Haloalkanes & Haloarenes Formula Sheet — NEET Chemistry

Every key Haloalkanes & Haloarenes formula, definition and fact for NEET Chemistry in one place — with common examiner traps and worked examples. Free to read; blurt from memory, then check your gaps.

Syllabus — topics coveredNCERT · 4 sub-topics

Methods of preparation of haloalkanes and haloarenes
Nature of C-X bond; physical and chemical properties
Substitution reactions and mechanisms
Uses and environmental effects — chloroform, iodoform, freons, DDT

Classification, Nomenclature & the C–X Bond

Haloalkanes & haloarenes: Compounds where halogen (

X = F, Cl, Br, I

) replaces H of a hydrocarbon. They differ by the carbon bearing X:

sp^{3}

(alkyl, allylic, benzylic) vs

sp^{2}

(vinylic, aryl) — a distinction that controls almost all their chemistry.

Bond	Length / pm	Enthalpy / kJ mo $l^{- 1}$
C–F	139	452
C–Cl	178	351
C–Br	193	293
C–I	214	234

Classifying the carbon

▸ $Alkyl$ ( $s p^{3}$ ): $1^{\circ}$ , $2^{\circ}$ or $3^{\circ}$ by the carbon holding X.
▸ $Allylic / benzylic$ : X on an $s p^{3}$ C next to a $C = C$ / ring.
▸ $Vinylic / aryl$ ( $s p^{2}$ ): X on a $C = C$ carbon / aromatic ring.
▸ $gem$ (both X on one C) vs $vic$ (on adjacent C).

★ Remember · The C–X bond is polar

X is more electronegative, so

C δ + - X δ -

. The

δ +

carbon invites

nucleophiles

(substitution), and the

δ -

halogen leaves as

X^{-}

🎯 Exam · Reactivity order R–I > R–Br > R–Cl ≫ R–F

Down the group the

C - X

bond gets

longer and weaker

(C–F 452 → C–I 234 kJ mo

l^{- 1}

), so the bond breaks more easily —

R - I

is the most reactive,

R - F

the least.

⚠️ Watch out · Naming gem vs vic dihalides

C H_{3} CHC l_{2}

= 1,1-dichloroethane (gem, 'ethylidene chloride');

C H_{2} ClC H_{2} Cl

= 1,2-dichloroethane (vic, 'ethylene dichloride'). Use

o / m / p

only in common names of arenes — IUPAC uses 1,2/1,3/1,4.

🚫 Examiner Trap · Examiner traps

(1) C

-

X bond strength falls down the group (C

-

>

-

>

-

>

-

I), so reactivity is the REVERSE: R

-

>

-

>

-

≫

-

F. (2) C

-

F is the shortest/strongest yet most polar — don't equate 'most polar' with 'most reactive'. (3)

s p^{2}

-

X (vinyl/aryl) is shorter and stronger than

s p^{3}

(alkyl). (4) gem

=

two X on the SAME carbon; vic

=

on ADJACENT carbons.

Preparation of Haloalkanes & Haloarenes

★ Remember · From alcohols

R - OH + HX

(

ZnC l_{2}

), or

+ P X_{3}

PC l_{5}

, or

+ SOC l_{2}

Thionyl chloride is best

R - OH + SOC l_{2} \to R - Cl + S O_{2} ↑ + HCl ↑

— the gaseous by-products escape, leaving pure

R - Cl

From hydrocarbons & alkenes

▸Free-radical halogenation of alkanes (gives a mixture).
▸Aryl halides: arene $+ X_{2}$ /anhyd. $FeC l_{3}$ (electrophilic substitution).
▸From alkenes: $+ HX$ (Markovnikov / peroxide); $+ X_{2} \to$ vicinal dihalide.

🎯 Exam · Halogen exchange

Finkelstein

R - X + NaI dry acetone R - I

(NaCl/NaBr precipitate drives it forward).

Swarts

R - Cl/Br + AgF (or H g_{2} F_{2}, Sb F_{3}) \to R - F

★ Remember · Aryl halides from diazonium salts

Sandmeyer

Ar N_{2}^{+} X^{-} C u_{2} X_{2} Ar - X + N_{2}

(X = Cl, Br). For iodide, just warm the diazonium salt with KI. Fluoroarenes are made by the Balz–Schiemann route, not direct halogenation.

🚫 Examiner Trap · Examiner traps

(1) Alcohol

+

SOC

l_{2}

is the

purest

route (by-products S

O_{2} ↑

+

HCl

↑

escape). (2) Lucas reagent reactivity

3^{\circ} > 2^{\circ} > 1^{\circ}

(carbocation order). (3)

Finkelstein

makes R

-

I (NaI/dry acetone — NaCl/NaBr precipitate drives it);

Swarts

makes R

-

F (AgF/Sb

F_{3}

). (4) Markovnikov vs peroxide (anti-Mark.) decides which carbon gets X from an alkene.

Nucleophilic Substitution — S_N1 vs S_N2

Nucleophilic substitution: A nucleophile (

N u^{-}

) attacks the

δ +

carbon and the halogen leaves:

N u^{-} + R - X \to R - Nu + X^{-}

. It runs by two limiting mechanisms —

S_{N} 1

and

S_{N} 2

★ Remember ·

S_{N}

2 — bimolecular

One

concerted step, rate

= k [RX] [Nu]

(2nd order). The nucleophile attacks from the

back

, so the centre

inverts

. Crowding slows it:

C H_{3} X > 1^{\circ} > 2^{\circ} > 3^{\circ}

🎯 Exam ·

S_{N}

1 — unimolecular

Two

steps via a carbocation; rate

= k [RX]

(1st order, depends only on RX). Favoured by

3^{\circ} > 2^{\circ} > 1^{\circ}

and polar protic solvents; allylic/benzylic halides react fast (resonance-stabilised cation).

Which mechanism wins?

▸ $Methyl / 1°$ : $S_{N} 2$ (open to backside attack).
▸ $3°$ : $S_{N} 1$ (stable carbocation, too crowded for $S_{N} 2$ ).
▸ $2°$ : either, depending on nucleophile and solvent.
▸Polar protic solvent, weak nucleophile, good ionising power → $S_{N} 1$ .

★ Remember · Leaving group still matters

For a given R, both mechanisms follow

R - I > R - Br > R - Cl ≫ R - F

— a better (weaker-bonded, more polarisable) leaving group reacts faster.

🚫 Examiner Trap · Examiner traps

(1) S

_{N}

1 rate depends on substrate ONLY (

k [RX]

); S

_{N}

2 on BOTH (

k [RX] [Nu]

). (2) Order is OPPOSITE: S

_{N}

1 favours

3^{\circ}

(stable cation), S

_{N}

2 favours

1^{\circ}

H_{3}

(least steric). (3) S

_{N}

\Rightarrow

inversion (one product); S

_{N}

\Rightarrow

racemisation (planar cation). (4) Polar PROTIC solvent + good leaving group push S

_{N}

1; polar APROTIC favours S

_{N}

Stereochemistry & Optical Activity

Chirality & optical activity: A carbon bearing

four different groups

is an asymmetric (stereo)centre, making the molecule

chiral

— non-superimposable on its mirror image. Chiral molecules are

optically active

: they rotate plane-polarised light (d/+ clockwise, l/− anticlockwise).

A pair of enantiomers — non-superimposable mirror images about a chiral carbon (4 different groups).

Key terms

▸ $Enantiomers$ : a mirror-image pair — identical physical properties, $opposite$ optical rotation.
▸ $Racemic$ ( $\pm$ or dl): an equimolar mix of enantiomers — $optically inactive$ (rotations cancel).
▸ $Racemisation$ : a chiral compound converting into the racemic mixture.

🎯 Exam ·

S_{N}

2 → inversion

Backside attack flips the stereocentre like an

umbrella in the wind

(Walden inversion). An optically active substrate gives a

single, inverted

product — e.g.

(-)

-2-bromooctane →

(+)

-octan-2-ol.

★ Remember ·

S_{N}

1 → racemisation

The intermediate carbocation is

planar

(

s p^{2}

), so the nucleophile attacks from

both faces

~equally → a ~50:50 mixture of the two enantiomers, i.e. a

racemic

(optically inactive) product.

⚠️ Watch out · Retention

no bond to the stereocentre is broken

, configuration is

retained

. The sign of rotation (

+

-

) is

not

a reliable guide to absolute configuration — they are independent.

🚫 Examiner Trap · Examiner traps

(1) Chirality needs a carbon with

four different

groups (no plane of symmetry). (2) A

racemic

(

\pm

) mix is 50:50 enantiomers and optically INACTIVE (external compensation). (3) Enantiomers share all physical properties EXCEPT direction of rotation (and behaviour in chiral environments). (4) S

_{N}

2 gives inversion (Walden); S

_{N}

1 gives partial racemisation — link mechanism to optical outcome.

Elimination & Reaction with Metals

★ Remember · β-Elimination (dehydrohalogenation)

Alcoholic KOH removes an H from the

β

-carbon and X from the

α

-carbon to form an alkene.

Saytzeff (Zaitsev) rule

: the

more substituted

alkene predominates — 2-bromopentane → pent-2-ene (81%) over pent-1-ene (19%).

🎯 Exam · Elimination vs substitution

Both compete for an alkyl halide with β-H's. A

bulky/strong base

and heat favour

elimination

; a good nucleophile in a suitable solvent favours

substitution

. Roughly:

1^{\circ} \to S_{N} 2

3^{\circ} \to S_{N} 1

/elimination.

Grignard reagent

R - X + Mg dry ether R - MgX

C–Mg is polar (

R^{δ -}

); very reactive — water/alcohols/amines destroy it to give RH, so it must be kept anhydrous.

Coupling with sodium

▸ $Wurtz$ : $2 R - X + 2 Na dry ether R - R$ (symmetrical alkane).
▸ $Wurtz-Fittig$ : $Ar - X + R - X + Na \to$ an alkylarene.
▸ $Fittig$ : $2 Ar - X + Na \to$ a biaryl (e.g. biphenyl).

🚫 Examiner Trap · Examiner traps

(1) Elimination follows

Saytzeff/Zaitsev

— the MORE substituted alkene dominates. (2) Same R

-

X can give substitution OR elimination: alc. KOH

\Rightarrow

elimination, aq. KOH

\Rightarrow

substitution; bulky base / high T /

3^{\circ}

favour elimination. (3) Grignard is destroyed by ANY acidic H (water, alcohol,

-

NH,

-

COOH)

\to

alkane. (4) Wurtz fails for unsymmetrical alkanes (mixture).

Haloarenes — Structure & Reactivity

Haloarenes: Aryl halides have X on an

s p^{2}

aromatic carbon. They are

far less reactive

to nucleophilic substitution than haloalkanes — the

C - X

bond is reinforced by the ring.

Resonance gives the haloarene C–X bond partial double-bond character, so it resists substitution.

🎯 Exam · Why so unreactive to nucleophiles?

(1)

Resonance

gives

C - X

partial double-bond character; (2) the

s p^{2}

carbon makes

C - X

shorter and stronger

(169 vs 177 pm); (3) the

phenyl cation

is unstable, ruling out

S_{N} 1

; (4) the electron-rich ring

repels

nucleophiles.

★ Remember · Nucleophilic substitution needs force or activation

Chlorobenzene → phenol only at

623 K / 300 atm

NaOH (Dow). An

electron-withdrawing

group (

- N O_{2}

) at o/p activates it (p-nitro at 443 K, 2,4-dinitro at 368 K) by stabilising the carbanion intermediate — m-

N O_{2}

does not help.

🎯 Exam · Electrophilic substitution

Haloarenes still undergo halogenation, nitration, sulphonation and Friedel–Crafts, but

more slowly

(ring deactivated by

- I

). The halogen is

ortho/para-directing

(resonance raises o/p density) yet

deactivating

overall — e.g. chlorobenzene → 1,4-dichlorobenzene (major).

⚠️ Watch out · Reactivity vs orientation

For halogens the

inductive

(

- I

) effect controls

reactivity

(net deactivation) while the

resonance

effect controls

orientation

(o/p). The two effects point in opposite directions — don't conflate them.

🚫 Examiner Trap · Examiner traps

(1) Haloarenes RESIST nucleophilic substitution — C

-

X has partial double-bond (resonance), shorter/stronger

s p^{2}

-

X, unstable phenyl cation, and the ring repels nucleophiles. (2) NS needs harsh conditions UNLESS

o/p-NO_{2}

groups activate it (m-N

O_{2}

does NOT). (3) In EAS the halogen is

o / p

-directing but

deactivating

(

-

I withdraws, lone pair directs). (4) Don't confuse haloarene (unreactive) with allyl/benzyl halide (very reactive).

Polyhalogen Compounds — Uses & Hazards

Polyhalogen compounds: Carbon compounds with

more than one halogen

. Many are industrially useful solvents, propellants and refrigerants — but several are toxic or environmentally damaging.

Compound	Name	Uses & hazards
$C H_{2} C l_{2}$	Dichloromethane	solvent, paint remover; harms CNS, skin/eye burns
$CHC l_{3}$	Chloroform	solvent, old anaesthetic; air+light $\to$ phosgene $COC l_{2}$ (store dark)
$CH I_{3}$	Iodoform	old antiseptic; replaced (bad smell)
$CC l_{4}$	Carbon tetrachloride	fire extinguisher, solvent; liver damage, ozone depletion
CFCs (Freon)	Chlorofluorocarbons	refrigerants/propellants; deplete the ozone layer
DDT	dichlorodiphenyltrichloroethane	first chlorinated insecticide; persistent, bioaccumulates — banned widely

★ Remember · Chloroform → phosgene

CHC l_{3}

(an old anaesthetic) is slowly oxidised by air and light to poisonous

phosgene

2 CHC l_{3} + O_{2} light 2 COC l_{2} + 2 HCl

. It is therefore stored in

dark bottles filled to the brim

to exclude air.

Uses at a glance

▸ $C H_{2} C l_{2}$ — solvent / paint remover; $CC l_{4}$ — fire extinguisher, solvent.
▸ $CH I_{3}$ (iodoform) — former antiseptic (free iodine).
▸Freons (CFCs) — refrigerants and aerosol propellants.

🎯 Exam · Environmental impact

Freons (CFCs)

are inert enough to reach the stratosphere, where they

deplete the ozone layer

DDT

— the first chlorinated insecticide — is

persistent

(non-biodegradable), bioaccumulates in fat, and is now banned/restricted in many countries.

⚠️ Watch out · Health hazards

These are not benign solvents:

CHC l_{3}

and

CC l_{4}

damage the

liver

(and

CC l_{4}

is carcinogenic), and

C H_{2} C l_{2}

harms the central nervous system and burns skin/eyes.

🚫 Examiner Trap · Examiner traps

(1) CHC

l_{3}

oxidises in air

+

light to poisonous

phosgene

(COC

l_{2}

) — store in dark, full bottles with a little ethanol. (2) CC

l_{4}

and CFCs

deplete the ozone layer

(Cl radicals). (3) Iodoform's antiseptic action is due to liberated FREE iodine, not the compound itself. (4) DDT is persistent and bio-accumulates

\Rightarrow

banned in many countries.

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This Haloalkanes & Haloarenes formula sheet covers all the high-yield Chemistry formulas, definitions and facts you need for NEET, across Methods of preparation of haloalkanes and haloarenes, Nature of C-X bond; physical and chemical properties, Substitution reactions and mechanisms, Uses and environmental effects — chloroform, iodoform, freons, DDT — each shown with the key result and, where useful, a worked example.

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