Chapter 8 Flashcards
Haloalkanes
Contain a halogen covalently bonded to an sp^3 hybridized carbon
Haloalkene
A compound containing a halogen atom bonded to one of the carbons of a C=C bond
Haloarenes
A compound containing a halogen atom bonded to a benzene ring. Given the symbol Ar-X
How are halogens named in the IUPAC system?
Named fluoro-, chloro-, bromo, and iodo and are listed in alphabetical order with other substituents.
Alkenyl or vinylic halides
Common name for haloalkenes
Alkyl halide
Common name for haloalkanes
Haloforms
A compound of the type CHX3
Most common haloalkane solvent
DCM
Perhaloalkane
Hydrocarbons in which all hydrogens are replaced by halogens
The van der Waals radius of fluorine is _____ than that of hydrogen.
slightly greater
Only ______ has a larger van der Waals radius than methyl
iodine
Chloro-, bromo-, and iodoalkanes have _____ boiling points than alkanes.
higher
Why is the boiling point of chloro-, bromo-, and iodoalkanes higher than alkanes?
Because of the greater polarizability of the unshared electrons of the halogen atom
Polarizability
The distortion of the distribution of the electron density around an atom that is interacting with another atom or ion
The electron density on larger, less electronegative atoms is _____ polarizable than that of electrons on more electronegative atoms with smaller atomic radii.
more
Boiling points of fluoralkanes are generally ______ to those of alkanes of similar size and shape, because:
comparable
of the uniquely low polarizability of the valence electrons of fluorine
The density of liquid haloalkanes is ______ than that of hydrocarbons of comparable molecular weight because:
more
the halogen’s larger mass-to-volume ratio
van der Waals forces
A group of intermolecular attractive forces including dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole forces
van der Waals radius
The minimum distance of approach to an atom that does not cause nonbonded interaction strain.
C-X bonds are _____ than C-H bonds
weaker
Radical
Any chemical species that contains one or more unpaired electrons
How are radicals produced?
From a molecule by cleavage of a bond in such a way that each atom or fragment participating in the bond retains one electrons
Homolytic bond cleavage
Cleavage of a bond so that each fragment retains one electron, producing radicals.
Heterolytic bond cleavage
Cleavage of a bond so that one fragment retains both electrons and the other retains none
As the size of the halogen atom increases, the C-X bond length ______ and its strength ______.
increases, decreases
Free-radical halogenation of alkanes
Uses Cl2 or Br2 and light or heat to produce haloalkanes
Free-radical halogenation is _______ and replaces H atoms with the halogen in the order:
regioselective
allylic > tertiary > secondary > methyl
Radical intermediate stability order
Allylic > tertiary > secondary > primary
Radical intermediates are analogous to carbocations in the sense:
that they are stabilized by the same interactions, namely resonance delocalization and hyperconjugation with attached alkyl groups
Regioselectivity of halogenation is greater for _________ than for _________.
bromination, chlorination
The overall energetics of a reaction can be calculated by:
adding all the BDEs of the bonds broken minus the BDEs of the bonds make in the reaction
When using BDEs to calculate overall energetics, ________ reactions have negative values.
exothermic
Chlorination and bromination of alkanes are regioselective in the order:
Tertiary H > secondary H > primary H
Chlorination and bromination of alkanes involves:
a radical chain process
Many of the simpler low-molecular-weight haloalkanes are prepared by:
halogenation of alkanes
Halogenation of alkanes is a _________ reaction.
substitution
In the case of chloromethane, a large excess of _______ drives the reaction to completes halogenation. For tetrachloromethane, a large excess of _______ drives the reaction to completion.
methane, chlorine
In all cases, monosubstituted products are only obtained by using an excess of ______.
alkane
The reaction of bromine with an alkane occurs in the order:
tertiary > secondary > primary hydrogen
Chlorination of alkanes is _______ regioselective than bromination.
less
BDE values for saturated hydrocarbons depend on _________ and are in the order ________.
the type of hydrogen being abstracted
methane > primary > secondary > tertiary
In general, interactions that are stabilizing to carbocations are also stabilizing to _______.
radicals
A radical chain mechanism consists of three types of steps:
chain initiation, chain propagation, and chain termination
In chain initiation:
radicals are formed from nonradical compounds
In a chain propagation step:
a radical and a molecule react to give a new radical and a new molecule
When summed, chain propagation steps give:
the observed stoichiometry of the reaction
Chain length
The number of time a cycle of chain propagation steps repeats
In a chain termination step:
radicals are destroyed, as two radicals collide to make a new bond
Simple alkyl radicals are _______ with bond angles of _____ about the carbon with the unpaired electron.
planar, 120 degrees
Heats of reaction for a radical reaction and for individual chain initiation, propagation, and termination steps can be calculated from:
BDEs
According to Hammond’s Postulate:
the structure of the transition state of an exothermic reaction step occurs early and looks more like the reactants of that step than like the products; thus, changes in the reaction have a large effect on the rate
The structure of the transition state of an endothermic reaction step occurs _____ and looks more than the ______ of that step than the ______, and changed in _____ have a large effect on rate
later, products, reactants, products
Hammond’s postulate accounts for the fact that bromination of an alkane:
is more regioselective that chlorination
For both bromination and chlorination of alkanes, the rate-determining step is:
hydrogen abstraction to form an alkyl radical
Hydrogen abstraction is _____ for bromination and ______ for chlorination.
endothermic, exothermic
Lone pair repulsion is one factor that:
weakens sigma bonds
Energy to cause bond cleavage and generation of radicals can be supplied by:
either heat or light
Dissociation of Cl2 and Br2 can be brought about by heating at temperatures above ______.
350 degrees C
Compounds containing oxygen-oxygen single bonds in peroxides and hydroperoxides are cleaved to radicals at considerable _____ temperatures than those required to rupture carbon-carbon bonds
lower
Chlorine is homolytically dissociated by:
heat or light
How is stoichiometry determined in chain propagation?
Adding the chain propagations steps together and cancelling structures that appear on both sides of the equation gives the balanced equation.
What are the most important chain termination reactions during halogenation of alkanes?
radical couplings and disproportionation
The first three possible chain termination steps involve:
coupling of radicals to form a new covalent bond
What step of chain termination is disproportionation and what happens during it?
Step 4
Involves transfer of a hydrogen atom from the beta position of one radical to another radical and formation of an alkane and alkene.
When radical halogenation produces a chiral center or takes place at a hydrogen of an existing chiral center, the product is:
an equal mixture of R and S isomers
Allylic substitution
Any reaction in which an atom or group of atoms is substituted for another atom or group of atoms at a carbon adjacent to a carbon-carbon double bond
Allylic halogenation proceeds by:
a radical chain mechanism
Bromination using NBS is initiated by:
light
Bromination using NBD involves:
a radical chain process, with a resonance delocalized allyl radical intermediate
Under high temperatures, the concentration of bromine radicals becomes much _____ than at room temp.; this greatly _______ the substitution reaction, which occurs by _____________. At room temp., there are far _____ radicals and __________ is observed.
higher, accelerates, radical halogenation mechanism, fewer, electrophilic addition
Vinylic C-H bonds are never abstracted in:
homolytic reactions
An ______ radical is more stable than a tertiary alkyl radical
The allyl radical involves the formation of _____ molecular orbitals by overlap of _____ 2p atomic orbitals.
three, three
The lowest energy MO has ____ node(s), the next MO has ____ node(s), and the highest energy MO has _____ node(s)
zero, one, two
The lone electron of the allyl radical is associated with the ____________ MO
pi-nonbonding
In the ground state of an allyl radical, _____ electrons lie in the pi-bonding MO and the _____ lies in the pi-nonbonding MO; the anti-bonding MO is _______.
two, third, unoccupied
Autoxidation of unsaturated compounds is an important process in:
aging and degradation of materials
Autoxidation involves:
Reaction of a CH bond, especially an allylic one, with oxygen under radical initiation conditions.
The primary product of autoxidation is:
hydroperoxide
The mechanism for autoxidation involves:
a radical chain process in which resonance-delocalized allylic radical intermediates react with molecular oxygen to give a peroxy radical that continues the radical chain
Autoxidation
Air oxidation of materials such as unsaturated fatty acids
Autoxidation takes places by a radical chain mechanism very simalar to that for:
allylic bromination
Autoxidation begins when a ________, which is formed by either _____________ or by ______________, abstracts a _____ allylic hydrogen to form a radical.
radical initiator,
light activation of an impurity in the oil,
thermal decomposition of peroxide impurities,
doubly
Some autoxidation products degrade to _________ and _______
short-chain aldehydes, carboxylic acids
Radical inhibitors
A compound such as a phenol that selectively reacts with radicals to remove them from a chain reaction and terminate the chain.
Addition of HBr to alkene in the presence of peroxides can lead to:
non-Markovnikov addition because a free-radical chain mchanism operates
Addition of HBr to alkenes in the presence of peroxides is particularly usefil for making:
primary haloalkanes from terminal alkanes
Non-Markovnikov addition of HBr occurs by a radical mechanism in the presence of peroxides, in which:
a Bromine radical reacts with the pi bond of the alkene to createa a radical intermediate that abstracts H radical from H-Br to continue the chain process
The addition of the bromine radical to the alkene could occur at either C of the double bond, but it is dominated by addition that gives:
the more stable carbon-based radical
Non-Markovnikov addition occurs only with ___. ___ and ___ always add to alkenes according to Markovnikov’s rule.
HBr, HCl, HI
