Chem 334 - Fall 1998 - Organic Chemistry I
Portland State University - Dr. Carl C. Wamser

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Chapter 7 - Alkyl Halides

Structure

• alkyl halide: a compound containing a halogen atom
  covalently bonded to an sp3 hybridized carbon atom
• given the symbol R-X

Vinylic and Aryl Halides

• if the halogen is bonded to an sp2 hybridized carbon, it is a called a vinylic halide
• if it is bonded to a benzene ring, it is called an aryl halide, given the symbol Ar-X
• we will not study vinylic and aryl halides in this chapter

Nomenclature

• halogen substituents are indicated by the prefixes fluoro-, chloro-, bromo-, and iodo- and listed in alphabetical order with other substituents
• for haloalkenes, numbering is determined by the location of the C=C double bond
• common names - name the alkyl group followed by the name of the halide
  e.g., methyl chloride or chloromethane, allyl bromide or 3-bromopropene
• several polyhaloalkanes are common solvents and are generally referred to by their common names
  e.g., chloroform or trichloromethane, Freon 12 or dichlorodifluoromethane
• hydrocarbons in which all hydrogens are replaced by halogens are commonly named as perhaloalkanes or perhaloalkenes
  e.g., perfluoroethylene or 1,1,2,2-tetrafluoroethene

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
• dipole moments of all the haloalkanes are comparable
  • see the table in your text

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
  van der Waals radii
  • see the table in your text
• notice that
  F is only slightly larger than H
  among the halogens, only I is larger than CH3

Boiling Points

• among constitutional isomers, branched isomers
  have a more compact shape,
  decreased area of contact,
  decreased van der Waals attractive forces between neighbors,
  and lower boiling points
  • 1-bromobutane (n-butyl bromide) bp = 100°
  • 2-bromo-2-methylpropane (t-butyl bromide) bp = 72°
• 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
  • ethane bp = -89°
  • bromomethane bp = 4°
• 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
  • 2-methylpropane bp = - 1°
  • 2-fluoropropane bp = - 11°

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

Bond Lengths, Strengths

• C-F bonds are stronger than C-H bonds
• C-Cl, C-Br, and C-I bonds are weaker
  • see table in your text

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

• 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 of 2° hydrogen over a 1° hydrogen is high for bromination
  • propane + Br2 --> 1-bromopropane (8%) + 2-bromopropane (92%)
• regioselectivity is not as high for chlorination
  • propane + Cl2 --> 1-chloropropane (8%) + 2-chloropropane (92%)
• regioselectivity is 3° > 2° > 1°
  for bromination - approximately 1600:80:1
  for chlorination - approximately 5:4:1
• example: draw all monobromination products for isobutane
  and predict the % of each for a given reaction

Energetics

• using BDE data (Appendix 3), calculate the heat of reaction, delta H°, for the halogenation of an alkane
• delta H = sum of bonds broken - sum of bonds made

Mechanism - A radical chain 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

Mechanism - Formation of radicals

• the order of stability of alkyl radicals is 3° > 2° > 1° > methyl
• radical initiators like peroxides (ROOR) have weak bonds, eaily broken

Mechanism - Chain initiation

• a step in a radical chain reaction characterized by formation of radicals from nonradical compounds
  • Cl2 --> 2 Cl.

Mechanism - Chain propagation

• a step in a radical chain reaction characterized by reaction of a radical and a molecule to form a new radical
  • Cl. + CH3CH3 --> CH3CH2. + HCl
  • CH3CH2. + Cl2 --> CH3CH2Cl + Cl.
• notice that the radicals recycle
• chain length, n: the number of times the cycle of chain propagation steps repeats in a chain reaction

Mechanism - Chain termination

• a step in a radical chain reaction that involves destruction of radicals
  • 2 Cl. --> Cl2
  • CH3CH2. + Cl. --> CH3CH2Cl
  • 2 CH3CH2. --> CH3CH2CH2CH3

Chain Propagation Steps

• for any set of chain propagation steps, their
  equations add to the observed stoichiometry
  energies add to the observed delta 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)
• how do we account for the greater regioselectivity of
  bromination (1600:80:1) compared with chlorination (5:4:1)?

Hammond's Postulate

• What does a transition state look like?
  • a transition state looks like something between reactants and products
    but closer in structure to whichever it is closer in energy to
• Hammond's Postulate: the structure of the transition state
  for an exothermic reaction looks more like the reactants of that step
  for an endothermic reaction looks more like the products of that step
• this postulate applies equally well to the transition state for a one-step reaction
  and to each transition state in a multi-step reaction
• in halogenation of an alkane, the rate-limiting step is hydrogen abstraction
  this step is exothermic for chlorination and endothermic for bromination
• 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, and
  regioselectivity is only slightly influenced by radical stability
• 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

• 3° > 2° > 1° > methyl
  • 3° C-H bond 93 kca/mol
  • 2° C-H bond 96 kca/mol
  • 1° C-H bond 100 kca/mol
• regioselectivity is in the same order
• the order is like carbocation stability
  and for a similar reason -
  the radical center is electron-deficient
  and needs electron donation

Stereochemistry

• when radical halogenation produces a stereocenter
  or takes place at a hydrogen on an existing stereocenter,
  the product is an R,S mixture
  • butane + Br2 --> racemic 2-bromobutane
• for simple alkyl radicals, the carbon bearing the radical is sp2 hybridized
  and the unpaired electron occupies the unhybridized 2p orbital
• since the radical is achiral, the Br attaches equally to either side

Allylic Halogenation

• allylic carbon: a carbon adjacent to a C=C double bond
• allylic hydrogen: a hydrogen on an allylic carbon
• an allylic C-H bond is weaker than a vinylic C-H bond
  • typical alkyl C-H 93-100 kcal/mol
  • allylic C-H 86 kcal/mol
  • vinylic or aryl C-H 106 kcal/mol

Allylic Bromination

• allylic bromination using NBS

• the Br2 necessary for radical halogenation
  is provided by reaction of NBS with HBr
• a radical chain mechanism

The Allyl Radical

• a hybrid of two equivalent contributing structures

Organometallic Compounds

• a compound that contains a metal bonded to a carbon atom
• Grignard reagents are formed by reaction of an alkyl halide
  with magnesium metal in diethyl ether or tetrahydrofuran (THF)
• organolithium reagents are formed by reaction of an alkyl halide
  with lithium metal in a hydrocarbon solvent such as pentane
• carbon-metal bonds are polar covalent with negative carbon
• polarity (ionic vs. covalent character) depends on the metal electronegativity
• organometallics are strong bases and react with any proton donor stronger than
  the alkane from which they are derived
• classes of proton donors that react with Grignard and organolithium reagents are
  alcohols, carboxylic acids, amines, thiols, alkynes, phenols, water

Gilman Reagents

• prepared from an organolithium reagent and copper(I) iodide
• Gilman reagents can be used to form new carbon-carbon bonds
  by cross-coupling with alkyl or vinylic halides