CH 8 - Alkyl Halides

haloalkane (alkyl halide)

a compound containing a halogen atom covalently bonded to an sp3 hybridized carbon; given the symbol RX

haloalkene (vinylic halide)

a compound containing a halogen atom bonded to an sp2 hybridized alkyl carbon

haloarene (aryl halide)

a compound containing a halogen atom bonded to a benzene ring; given the symbol ArX

halide nomenclature - treat halogen atoms as

if they are substituents

halide nomenclature - indiciate halogen substituents by

the prefixes fluoro-, chloro-, bromo-, and iodo- and list them in alphabetical order order

common names - 2-bromobutane =

sec-butyl bromide

common names - chloroethane

vinyl chloride

3-chloropropene

allyl chloride

common names - dichloromethane

methylene chloride

common names - trichloromethane

chloroform

common names - all subusituted

per- fluoropropane

the dipole moment of an RX depends on

the size of partial charges, the distance between them, and the polarizability of the electrons on the halogen

polarizability

a measure of the ease of distortion of the distribution of electron density about an atom in response to interaction with other molecules and ions

fluorine has a

very low polarizability

iodine has a very

high polarizability

an increase of the electronegativity of the halogen typically means

an increase in the dipole moment

• F is an exception

haloalkanes are associated in the

liquid state by van der Waals forces

van der Waals forces

a group of intermolecular attractive forces including

• dipole and dispersion forces

for an alkane and a haloalkane of comparable size and shape, the haloalkane has

the higher boiling point

• due to greater polarizability of the halogen atom

among constitutional isomers, branched isomers have

• a more compact shape

• decreases area of contact

• decreased van der Waals attractive forces between neighbors

• and lower boiling points

C-F bonds are stronger than

C-H bonds

C-Cl, C-Br, and C-I bonds are

weaker than C-H bonds

halogenation of alkanes - if a mixture of methane and chlorine is heated or exposed to light, a reaction begins

at once with the evolution of heat

halogenation of alkanes - bromination regioselectivity

high

halogenation of alkanes - chlorination regioselectivity

not very high

halogenation of alkanes - Lower bond energy =

more stable radical

mechanism of radial halogenation - chain initiation

a step in a chain reaction characterized by formation of reactive intermediates (radicals, anions, or cations) from nonradical or non charged molecules

mechanism of radial halogenation - chain propagation

a step in a chain reaction characterized by the reaction of a radical and a molecule to form a new radical and a new molecule

mechanism of radial halogenation - chain termination

a step in a chain reaction that involved destruction of reactive intermediates

hammond’s postulate - the structure of the transition state → for an exothermic step resembles

the reactants of that step more than products

hammond’s postulate - the structure of the transition state → for an endothermic step resembles

the products of that step more than reactants

Hammonds postulate applies equally well to the transition state for

one-step reactions and do each transition state in a multi-step reaction

radical halogenation stereochemistry - when radical halogenation produces a chiral center or takes place at a hydrogen on a chiral center, the product is

a racemic mixture of R and S enantiomers

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

breaking an allylic C-H bond generates a

resonance-stabilized radical

allylic bromination

• NBS

• very high selectivity

• halogen attaches to the allylic carbon

radical addition of HBr to alkenes → no peroxides =

Markovnikov addition

radical addition of HBr to alkenes with peroxides →

non-markovnikov addition