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Chem 260 > Chapter 6 > Flashcards

Chapter 6 Flashcards

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1
Q

The most characteristic reaction of alkenes is an ________ to the C=C bond, breaking the ___ bond and producing __ new sigma bonds to __ new atoms or groups of atoms.

A

addition reaction, pi, two, two

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2
Q

Reative intermediate

A

Corresponds to the energy minimum between two transition states

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3
Q

Rate-determining step

A

The step that corresponds to the highest energy barrier

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4
Q

The most common reactive intermediates are:

A

carbocations, carbon radicals, and carbanions

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5
Q

Peaks on the reaction coordinate diagrams

A

transition states

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6
Q

Troughs on reaction coordinate diagrams

A

intermediates

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7
Q

Calculating the reaction enthalpy can be used to predict the ______ of a reaction, especially when the reaction entropy is relatively ____.

A

thermodynamics, low

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8
Q

Lewis acid

A

Has an empty orbital and accepts an electron pair from the lewis base

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9
Q

Lewis base

A

Has the donor electron pair

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10
Q

Bronsted-Lowry acid-base reactions are often referred to as:

A

proton transfers

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11
Q

Nucleophile

A

Electron rich species

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12
Q

Electrophile

A

Electron deficient species

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13
Q

The region of the molecule that is electron-rich is referred to as:

A

nucleophilic

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14
Q

The electron-poor region of a molecule is referred to as:

A

electrophilic

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15
Q

Weak bonds are _____

A

electrophilic

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16
Q

Weak bonds have low-energy empty antibonding orbitals that can:

A

accept electron density from a nucleophile

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17
Q

Electrophilic addition reactions occur when:

A

electrophiles add to a pi bond

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18
Q

Bronsted-Lowry acids can be considered

A

electrophiles

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19
Q

The rate-determining step in electrophilic addition reactions is:

A

the making of the first new bond between the alkene pi bond and the electrophile

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20
Q

Carbocations are _____ with bond angles of ____

A

planar, 120 degrees

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21
Q

Carbocations are stabilized by:

A

the electron-releasing inductive effect of alkyl groups bonded to the cationic carbon and by hyperconjugation

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22
Q

Hyperconjugation

A

A stabilizing interaction tat involves overlap of pi bonding electron density from adjacent alkyl groups with the empty 2p orbital of the cationic carbon atom

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23
Q

Carbocations can

A

rearrange

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24
Q

The driving force for carbocation rearrangement is:

A

conversion to a more stable secondary or tertiary carbocation

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25
Q

Rearrangement is by;

A

a 1,2 shift in which an atom or a group of atoms with its bonding electrons moves from an adjacent carbon to an electron-deficient electron

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26
Q

Regioselective reaction

A

a reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions

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27
Q

According to Markovnikov’s rule:

A

In the addition of HX or H2O to an unsymmetrical alkene, hydrogen adds to the carbon of the double bond having the greater number of hydrogens and X or OH adds to the more substituted carbon.

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28
Q

Stereoselective reaction

A

A reaction in which one stereoisomer is formed in preference to all other that might be formed

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29
Q

Anti addition

A

Addition of new atoms or groups of atoms from opposite faces of a double bond

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30
Q

In cyclic systems, anti addition is equivalent to:

A

trans coplanar

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31
Q

Syn addition

A

The addition of atoms or groups o atoms to the same face of a double bond

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32
Q

The key step in electrophilic addition mechanisms:

A

Involves the pi electrons of the alkene reacting as a nucleophile with an electrophilic species

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33
Q

HX is used to convert alkenes to ________ in an electrophilic addition reaction

A

haloalkanes

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34
Q

The two step mechanism for the addition of hydrogen halides involves:

A

Initial protonation of the alkene pi bond to form a carbocation, which reacts with X- to give the product haloalkane

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35
Q

The addition of HX does or does not follow Markovnikov’s rule

A

does

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36
Q

Are carbocation rearrangements possible in the addition of HX?

A

yes

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37
Q

What hydrogen halides are often involved in addition of HX reactions?

A

HCl, HBr, and HI

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38
Q

The addition of HX may be carried out either with ____ reagents or in the presence of ______

A

pure, polar solvents

39
Q

HCl reacts _____ compared to HBr and HI

A

sluggishly

40
Q

What are the steps for the mechanism of the addition of HX?

A

1) add a proton
2) make a new bond between a nucleophile and an electrophile

41
Q

Describe step one for the addition of HX: add a proton

A

Addition begins with the transfer of a froton from HX to the alkene.

42
Q

Describe step two for the addition of HX: make a new bond between a NU and an EL

A

Reaction of the cation with X- completes the valence shell of carbon

43
Q

What is the rate-determining step for the addition of HX?

A

add a proton

44
Q

Water, in the presence of an acid-catalyst, converts an alkene to an:

A

alcohol

45
Q

The mechanism for acid-catalyzed hydration involves:

A

Initial protonation of the alkene pi bond to give a carbocation, which reacts with the nucleophile water to create a second intermediate, which loses a proton to give the alcohol product

46
Q

The OH group in acid-catalyzed hydrate becomes bonded to:

A

the more highly substituted carbon of the alkene

47
Q

Are carbocation rearrangements possible during acid-catalyzed hydration?

A

yes

48
Q

What is the most common acid used in acid-catalyzed hydration?

A

H2SO4

49
Q

What are the steps for the mechanism of acid-catalyzed hydration?

A

1) add a proton
2) make a new bond between a NU and a EL
3) Take a proton away

50
Q

Describe step 1 for the mechanism of acid catalyzed hydration: add a proton

A

Addition of a proton to the alkene from H3O+ gives a carbocation intermediate

51
Q

Describe step 2 for the mechanism of acid catalyzed hydration: make a new bond between a NU and a EL

A

The carbocation completes its valence shell by forming a new covalent bond with an unshared pair of electrons of the oxygen atom of water , giving an oxonium ion

52
Q

Describe step 3 for the mechanism of acid catalyzed hydration: take a proton away

A

Loss of a proton from the relatively acidic oxonium ion to water gives the alcohol and generates a new acid catalyst

53
Q

Cl2 or Br2 is used to convert an alkene to:

A

a vicinal dihalide

54
Q

The mechanism for the addition of Br2 or Cl2 involves:

A

attack by the alkene pi bond on one atom of X2 to give a bridged halonium ion intermediate that is, in turn, attacked by X- from the backside to give a vicinal dihalide

55
Q

Can rearrangements occur in the addition of Br2 or Cl2?

A

no

56
Q

The addition of X2 displays ___ addition

A

anti

57
Q

The reaction for the addition of X2 is stereospecific because:

A

Z alkenes give different products than E alkenes

58
Q

Halogenation is generally carried out with ___ reagents or with them being mixed with an ________ such as _____.

A

pure, inert solvent, CH2Cl2

59
Q

What are the steps for the mechanism of the addition of X2?

A

1) make a new bond between a NU and an EL
2) make anew bond between a NU and an EL

60
Q

Describe step 1 for the mechanism for the addition of X2:

A

Reaction is initiated by interaction of the pi bond electrons of the alkene with X that acts as an EL to form an intermediate in which X bears the pos. charge

61
Q

Describe step 2 for the mechanism for the addition of X2:

A

Attack of X ion (a NU) on C from the side opp. the X+ ion opens the three-membered rind to give the anti product

62
Q

Addition of X2 to cyclohexene and its derivatives gives a _______ product because:

A

trans diaxial
only axial positions on adjacent atoms of a cyclohexane ring are anti and coplanar

63
Q

Te initial trans diaxial conformation is in equilibrium with the ________ conformation

A

trans equatorial

64
Q

Cl2 or Br2 in the presence of H2O is used to convert alkenes to

A

halohydrins

65
Q

Halohydrin

A

A compound containing a halogen atom and a hydroxyl group on adjacent carbons

66
Q

The mechanism for the addition of HOX involves:

A

attack by the alkene pi bond on one atom of X2 to give a bridged halonium ion intermediate that is, in turn, attacked by H2O from the backside to give a new intermediate, which loses proton to give a halohydrin

67
Q

Are rearrangements observed in the addition of HOX?

A

no

68
Q

The addition of HOX displays ___ addition and gives ______________ regioselectivity in that the -OH group becomes bonded to the ____ substituted alkene carbon.

A

anti, Markovnikov, more highly

69
Q

What are the steps of the mechanism for the addition of HOX?

A

1) make a new bond between a NU and an EL
2) make a new bond between a NU and an EL
3) take a proton away

70
Q

Oxymercuration-reduction is used to convert alkenes to:

A

alcohols

71
Q

Oxymercuration-reduction displays _________ regioselectivity in that the -OH group bonds to the _____ substituted alkene C.

A

Markovnikov, more highly

72
Q

Are rearrangements observed in oxymercuration-reduction?

A

no

73
Q

What is the stereoselectivity for oxymercuration-reduction?

A

A mixture of syn and anti addition

74
Q

What are the reagents in oxymercuration-reduction?

A
  1. Hg(OAc)2, H2O
  2. NaBH4
75
Q

Non-Markovnikov hydration of an alkene can be achieved by:

A

hydroboration-oxidation

76
Q

BH3 followed by H2O2 converts alkenes into:

A

alcohols with non-Markovnikov regioselectivity

77
Q

Stereochemistry for hydroboration-oxidation:

A

syn addition

78
Q

Are rearrangements possible during hydroboration-oxidation?

A

no

79
Q

Why is hydroboration oxidation syn stereoselective?

A

The H and B add from the sam face

80
Q

What are the reagents for hydrobroation-oxidation?

A
  1. BH3
  2. H2O2, NaOH
81
Q

OsO4 followed by NaHSO3 converts alkenes into:

A

vicinal diols

82
Q

Oxidation of an alkene to a glycol occurs with ____ stereoselectivity and ______ rearrangement

A

syn, without

83
Q

What are the reagents involved in oxidation of an alkene to a glycol?

A
  1. OsO4
  2. NaHSO3, H2O
84
Q

Ozone followed by dimethylsulfide cleaves the C=C bond of alkenes and gives:

A

two carbonyl groups in its place

85
Q

In the presence of an appropriate transition metal, H2 reduces alkenes to _______ with __ stereoselectivity and ____ rearrangement

A

alkanes, syn, without

86
Q

Can trans isomers form during catalytic reduction to form a racemic mixture?

A

yes

87
Q

What are the reagents for catalytic reduction?

A

H2/Pt or H2/Pd

88
Q

The reduction of an alkene to an alkane is an _______ process.

A

exothermic

89
Q

Heats of hydrogenation depend on:

A

the degree of substitution of the C=C double bond. The greater the substitution, the lower the heat of hydrogenation.

90
Q

The heat of hydrogenation of trans-alkene is ___ than that of a cis-alkene.

A

lower

91
Q

Optically active product can never be formed by:

A

the reaction of achiral starting material in an achiral environment

92
Q

Optically active products may be formed by:

A

achiral starting materials in a chiral environment

93
Q

______ products are optically inactive

A

achiral

Chem 260 (10 decks)

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