Haloalkanes and Haloarenes: NEET notes

1. Classification

NCERT §6.1

• Swapping one or more hydrogen atoms of a hydrocarbon for halogen gives a haloalkane (alkyl halide) when the parent is aliphatic and a haloarene (aryl halide) when it is aromatic. In haloalkanes the halogen sits on an sp³ carbon; in haloarenes it sits on an sp² carbon of an aryl ring.
• Halogen compounds matter in medicine: the chlorine-containing antibiotic chloramphenicol treats typhoid, the body's iodine hormone thyroxine prevents goiter, chloroquine treats malaria and halothane is a surgical anaesthetic. Some fully fluorinated compounds are being studied as blood substitutes.
• By the number of halogen atoms, compounds are mono-, di- or polyhalogen (tri-, tetra- and so on).
• Compounds with an sp³ C–X bond: alkyl halides (R–X, general formula CₙH₂ₙ₊₁X), allylic halides (X on the sp³ carbon next to a C=C) and benzylic halides (X on the sp³ carbon joined to an aromatic ring).
• Alkyl halides are called primary (1°), secondary (2°) or tertiary (3°) according to whether the carbon holding the halogen is itself primary, secondary or tertiary.
• Compounds with an sp² C–X bond: vinylic halides (X on a carbon of a C=C) and aryl halides (X directly on a carbon of an aromatic ring).
• Halogenated compounds persist in the environment because soil bacteria break them down poorly.

2. Nomenclature

NCERT §6.2

• Common names put the alkyl group first and then the halide (methyl chloride). IUPAC names treat the compound as a hydrocarbon carrying a halo substituent (chloromethane).
• For monohalobenzenes the common and IUPAC names coincide. Dihalobenzenes take o-, m-, p- in common names but the locants 1,2-, 1,3- and 1,4- in IUPAC names.
• Dihalides with both halogens on one carbon are geminal (gem-dihalides, common name alkylidene halides); with the halogens on neighbouring carbons they are vicinal (vic-dihalides, alkylene dihalides). IUPAC calls both dihaloalkanes.
• Name pairs to know: sec-butyl chloride = 2-chlorobutane; neo-pentyl bromide = 1-bromo-2,2-dimethylpropane; tert-butyl bromide = 2-bromo-2-methylpropane; vinyl chloride = chloroethene; allyl bromide = 3-bromopropene.
• More pairs: methylene chloride = dichloromethane; chloroform = trichloromethane; bromoform = tribromomethane; carbon tetrachloride = tetrachloromethane; n-propyl fluoride = 1-fluoropropane; benzyl chloride = chlorophenylmethane; o-chlorotoluene = 1-chloro-2-methylbenzene (or 2-chlorotoluene).
• C₅H₁₁Br has eight structural isomers. Primary: 1-bromopentane, 1-bromo-3-methylbutane, 1-bromo-2-methylbutane, 1-bromo-2,2-dimethylpropane. Secondary: 2-bromopentane, 3-bromopentane, 2-bromo-3-methylbutane. Tertiary: 2-bromo-2-methylbutane.

3. The carbon-halogen bond

NCERT §6.3

• A halogen is more electronegative than carbon, so the C–X bond is polar: carbon carries a partial positive charge (δ+) and the halogen a partial negative charge (δ−).
• Down the group the halogen atom grows (F smallest, I largest), so the C–X bond lengthens from C–F to C–I.
• Bond lengths in the methyl halides: CH₃–F 139 pm, CH₃–Cl 178 pm, CH₃–Br 193 pm, CH₃–I 214 pm.
• C–X bond enthalpies fall as the bond lengthens: 452 (C–F), 351 (C–Cl), 293 (C–Br) and 234 (C–I) kJ mol⁻¹.
• Dipole moments: CH₃F 1.847 D, CH₃Cl 1.860 D, CH₃Br 1.830 D, CH₃I 1.636 D. The largest is for CH₃Cl, not CH₃F: F has the bigger charge separation but the far shorter bond.
• The δ+ carbon is the site a nucleophile attacks, and the weaker, longer C–I bond breaks most easily; this explains the reactivity order R–I > R–Br > R–Cl > R–F met later.

4. Preparing haloalkanes

NCERT §6.4

• From alcohols: the –OH group is replaced by halogen using concentrated halogen acids, phosphorus halides (PCl₃, PCl₅, PBr₃, PI₃) or thionyl chloride (SOCl₂). Alcohols are easy to obtain, so this is the preferred route.
• SOCl₂ is the reagent of choice for chlorides: the by-products SO₂ and HCl are gases that leave the mixture, so the alkyl chloride is obtained pure.
• With HCl, primary and secondary alcohols need ZnCl₂ as a catalyst, while a tertiary alcohol reacts on simple shaking with concentrated HCl at room temperature. With a given halogen acid the order of reactivity is 3° > 2° > 1°.
• Bromides are made with constant boiling HBr (48%). Iodides form in good yield when the alcohol is heated with NaI or KI in 95% orthophosphoric acid; H₂SO₄ is avoided because it turns KI into HI and then oxidises HI to I₂. PBr₃ and PI₃ are made in situ from red phosphorus with Br₂ or I₂.
• None of these methods works for aryl halides: the C–O bond of a phenol has partial double bond character and is too strong to break.
• Free radical chlorination or bromination of an alkane gives a hard-to-separate mixture of isomeric mono- and polyhalo products, so the yield of any single one is low. (CH₃)₂CHCH₂CH₃ has four kinds of hydrogen and gives four monochloro isomers.
• From alkenes: HCl, HBr or HI add to give an alkyl halide; propene can give two products, and Markovnikov's rule picks the major one. Br₂ in CCl₄ adds to give a colourless vic-dibromide, and the loss of the reddish brown bromine colour is a test for a C=C bond.
• Finkelstein reaction: an alkyl chloride or bromide with NaI in dry acetone gives the alkyl iodide. NaCl or NaBr is insoluble in dry acetone and precipitates, which pulls the equilibrium forward (Le Chatelier's principle).
• Swarts reaction: heating an alkyl chloride or bromide with a metallic fluoride (AgF, Hg₂F₂, CoF₂ or SbF₃) gives the alkyl fluoride.

5. Preparing haloarenes

NCERT §6.5

• Aryl chlorides and bromides come from electrophilic substitution of an arene with Cl₂ or Br₂ and a Lewis acid catalyst such as iron or iron(III) chloride.
• When ortho and para isomers form, they are easy to separate because their melting points differ widely.
• Iodination is reversible, so an oxidising agent (HNO₃ or HIO₄) is added to destroy the HI formed. Fluoroarenes cannot be made this way because fluorine is far too reactive.
• Sandmeyer's reaction: a primary aromatic amine in cold aqueous mineral acid is treated with NaNO₂ to give a diazonium salt; mixing the freshly made salt with CuCl or CuBr puts –Cl or –Br in place of the diazonium group.
• Iodoarenes need no copper salt: shaking the diazonium salt with potassium iodide is enough.

6. Physical properties

NCERT §6.6

• Pure alkyl halides are colourless, but bromides and iodides turn coloured in light. Many volatile halogen compounds smell sweet.
• At room temperature CH₃Cl, CH₃Br, C₂H₅Cl and some chlorofluoromethanes are gases; the larger members are liquids or solids.
• Being polar and heavier, halogen derivatives have stronger dipole-dipole and van der Waals attractions than the parent hydrocarbons, so chlorides, bromides and iodides boil much higher than hydrocarbons of similar molecular mass.
• For one alkyl group, boiling points run RI > RBr > RCl > RF, because a bigger, heavier halogen gives larger van der Waals forces. Among isomers, more branching means a lower boiling point: 2-bromo-2-methylpropane boils lowest of the three C₄H₉Br isomers NCERT compares.
• Isomeric dihalobenzenes boil at nearly the same temperature, but the para isomer melts higher than ortho and meta because its symmetry packs better into the crystal lattice.
• Haloalkanes are only very slightly soluble in water: dissolving must break the haloalkane-haloalkane attractions and water's hydrogen bonds, and the new haloalkane-water attractions release less energy. In organic solvents the attractions formed and broken are similar, so they dissolve.
• Bromo, iodo and polychloro compounds are denser than water. Density rises with more carbon atoms, more halogen atoms and heavier halogen: n-C₃H₇Cl 0.89, n-C₃H₇Br 1.335, n-C₃H₇I 1.747, CH₂Cl₂ 1.336, CHCl₃ 1.489, CCl₄ 1.595 g/mL.

7. Nucleophilic substitution and SN2

NCERT §6.7.1

• Haloalkane reactions fall into three groups: nucleophilic substitution, elimination, and reaction with metals.
• An electron-rich nucleophile attacks the δ+ carbon of the C–X bond and the halogen departs as a halide ion, the leaving group. The haloalkane is the substrate.
• Products with common nucleophiles: OH⁻ (NaOH/KOH) or H₂O gives an alcohol; R′O⁻ an ether; I⁻ an alkyl iodide; NH₃ a primary amine, R′NH₂ a secondary and R′R″NH a tertiary amine; KCN a nitrile; AgCN an isonitrile; KNO₂ an alkyl nitrite; AgNO₂ a nitroalkane; R′COOAg an ester; LiAlH₄ (H⁻) a hydrocarbon; R′⁻M⁺ an alkane RR′.
• Cyanide and nitrite are ambident nucleophiles, each with two donor sites. Cyanide can bond through C (alkyl cyanide) or N (isocyanide); nitrite through O (alkyl nitrite) or N (nitroalkane).
• KCN is mostly ionic, so free CN⁻ attacks mainly through carbon, since a C–C bond is more stable than C–N: the product is the alkyl cyanide. AgCN is mostly covalent, leaving nitrogen to donate: the main product is the isocyanide.
• SN2 (substitution nucleophilic bimolecular): CH₃Cl + OH⁻ → CH₃OH + Cl⁻ follows second order kinetics; the rate depends on both [CH₃Cl] and [OH⁻]. Hughes and Ingold proposed the mechanism in 1937.
• It is one step with no intermediate. OH⁻ attacks from the side opposite the leaving group; as the C–O bond forms, the C–Cl bond weakens. In the transition state carbon is partly bonded to five groups and its three H atoms lie in one plane. This transition state cannot be isolated.
• The three substituents flip over like an umbrella blown inside out, so the product has an inverted configuration (inversion of configuration).
• Because the nucleophile must reach the back of the carbon, bulky groups on or near it slow SN2 sharply. Methyl halides react fastest (only three small H atoms) and tertiary halides slowest: CH₃X > 1° > 2° > 3°.

8. SN1 and reactivity order

NCERT §6.7.1

• SN1 (substitution nucleophilic unimolecular) reactions are usually run in polar protic solvents such as water, alcohol or acetic acid.
• (CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻ follows first order kinetics: the rate depends on [tert-butyl bromide] only, not on [OH⁻].
• Step I: the polarised C–Br bond breaks slowly and reversibly into a carbocation and Br⁻; the energy comes from solvation of the halide ion by the protons of the protic solvent. Step II: the nucleophile attacks the carbocation quickly. The slow step sets the rate.
• The more stable the carbocation, the faster it forms and the faster the reaction. Tertiary carbocations are the most stable, so 3° halides react fastest: SN1 order 3° > 2° > 1° > CH₃X, the reverse of SN2.
• Allylic and benzylic halides are very reactive in SN1 because their carbocations are stabilised by resonance.
• For a given alkyl group, both mechanisms follow R–I > R–Br > R–Cl >> R–F; the large iodide ion leaves most readily.
• Bromobutanes: SN1 order n-C₄H₉Br < (CH₃)₂CHCH₂Br < CH₃CH₂CH(Br)CH₃ < (CH₃)₃CBr, and SN2 the exact reverse. Of the two primary bromides, (CH₃)₂CHCH₂Br is faster in SN1 because (CH₃)₂CH– has a larger electron-donating inductive effect.
• Benzylic bromides: SN1 order C₆H₅C(CH₃)(C₆H₅)Br > C₆H₅CH(C₆H₅)Br > C₆H₅CH(CH₃)Br > C₆H₅CH₂Br, and SN2 the reverse. Two phenyl groups stabilise a cation by resonance, but a phenyl group is bulkier than methyl, which slows SN2.

9. Stereochemistry of substitution

NCERT §6.7.1

• Plane polarised light is made by passing ordinary light through a Nicol prism. Optically active compounds rotate its plane; a polarimeter measures the angle. Clockwise rotation is dextrorotatory, d or (+); anticlockwise is laevorotatory, l or (−). Such (+) and (−) forms are optical isomers.
• Pasteur (1848) found crystals that were mirror images, whose solutions rotated light by equal amounts in opposite directions. In 1874 van't Hoff and Le Bel independently proposed that carbon's four bonds point to the corners of a tetrahedron.
• A carbon holding four different groups is an asymmetric carbon (stereocentre); its molecule and mirror image cannot be superimposed. Such objects are chiral and optically active; objects superimposable on their mirror image are achiral and optically inactive.
• Propan-2-ol has no asymmetric carbon and is achiral; butan-2-ol has one and is chiral. Other chiral examples: 2-chlorobutane, 2,3-dihydroxypropanal, bromochloroiodomethane (BrClCHI) and 2-bromopropanoic acid.
• Enantiomers are non-superimposable mirror-image stereoisomers. They share melting point, boiling point and refractive index, and differ only in the direction they rotate plane polarised light.
• An equal mixture of two enantiomers has zero rotation: a racemic mixture, written dl or (±), e.g. (±)-butan-2-ol. Turning one enantiomer into a racemic mixture is racemisation. The sign of rotation does not tell the actual (absolute) configuration.
• Retention: if no bond to the stereocentre breaks, the arrangement around it is kept. Heating (−)-2-methylbutan-1-ol with concentrated HCl keeps the configuration, yet the chloride has the opposite sign of rotation, because a different compound can rotate light differently.
• When a bond to the asymmetric carbon itself breaks, three outcomes are possible: retention (same arrangement), inversion (mirror arrangement) or racemisation (a 50:50 mixture, optically inactive).
• SN2 gives inversion, because the nucleophile enters opposite the halogen: (−)-2-bromooctane with NaOH gives (+)-octan-2-ol, the –OH on the side away from where Br was.
• SN1 gives racemisation: the sp² carbocation is planar and achiral, so the nucleophile adds from either face, giving both configurations. Hydrolysis of optically active 2-bromobutane gives (±)-butan-2-ol.

10. Elimination and reactions with metals

NCERT §6.7.1

• The carbon bearing the halogen is the α-carbon and its neighbour is the β-carbon. Heating a haloalkane that has a β-hydrogen with alcoholic KOH removes H from the β-carbon and X from the α-carbon to give an alkene: β-elimination, or dehydrohalogenation.
• Zaitsev (Saytzeff) rule (1875): when more than one alkene can form, the major product is the one with more alkyl groups on the doubly bonded carbons. 2-Bromopentane gives mainly pent-2-ene.
• Substitution and elimination compete. A bulky nucleophile tends to act as a base and pull off a proton. Primary halides favour SN2; secondary halides go SN2 or elimination depending on the strength of the base or nucleophile; tertiary halides go SN1 or elimination depending on the stability of the carbocation or of the more substituted alkene.
• Aqueous KOH mainly substitutes (alcohol); alcoholic KOH mainly eliminates (alkene).
• With certain metals, most organic chlorides, bromides and iodides give organometallic compounds, which hold a carbon-metal bond.
• Grignard reagents, RMgX, form from a haloalkane and magnesium in dry ether; Victor Grignard reported them in 1900 and shared the 1912 Nobel Prize in Chemistry with Paul Sabatier.
• In RMgX the C–Mg bond is covalent but highly polar, with carbon drawing electrons from magnesium; the Mg–X bond is essentially ionic.
• Grignard reagents react with any proton source to give hydrocarbons; even water, alcohols and amines are acidic enough (RMgX + H₂O → RH + Mg(OH)X). Moisture must therefore be kept out, which is why dry ether is used; the reaction is also a way to turn a halide into a hydrocarbon.
• Wurtz reaction: an alkyl halide with sodium in dry ether gives an alkane with twice the number of carbon atoms (2RX + 2Na → R–R + 2NaX).

11. Reactions of haloarenes

NCERT §6.7.2

• Aryl halides are far less reactive than alkyl halides towards nucleophilic substitution, for four reasons listed below.
• Resonance: the halogen's lone pairs conjugate with the ring's π electrons, giving the C–X bond partial double bond character that is harder to break.
• Hybridisation: the sp² carbon has more s-character, is more electronegative and holds the bonding pair tighter than an sp³ carbon. C–Cl is 169 pm in a haloarene against 177 pm in a haloalkane, and the shorter bond is harder to break.
• A phenyl cation from self-ionisation gets no resonance stabilisation, which rules out SN1; and the electron-rich nucleophile is repelled by the electron-rich ring.
• Chlorobenzene becomes phenol only under harsh conditions: aqueous NaOH at 623 K and 300 atmospheres.
• An –NO₂ group ortho or para to the halogen raises reactivity by withdrawing electron density and stabilising, through resonance, the negative charge in the carbanion intermediate. A meta –NO₂ has no effect, because none of the resonance structures then puts the negative charge on the carbon carrying –NO₂.
• Haloarenes undergo the usual electrophilic substitutions of benzene: halogenation, nitration, sulphonation and Friedel-Crafts reactions. The halogen is slightly deactivating yet ortho, para-directing.
• Why both: the –I effect withdraws electrons and deactivates the ring, so reactions are slower and need harsher conditions than with benzene. Resonance releases electrons to the ortho and para positions and stabilises attack there. The stronger inductive effect sets reactivity; resonance sets orientation.
• Wurtz-Fittig reaction: an alkyl halide and an aryl halide with sodium in dry ether give an alkylarene. Fittig reaction: aryl halides alone with sodium in dry ether join two aryl groups (e.g. biphenyl).

12. Polyhalogen compounds

NCERT §6.8

• Carbon compounds with more than one halogen atom are polyhalogen compounds; many are used in industry and agriculture.
• Dichloromethane (methylene chloride) is a paint remover, aerosol propellant, process solvent for making drugs, and metal cleaning and finishing solvent. It harms the central nervous system: low levels in air slightly impair hearing and vision; higher levels bring dizziness, nausea, tingling and numbness in fingers and toes. On skin it burns and reddens; in the eye it can burn the cornea.
• Chloroform (trichloromethane) dissolves fats, alkaloids and iodine; its main use today is making the refrigerant freon R-22. Once a general anaesthetic, it has been replaced by safer ones such as ether.
• Breathing about 900 ppm chloroform briefly causes dizziness, fatigue and headache; long exposure can damage the liver (where it is metabolised to phosgene) and kidneys. In light, air slowly oxidises it to the very poisonous carbonyl chloride (phosgene), so it is kept in closed, dark bottles filled to the top.
• Iodoform (triiodomethane) was used as an antiseptic, but the action came from free iodine it released, not from iodoform itself; its unpleasant smell led to iodine-containing replacements.
• Carbon tetrachloride (tetrachloromethane) is used to make refrigerants and aerosol propellants, as a feedstock for chlorofluorocarbons, in pharmaceutical manufacturing and as a solvent; until the mid 1960s it was also a cleaning fluid, spot remover and fire extinguisher.
• CCl₄ exposure can cause dizziness, nausea and vomiting with lasting nerve damage and, in severe cases, stupor, coma or death; it can make the heart beat irregularly or stop, and some evidence links it to liver cancer. Released into air, it rises and depletes the ozone layer, which raises UV exposure (skin cancer, eye disease, possible immune disruption).
• Freons are the chlorofluorocarbons of methane and ethane: extremely stable, unreactive, non-toxic, non-corrosive, easily liquefied gases. Freon 12 (CCl₂F₂), a common industrial one, is made from CCl₄ by the Swarts reaction. By 1974 world output was about 2 billion pounds a year.
• Freons used in aerosols, refrigeration and air conditioning eventually escape and diffuse unchanged into the stratosphere, where they start radical chain reactions that disturb the natural ozone balance.
• DDT (p,p′-dichlorodiphenyltrichloroethane), a chlorinated insecticide, was made in 1873; Paul Muller of Geigy found its insecticidal power in 1939 and received the 1948 Nobel Prize in Physiology or Medicine. It was widely used against malaria mosquitoes and typhus-carrying lice after World War II.
• Trouble with DDT: insects became resistant and it is highly toxic to fish. It is chemically stable and fat-soluble, is metabolised slowly and is stored in fatty tissue, so it builds up over time. The USA banned it in 1973, though some other countries still use it.

Must-know facts

1. Haloalkane: X on sp³ carbon; haloarene: X on sp² carbon of an aromatic ring. Allylic and benzylic halides are sp³; vinylic and aryl halides are sp².
2. gem-dihalide = both X on one carbon (alkylidene halide); vic-dihalide = X on adjacent carbons (alkylene dihalide).
3. C₅H₁₁Br has 8 structural isomers: 4 primary, 3 secondary and 1 tertiary (2-bromo-2-methylbutane).
4. C–X bond length: 139, 178, 193, 214 pm (F → I); bond enthalpy 452, 351, 293, 234 kJ mol⁻¹; dipole moment largest for CH₃Cl (1.860 D).
5. SOCl₂ gives pure alkyl chlorides because SO₂ and HCl escape as gases.
6. Alcohol reactivity with HX is 3° > 2° > 1°; 1° and 2° need ZnCl₂ with HCl.
7. Finkelstein: RCl/RBr + NaI in dry acetone → RI (NaCl/NaBr precipitates). Swarts: RCl/RBr + AgF, Hg₂F₂, CoF₂ or SbF₃ → RF.
8. Sandmeyer: ArN₂⁺ + CuCl/CuBr → ArCl/ArBr; ArN₂⁺ + KI → ArI without copper.
9. Boiling point: RI > RBr > RCl > RF; branching lowers it. p-Dihalobenzene has the highest melting point of its isomers (symmetry).
10. KCN → alkyl cyanide (RCN); AgCN → isocyanide (RNC). KNO₂ → alkyl nitrite; AgNO₂ → nitroalkane.
11. SN2: rate = k[RX][Nu⁻], one step, backside attack, inversion; CH₃X > 1° > 2° > 3°.
12. SN1: rate = k[RX], carbocation intermediate, polar protic solvent, racemisation; 3° > 2° > 1°; allylic and benzylic fast.
13. Leaving group ability for both mechanisms: R–I > R–Br > R–Cl >> R–F.
14. (−)-2-Bromooctane + NaOH (SN2) → (+)-octan-2-ol with inversion; optically active 2-bromobutane hydrolysis (SN1) → (±)-butan-2-ol.
15. Racemic mixture = 50:50 enantiomers, zero rotation, written (±) or dl. Enantiomers differ only in the sign of rotation.
16. Zaitsev: more substituted alkene is major; 2-bromopentane → pent-2-ene. Alcoholic KOH eliminates; aqueous KOH substitutes.
17. Grignard RMgX made in dry ether; any proton source (water, alcohol, amine) turns it into RH.
18. Wurtz: 2RX + 2Na → R–R; Wurtz-Fittig: RX + ArX + Na → Ar–R; Fittig: 2ArX + 2Na → Ar–Ar.
19. Aryl C–Cl 169 pm vs alkyl C–Cl 177 pm; chlorobenzene → phenol needs NaOH, 623 K, 300 atm; o/p –NO₂ activates, m –NO₂ does not.
20. Halogen on benzene: deactivating (–I) but ortho, para-directing (resonance). Freon 12 = CCl₂F₂; chloroform is stored in dark bottles filled to the top to stop phosgene forming.

Common traps

Formulas

Key terms

Haloalkane (alkyl halide)

A compound with halogen on an sp³ carbon of an alkyl group.

Haloarene (aryl halide)

A compound with halogen bonded straight to an sp² carbon of an aromatic ring.

Allylic / benzylic halide

Halogen on an sp³ carbon next to a C=C / next to an aromatic ring.

Vinylic halide

Halogen on one of the sp² carbons of a C=C.

gem- / vic-dihalide

Both halogens on the same carbon / on neighbouring carbons.

Nucleophile

An electron-rich species that attacks an electron-poor centre.

Leaving group

The group, here a halide ion, that departs with the bonding pair.

Ambident nucleophile

A nucleophile with two different atoms that can attack, such as CN⁻ or NO₂⁻.

Carbocation

A planar, sp² carbon ion carrying a positive charge, formed in the slow step of SN1.

Chiral

Not superimposable on its own mirror image.

Stereocentre (asymmetric carbon)

A carbon joined to four different groups.

Enantiomers

A pair of stereoisomers that are non-superimposable mirror images.

Racemic mixture

A 50:50 mix of two enantiomers, with zero net optical rotation.

Inversion of configuration

The product's arrangement around the stereocentre is the mirror of the reactant's.

Retention of configuration

The arrangement around the stereocentre is unchanged by the reaction.

β-Elimination

Loss of H from the β-carbon and X from the α-carbon to give an alkene.

Grignard reagent

An alkyl or aryl magnesium halide, RMgX, made in dry ether.

Freons

Chlorofluorocarbons of methane and ethane, such as CCl₂F₂.