups attached to the doubly bonded carbon atoms. eg,
Some Important Reaction of Haloalkanes
(i) R – X AgNO2
(ii) RX
(iii) RX
(iv) RX
R – NO2 + Ag X
NaNO2 or KNO2
KCN or NaCN
AgCN
R – ONO + Kx or NaX
RCN + KX or NaX
RNC + AgX
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
4
SARASWATI
CHEMISTRY POINT
RNH3 + R – NH – R + R3N + [R4N+]Cl⁻
(v) RX + NH3
[Hofmann ammonolysis reaction]
Dry ether
(vi) RX + Mg
RMgX
Grignard reagent
 Chemical Properties of Haloarenes
Haloarenes give three types of reactions:
Nucleophilic Substitution Reactions
Haloarenes are less reactive than Haloalkanes towards nucleophilic substitution reaction.
For example;
Cl
Cl
(i)NaOH,623K,300atm

(ii)
H⁺
This is known as Dow’s process.
Cl
(i)

Cl
NaOH,443K
(ii)
H⁺
NO2
NO2
Cl
Cl
NO2
NO2
(i)

(ii)
NaOH, 368K
H⁺
NO2
NO2
Cl
Cl
O2N

NO2
O2N
(i)
Warm
(ii)
H₂O
NO2
NO2
NO2
2, 4, 6 (-) trinitrophenol
or picric acid
Electrophilic Substitution Reactions
Haloarenes undergo usual electrophilic substitution reactions of benzene nucleus such as
halogenation, nitration, sulphonation and Friedel-Crafts’ alkylation and Acylation, eg.

Halogenation
Cl
Cl
Cl
Cl
+ Cl2
(i) AlCl3 (anhyd)
+
Cl
o-dichloro
p-dichloro
benzene (minor)
benzene (major)

Nitration
Cl
PITAM PURA & ROHINI
Cl
SUDHIR PANWAR
Cl
9871621367 / 9213145146 / 9899899326
5
SARASWATI
CHEMISTRY POINT
NO2
HNO3 (conc.)
+
H2 SO4 (conc.)
1–chloro–4–nitrobenzene
NO2
(minor)
1–chloro–4–nitrobenzene
(major)

Sulphonation
Cl
Cl
Cl
SO3H
H₂SO₄(conc.)
+
∆
SO3 H
2–chlorobenzene
Sulphonic acid(minor)
4–chlorobenzene
Sulphonic acid (major)

Friedel-Crafts alkylation
Cl
Cl
Cl
CH3
+ CH3Cl
AlCl₃ (anhyd.)
+
1–chloro methyl
Benzene (minor)
CH3
4–chloro methyl
Benzene (major)
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
6
SARASWATI

Friedel-Crafts Acylation
Cl
CHEMISTRY POINT
Cl
Cl
COCH3
+ CH3COCl
AlCl₃ (anhyd.)
+
2–chloro acetophenone
(minor)
COCH3
4–chloro acetophenone
(major)
Reactions with Metals
 Wurtz-Fittig reaction A mixture of haloarenes and Haloalkanes, when treated with Na in dry ether gives alkyl
arene eg.
X
R
Ether
+ 2Na + RX
+ NaX

Alkyl benzene
Fittig reaction Haloarenes on treatment with Na in dry ether give analogous compounds, eg,
X
R
2
+ 2Na Ether
+ 2NaX
diphenyl
 Polyhalogen Compounds
Dichloromethane
 CH2Cl2 is widely used as a solvent.
 It is used as a paint remover.
 It is used as a propellant in aerosols.
 It is used as a process solvent in the manufacture of drugs.
Trichloromethane
 Chemically, chloroform is employed as a solvent for fats, alkaloids, iodine and other substances.
 In previous time, it was used as a general anaesthetic but now a day it is replaced by less toxic and
safe anaesthetic, ether.
 The major use of chloroform today is in the production of the Freon refrigerant R – 22.
 Chloroform is stored in closed dark coloured bottles, filled completely so that air is kept out because
it is oxidised by air in the presence of light.
2CHCl3 + O2 Light
(air)
2COCl2
+ 2HCl
phosgene
(poisonous gas)
Tetrachloromethane
 It is produced in large quantities for use in the manufacture of refrigerants and propellants for aerosol
cans.
 It is also used as feed stock in the synthesis of chlorofluorocarbons and other chemicals.
 It is widely used as industrial solvent for oils, fats, resins, loacquers etc.
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
7
SARASWATI
CHEMISTRY POINT
 It is used as fire extinguisher but after using it to extinguish fire; the room should be ventilated to
remove Phosgene.
Freons
 They are extremely stable, unreactive non-toxic, non-corrosive and easily liquefiable gases.
 It is manufactured from tetrachloromethane by Swarts reaction and used for aerosol propellants,
refrigeration and air conditioning purposes.
 Freons deplete the protective ozone layer surrounding our planet, so their use has been banned in may
Countries.
DDT (Dichlorodiphenyltrichloro methane)

Preparation
—H
Cl—
H
+ O = C – CCl3
—H
Cl—
H2SO4 (conc.)
Chloral
chlorobenzene
Cl—
2
1
CHCCl3 + H2O
Cl—
(DDT)
p, p’ -dichlorodiphenyltrichloroethane
 It is a powerful insecticide but is not easily biodegradable. Therefore, its long term effects could be
potentially dangerous and its use is banned in many countries.
Solved Questions
Q1. Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN form
isocyanides as the chief product. Explain.
Ans: KCN is a ionic compound and provides cyanide ions in solution. Although both carbon and nitrogen
atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not
through nitrogen atom since C – C bond is more stable than C – N bond. However, AgCN is mainly covalent
in nature and nitrogen is free to donate electron pair forming isocyanide as the main product.
Q2. Among the isomeric alkanes of molecular formula C5H12. Identify the one that on photochemical
chlorination yields.
(i) A single monochloride (ii) Three isomeric monochlorides (iii) Four isomeric monochlorides.
Ans: (i)
CH3
CH3
C
CH3
CH3
Neopentane
All the H atoms are equivalent. Therefore replacement of any one of them will give the same product.
1
2
3
2
1
(ii) CH3 CH2 CH2 CH2 CH3
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
8
SARASWATI
CHEMISTRY POINT
n-Pentane
There are three groups of equivalent hydrogen designated as 1, 2 and 3. The replacement of any one
of the equivalent hydrogen will give the same product. Therefore, three isomeric monochlorides are
possible.
1
2
3
4
(iii) CH3 – CH – CH2 – CH3
1
CH3
There are four types of equivalent hydrogen designed as 1, 2, 3 and 4. Therefore, four isomeric
monochlorides are possible.
Q3. Arrange each set of compounds in order of increasing boiling points.
(a) Bromomethane, bromoform, chloromethane, dichloromethane.
(b) 1-Chloropropane, isopropyl chloride, 1-Chlorobutane.
Ans: (a) we know that boiling point is directly proportional to size of halogen in same alkyl group,
compound directly proportional to the molecular mass. The boiling points increase as the number of
halogen atoms increases. Combining all the arguments presented above, the boiling points of the
four compounds discussed above increases in the order:
Chloromethane < Bromomethane < dibromomethane < 1-chlorobutane.
(b) For the same halogen, boiling point is directly proportional to the size of the alkyl group increases
directly proportional to the molecular mass. Combining these two arguments, the boiling points of
the three compounds discussed above increase in the order:
Isopropyl chloride < 1-chloropropane < 1-chlorobutane.
Q4. Explain why:
(i) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride.
(ii) Alkyl halides, though polar, are immiscrible with water.
(iii) Grignard reagents should be prepared under anhydrous conditions.
Ans: (i) In chlorobenzene the electron withdrawing inductive effect of Cl atom is opposed by electron
releasing resonance effect. Therefore, it has relatively lower dipole moment. On the other had in
cyclohexyl chloride there is only electron withdrawing inductive effect of Cl atom due to which it
has higher dipole moment.
(ii) Alkyl halides cannot form H-bonds with water molecules and hence are insoluble in water.
(iii) Grignard reagents react with water and get decomposed
RMgX + H2O → R – H + MgX(OH)
Hence, they have to be prepared under anhydrous conditions.
Q5. p-Dichlorobenzene has higher m.p. than those of o-and m- isomers. Discuss.
Ans: p-Dichlorobenzene has melting point than o- and m- isomers due to its symmetry which leads to
more close packing of its molecules in the crystal lattice and consequently stronger intermolecular
attractive forces.
Cl–
–Cl
More symmetrical molecular attractive forces.
Q6. The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in
presence of alcoholic KOH alkenes are major products. Explains
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
9
SARASWATI
CHEMISTRY POINT
Ans: When an alkyl halides is treated with some nucleophile/base, there is always competition between
substitution and elimination to take place. In the present case, substitution would result in alcohol as
product whereas elimination would result in alkene as product. In the presence of solvent of low
polarity such as alcohol elimination reaction is favoured whereas in the presence of solvent of high
polarity substitution is favoured.
Q7. What happens when
(i) n-Butyl chloride is treated with alcoholic KOH.
(ii) Bromobenzene is treated with Mg in the presence of dry ether.
(iii) Chlorobenzene is subjected to hydrolysis.
(iv) Ethyl chloride is treated with (aq) KOH.
(v) Mehtyl bromide is treated with sodium in the presence of dry ether.
(vi) Methyl chloride is treated with KCN.
Ans: (i) n-Butyl chloride undergoes -elimination to yield but-I-ene.
CH3CH2CH2CH2Cl alc. KOH
CH3CH2CH = CH2
n-Butyl Chloride
But-1-Ene
(ii) Grignard reagent is formed
(iv) It undergoes nucleophilic substitution to yield ethyl alcohol.
(v) Methylbromide undergoes Wurtz reaction to yield ethane.
(vi) Methyl cyanide is formed by nucleophilic substitution.
Q8. Differentiate between retention and inversion.
Ans: If the relative configuration of the atoms/groups around a chiral centre in an optically active molecule
remains the same before and after the reaction, the reaction is said to proceed with retention of
configuration. On the other hand, if the relative configuration of the atoms/ groups around a
stereocentre in the product is opposite to that in the reactant, the reaction is said to proceed with
inversion of configuration. For examples,
Q9. Explain the following in one or two sentences.
(i) Allyl chloride is hydrolysed more readily than n-propyl chloride.
(ii) Vinyl chloride is hydrolysed more slowly than ethyl chloride.
Ans: (i) Allyl chloride readily undergoes ionization to produce resonance stabilized allyl carbocation. Since
carbocations are reactive species, therefore, allyl cation readily combines with OH⁻ ions to form allyl
alcohol.
CH2 = CH – CH2 – Cl Ionization CH2 = CH – CH2 + Cl⁻
Allyl chloride
𝑂𝐻⁻
CH2 = CH – CH2OH
CH2 – CH = CH2
𝐹𝑎𝑠𝑡
In contrast, n-propyl chloride does not undergo ionization to produce n-propyl carbocation and hence
allyl chloride is hydrolysed more readily than n-propyl chloride.
(ii) Vinly chloride may be represented as a resonance hybrid of the following two structures:
CH2 = CH – Cl  :CH2 – CH = Cl:
As a result of resonance, the carbon-chloride bond acquires some double bond character. In contrast,
in ethyl chloride, the carbon-chlorine bond is a pure single bond. Thus vinyl chloride undergoes
hydrolysis more slowly than ethyl chloride.
Q10. Account for the following:
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
10
SARASWATI
CHEMISTRY POINT
Haloalkanes undergo nucleophilic substitutions whereas haloarenes undergo electrophilic
substitutions.
Ans: Haloalkanes ae more polar than haloarenes. Consequently the carbon atom carrying the halogen in
Haloalkanes is more electron-deficient than that in haloarenes. As a result Haloalkanes undergo
nucleophilic substitutions more readily than haloarenes.
In contrast, haloarenes contain a benzene ring. Since the typical reactions o benzene are electrophilic
substitutions, therefore, haloarenes undergo electrophilic substitutions which do not contain a benzene
rig do not undergo electrophilic substitutions.
Q11. Neopentyl bromide undergoes nucleophilic substitution reactions very slowly. Why?
Neopentyl bromide undergoes SN2 reaction slowly due to
(a) Bulky Neopentyl group difficult for a Nucleophile to attack from backside at C of C – Br bond.
(b) Cleavage of C – Br bond gives 1˚ carbocation which is less stable.
Q12. Describe nucleophilic substitution reaction.
Ans: The reactions involving the displacement of one nucleophile by another nucleophile is known as
nucleophilic substitution reaction. There are two different mechanisms possible for nucleophilic
aliphatic substitution.
(i) Unimolecular nucleophilic substitution – According to these mechanisms, the reaction takes place
in two steps. The first step involves the heterolytic cleavage of C – X bond resulting in the
formation of carbocation. The formation of carbocation is the rate determining and this step
involves only RX and is unimolecular reaction. As the stability order of the carbocation is 3° > 2° >
1°, the reactivity of R towards SN1 mechanism follows a order
3° RX > 2°RX > 1°RX
The second step is the fast reaction between the carbocation and the nucleophile. The reaction is
summarized as follow.
R
R
R’
C
X
slow
R
⊕
C
R”
R’
Nu
fast
R
C
Nu
R”
R
R’
+
C
R”
R
Nu
The rate of reaction depends only on the concentration of RX and independent of nucleophile.
(ii) Bimolecular nucleophilic substitution – According to this mechanism, the reaction proceeds via
transitions state in which the bonds are half broken and half-formed. In this, the nucleophile attacks
at the C of C – X bond from the backside which has to be less crowded. Therefore, transition state is
formed and is the rate determining step. This then break down fast to give the desired precuts.
The reactivity order of RX towards SN2 mechanism is
3° RX < 2°RX < 1°RX < CH3X
The reaction is summarized as follows:
The rate of reaction depends on the concentrations of both RX and nucleophile.
Q13. Why is thionyl chloride considered as the best reagent to convert alcohol into alkyl chlorides?
Ans: Thionyl Chloride is considered the best reagent because both the byproducts are gaseous which escape
from the reaction mixture. This makes the purification of the chloro compounds easy.
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
11
SARASWATI
CHEMISTRY POINT
Q14. What happen when CHCl3 react with oxygen in presence of sunlight?
Ans: Chloroform is slowly oxidised by air in the presence of sunlight to form poisonous phosgene gas.
CHCl3 = ½ O2 sunlight COCl2 + HCl
For this reason, chloroform is stored in dark-coloured bottles. The bottle also contains small amount
of ethyl alcohol.
Q15. Explain the terms: (i) Plane-polarized light, (ii) Optical activity, (iii) Mirror plane, (iv)
Asymmetric molecule, (v) super-imposable mirror images.
Ans: (i) Plane-polarized light: A beam of light which has oscillations in one plane only is called plane
polarized light.
(ii) Optical activity: The substances which rotate the plane polarized light are called optically active
substance. For examples: Lactic acid, tartaric acid, glucose, fructose etc. This property of a substance by
virtue of which it rotates the plane of polarized light, is called optical activity.
(iii) Mirror Plane: It is a plane that bisects a molecule in such a way that the two halves of molecules are
mirror image of each other. It is also called mirror plane or plane of symmetry.
(iv) Asymmetric molecule: A molecule is said to be asymmetric if it does not possess any symmetry.
Asymmetric molecules are optically active. In the organic chemistry, a molecule becomes asymmetric if
all the four substituent attached to a carbon atom are different.
(v) Super-imposable mirror images: Symmetrical objects are identical to their mirror images. The
symmetrical objects and its mirror images are called super –imposable mirror images of one another.
Q16. Point out the difference between (i) chirality and chiral centre, (ii) racemic modification and
meso compound.
Ans: (i) Chirality and Chiral Centre
Q17. Why enantiomers are also called optical isomers?
Ans: Enantiomers rotate the plane of polarized light either towards right or towards left.
Q18. What is racemic mixture?
Ans: A mixture containing equimolar amounts of two enantiomers.
Q19. Optically active 2-iodobutane on treatment with NaI in acetone gives a product which does not
show optical activity. Explain.
Ans: In the reaction with Na the C–I bond first cleaves and then reforms. This leads to the formation of
racemic mixture which is optically inactive.
CH3
H
C
CH3
I
-I⁻
C2H5
H
C⊕
CH3
+I⁺
(NaI)
C2H6
H
C
CH3
I
C2H5
+
I
C
H
C2H5
Racemic mixture (±)
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
12
SARASWATI
CHEMISTRY POINT
Unsolved Questions
1. Give the IUPAC name of the following compound.
CH2 = C – CH2Br
2.
3.
4.
5.
6.
7.
8.
9.
CH3
Write the IUPAC name of the following compound (CH3)3 CCH2Br.
Write the IUPAC name of the following compound CH2 = CHCH2Br.
Write the structure of 1, 4-Dibromobut-2-ene.
Write the structure of 2- (2-Bromophenyl) butane
Write the structure of 2- (2-Chlorophenyl)-1-iodooctane
Write the structure of the following compound: 1-Bromo-4-sec. Butyl-2-methyl benzene.
Which will react faster in SN2 displacement, 1-Bromopentane or 2-Bromopentane & why?
Write the IUPAC name of the following.
H
CH3
H
H
Br
H
10. A solution of KOH hydrolysis CH3CHClCH2CH3 and CH3CH2CH2CH2Cl. Which one of these is more
easily hydrolyzed?
11. Give the IUPAC name of the following.
H3C
CH3
H
H
Br
H3C
12. Write the structure of the compound 4-tert.butyl-3-iodoheptane.
13. Write the IUPAC name of the following compound.
CH3
H3C – C – CH2Cl
CH3
14. Write the structure of the compound 1-Chloro-4 ethylcyclohexane.
15. Draw the structure of the following compound: 4-Bromo-3-methylpent-2-ene.
16. Write the structure of the following organic compound: 2-Chloro-3-methylpentane.
17. Write a chemical reaction in which the iodide ion replaces the diazonium group in a diazonium salt.
18. Write the IUPAC name of ClCH2C ≡ CCH2Br.
19. Write IUPAC name of
H3C
CH3
H
H
PITAM PURA & ROHINI
Br
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
13
SARASWATI
CHEMISTRY POINT
H
20. Which one of the two compounds CH3Br and CH3I will react faster in an SN2 reaction with –OH?
21. Explain as to why
i. Alkyl halides, though polar are immiscible with water?
ii. Grignards reagent should be prepared under anhydrous conditions?
22. What are ambident nucleophiles? Explain giving an example.
23. Which one in the following pairs of substances undergoes SN2 substitution reaction faster and why?
(i)
CH2Cl Or
(ii)
Cl
| Or
Cl
24. Which one in the following pairs undergoes S N2 substitution reaction faster and why?
Cl
(i)
Cl
Or
Cl
(ii)
Or
Cl
25. Complete the following reaction equations.
CH3
(i)
+ Hl
(ii) CH3CH2CH = CH2 + HBr
(iii)
H + HBr
H
H
26. Suggest a possible mechanism for the following reaction:
n-m + BuBr + KCN EtOH, H2O n-BuCN
27. Suggest a possible reason for the following observations.
(i) The order of reactivity of haloalkanes is Rl > RBr > RCl.
(ii) Neo-pentyl chloride, (CH3)3C – CH3Cl does not follow SN2 mechanism.
28. Complete the following reaction equation.
(i) C6H5N2Cl + Kl
H
H
(ii)
C=C
+ Br2 CCl4
H
H
29. Discuss the mechanism of SN1 reaction of haloalkanes.
30. Give one example of
(i) Wurtz reaction
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
14
SARASWATI
CHEMISTRY POINT
(ii) Wurtz fittig reaction
31. What happens when iodoform is heated with silver powder? Write chemical reaction.
32. What type of reaction is known as nucleophilic substitution reaction? Give on e example.
33. Write IUPAC name of the following compound.
F
Cl
(i) CH3 – C – CH2 – C – CH3
C2H5
C2H5
C2H5
C2H5
(ii) CH3 – C – CH2 – C – CH3
NO2
34. Although chlorine is an electron withdrawing group, yet it is ortho-para-directing in electrophilic
aromatic substitution reactions. Explain, why is it so?
35. Answer the following questions.
(i) What is meant by chirality of a compound? Give an example.
(ii) Which one of the following compound is more easily hydrolyzed by KOH and why?
CH3CHClCH2CH3 or CH3CH2CH2CH2Cl
(iii) Which one undergoes SN2 substitution reaction faster and why?
| Or
Cl
(iv) Haloalkanes easily dissolve is organic solvents. Why?
(v) What is known as racemic mixture? Give an example.
(vi) Of the two bromo derivatives C6H5CH(CH3)Br and (C6H5)CH (C6H5)Br, which on is more reactive in
SN1 substitution reaction and why?
36. Rearrange the compounds of each of the following sets in order of reactivity towards SN2
displacement.
(i) 2-Bromo-2-methyl butane, 1-Bromopentane, 2-Bromopentane.
(ii) 1-Bromo-3-methyl butane, 2-Bromo-2-methyl butane, 3-Bromo-2-methyl butane.
(iii) 1-Bromobutane, 1-Bromo-2, 2-Dimethyl propane, 1-Bromo-2-methyl butane 1-Bromo-3-methyl
butane
37. (i) Write the mechanism of the following reaction.
n-BuBr + KCN EtOH, H2O n-BuCN
(ii) Why is the dipole moment of choro benzene lower than that of cyclo hexyl chloride?
38. Write a chemical test to distinguish between
(a) Chlorobenzene and benzyl chloride
(b) Chloroform and carbon tetrachloride
(c) Why is methyl chloride hydrolysed more easily than Chlorobenzene?
39. (i) State one use of DDT and iodoform.
(ii) Which compound in the following couples will react faster in SN2 displacement and why?
(a) 1-Bromopentane, 2-Bromopentane
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
15
SARASWATI
CHEMISTRY POINT
(b) 1-Bromo-2-Methylbutane, 2-Bromo-2-methyl butane.
40. How would you differentiate between SN1 and SN2 mechanism of substitution reactions? Give one
example of each.
41. Explain why
(i) The dipole moment of Chlorobenzene is lower than that of cyclohexyl chloride?
(ii) Alkyl halides though polar are immiscible with water.
(iii) In the pair, (CH3)3 – C – Cl and CH3Cl, CH3Cl will react faster in SN2 reaction with –OH.
42. Give reasons for the following observations.
(i) P-Dichlorobenzene has higher melting point than those of o and m-isomers.
(ii) Haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.
(iii) The treatment of alkyl chloride with aqueous KOH leads to the formation of alcohol but in the
presence of alcoholic KOH, alkene is the major product.
PITAM PURA & ROHINI
SUDHIR PANWAR
9871621367 / 9213145146 / 9899899326
16