Organic Chemistry

Organic Chemistry William H. Brown & Christopher Foote

Alkyl Halides Chapter 7 Chapter 7

Structure • Alkyl halide:a compound containing a halogen covalently bonded to an sp3 hybridized carbon; given the symbol RX • Vinylic halide: a compound containing a halogen bonded to an sp2 hybridized carbon • Aryl halide: a compound containing a halogen bonded to a benzene ring; given the symbol ArX • (we do not study vinylic or aryl halides in this chapter)

Nomenclature • number the parent chain to give the substituent encountered first the lowest number, whether it is halogen or an alkyl group • halogen substituents are indicated by the prefixes fluoro-, chloro-, bromo-, and iodo- and listed in alphabetical order with other substituents • the location of each halogen on the parent chain is given by a number preceding the name of the halogen • in haloalkenes, numbering of the parent hydrocarbon is determined by the location of the carbon-carbon double bond; give it the lowest number

Nomenclature • Examples

Nomenclature • Common names: name the alkyl group followed by the name of the halide

Nomenclature • Several polyhaloalkanes are common solvents and are generally referred to by their common or trivial names

Nomenclature • Hydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes

Dipole Moments • Dipole moment of RX depends on: • the sizes of the partial charges, • the distance between them, and • the polarizability of the unshared electrons on halogen

Boiling Points • For an alkane and an alkyl halide of comparable size and shape, the alkyl halide has the higher boiling point • the difference is due almost entirely to the greater polarizability of the three unshared pairs of electrons on halogen compared with the polarizability of shared electron pairs of the hydrocarbon

Boiling Points • Among constitutional isomers, branched isomers have • a more compact shape • decreased area of contact and decreased van der Waals attractive forces between neighbors • lower boiling points

Boiling Points • Boiling points of alkyl fluorides are lower than those of hydrocarbons of comparable molecular weight • the difference is due to the small size of fluorine, the tightness with which its electrons are held, and their particularly low polarizability

Density • The densities of liquid alkyl halides are greater than those of hydrocarbons of comparable molecular weight • a halogen has a greater mass per volume than a methyl or methylene group • Some liquid alkyl chlorides are less dense than water • All liquid alkyl bromides and iodides are more dense than water

van der Waals forces • van der Waals forces: a group of intermolecular forces, including • dipole-dipole • dipole-induced dipole • induced dipole - induced dipole (dispersion forces) • As atoms or molecules are brought closer together, van der Waals attractive forces are overcome by repulsive forces between electron clouds of adjacent atoms

van der Waals radii • energy minimum is where net attractive forces are the strongest • nonbonded interatomic and intermolecular distances at these minima can be measured by x-ray crystallography • each atom or group of atoms can be assigned an atomic or molecular radius called a van der Waals radius

Bond Lengths, Strengths • C-F bonds are stronger than C-H bonds; C-Cl, C-Br, and C-I bonds are weaker

Halogenation of Alkanes • If a mixture of methane and chlorine is kept in the dark at room temperature, no change occurs • If the mixture is heated or exposed to visible or ultraviolet light, reaction begins at once with the evolution of heat

Halogenation of Alkanes • What occurs is a substitution reaction, in this case, substitution of a chlorine atom for a hydrogen atom in methane • Substitution:a reaction in which an atom or group of atoms is replaced by another atom or group of atoms

Regioselectivity • Regioselectivity is high for bromination, but not as high for chlorination

Regioselectivity • Regioselectivity is 3° > 2° > 1° • for bromination, approximately 1600:80:1 • for chlorination, approximately 5:4:1 Example:draw all monobromination products and predict the % of each for this reaction

Energetics • Bond Dissociation Energies (BDE)

Energetics • Using BDE, we can calculate the heat of reaction, H°, for the halogenation of an alkane

Mechanism • Radical:any chemical species that contains one or more unpaired electrons • radicals are formed by homolytic cleavage of a bond • a barbed curved (fishhook) arrow is used to show the change in position of a single electron

Formation of Radicals • the order of stability of alkyl radicals is 3° > 2° > 1° > methyl

Mechanism • Chain initiation: a step in a chain reaction characterized by formation of reactive intermediates (radicals, anions, or cations) from nonradical or noncharged compounds

Mechanism • Chain propagation: a step in a chain reaction characterized by reaction of a reactive intermediate and a molecule to form a new reactive intermediate and a new molecule • Chain length:the number of times the cycle of chain propagation steps repeats in a chain reaction

Mechanism • Chain termination: a step in a chain reaction that involves destruction of reactive intermediates

Chain Propagation Steps • For any set of chain propagation steps, their • equations add to the observed stoichiometry • energies add to the observed H°

Regioselectivity? • The regioselectivity of chlorination and bromination can be accounted for in terms of the relative stabilities of alkyl radicals (3° > 2° > 1° > methyl) • But how do we account for the greater regioselectivity of bromination (1600:80:1) compared with chlorination (5:4:1)

Hammond’s Postulate • Hammond’s Postulate: the structure of the transition state • for an exothermic step looks more like the reactants of that step than like the products • for an endothermic step looks more like the products of that step than like the reactants • This postulate applies equally well to the transition state for a one-step reaction and to each transition state in a multi-step reaction

Hammond’s Postulate

Hammond’s Postulate • In halogenation of an alkane, the rate-determining step is hydrogen abstraction • this step is exothermic for chlorination and endothermic for bromination

Hammond’s Postulate • Because hydrogen abstraction for chlorination is exothermic: • the transition state resembles the alkane and a chlorine atom • there is little radical character on carbon in the transition state • regioselectivity is only slightly influenced by radical stability

Hammond’s Postulate • Because hydrogen abstraction for bromination is endothermic: • the transition state resembles an alkyl radical and HBr • there is significant radical character on carbon in the transition state, and • regioselectivity is greatly influenced by radical stability • radical stability is 3° > 2° > 1° > methyl, and regioselectivity is in the same order

Hammond’s Postulate

Stereochemistry • When radical halogenation produces a stereocenter or takes place at a hydrogen on an existing stereocenter, the product is an R,S mixture

Stereochemistry • for simple alkyl radicals, the carbon bearing the radical is sp2 hybridized and the unpaired electron occupies the unhybridized 2p orbital

Allylic Halogenation • Allylic carbon:a C adjacent to a C-C double bond • Allylic hydrogen: an H on an allylic carbon • an allylic C-H bond is weaker than a vinylic C-H bond

Allylic Bromination • Allylic bromination using NBS

Allylic Bromination • A radical chain mechanism • Chain initiation • Chain propagation

Allylic Bromination • chain termination • Br2 is provided by the reaction of NBS with HBr

The Allyl Radical • A hybrid of two equivalent contributing structures

Prob 7.6 Write the IUPAC name for each compound.

Prob 7.11 Name and draw structural formulas for all monohalogenation products of each reaction.

Prob 7.18 Account for the differences between the reactions of each compound with bromine.

Prob 7.21 Estimate the BDE of each indicated bond.

Prob 7.22 Propose a radical chain mechanism for this reaction.

Prob 7.27 Propose reagents and conditions to bring about each conversion.

Alkyl Halides End Chapter 7

Organic Chemistry