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Chem 260 > Chapter 8 > Flashcards

Chapter 8 Flashcards

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1
Q

Haloalkanes

A

Contain a halogen covalently bonded to an sp^3 hybridized carbon

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2
Q

Haloalkene

A

A compound containing a halogen atom bonded to one of the carbons of a C=C bond

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3
Q

Haloarenes

A

A compound containing a halogen atom bonded to a benzene ring. Given the symbol Ar-X

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4
Q

How are halogens named in the IUPAC system?

A

Named fluoro-, chloro-, bromo, and iodo and are listed in alphabetical order with other substituents.

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5
Q

Alkenyl or vinylic halides

A

Common name for haloalkenes

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6
Q

Alkyl halide

A

Common name for haloalkanes

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7
Q

Haloforms

A

A compound of the type CHX3

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8
Q

Most common haloalkane solvent

A

DCM

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9
Q

Perhaloalkane

A

Hydrocarbons in which all hydrogens are replaced by halogens

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10
Q

The van der Waals radius of fluorine is _____ than that of hydrogen.

A

slightly greater

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11
Q

Only ______ has a larger van der Waals radius than methyl

A

iodine

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12
Q

Chloro-, bromo-, and iodoalkanes have _____ boiling points than alkanes.

A

higher

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13
Q

Why is the boiling point of chloro-, bromo-, and iodoalkanes higher than alkanes?

A

Because of the greater polarizability of the unshared electrons of the halogen atom

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14
Q

Polarizability

A

The distortion of the distribution of the electron density around an atom that is interacting with another atom or ion

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15
Q

The electron density on larger, less electronegative atoms is _____ polarizable than that of electrons on more electronegative atoms with smaller atomic radii.

A

more

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16
Q

Boiling points of fluoralkanes are generally ______ to those of alkanes of similar size and shape, because:

A

comparable
of the uniquely low polarizability of the valence electrons of fluorine

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17
Q

The density of liquid haloalkanes is ______ than that of hydrocarbons of comparable molecular weight because:

A

more
the halogen’s larger mass-to-volume ratio

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18
Q

van der Waals forces

A

A group of intermolecular attractive forces including dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole forces

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19
Q

van der Waals radius

A

The minimum distance of approach to an atom that does not cause nonbonded interaction strain.

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20
Q

C-X bonds are _____ than C-H bonds

A

weaker

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21
Q

Radical

A

Any chemical species that contains one or more unpaired electrons

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22
Q

How are radicals produced?

A

From a molecule by cleavage of a bond in such a way that each atom or fragment participating in the bond retains one electrons

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23
Q

Homolytic bond cleavage

A

Cleavage of a bond so that each fragment retains one electron, producing radicals.

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24
Q

Heterolytic bond cleavage

A

Cleavage of a bond so that one fragment retains both electrons and the other retains none

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25
Q

As the size of the halogen atom increases, the C-X bond length ______ and its strength ______.

A

increases, decreases

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26
Q

Free-radical halogenation of alkanes

A

Uses Cl2 or Br2 and light or heat to produce haloalkanes

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27
Q

Free-radical halogenation is _______ and replaces H atoms with the halogen in the order:

A

regioselective
allylic > tertiary > secondary > methyl

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28
Q

Radical intermediate stability order

A

Allylic > tertiary > secondary > primary

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29
Q

Radical intermediates are analogous to carbocations in the sense:

A

that they are stabilized by the same interactions, namely resonance delocalization and hyperconjugation with attached alkyl groups

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30
Q

Regioselectivity of halogenation is greater for _________ than for _________.

A

bromination, chlorination

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31
Q

The overall energetics of a reaction can be calculated by:

A

adding all the BDEs of the bonds broken minus the BDEs of the bonds make in the reaction

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32
Q

When using BDEs to calculate overall energetics, ________ reactions have negative values.

A

exothermic

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33
Q

Chlorination and bromination of alkanes are regioselective in the order:

A

Tertiary H > secondary H > primary H

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34
Q

Chlorination and bromination of alkanes involves:

A

a radical chain process

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35
Q

Many of the simpler low-molecular-weight haloalkanes are prepared by:

A

halogenation of alkanes

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36
Q

Halogenation of alkanes is a _________ reaction.

A

substitution

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37
Q

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.

A

methane, chlorine

38
Q

In all cases, monosubstituted products are only obtained by using an excess of ______.

A

alkane

39
Q

The reaction of bromine with an alkane occurs in the order:

A

tertiary > secondary > primary hydrogen

40
Q

Chlorination of alkanes is _______ regioselective than bromination.

A

less

41
Q

BDE values for saturated hydrocarbons depend on _________ and are in the order ________.

A

the type of hydrogen being abstracted
methane > primary > secondary > tertiary

42
Q

In general, interactions that are stabilizing to carbocations are also stabilizing to _______.

A

radicals

43
Q

A radical chain mechanism consists of three types of steps:

A

chain initiation, chain propagation, and chain termination

44
Q

In chain initiation:

A

radicals are formed from nonradical compounds

45
Q

In a chain propagation step:

A

a radical and a molecule react to give a new radical and a new molecule

46
Q

When summed, chain propagation steps give:

A

the observed stoichiometry of the reaction

47
Q

Chain length

A

The number of time a cycle of chain propagation steps repeats

48
Q

In a chain termination step:

A

radicals are destroyed, as two radicals collide to make a new bond

49
Q

Simple alkyl radicals are _______ with bond angles of _____ about the carbon with the unpaired electron.

A

planar, 120 degrees

50
Q

Heats of reaction for a radical reaction and for individual chain initiation, propagation, and termination steps can be calculated from:

A

BDEs

51
Q

According to Hammond’s Postulate:

A

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

52
Q

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

A

later, products, reactants, products

53
Q

Hammond’s postulate accounts for the fact that bromination of an alkane:

A

is more regioselective that chlorination

54
Q

For both bromination and chlorination of alkanes, the rate-determining step is:

A

hydrogen abstraction to form an alkyl radical

55
Q

Hydrogen abstraction is _____ for bromination and ______ for chlorination.

A

endothermic, exothermic

56
Q

Lone pair repulsion is one factor that:

A

weakens sigma bonds

57
Q

Energy to cause bond cleavage and generation of radicals can be supplied by:

A

either heat or light

58
Q

Dissociation of Cl2 and Br2 can be brought about by heating at temperatures above ______.

A

350 degrees C

59
Q

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

A

lower

60
Q

Chlorine is homolytically dissociated by:

A

heat or light

61
Q

How is stoichiometry determined in chain propagation?

A

Adding the chain propagations steps together and cancelling structures that appear on both sides of the equation gives the balanced equation.

62
Q

What are the most important chain termination reactions during halogenation of alkanes?

A

radical couplings and disproportionation

63
Q

The first three possible chain termination steps involve:

A

coupling of radicals to form a new covalent bond

64
Q

What step of chain termination is disproportionation and what happens during it?

A

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.

65
Q

When radical halogenation produces a chiral center or takes place at a hydrogen of an existing chiral center, the product is:

A

an equal mixture of R and S isomers

66
Q

Allylic substitution

A

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

67
Q

Allylic halogenation proceeds by:

A

a radical chain mechanism

68
Q

Bromination using NBS is initiated by:

A

light

69
Q

Bromination using NBD involves:

A

a radical chain process, with a resonance delocalized allyl radical intermediate

70
Q

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.

A

higher, accelerates, radical halogenation mechanism, fewer, electrophilic addition

71
Q

Vinylic C-H bonds are never abstracted in:

A

homolytic reactions

72
Q

An ______ radical is more stable than a tertiary alkyl radical

A
73
Q

The allyl radical involves the formation of _____ molecular orbitals by overlap of _____ 2p atomic orbitals.

A

three, three

74
Q

The lowest energy MO has ____ node(s), the next MO has ____ node(s), and the highest energy MO has _____ node(s)

A

zero, one, two

75
Q

The lone electron of the allyl radical is associated with the ____________ MO

A

pi-nonbonding

76
Q

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 _______.

A

two, third, unoccupied

77
Q

Autoxidation of unsaturated compounds is an important process in:

A

aging and degradation of materials

78
Q

Autoxidation involves:

A

Reaction of a CH bond, especially an allylic one, with oxygen under radical initiation conditions.

79
Q

The primary product of autoxidation is:

A

hydroperoxide

80
Q

The mechanism for autoxidation involves:

A

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

81
Q

Autoxidation

A

Air oxidation of materials such as unsaturated fatty acids

82
Q

Autoxidation takes places by a radical chain mechanism very simalar to that for:

A

allylic bromination

83
Q

Autoxidation begins when a ________, which is formed by either _____________ or by ______________, abstracts a _____ allylic hydrogen to form a radical.

A

radical initiator,
light activation of an impurity in the oil,
thermal decomposition of peroxide impurities,
doubly

84
Q

Some autoxidation products degrade to _________ and _______

A

short-chain aldehydes, carboxylic acids

85
Q

Radical inhibitors

A

A compound such as a phenol that selectively reacts with radicals to remove them from a chain reaction and terminate the chain.

86
Q

Addition of HBr to alkene in the presence of peroxides can lead to:

A

non-Markovnikov addition because a free-radical chain mchanism operates

87
Q

Addition of HBr to alkenes in the presence of peroxides is particularly usefil for making:

A

primary haloalkanes from terminal alkanes

88
Q

Non-Markovnikov addition of HBr occurs by a radical mechanism in the presence of peroxides, in which:

A

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

89
Q

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:

A

the more stable carbon-based radical

90
Q

Non-Markovnikov addition occurs only with ___. ___ and ___ always add to alkenes according to Markovnikov’s rule.

A

HBr, HCl, HI

Chem 260 (10 decks)

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