Haloalkanes and Haloarenes — JEE Mains Chemistry Formulas and Traps

Classification, Structure and Physical Properties

Types of organic halides and the aryl C–X bond

Class	Carbon that holds X	Example	What follows
Alkyl, 1°	sp³ carbon bonded to one other carbon	$\mathrm{CH_3CH_2CH_2Cl}$	Reacts mainly by SN2
Alkyl, 3°	sp³ carbon bonded to three other carbons	$\mathrm{(CH_3)_3C{-}Cl}$	Reacts mainly by SN1 or elimination
Allylic	sp³ carbon next to a C=C	$\mathrm{CH_2{=}CH{-}CH_2{-}Cl}$	Ionises easily: the allyl cation is resonance-stabilised
Benzylic	sp³ carbon attached to a benzene ring	$\mathrm{C_6H_5CH_2Cl}$ , $\mathrm{C_6H_5CH(Cl)CH_3}$	Ionises easily: the benzyl cation is resonance-stabilised
Vinylic	sp² carbon of a C=C	$\mathrm{CH_2{=}CHCl}$	Partial C=Cl character; no SN1 or SN2 under normal conditions
Aryl	sp² carbon of a benzene ring	$\mathrm{C_6H_5Cl}$	Partial C=Cl character; substituted only under harsh conditions or with nitro groups ortho or para
Geminal dihalide	Both X on one carbon	Ethylidene chloride, $\mathrm{CH_3CHCl_2}$ (1,1-dichloroethane)	Common name ends in -idene
Vicinal dihalide	X on two adjacent carbons	Ethylene dichloride, $\mathrm{ClCH_2CH_2Cl}$ (1,2-dichloroethane)	Common name ends in -ene

Allylic and benzylic halides are sp³ at the C–X carbon; vinylic and aryl halides are sp².

Boiling point, melting point, density and polarity of halides

Property	Trend	Reason
Boiling point, changing the halogen	$\mathrm{RI > RBr > RCl > RF}$	A larger, more polarisable halogen gives stronger van der Waals forces
Boiling point, longer chain	$\mathrm{CH_3Cl < C_2H_5Cl < n{-}C_3H_7Cl}$	A larger surface gives stronger London forces
Boiling point, branched isomers	Falls with branching	A branched molecule is more nearly spherical, with less contact area
Melting point of dichlorobenzenes	para (323 K) > ortho (256 K) > meta (249 K)	The symmetric para isomer packs best in the crystal
Boiling point of dichlorobenzenes	ortho (453 K) > para (448 K) > meta (446 K)	The ortho isomer has the largest dipole
Density	$\mathrm{CH_2Cl_2 < CHCl_3 < CCl_4}$ ; iodides densest	More and heavier halogen atoms in about the same volume
Dipole moment of $\mathrm{CH_3X}$	$\mathrm{CH_3Cl > CH_3F > CH_3Br > CH_3I}$	Charge × bond length is largest for C–Cl; the C–F bond is very short
cis against trans 1,2-dihaloethene	cis is polar and boils higher; trans has almost no dipole	In the trans isomer the two C–X dipoles point opposite ways and cancel

Symmetry raises the melting point; polarity and size raise the boiling point.

Polyhalogen compounds: formulas and uses

Compound	Formula	Use or fact
Dichloromethane (methylene chloride)	$\mathrm{CH_2Cl_2}$	Paint remover, solvent and aerosol propellant
Trichloromethane (chloroform)	$\mathrm{CHCl_3}$	Solvent; stored in dark, full bottles because air and light turn it into phosgene
Triiodomethane (iodoform)	$\mathrm{CHI_3}$	Antiseptic, through the free iodine it releases
Tetrachloromethane (carbon tetrachloride)	$\mathrm{CCl_4}$	Fire extinguisher (earlier), solvent, feedstock for freons
Freon-12 (dichlorodifluoromethane)	$\mathrm{CCl_2F_2}$	Refrigerant and aerosol propellant; a CFC with 2 Cl
DDT (p,p′-dichlorodiphenyltrichloroethane)	$\mathrm{C_{14}H_9Cl_5}$	Non-biodegradable insecticide; 5 Cl
Gammaxene (lindane, BHC)	$\mathrm{C_6H_6Cl_6}$	Insecticide; 6 Cl
Chloropicrin (trichloronitromethane)	$\mathrm{CCl_3NO_2}$	Insecticide and war gas; 3 Cl
Chloral (trichloroethanal)	$\mathrm{CCl_3CHO}$	Raw material for DDT; 3 Cl

Learn each compound with its formula: most questions ask for a use or an atom count.

Common traps

Allylic is not vinylic

In an allyl halide such as

\mathrm{CH_2{=}CH{-}CH_2Cl}

the halogen is on the sp³ carbon next to the double bond. A statement that allylic halides carry X on an sp² carbon is false; that describes a vinylic halide.

Ethylidene and ethylene dichloride

The -idene name puts both halogens on one carbon: ethylidene chloride is 1,1-dichloroethane. Ethylene dichloride has them on adjacent carbons: 1,2-dichloroethane.

Resonance makes the aryl C–Cl bond shorter and less polar

Delocalisation of a chlorine lone pair into the ring gives the bond partial double-bond character. The bond gets shorter, not longer, and chlorine carries less negative charge than in an alkyl chloride.

Symmetry raises the melting point, not the boiling point

1,4-Dichlorobenzene melts far above its isomers because it packs well, yet it boils below 1,2-dichlorobenzene, which has the larger dipole. Do not carry the melting-point order over to boiling points.

Methyl fluoride is not the most polar methyl halide

Fluorine is the most electronegative halogen, but the C–F bond is so short that the dipole moment of

\mathrm{CH_3F}

is slightly below that of

\mathrm{CH_3Cl}

.

Iodides are the densest, not the lightest

For the same alkyl group, density rises from chloride to bromide to iodide. An order that puts the iodo compound lowest has it backwards.

A freon needs both chlorine and fluorine

Freons are chlorofluorocarbons. A compound with fluorine but no chlorine, such as

\mathrm{C_2F_4}

or

\mathrm{C_2HF_3}

, is not a freon.

Chloroform is kept full and dark

Chloroform is stored in dark bottles filled to the brim to keep out air and light, which oxidise it to phosgene. The storage rule is a common statement question.

Preparation of Haloalkanes and Haloarenes

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Haloalkanes from alcohols, alkenes and hydrocarbons

Alcohol to alkyl chloride with thionyl chloride; alcohol reactivity with HX

\mathrm{R{-}OH + SOCl_2 \longrightarrow R{-}Cl + SO_2\uparrow + HCl\uparrow} \qquad \text{reactivity with HX: } 3^\circ > 2^\circ > 1^\circ

Named reactions that make or couple organic halides

Reaction	Reagent	Change	Example
Finkelstein	NaI in dry acetone	R–Cl or R–Br → R–I	$\mathrm{CH_3CH_2Br \to CH_3CH_2I}$
Swarts	AgF, $\mathrm{Hg_2F_2}$ , $\mathrm{CoF_2}$ or $\mathrm{SbF_3}$	R–Cl or R–Br → R–F	$\mathrm{CH_3Br + AgF \to CH_3F}$
Sandmeyer	$\mathrm{Cu_2Cl_2/HCl}$ , $\mathrm{Cu_2Br_2/HBr}$ or CuCN/KCN	$\mathrm{ArN_2^+}$ → ArCl, ArBr or ArCN	Benzenediazonium chloride → chlorobenzene
Gattermann	Copper powder with HCl or HBr	$\mathrm{ArN_2^+}$ → ArCl or ArBr	Benzenediazonium chloride → bromobenzene
Iodide from a diazonium salt	KI (no copper)	$\mathrm{ArN_2^+}$ → ArI	Benzenediazonium chloride → iodobenzene
Wurtz-Fittig	Na in dry ether	ArX + RX → Ar–R	Chlorobenzene + methyl chloride → toluene
Fittig	Na in dry ether	2 ArX → Ar–Ar	Chlorobenzene → biphenyl

Sandmeyer uses copper(I) salts; Gattermann uses copper powder.

Common traps

Only HBr shows the peroxide effect

A peroxide reverses the addition of HBr only. HCl and HI still add by Markovnikov's rule when a peroxide is present.

Phenol does not give an aryl halide with HX

The C–O bond of phenol has partial double-bond character and does not break. Aryl halides are made by ring halogenation or from diazonium salts. Phenol does not react violently with halogen acids either.

Light chlorinates the side chain, iron(III) chloride the ring

Toluene with

\mathrm{Cl_2}

and light gives benzyl chloride; with

\mathrm{Cl_2}

and

\mathrm{FeCl_3}

in the dark it gives 2- and 4-chlorotoluene.

Gattermann makes aryl chlorides and bromides, not cyanides

In the NCERT scheme the Gattermann reaction uses copper powder with HCl or HBr and gives ArCl or ArBr. The aryl cyanide comes from the Sandmeyer reaction with CuCN. A statement that both reactions give aryl cyanides is false by this scheme.

Finkelstein runs because the salt precipitates

NaI dissolves in dry acetone but NaCl and NaBr do not. Their precipitation removes a product and drives the exchange forward; in water the reaction would not go to completion.

SN1 and SN2: Mechanism, Kinetics and Stereochemistry

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SN1 and SN2 compared: steps, rate law and conditions

Feature	SN1	SN2
Steps	Two: slow ionisation, then fast attack	One, concerted
Rate law	rate = $k[\mathrm{RX}]$ , first order	rate = $k[\mathrm{RX}][\mathrm{Nu^-}]$ , second order
Intermediate	Planar carbocation	None; a five-coordinate transition state
Stereochemistry at a chiral carbon	Racemisation (mostly)	Inversion (Walden inversion)
Substrate order	3° > 2° > 1° > $\mathrm{CH_3X}$	$\mathrm{CH_3X}$ > 1° > 2° > 3°
Nucleophile	Weak, often the solvent	Strong, at high concentration
Best solvent	Polar protic: water, alcohols	Polar aprotic: acetone, DMSO, DMF
Rearrangement	Possible, by a hydride or methyl shift	Never
Leaving group	I > Br > Cl > F	I > Br > Cl > F

A secondary halide can go either way; the nucleophile and the solvent decide.

Common traps

A secondary halide does not have one fixed mechanism

A strong nucleophile at high concentration in an aprotic solvent pushes a secondary halide towards SN2; a weak nucleophile such as the solvent pushes it towards SN1. Read the conditions before choosing.

Adding more nucleophile does not speed SN1

The slow step of SN1 is ionisation of the halide, so rate =

k[\mathrm{RX}]

. Doubling the nucleophile leaves the rate unchanged.

Polar solvents do not speed every substitution

A polar solvent helps when the transition state carries more charge than the reactants. For

\mathrm{HO^-}

attacking a neutral halide the charge is spread out in the transition state, so a less polar solvent is faster.

Inversion does not always change R to S

Inversion is a change in the 3-D arrangement. The R/S letter changes only when the incoming group has the priority rank of the leaving group. Assign the priorities of the product again before writing the label.

Tosylation keeps the configuration

TsCl reacts at the O–H bond, so the stereocentre is untouched. The inversion comes only in the next step, when a nucleophile displaces the tosylate.

Reactivity Order in Nucleophilic Substitution

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SN1 reactivity: ranking halides by carbocation stability

SN1 order of alkyl halides; stability of phenyl-substituted cations

\text{SN1: } 3^\circ > 2^\circ > 1^\circ > \mathrm{CH_3X} \qquad \mathrm{(C_6H_5)_3C^+ > (C_6H_5)_2CH^+ > C_6H_5CH_2^+}

SN2 reactivity: crowding, neighbouring groups and benzylic halides

SN2 order of alkyl halides; leaving-group order

\text{SN2: } \mathrm{CH_3X} > 1^\circ > 2^\circ > 3^\circ \qquad \mathrm{R{-}I > R{-}Br > R{-}Cl > R{-}F}

Halides that cannot ionise: vinylic, aryl and bridgehead

Halide	Cation it would give	SN1 and the AgNO₃ test
$\mathrm{(CH_3)_3C{-}Cl}$	3° cation, stabilised by hyperconjugation	Fast; AgCl precipitates at once
$\mathrm{C_6H_5CH_2Cl}$	Benzyl cation, stabilised by resonance	Fast; AgCl precipitates
$\mathrm{CH_2{=}CHCH_2Cl}$	Allyl cation, stabilised by resonance	Fast; AgCl precipitates
$\mathrm{CH_3CH_2CH_2CH_2Cl}$	1° cation, unstable	Very slow; precipitate only on long warming
$\mathrm{CH_2{=}CHCl}$	Vinyl cation, charge on an sp carbon	No SN1; no precipitate
$\mathrm{C_6H_5Cl}$	Phenyl cation, empty orbital in the ring plane	No SN1; no precipitate
1-Bromobicyclo[2.2.2]octane	Bridgehead cation that cannot become planar	Extremely slow; no practical SN1
3-Bromocyclopropene	Cyclopropenyl cation, aromatic with 2 π electrons	Ionises readily; AgBr precipitates

Being tertiary is not enough: the cation must also be able to become planar.

Common traps

A para-chloro group slows SN1

Chlorine donates a lone pair (+R) but withdraws more strongly through its −I effect, so 4-chlorobenzyl chloride ionises a little more slowly than benzyl chloride. It is still much faster than 4-nitrobenzyl chloride.

Primary halides can still go SN1

The rule 3° > 2° > 1° applies to plain alkyl halides. A primary halide whose cation is stabilised by resonance, such as benzyl chloride, allyl chloride or

\mathrm{CH_3OCH_2Cl}

, ionises readily.

A tertiary bridgehead halide does not ionise

A halogen at the bridgehead of a small cage is tertiary, yet it barely reacts by SN1: the cage holds the carbon pyramidal, so the cation cannot flatten.

Some cyclic halides ionise because the cation is aromatic

3-Bromocyclopropene and 7-bromocycloheptatriene look like awkward substrates, but their cations are aromatic. They precipitate silver bromide readily.

Primary does not guarantee fast SN2

Neopentyl halides are primary, but the tert-butyl group next door blocks the back of the carbon. They react far more slowly than 1-halobutanes.

Benzylic halides are fast by both mechanisms

Benzyl halides ionise easily (SN1) and also react fast by SN2, because the ring stabilises both the cation and the SN2 transition state. Do not assume that fast SN1 means slow SN2.

Nucleophiles and Ambident Reagents

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Ranking nucleophiles: charge, basicity, size and solvent

Halide nucleophilicity in protic and in aprotic solvents

\text{protic: } \mathrm{I^- > Br^- > Cl^- > F^-} \qquad \text{aprotic: } \mathrm{F^- > Cl^- > Br^- > I^-}

Ambident nucleophiles and the reagent-to-product table

Reagent with R–X	Attacking atom	Product	Class of product
Aqueous NaOH or KOH	O	$\mathrm{R{-}OH}$	Alcohol
$\mathrm{NaOR'}$	O	$\mathrm{R{-}O{-}R'}$	Ether (Williamson synthesis)
NaI in acetone	I	$\mathrm{R{-}I}$	Alkyl iodide
$\mathrm{NH_3}$	N	$\mathrm{R{-}NH_2}$ , then further alkylation	Amine
KCN (alcoholic)	C	$\mathrm{R{-}C{\equiv}N}$	Nitrile (alkyl cyanide)
AgCN	N	$\mathrm{R{-}N{\equiv}C}$	Isocyanide (isonitrile)
$\mathrm{KNO_2}$	O	$\mathrm{R{-}O{-}N{=}O}$	Alkyl nitrite
$\mathrm{AgNO_2}$	N	$\mathrm{R{-}NO_2}$	Nitroalkane
$\mathrm{R'COOAg}$	O	$\mathrm{R'COOR}$	Ester
$\mathrm{LiAlH_4}$	H (hydride)	$\mathrm{R{-}H}$	Alkane

Both silver salts bond through nitrogen: AgCN gives the isocyanide and AgNO₂ the nitroalkane.

Common traps

The solvent reverses the halide order

In water or an alcohol, iodide is the best halide nucleophile and fluoride the worst. In DMSO or DMF the order follows basicity and fluoride is the best. Check the solvent before ranking.

Basicity ranks only donors of the same atom

Hydroxide is a stronger base than hydrogen sulfide ion, yet

\mathrm{HS^-}

is the better nucleophile in water. Use basicity to compare two oxygen nucleophiles, not an oxygen with a sulfur.

A strong base can be a poor nucleophile

Potassium tert-butoxide is too bulky to reach a carbon from behind. It takes a β-hydrogen instead and gives an alkene.

Potassium nitrite gives the nitrite, silver nitrite the nitro compound

\mathrm{KNO_2}

→

\mathrm{R{-}O{-}N{=}O}

and

\mathrm{AgNO_2}

→

\mathrm{R{-}NO_2}

. This pair is easy to swap; remember that both silver salts bond through nitrogen.

AgCN is not ionic

KCN is largely ionic, which leaves carbon free to attack. AgCN is largely covalent, which is why it gives the isocyanide. A reason stating that both salts are highly ionic is false.

Aryl halogens survive while the side chain reacts

When a benzene ring carries both ring halogens and a

\mathrm{CH_2Cl}

group, a nucleophile such as cyanide replaces only the benzylic chlorine. Aryl C–X bonds do not undergo ordinary substitution.

Elimination Versus Substitution

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Substitution or elimination: reading the reagent, solvent and halide

Dehydrohalogenation with alcoholic KOH

\mathrm{R{-}CH_2{-}CH_2{-}X + KOH \xrightarrow{\text{ethanol},\ \Delta} R{-}CH{=}CH_2 + KX + H_2O}

Zaitsev rule and counting the alkenes from dehydrohalogenation

Stability order of alkenes (Zaitsev)

\text{tetrasubstituted} > \text{trisubstituted} > \text{disubstituted} > \text{monosubstituted}

Common traps

A bulky alkoxide gives the alkene, not the ether

Potassium tert-butoxide is an alkoxide, but it is too crowded to attack carbon. With a tertiary or secondary halide it removes a β-hydrogen and gives the alkene.

No β-hydrogen, no elimination

A halide such as

\mathrm{(CH_3)_3C{-}CH_2Br}

has no hydrogen on the carbon next to the C–Br carbon. Alcoholic KOH cannot give an alkene from it by E2.

Count cis and trans separately

A question asking for isomeric alkenes counts geometrical isomers. An alkene such as hept-3-ene counts as two, cis and trans.

Conjugation beats the Zaitsev count

When one possible C=C lies next to a benzene ring, it is the major product even if another alkene carries more alkyl groups. Check for conjugation before counting substituents.

A dihalide with excess base gives a diene

Excess alcoholic KOH removes both HX molecules. The two new double bonds form in conjugation, with each other and with any ring, wherever the structure allows.

Haloarenes and Reactions with Metals

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Substitution on the haloarene ring: nucleophilic and electrophilic

Chlorobenzene to phenol (Dow process)

\mathrm{C_6H_5Cl \xrightarrow[\text{(ii) } H^+]{\text{(i) NaOH, 623 K, 300 atm}} C_6H_5OH}

Reactions of organic halides with magnesium and sodium

Reactants	Conditions	Product	Name
R–X + Mg	Dry ether	R–MgX	Grignard reagent
R–MgX + $\mathrm{H_2O}$	Any trace of water	R–H + Mg(OH)X	Hydrolysis: the reason the ether must be dry
R–MgX + $\mathrm{D_2O}$	Heavy water	R–D	Deuterium labelling at the old C–X carbon
A dibromide + excess Mg	Dry ether	Both C–Br become C–MgBr	Di-Grignard reagent
2 R–X + 2 Na	Dry ether	R–R	Wurtz reaction
$\mathrm{BrCH_2CH_2CH_2Br}$ + Zn or Na	Heat	Cyclopropane	Ring closure (intramolecular Wurtz)
ArX + RX + 2 Na	Dry ether	Ar–R	Wurtz-Fittig reaction
2 ArX + 2 Na	Dry ether	Ar–Ar	Fittig reaction

Magnesium makes a carbon nucleophile; sodium joins two carbon groups.

Common traps

Halogens deactivate yet direct ortho and para

A halogen on benzene makes electrophilic substitution slower than on benzene itself (−I), but its lone pair places the new group ortho or para (+R). Deactivating does not mean meta-directing here.

Aryl substitution is neither SN1 nor SN2

The nucleophile adds to the ring to give an anionic intermediate and the halide then leaves. There is no backside attack and no aryl cation.

Two different halides give a mixture in the Wurtz reaction

\mathrm{CH_3Br}

and

\mathrm{C_2H_5Br}

with sodium give ethane, propane and butane together. The Wurtz reaction is useful only for symmetric alkanes.

Water is not the only thing that destroys a Grignard reagent

Any compound with an acidic hydrogen, such as an alcohol, an amine or water, converts R–MgX into R–H. A Grignard reagent cannot be made from a halide that also carries an OH or NH₂ group.

Haloalkanes and Haloarenes formulas

Classification, Structure and Physical Properties

Types of organic halides and the aryl C–X bond

Boiling point, melting point, density and polarity of halides

Polyhalogen compounds: formulas and uses

Common traps

Preparation of Haloalkanes and Haloarenes

Haloalkanes from alcohols, alkenes and hydrocarbons

Named reactions that make or couple organic halides

Common traps

SN1 and SN2: Mechanism, Kinetics and Stereochemistry

SN1 and SN2 compared: steps, rate law and conditions

Common traps

Reactivity Order in Nucleophilic Substitution

SN1 reactivity: ranking halides by carbocation stability

SN2 reactivity: crowding, neighbouring groups and benzylic halides

Halides that cannot ionise: vinylic, aryl and bridgehead

Common traps

Nucleophiles and Ambident Reagents

Ranking nucleophiles: charge, basicity, size and solvent

Ambident nucleophiles and the reagent-to-product table

Common traps

Elimination Versus Substitution

Substitution or elimination: reading the reagent, solvent and halide

Zaitsev rule and counting the alkenes from dehydrohalogenation

Common traps

Haloarenes and Reactions with Metals

Substitution on the haloarene ring: nucleophilic and electrophilic

Reactions of organic halides with magnesium and sodium

Common traps

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