Cheat Sheet: Haloalkanes and Haloarenes | Chemistry Class 12 - NEET PDF Download

 Page 1


Haloalkanes and Haloarenes Cheat Sheet (Class 12 CBSE)
A concise guide with clear explanations for quick revision
1 Introduction
Haloalkanes and haloarenes are organic
compounds containing halogen atoms,
widely studied for their reactivity and ap-
plications.
• Haloalkanes : Contain halogen (F , Cl,
Br , I) bonded to an sp
3
h ybridized
carbon in an alkyl group. Gener al
formula: C
n
H
2n+1
X. Example: CH
3
Cl
(chloromethane).
• Haloarenes : Contain halogen bonded to
an sp
2
h ybridized carbon in an aromatic
ring. Example: C
6
H
5
Br (bromobenzene).
• Nomenclature :
– Haloalkanes: Name the alkane and
prefix with halogen (e.g., CH
3
CH
2
Cl is
chloroethane).
– Haloarenes: Name the ha logen as
a prefix to benzene (e.g., C
6
H
5
Cl is
chlorobenzene).
• C–X Bond : Polar due to electronegativity
difference (C is d
+
, X isd
-
). Bond strength
decreases as: C–F > C–Cl > C–Br > C–I, af-
fecting reactivity .
2 Classification
Haloalkanes are classified based on the
number of halogens or the type of carbon
to which the halogen is attached.
• B y Number of Halogens :
– Monohaloalkanes: One halogen (e.g.,
CH
3
Cl).
– Dihaloalkanes: Two halogens (e.g.,
CH
2
Cl
2
).
– Trihaloalkanes: Three halogens (e.g.,
CHCl
3
).
• B y Carbon Attachment :
– Primary (1
?
): Halogen on carbon with
one carbon neighbor (e.g., C H
3
CH
2
Cl).
– Secondary (2
?
): Halogen on car-
bon with two carbon neighbors (e.g.,
(CH
3
)
2
CHCl).
– T ertiary (3
?
): Halogen on carbon
with three carbon neighbors (e.g.,
(CH
3
)
3
CCl).
3 Prepar ation of Haloalkanes
Haloalkanes can be synthesized from alco-
hols, alk enes, or other halides through var-
ious methods.
1. From Alcohols : Alcohols react with h y-
drogen halides or other reagents to re-
place –OH with –X.
• R–OH + HX? R–X + H
2
O (HX = HCl,
HBr; ZnCl
2
catalyst for HCl). Example:
CH
3
CH
2
OH + HBr? CH
3
CH
2
Br .
• R–OH + SOCl
2
? R–Cl + SO
2
+ HCl (with
p yridine). Efficient for chlorides, pro-
duces gaseous b yproducts.
2. From Alk enes : Alk enes undergo addi-
tion or substitution reactions.
• A ddition of HX: R–CH=CH
2
+ HX ?
R–CHX–CH
3
(follows Mark ovnik ov’ s
rule). Example: CH
2
=CH
2
+ HBr?
CH
3
CH
2
Br .
• Allylic Halogenation: CH
3
–CH=CH
2
+
Cl
2
(high temp.)? CH
2
=CH–CH
2
Cl.
3. Halogen Exchange : Replace one halo-
gen with another .
• Fink elstein Reaction: R–X + NaI? R–I
+ NaX (X = Cl, Br; in acetone). Example:
CH
3
Br + NaI? CH
3
I + NaBr .
• S warts Reaction: R–X + A gF? R–F +
A gX (X = Cl, Br). Example: CH
3
Br + A gF
? CH
3
F + A gBr .
1
Page 2


Haloalkanes and Haloarenes Cheat Sheet (Class 12 CBSE)
A concise guide with clear explanations for quick revision
1 Introduction
Haloalkanes and haloarenes are organic
compounds containing halogen atoms,
widely studied for their reactivity and ap-
plications.
• Haloalkanes : Contain halogen (F , Cl,
Br , I) bonded to an sp
3
h ybridized
carbon in an alkyl group. Gener al
formula: C
n
H
2n+1
X. Example: CH
3
Cl
(chloromethane).
• Haloarenes : Contain halogen bonded to
an sp
2
h ybridized carbon in an aromatic
ring. Example: C
6
H
5
Br (bromobenzene).
• Nomenclature :
– Haloalkanes: Name the alkane and
prefix with halogen (e.g., CH
3
CH
2
Cl is
chloroethane).
– Haloarenes: Name the ha logen as
a prefix to benzene (e.g., C
6
H
5
Cl is
chlorobenzene).
• C–X Bond : Polar due to electronegativity
difference (C is d
+
, X isd
-
). Bond strength
decreases as: C–F > C–Cl > C–Br > C–I, af-
fecting reactivity .
2 Classification
Haloalkanes are classified based on the
number of halogens or the type of carbon
to which the halogen is attached.
• B y Number of Halogens :
– Monohaloalkanes: One halogen (e.g.,
CH
3
Cl).
– Dihaloalkanes: Two halogens (e.g.,
CH
2
Cl
2
).
– Trihaloalkanes: Three halogens (e.g.,
CHCl
3
).
• B y Carbon Attachment :
– Primary (1
?
): Halogen on carbon with
one carbon neighbor (e.g., C H
3
CH
2
Cl).
– Secondary (2
?
): Halogen on car-
bon with two carbon neighbors (e.g.,
(CH
3
)
2
CHCl).
– T ertiary (3
?
): Halogen on carbon
with three carbon neighbors (e.g.,
(CH
3
)
3
CCl).
3 Prepar ation of Haloalkanes
Haloalkanes can be synthesized from alco-
hols, alk enes, or other halides through var-
ious methods.
1. From Alcohols : Alcohols react with h y-
drogen halides or other reagents to re-
place –OH with –X.
• R–OH + HX? R–X + H
2
O (HX = HCl,
HBr; ZnCl
2
catalyst for HCl). Example:
CH
3
CH
2
OH + HBr? CH
3
CH
2
Br .
• R–OH + SOCl
2
? R–Cl + SO
2
+ HCl (with
p yridine). Efficient for chlorides, pro-
duces gaseous b yproducts.
2. From Alk enes : Alk enes undergo addi-
tion or substitution reactions.
• A ddition of HX: R–CH=CH
2
+ HX ?
R–CHX–CH
3
(follows Mark ovnik ov’ s
rule). Example: CH
2
=CH
2
+ HBr?
CH
3
CH
2
Br .
• Allylic Halogenation: CH
3
–CH=CH
2
+
Cl
2
(high temp.)? CH
2
=CH–CH
2
Cl.
3. Halogen Exchange : Replace one halo-
gen with another .
• Fink elstein Reaction: R–X + NaI? R–I
+ NaX (X = Cl, Br; in acetone). Example:
CH
3
Br + NaI? CH
3
I + NaBr .
• S warts Reaction: R–X + A gF? R–F +
A gX (X = Cl, Br). Example: CH
3
Br + A gF
? CH
3
F + A gBr .
1
4 Prepar ation of Haloarenes
Haloarenes are prepared via electrophilic
substitution or from diazonium salts.
1. From Benzene : Halogenation via elec-
trophilic substitution.
• C
6
H
6
+ X
2
? C
6
H
5
X + HX (X
2
= Cl
2
,
Br
2
; catalyst: F eCl
3
or AlCl
3
). Example:
C
6
H
6
+ Cl
2
? C
6
H
5
Cl.
2. From Diazonium Salts : Diazonium
salts (C
6
H
5
N
+
2
Cl
-
) are versatile interme-
diates.
• Sandmeyer Reaction: C
6
H
5
N
2
Cl + CuX
? C
6
H
5
X + N
2
+ CuCl (X = Cl, Br). Ex-
ample: C
6
H
5
N
2
Cl + CuCl? C
6
H
5
Cl.
• Gattermann Reaction: C
6
H
5
N
2
Cl +
Cu/HX? C
6
H
5
X + N
2
+ CuCl. Similar
to Sandmeyer .
• Balz-Schiemann Reaction: C
6
H
5
N
2
BF
4
? C
6
H
5
F + N
2
+ BF
3
. Used for fluo-
roarenes.
3. From Phenol : C
6
H
5
OH + PCl
5
? C
6
H
5
Cl
+ POCl
3
+ HCl.
5 Ph ysical Properties
Ph ysical properties depend on molecular
weight, polarity , and structure.
• Haloalkanes :
– Boiling Point : Increases with molecu-
lar weight: R–I > R–Br > R–Cl > R–F . Ex-
ample: CH
3
I has a higher boiling point
than CH
3
F .
– Solubility : Insoluble in water (weak
polarity), soluble in organic solvents.
– Density : Increases with heavier halo-
gens: CH
3
I > CH
3
Br > CH
3
Cl.
• Haloarenes : Higher boiling points than
haloalkanes due to aromatic ring. Insol-
uble in water , soluble in organic solvents.
6 Chemical Reactions of Haloalka-
nes
Haloalkanes undergo substitution, elimi-
nation, or reactions with metals due to the
polar C–X bond. Nucleophilic Substitu-
tion
• Mechanisms :
– S
N
1: Two-step, forms carbocation. F a-
vored for 3
?
> 2
?
> 1
?
. Example:
(CH
3
)
3
CBr? (CH
3
)
3
C
+
? product.
– S
N
2: One-step, backside attack. F a-
vored for 1
?
> 2
?
> 3
?
. Example: CH
3
Br
+ OH
-
? CH
3
OH + Br
-
.
• Reactions :
– R–X + OH
-
? R–OH + X
-
(forms alco-
hol).
– R–X + CN
-
? R–CN + X
-
(forms nitrile).
– R–X + NH
3
? R–NH
2
+ HX (forms
amine).
– R–X + A gNO
2
? R–ONO + A gX (forms ni-
trite).
Elimination
• E1 : Two-step, carbocation intermediate,
3
?
> 2
?
> 1
?
.
• E2 : One-step, requires strong base (e.g.,
alc. K OH), 1
?
> 2
?
> 3
?
.
• Example: CH
3
CH
2
Br + alc. K OH ?
CH
2
=CH
2
+ HBr .
With Metals
• Grignard Reagent : R–X + Mg? R–MgX
(in dry ether). Used in organic synthesis.
• Wurtz Reaction : 2R–X + 2Na? R–R +
2NaX. F orms alkanes.
7 Chemical Reactions of Haloarenes
Haloarenes are less reactive due to reso-
nance and sp
2
C–X bond.
2
Page 3


Haloalkanes and Haloarenes Cheat Sheet (Class 12 CBSE)
A concise guide with clear explanations for quick revision
1 Introduction
Haloalkanes and haloarenes are organic
compounds containing halogen atoms,
widely studied for their reactivity and ap-
plications.
• Haloalkanes : Contain halogen (F , Cl,
Br , I) bonded to an sp
3
h ybridized
carbon in an alkyl group. Gener al
formula: C
n
H
2n+1
X. Example: CH
3
Cl
(chloromethane).
• Haloarenes : Contain halogen bonded to
an sp
2
h ybridized carbon in an aromatic
ring. Example: C
6
H
5
Br (bromobenzene).
• Nomenclature :
– Haloalkanes: Name the alkane and
prefix with halogen (e.g., CH
3
CH
2
Cl is
chloroethane).
– Haloarenes: Name the ha logen as
a prefix to benzene (e.g., C
6
H
5
Cl is
chlorobenzene).
• C–X Bond : Polar due to electronegativity
difference (C is d
+
, X isd
-
). Bond strength
decreases as: C–F > C–Cl > C–Br > C–I, af-
fecting reactivity .
2 Classification
Haloalkanes are classified based on the
number of halogens or the type of carbon
to which the halogen is attached.
• B y Number of Halogens :
– Monohaloalkanes: One halogen (e.g.,
CH
3
Cl).
– Dihaloalkanes: Two halogens (e.g.,
CH
2
Cl
2
).
– Trihaloalkanes: Three halogens (e.g.,
CHCl
3
).
• B y Carbon Attachment :
– Primary (1
?
): Halogen on carbon with
one carbon neighbor (e.g., C H
3
CH
2
Cl).
– Secondary (2
?
): Halogen on car-
bon with two carbon neighbors (e.g.,
(CH
3
)
2
CHCl).
– T ertiary (3
?
): Halogen on carbon
with three carbon neighbors (e.g.,
(CH
3
)
3
CCl).
3 Prepar ation of Haloalkanes
Haloalkanes can be synthesized from alco-
hols, alk enes, or other halides through var-
ious methods.
1. From Alcohols : Alcohols react with h y-
drogen halides or other reagents to re-
place –OH with –X.
• R–OH + HX? R–X + H
2
O (HX = HCl,
HBr; ZnCl
2
catalyst for HCl). Example:
CH
3
CH
2
OH + HBr? CH
3
CH
2
Br .
• R–OH + SOCl
2
? R–Cl + SO
2
+ HCl (with
p yridine). Efficient for chlorides, pro-
duces gaseous b yproducts.
2. From Alk enes : Alk enes undergo addi-
tion or substitution reactions.
• A ddition of HX: R–CH=CH
2
+ HX ?
R–CHX–CH
3
(follows Mark ovnik ov’ s
rule). Example: CH
2
=CH
2
+ HBr?
CH
3
CH
2
Br .
• Allylic Halogenation: CH
3
–CH=CH
2
+
Cl
2
(high temp.)? CH
2
=CH–CH
2
Cl.
3. Halogen Exchange : Replace one halo-
gen with another .
• Fink elstein Reaction: R–X + NaI? R–I
+ NaX (X = Cl, Br; in acetone). Example:
CH
3
Br + NaI? CH
3
I + NaBr .
• S warts Reaction: R–X + A gF? R–F +
A gX (X = Cl, Br). Example: CH
3
Br + A gF
? CH
3
F + A gBr .
1
4 Prepar ation of Haloarenes
Haloarenes are prepared via electrophilic
substitution or from diazonium salts.
1. From Benzene : Halogenation via elec-
trophilic substitution.
• C
6
H
6
+ X
2
? C
6
H
5
X + HX (X
2
= Cl
2
,
Br
2
; catalyst: F eCl
3
or AlCl
3
). Example:
C
6
H
6
+ Cl
2
? C
6
H
5
Cl.
2. From Diazonium Salts : Diazonium
salts (C
6
H
5
N
+
2
Cl
-
) are versatile interme-
diates.
• Sandmeyer Reaction: C
6
H
5
N
2
Cl + CuX
? C
6
H
5
X + N
2
+ CuCl (X = Cl, Br). Ex-
ample: C
6
H
5
N
2
Cl + CuCl? C
6
H
5
Cl.
• Gattermann Reaction: C
6
H
5
N
2
Cl +
Cu/HX? C
6
H
5
X + N
2
+ CuCl. Similar
to Sandmeyer .
• Balz-Schiemann Reaction: C
6
H
5
N
2
BF
4
? C
6
H
5
F + N
2
+ BF
3
. Used for fluo-
roarenes.
3. From Phenol : C
6
H
5
OH + PCl
5
? C
6
H
5
Cl
+ POCl
3
+ HCl.
5 Ph ysical Properties
Ph ysical properties depend on molecular
weight, polarity , and structure.
• Haloalkanes :
– Boiling Point : Increases with molecu-
lar weight: R–I > R–Br > R–Cl > R–F . Ex-
ample: CH
3
I has a higher boiling point
than CH
3
F .
– Solubility : Insoluble in water (weak
polarity), soluble in organic solvents.
– Density : Increases with heavier halo-
gens: CH
3
I > CH
3
Br > CH
3
Cl.
• Haloarenes : Higher boiling points than
haloalkanes due to aromatic ring. Insol-
uble in water , soluble in organic solvents.
6 Chemical Reactions of Haloalka-
nes
Haloalkanes undergo substitution, elimi-
nation, or reactions with metals due to the
polar C–X bond. Nucleophilic Substitu-
tion
• Mechanisms :
– S
N
1: Two-step, forms carbocation. F a-
vored for 3
?
> 2
?
> 1
?
. Example:
(CH
3
)
3
CBr? (CH
3
)
3
C
+
? product.
– S
N
2: One-step, backside attack. F a-
vored for 1
?
> 2
?
> 3
?
. Example: CH
3
Br
+ OH
-
? CH
3
OH + Br
-
.
• Reactions :
– R–X + OH
-
? R–OH + X
-
(forms alco-
hol).
– R–X + CN
-
? R–CN + X
-
(forms nitrile).
– R–X + NH
3
? R–NH
2
+ HX (forms
amine).
– R–X + A gNO
2
? R–ONO + A gX (forms ni-
trite).
Elimination
• E1 : Two-step, carbocation intermediate,
3
?
> 2
?
> 1
?
.
• E2 : One-step, requires strong base (e.g.,
alc. K OH), 1
?
> 2
?
> 3
?
.
• Example: CH
3
CH
2
Br + alc. K OH ?
CH
2
=CH
2
+ HBr .
With Metals
• Grignard Reagent : R–X + Mg? R–MgX
(in dry ether). Used in organic synthesis.
• Wurtz Reaction : 2R–X + 2Na? R–R +
2NaX. F orms alkanes.
7 Chemical Reactions of Haloarenes
Haloarenes are less reactive due to reso-
nance and sp
2
C–X bond.
2
• Electrophilic Aromatic Substitution :
Halogens are o,p-directing but deactivat-
ing. Example: C
6
H
5
Cl + HNO
3
/H
2
SO
4
? o-
and p-chloronitrobenzene.
• Nucleophilic Aromatic Substitution :
– Dow’ s Process: C
6
H
5
Cl + NaOH (350°C,
300 atm)? C
6
H
5
OH + NaCl.
– With Strong Nucleophiles: C
6
H
5
X +
KNH
2
? C
6
H
5
NH
2
(via benzyne inter-
mediate).
• Wurtz-Fittig : C
6
H
5
X + 2Na + R–X? C
6
H
5
–
R + 2NaX.
• Fittig : 2C
6
H
5
X + 2Na ? C
6
H
5
–C
6
H
5
+
2NaX.
8 Polyhalogen Compounds
These compounds have multiple halogens
and specific applications.
• Dichloromethane (CH
2
Cl
2
) : Solvent,
paint remover , toxic.
• Chloroform (CHCl
3
) :
– Prep: CH
3
OH + Cl
2
? CHCl
3
.
– Uses: Solvent, former anesthetic
(toxic). T est: Carb ylamine (foul smell).
• Iodoform (CHI
3
) :
– Prep: CH
3
CH
2
OH + I
2
+ NaOH? CHI
3
.
– Uses: Antiseptic.
• Carbon T etr achloride (CCl
4
) : Fire extin-
guisher , solvent, ozone-depleting.
• Freons (e.g., CF
2
Cl
2
) : Refriger ants,
harm ozone la yer .
• DD T : Insecticide, persistent pollutant.
9 Stereochemistry
Haloalkanes can exhibit stereoisomerism
due t o chir al centers.
• Chir ality : Compounds with a chir al car-
bon (e.g., CH
3
CHClCH
2
CH
3
) show optical
isomerism.
• S
N
1 : Leads to r acemization due to planar
carbocation.
• S
N
2 : Causes inversion of configur ation
(W alden inversion).
10 K ey Reagents
• alc. K OH : Promotes elimination (alk ene
formation).
• aq. K OH : Promotes substitution (alcohol
formation).
• NaI/acetone : Fink elstein reaction (io-
dide formation).
• A gF: S warts reaction (fluoride forma-
tion).
• CuCl/HCl : Sandmeyer reaction
(chloro/bromoarenes).
11 Exam Tips
• Reactivity Orders :
– S
N
2: CH
3
X > 1
?
> 2
?
> 3
?
.
– S
N
1: 3
?
> 2
?
> 1
?
> CH
3
X.
– C–X Bond Cleavage: R–I > R–Br > R–Cl >
R–F .
• Haloarenes are less reactive than
haloalkanes due to resonance.
• T ests :
– Chloroform: Carb ylamine test (foul
smell).
– Iodoform: Y ellow ppt with I
2
/NaOH for
CH
3
CO– or CH
3
CH(OH)– groups.
• Environmental Impact : CFCs and DD T
deplete ozone and pollute ecosystems.
3