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Alkyl Halides 3/12/2012 Primary, Secondary and Tertiary Alkyl halides Alkyl halides Alkyl: Halogen, X, is directly bonded to sp3 carbon A compound with a halogen atom bonded to one of the sp3 hybrid carbon atoms of an alkyl group + - C X 1 2 Nomenclature of Alkyl halides Substitutive nomenclature • Substitutive nomenclature of alkyl halides • functional class nomenclature treats the halogen as a halo- ( fluoro-, chloro-, – The alkyl group and the halide ( fluoride, chloride, bromo-, or iodo-) substituent on an alkane bromide, or iodide) are designated as separate chain. words. – The alkyl group is named on the basis of its • The carbon chain is numbered in the direction longest continuous chain beginning at the carbon that gives the substituted carbon the lower to which the halogen is attached. locant. Cl Cl Br H3C I Br CH I 3 1-Bromo-butane Iodomethane Chloro-cyclopentane n-Butyl bromide Methyl iodide Cyclopentyl Chloride 3 4 n-Butyl bromide Methyl iodide Cyclopentyl Chloride 1 3/12/2012 Naming Alkyl Halides using Substitutive nomenclature Substitutive Nomenclature • When the carbon chain bears both a halogen • Find longest chain, name it as parent chain and an alkyl substituent, the two substituents – (Contains double or triple bond if present) are considered of equal rank, and the chain is – Number from end nearest any substituent (alkyl or numbered so as to give the lower number to halogen) the substituent nearer the end of the chain. • Substitutive names are preferred, but functional class names are sometimes more convenient or more familiar and are frequently encountered in organic chemistry. 5 6 Naming Alkyl Halides using Common and IUPAC names of some Halides Substitutive Nomenclature Structure IUPAC Name Common Name • Naming if Two Halides or Alkyl Are Equally F Distant from Ends of Chain 1-Fluropropane n-Propyl fluoride (CH3CH2CH2F) • Begin at the end nearer the substituent whose name comes first in the alphabet 2-Chloro-butane Cl Sec Butyl Chloride [CH3CH2CH(Cl)CH3] Br 1-Bromo-2,2-dimethyl-propane Neo-pentyl bromide [(CH3)3CCH2Br] 7 8 2 3/12/2012 Common and IUPAC names of some Halides Common and IUPAC names of some Halides Structure IUPAC Name Common Name Structure IUPAC Name Common Name H Cl Cl Chloroethene Vinyl Chloride H H H C Cl Trichloromethane Chloroform (CH2=CHCl) Cl (CHCl3) H Br Br H Tribromoethane H 3-Bromoethene Allyl bromide H C Br Bromoform (CH =CHCH Br) 2 2 Br Cl (CHBr3) H C H Cl Dichloromethane Methylene Chloride Cl H C Cl Tetrachloromethane Carbon tetrachloride (CH Cl ) 2 2 Cl 9 (CCl4) 10 Q & A: Write IUPAC names of the Classes of Halides following halides • Alkyl: Halogen, X, is directly bonded to sp3 carbon. • Vinyl: X is bonded to sp2 carbon of alkene. H X X H H 11 12 3 3/12/2012 Classes of Halides Dihalides • Aryl: X is bonded to sp2 carbon on benzene • Geminal dihalide: two halogen atoms are ring. bonded to the same carbon X H C H X • Vicinal dihalide: two halogen atoms are bonded to adjacent carbons. X X H C C H H H 13 14 Physical Properties of Alkyl halides: Physical Properties of Alkyl halides: Bond length Bond length and energy • The C-X bond (where X denotes a halogen) of an alkyl • The electron density of the orbital decreases halide is formed from the overlap of an SP3 orbital of with increasing volume, the bond becomes carbon with an SP3 orbital of the halogen 3 3 longer and weaker as the size of the halogen • Fluorine uses a 2SP orbital, chlorine a 3SP orbital, increases. bromine a 4SP3 orbital, and iodine a 5 SP3 orbital. • Since halogen atoms are more electronegative than • Since the size of halogen atom increases as we carbon, the carbon halogen bond of alkyl halide is go down the group in the periodic table, polarised; the carbon atom bears a partial positive fluorine atom is the smallest and iodine atom, charge whereas the halogen atom bears a partial the largest. Consequently the carbon-halogen negative charge. bond length also increases from C—F to C—I. 15 16 4 3/12/2012 Physical Properties of Alkyl halides: Physical Properties of Alkyl Halides: Bond length and energy Colour and Smell Bond Bond C-X bond • Alkyl halides are colourless when pure. length/ppm (Å) energy/KJ mol-1 However, bromides and iodides develop CH3-F 139 (1.39) 452 colour when exposed to light. CH3-Cl 178 (1.78) 351 • Many volatile halogen compounds have sweet CH3-Br 193 (1.93) 293 smell. CH3-I 214 (2.14) 234 17 18 Physical Properties of Alkyl Halides: Physical Properties of Alkyl Halides: Boiling Points Boiling Points • The boiling point (bp) of a compound is the • If the molecules are held together by strong forces, it will take a lot of energy to pull the molecules away temperature at which the liquid form of the from each other, the compound will have a high boiling compound becomes a gas (vaporizes). In order point. for a compound to vaporize, the forces that • Boiling point of a compound depends on the strength hold the individual molecules close to each of the attractive forces between the individual molecules. other in the liquid must be overcome. • In contrast, if the molecules are held together by weak forces, only a small amount of energy will be needed to pull the molecules away from each other and the compound will have a low boiling point. 19 20 5 3/12/2012 Physical Properties of Alkyl Halides: Physical Properties of Alkyl Halides: Boiling Points Boiling Points • Both van der Waals forces and dipole–dipole interactions must be overcome in order for an alkyl halide to boil. • As the halogen atom increases in size, the size of its electron cloud increases. • As a result, both the van der Waals contact area and the polarizability of the electron cloud increase. 21 22 Physical Properties of Alkyl Halides: Physical Properties of Alkyl Halides: Boiling Points Boiling Points • For the same alkyl group, the boiling points of • Methyl chloride, methyl bromide, ethyl alkyl halides decrease in the order: chloride and some chlorofluoromethanes are RI> RBr> RCl> RF. gases at room temperature. • This is because with the increase in size and • Higher members are liquids or solids. mass of halogen atom, the magnitude of van der Waal forces increases. • The boiling points of isomeric haloalkanes decrease with increase in branching 23 24 6 3/12/2012 Physical Properties of Alkyl Halides: Physical Properties of Alkyl Halides: Boiling Points Solubilities in Water • Due to greater polarity of the carbon-halogen • The haloalkanes are only very slightly soluble in bond, as well as higher molecular mass as water. In order for a haloalkane to dissolve in compared to the parent hydrocarbon, the water, energy is required to overcome the intermolecular forces of attraction (dipole-dipole attractions between the haloalkane molecules and break the hydrogen bonds between water and van der Waals) are stronger in the halogen molecules. derivatives. • Less energy is released when new attractions are • That is why the boiling points of chlorides, set up between the haloalkane and the water bromides and iodides are considerably higher molecules as these are not as strong as the than those of the hydrocarbons of comparable original hydrogen bonds in water. As a result, the molecular mass. solubility of haloalkanes in water is low. 25 26 Physical Properties of Alkyl Halides: Physical Properties of Alkyl Halides: Solubilities in Water Solubilities in Water • In summary: • Alkyl halides have some polar character, but only the alkyl fluorides have an atom that can form a hydrogen bond with water. • Alkyl fluorides are the most water soluble of the alkyl halides. • The other alkyl halides are less soluble in water 27 28 7 3/12/2012 Physical Properties of Alkyl Halides: Densities of Alkyl Halides Solubilities in Organic Solvents • However, haloalkanes tend to dissolve in • Alkyl monofluorides and monochlorides less organic solvents because the new dense than water. intermolecular attractions between • Alkyl dichlorides, mono bromides, and haloalkanes and solvent molecules have much monoiodides more dense than water i.e. the same strength as the ones being broken in Bromo, iodo and polychloro derivatives of the separate haloalkane and solvent hydrocarbons are heavier than water. molecules. • The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms 29 30 Uses of Alkyl Halides Uses of Alkyl Halides • Solvents - degreasers and dry cleaning fluid • Propellants: • Reagents for synthesis of other compounds – One major use of CFCs has been as propellants in • Halothane CF3CHClBr, is an Anesthetic aerosol inhalers for drugs used to treat asthma. • CHCl3 used originally (toxic and carcinogenic) – The conversion of these devices and treatments • Freons, chlorofluorocarbons or CFC’s are used in from CFC to halocarbons that do not have the fridges but are harmful to ozone layer. CF2Cl2, same effect on the ozone layer is well under way. now replaced with CF2CHCl, not as harmful to ozone layer (i.e. it is ozone friendly). • Many Pesticides including DDT contain halogens. 31 32 8 3/12/2012 Uses of Alkyl Halides (halons): Fire Uses of Alkyl Halides: Fire extinguishing extinguishing • Definition of halons • At high temperatures, halons decompose to – A class of simple hydrocarbon derivatives in which release halogen atoms that combine readily with bromine, chlorine and fluorine are substituted for active hydrogen atoms, quenching the flame some or all of the hydrogen atoms. propagation reaction even when adequate fuel, – These compounds are used mainly as fire- oxygen and heat remains. extinguishing gases, the two best known being • Halons are able to "poison" the fire at much Halon 1211 (CF2BrCl) and Halon 1301 (CF3Br).
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