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ALKYL HALIDES

STRUCTURE

Alkyl halides (R−X) have an alkyl group joined to a halogen atom by a single bond.

As you go down the periodic table,the size of the halogen atom increasesthe C−X bond is longerthe C−X bond is weaker

The C−X bond is polar.

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INTERESTING ALKYL HALIDES

important solvent

general anesthetic

refrigerant and aerosol propellant

nonstick coating

plastic used in films, pipes, and insulation

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INTERESTING ALKYL HALIDES

pesticide

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They can be classified as 1⁰, 2⁰, or 3⁰ alkyl halides:

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NOMENCLATURE

The systematic (IUPAC) name treats the compound as a haloalkane, while the common name treats the compound as an alkyl halide.

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IUPAC names:1. Identify and name the parent.2. Identify and name the substituents.3. Number the parent chain and assign a locant to each substituent.4. If more than one of the same kind of halogen is present, use prefix di, tri, tetra.5. If there are several different halogens, number them and list them in alphabetical order.

6-bromo-2,3,3-trichloro-5-methylheptane

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Common names:1. name all the carbon atoms of the molecule as a single alkyl group.2. name the halogen bonded to the alkyl group.3. combine the names of the alkyl group and halide, separating the words with a space

sec-butyl bromide(2-bromobutane)

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PHYSICAL PROPERTIES

Their boiling points (melting points) are higher than those of the parent alkanes due to higher molar masses (London forces) and polarizability of X.

butane 0°C

butyl fluoride 33°C

butyl chloride 78°C

butyl bromide 102°C

butyl iodide 131°C

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BP (MP) of branched chains are lower than BP (MP) of unbranched isomers. They are essentially insoluble in water (no H-bonding possible; except for alkyl fluorides). The densities of liquid alkyl halides are greater than those of hydrocarbons of comparable molar mass. Alkyl fluorides and monochlorides usually less dense than water. Iodides, bromides and polychlorides usually more dense than water.

CH3Cl 0.92 g/mL CH3(CH2)6CH2F 0.8 g/mL

CH2Cl2 1.4 g/mL CH3Br 1.7 g/mL CH3I 2.3 g/mL

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PREPARATION OF ALKYL HALIDES

A. From alkanes R−Cl or R−Br can be obtained from alkanes by reaction with Cl2 (g) or Br2 (g), in the presence of heat or UV light. The reaction involves a radical chain mechanism. Not a good synthesis because a mixture of halogenated products is usually obtained (hard to control). Regioselective in the order 3⁰ H > 2⁰ H > 1⁰H

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B. From alkenes

Addition of bromine or chlorine gas at room temperature (anti-addition)

Addition of HCl, HBr, HI to alkenes gives Markovnikov product. It involves carbocation intermediate which may rearrange. If a peroxide is added (HBr only), the reaction gives the anti-Markovnikov product.

Allylic bromination of alkenes with NBS (radical halogenation)

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C. From alkynes

Alkynes react with two equivalent of hydrogen halide to produce geminal dihaloalkanes.

Dihalogenation of alkynes result in tetrahaloalkanes.

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D. From alcohols

Reaction of 1⁰, 2⁰ or 3⁰ alcohol with HX (X = Cl, Br, I) gives R−X.Methyl alcohols or 1⁰ alcohols usually SN2.2⁰ and 3⁰ alcohols undergo SN1 with formation of carbocation intermediate.

Thionyl chloride (SOCl2) and phosphorus pentachloride (PCl5) converts alcohols into alkyl chlorides

ROH + PCl5 → RCl + POCl3 + HClROH + SOCl2 → RCl + SO2 + HCl (heat, base to mop up HCl byproduct)

Phosphorus tribromide (PBr3) converts alcohols into alkyl bromides.

3ROH + PBr3 → 3RBr + P(OH)3

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E. From other halides

Proceeds via nucleophilic substitution reactions.

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REACTIONS OF ALKYL HALIDES

A. Nucleophilic substitution reactions Alkyl halides react with nucleophiles in substitution reactions (SN1 and SN2).

The SN2 reaction:

Nucleophile forms a new bond to carbon at the same time as the bond to the

halogen is broken.

The reaction is bimolecular: rate = k[Nu][RX]

Reaction leads to inversion (or change) of stereochemistry at a chiral centre.

Reactivity: CH3X > RCH2X > R2CHX > R3CX

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The SN1 reaction:

Initial cleavage of the carbon–halogen bond to form an intermediate carbocation.

The reaction is unimolecular: rate = k[RX]

Reaction leads to racemisation.

Reactivity: R3CX > R2CHX > RCH2X > CH3X

The better the leaving group, the faster the rate of both SN1 and SN2 reactions.

Leaving group ability: I− > Br− > Cl− > F−

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B. Elimination reactions Alkyl halides react with bases in elimination reactions (E1 and E2).

The E2 reaction:The C−C bond begins to form at the same time as the C−H and C−X bonds begin to

break.The reaction is bimolecular: rate = k[base][RX]RX must adopt an antiperiplanar shapeStereospecific

The E1 reaction:

Initial cleavage of the carbon–halogen bond to form an intermediate carbocation.The reaction is unimolecular, rate = k[RX]No particular geometry required

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C. Formation of organolithium and organomagnesium (Grignard reagents) compounds

They are prepared from alkyl (or aryl) halides. The reactions are carried out under anhydrous conditions. Order of reactivity: RI > RBr > RCl

The alkyl group behaves like a carbanion (nucleophile).

They are strong bases and react rapidly with protons to form hydrocarbons.

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Alkyl dihalides do not yield satisfactory Grignard reagents due to polymerization.

Grignard reagents form complexes with solvent (stabilization).

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SPECTROSCOPY

A. MS Mass spectra of alkyl chlorides and alkyl bromides show the presence of an important M + 2 peak.For Cl, M : M + 2 = 3:1 and for Br, M : M + 2 = 1:1 The most important fragmentation mechanism is the simple loss of the halogen atom, leaving a carbocation. They may also lose a molecule of hydrogen halide. B. IR Difficult, C−X absorption occurs at very low frequencies. C. NMR The α-hydrogen will be deshielded and its chemical shift will increase.