Halogen Derivatives of Alkanes — MHT-CET Chemistry Formulas and Traps

Classification, Nomenclature and Physical Properties

Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides

Class by the carbon bearing X

\text{sp}^3:\ \text{alkyl / allylic (next to C=C) / benzylic (on ring carbon)};\quad \text{sp}^2:\ \text{vinylic / aryl};\quad \text{sp}:\ \text{alkynyl}

Boiling Point and C–X Bond Strength

Two opposite orders

\text{b.p.: } \text{RI} > \text{RBr} > \text{RCl} > \text{RF};\qquad \text{bond strength: } \text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}

Chiral Haloalkanes and Naming From a Structure

Chirality test

\text{C}abcd,\ a \neq b \neq c \neq d \Rightarrow \text{chiral; two identical groups} \Rightarrow \text{achiral}

Common traps

Calling 1-bromo-2-phenylethane benzylic

The bromine carbon is bonded to CH₂, not to the ring. Benzylic needs X on the ring-attached carbon: PhCH₂Br, PhCHBrR, PhCBrR₂.

Using bond strength to rank boiling points

The strong C–F bond does not make CH₃F hard to boil — boiling breaks intermolecular forces, not bonds. Fluoromethane has the LOWEST boiling point and the STRONGEST bond.

Checking only the carbon that carries the halogen

In 2-chloro-3,4-dimethylhexane C3 and C4 are also stereocentres. Walk every branch-point carbon before you count.

Preparation of Alkyl Halides

Learn this subtopic in the notes

From Alcohols and Alkenes

Alcohol to halide

\text{R-OH} \xrightarrow{\text{HX / PCl}_5\text{ / SOCl}_2} \text{R-X};\qquad \text{alkene} + \text{HX} \xrightarrow{\text{Markovnikov}} \text{R-X}

Finkelstein and Swarts: Swapping the Halogen

The two exchanges

\text{Finkelstein: R-X} + \text{NaI} \to \text{R-I};\qquad \text{Swarts: R-X} + \text{AgF} \to \text{R-F}

Wurtz, Fittig and Wurtz–Fittig Coupling

Sodium coupling

\text{R-X} + \text{R'-X} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{R-R'} + 2\text{NaX}

Common traps

Adding HBr the wrong way round

Without peroxide the H goes where the hydrogens already are. On 1-methylcyclohexene that puts Br on C1 with the methyl — the tertiary product — not on C2.

Swapping the two names

Finkelstein — Iodide (NaI). Swarts — Fluoride (AgF). The pairing is asked more often than either reaction, and always with both names as options.

Calling the aryl + alkyl coupling 'Fittig'

Fittig needs TWO aryl halides and gives a biaryl. Aryl + alkyl is Wurtz–Fittig. The 2025 papers printed the name as 'Wurtzilite'; the intended reaction is the same.

Nucleophilic Substitution: SN1 and SN2

Learn this subtopic in the notes

SN1 Against SN2: Mechanism, Rate Order and Stereochemistry

Rate orders

\text{SN1: } 3^\circ > 2^\circ > 1^\circ > \text{CH}_3\text{X};\qquad \text{SN2: } \text{CH}_3\text{X} > 1^\circ > 2^\circ > 3^\circ

Which Nucleophile Gives Which Product

Ambident nucleophiles

\text{KCN} \to \text{R-C≡N};\quad \text{AgCN} \to \text{R-N≡C};\qquad \text{KNO}_2 \to \text{R-O-N=O};\quad \text{AgNO}_2 \to \text{R-NO}_2

Common traps

Expecting the 3°-fastest order for SN2

Crowding blocks the backside. SN2 is fastest on the LEAST substituted carbon; tertiary halides go SN1 (or eliminate). The two orders are exact opposites.

Writing the ester the wrong way round

The acyl part comes from the silver salt, the alkyl from the halide: silver ACETATE + ETHYL bromide is CH₃COO–C₂H₅, ethyl acetate — not ethyl-COO-methyl.

Elimination and Aromatic Nucleophilic Substitution

Learn this subtopic in the notes

Dehydrohalogenation and the Saytzeff Rule

β-Elimination

\text{R}_2\text{CH-CR}_2\text{X} \xrightarrow{\text{alc. KOH}} \text{R}_2\text{C=CR}_2 + \text{HX} \quad (\text{Saytzeff: more substituted alkene})

Nitro Groups Activate the C–X Bond of a Haloarene

Activation by nitro groups

\text{rate} \uparrow \text{ with each } -\text{NO}_2 \text{ at o/p};\qquad m\text{-NO}_2 \text{ has no effect}

Reactions of Haloarenes: Fittig and o/p Electrophilic Substitution

Halogen directs o/p

\text{C}_6\text{H}_5\text{Cl} \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} o\text{- and } p\text{-ClC}_6\text{H}_4\text{NO}_2;\qquad 2\text{ArX} + 2\text{Na} \to \text{Ar-Ar}

Common traps

Expecting the ether from ethoxide and a 2° halide

Ethoxide is a strong base as well as a nucleophile; on a secondary or tertiary halide it eliminates. 2-Ethoxypropane is the offered wrong answer; propene is the product.

Counting a meta nitro group as activating

Only ortho and para positions put the ring's negative charge next to the C–Cl carbon. m-Nitrochlorobenzene behaves almost like chlorobenzene — it is the 'most difficult' answer, not p-nitro.

Picking 'only para'

Para is the MAJOR product, but the ortho isomer forms too and the paper asks for the mixture. 'Only 1-chloro-4-nitrobenzene' is the planted option.

Polyhalogen Compounds: Freons, DDT, BHC and War Gases

Learn this subtopic in the notes

Freons, DDT and BHC

Compound	Formula	Use
Freon-12	$\text{CCl}_2\text{F}_2$	Refrigerant, aerosol propellant Dichlorodifluoromethane — two Cl, two F.
Refrigerant-22	$\text{CHClF}_2$	Refrigerant; made from chloroform
DDT	$(p\text{-ClC}_6\text{H}_4)_2\text{CH-CCl}_3$	Insecticide (banned) Both rings carry a PARA chlorine; the side carbon carries CCl₃, not CCl₂.
BHC (lindane)	$\text{C}_6\text{H}_6\text{Cl}_6$	Insecticide used in place of DDT
Chloroform	$\text{CHCl}_3$	Solvent; once an anaesthetic
Iodoform	$\text{CHI}_3$	Antiseptic

Freon-12 is the two-and-two halomethane; DDT is two para-chlorophenyls plus a trichloromethyl on one carbon.

Phosgene, Mustard Gas and Tear Gas

Agent	Formula	Cl per molecule	Type
Phosgene	$\text{COCl}_2$	2	Choking
Mustard gas	$(\text{ClCH}_2\text{CH}_2)_2\text{S}$	2	Blister (vesicant) One S per molecule.
Tear gas (chloropicrin)	$\text{CCl}_3\text{NO}_2$	3	Lachrymator Highest chlorine count of the three — the official key on the 2022 paper.

Tear gas has three chlorines; the other two have two each.

Common traps

Picking the DDT drawing without the ring chlorines

1,1,1-Trichloro-2,2-diphenylethane is offered every time. DDT's rings each carry a chlorine at the para position; the trichloromethyl alone is not enough.

Giving mustard gas the most chlorine

Mustard gas has two chlorines, one at each end. Chloropicrin has three on one carbon. The 2022 paper keyed tear gas.

Halogen Derivatives of Alkanes formulas

Classification, Nomenclature and Physical Properties

Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides

Boiling Point and C–X Bond Strength

Chiral Haloalkanes and Naming From a Structure

Common traps

Preparation of Alkyl Halides

From Alcohols and Alkenes

Finkelstein and Swarts: Swapping the Halogen

Wurtz, Fittig and Wurtz–Fittig Coupling

Common traps

Nucleophilic Substitution: SN1 and SN2

SN1 Against SN2: Mechanism, Rate Order and Stereochemistry

Which Nucleophile Gives Which Product

Common traps

Elimination and Aromatic Nucleophilic Substitution

Dehydrohalogenation and the Saytzeff Rule

Nitro Groups Activate the C–X Bond of a Haloarene

Reactions of Haloarenes: Fittig and o/p Electrophilic Substitution

Common traps

Polyhalogen Compounds: Freons, DDT, BHC and War Gases

Freons, DDT and BHC

Phosgene, Mustard Gas and Tear Gas

Common traps

More MHT-CET Chemistry formula sheets