Chapter 8 Flashcards
compounds of the type CHX3 are called
haloforms
C-X bonds with halogens are … with a partial… on halogen and a … on C
polarized; negative charge; partial positive charge
constitutional isomers with branched chains have … boiling points than their unbranched-chain isomers
lower
for an alkane and haloalkane of comparable size and shape, the … has a higher bp. this is due to the … and greater …
haloalkane; dipole moment; polarizability
the bps of fluoroalkanes are comparable to those of hydrocarbons of similar molecular weight. this is attributable to the … of F, its high …, and particularly low …
small size; electronegativity; polarizability
all liquid bromo and iodoalkanes are … than that of water
greater
with the exception of C-F bonds, C-X bonds are … than C-H bonds as measured by bond dissociation enthalpies, which are a measure of bond strengths
weaker
radicals are chemical species that contain one or more …
unpaired electrons
radicals are produced from a molecule by … of a bond in such a way that each atom/fragment participating in the bond retains …, a process called …
cleavage; one electron; hemolytic bond cleavage
as the size of the halogen atom increases, the C-X bond length … and its strength …
increases; decreases
halogenation of alkanes occurs via a … reaction. Monosubstituted products are only obtained using an …
substitution; excess of alkane
for hydrobromination of alkanes, the rxn is highly …
regioselective
Both Br and Cl are regioselective in H replacement in the order …, regioselectivity is far greater for Br than for Cl
3 > 2 > 2
in general, interactions that are stabilizing to … are also stabilizing to radicals
carbocations
halogenation of alkanes occur by a type of mechanism called a
radical chian mechanism
a radical chain mechanism involves three types of steps:
- …
- …
- ..
chain initiation; chain propagation; chain termination
chain initiation involves formation of … from …
radicals; nonradicals
chain propagation involves reaction of a … and a … to form a …
radical; molecule; new radical
chain termination involves … of radicals. there are two types of terminations: … and …
destruction; coupling; disproportionation
disproportionation involves transfer of a H from the … position of one radical to another radical and formation of an … and an …
beta; alkane; alkene
the number of times a cycle of chain propagation steps repeats is called …
chain length
Hammond’s postulate: the structure of the transition state for an exothermic rxn step is reached relatively … in the rxn, so it resembles the … of that step more than the … Conversely, the structure of the transition state for an endothermic rxn step is reached relatively …, so it resembles the … of that step more than the ..
early; reactants; products; late; products; reactants
changes in starting material energy affect the transition state of an … rxn more than changes in product energy. The converse is true for an .. rxn
exothermic; endothermic
the rate-determining step for the radical halogenation of alkanes is the … by a halogen radical
abstraction of a H
abstraction of H by Cl2 is …, which means that the TS is reached early in the course of the rxn. Therefore, it resembles the …, so there is little … character on C in this TS, and regioselectively in radical Cl is only slightly influenced by relative stabilities of radical intermediates
exothermic; reactants; radical
reaction between an alkene and NBC involves a …: a bromine in NBS and a H in the alkene exchange places
net double substitution
allylic bromination and chlorination proceed by a … as well
radical chain mechanism
an allyl radical is even more stable than a
tertiary alkyl radical
the conjugated system of the allyl radical involves the formation of … MOS by …
3; overlap of 3 2p orbitals
in the ground state of the allyl radical, 2 e- of the pi system lie in the pi bonding MO and the third lies in the pi nonbonding MO; the antibonding MO is unoccupied. because the lowest pi MO is at a lower energy than the isolated 2p atomic atoms, putting two eelectrons in this MO releases .., which accounts for the stability of the allyl radical
considerable energy
the rxn will occur to generate the alkene product that is more stable: in other words, with the more
highly substituted double bond
(radical initiated non-markovnikov addition of HBR to alkenes)
chain initiation: hemolytic cleavage of a dialkyl peroxide is induced by … or … to give two … radicals, one of which reacts with HBR by … to give an … and a ..
light; heat; alkoxy radicals; hydrogen abstraction; alcohol; bromine radical
(radical initiated non-markovnikov addition of HBR to alkenes) chain propagation: a bromine radical adds to the C-C double bond regioselectively to give the … C radical, which reacts with a a molecule of HBr to give the … and to generate a new …
more substituted; bromoalkene; bromine radical
(radical initiated non-markovnikov addition of HBR to alkenes) chain termination: combination of a … and a … and the combination of two …
carbon radical; bromine radical; bromine radicals
(radical initiated non-markovnikov addition of HBR to alkenes) the observed non-Mark regioselectivity of radical addition of HBr to an alkene is a combination of a … factor and an … factor
steric; electronic
the regiochemistry of polar and radical mechanisms are dominated by the rxns that proceed through the most stable
reactive intermediates
non-Mark addition occurs only with
HBr
