Chapter 6 Flashcards
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
2
Q
Reative intermediate
A
Corresponds to the energy minimum between two transition states
3
Q
Rate-determining step
A
The step that corresponds to the highest energy barrier
4
Q
The most common reactive intermediates are:
A
carbocations, carbon radicals, and carbanions
5
Q
Peaks on the reaction coordinate diagrams
A
transition states
6
Q
Troughs on reaction coordinate diagrams
A
intermediates
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
8
Q
Lewis acid
A
Has an empty orbital and accepts an electron pair from the lewis base
9
Q
Lewis base
A
Has the donor electron pair
10
Q
Bronsted-Lowry acid-base reactions are often referred to as:
A
proton transfers
11
Q
Nucleophile
A
Electron rich species
12
Q
Electrophile
A
Electron deficient species
13
Q
The region of the molecule that is electron-rich is referred to as:
A
nucleophilic
14
Q
The electron-poor region of a molecule is referred to as:
A
electrophilic
15
Q
Weak bonds are _____
A
electrophilic
16
Q
Weak bonds have low-energy empty antibonding orbitals that can:
A
accept electron density from a nucleophile
17
Q
Electrophilic addition reactions occur when:
A
electrophiles add to a pi bond
18
Q
Bronsted-Lowry acids can be considered
A
electrophiles
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
20
Q
Carbocations are _____ with bond angles of ____
A
planar, 120 degrees
21
Q
Carbocations are stabilized by:
A
the electron-releasing inductive effect of alkyl groups bonded to the cationic carbon and by hyperconjugation
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
23
Q
Carbocations can
A
rearrange
24
Q
The driving force for carbocation rearrangement is:
A
conversion to a more stable secondary or tertiary carbocation
25
Rearrangement is by;
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
26
Regioselective reaction
a reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions
27
According to Markovnikov's rule:
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.
28
Stereoselective reaction
A reaction in which one stereoisomer is formed in preference to all other that might be formed
29
Anti addition
Addition of new atoms or groups of atoms from opposite faces of a double bond
30
In cyclic systems, anti addition is equivalent to:
trans coplanar
31
Syn addition
The addition of atoms or groups o atoms to the same face of a double bond
32
The key step in electrophilic addition mechanisms:
Involves the pi electrons of the alkene reacting as a nucleophile with an electrophilic species
33
HX is used to convert alkenes to ________ in an electrophilic addition reaction
haloalkanes
34
The two step mechanism for the addition of hydrogen halides involves:
Initial protonation of the alkene pi bond to form a carbocation, which reacts with X- to give the product haloalkane
35
The addition of HX does or does not follow Markovnikov's rule
does
36
Are carbocation rearrangements possible in the addition of HX?
yes
37
What hydrogen halides are often involved in addition of HX reactions?
HCl, HBr, and HI
38
The addition of HX may be carried out either with ____ reagents or in the presence of ______
pure, polar solvents
39
HCl reacts _____ compared to HBr and HI
sluggishly
40
What are the steps for the mechanism of the addition of HX?
1) add a proton
2) make a new bond between a nucleophile and an electrophile
41
Describe step one for the addition of HX: add a proton
Addition begins with the transfer of a froton from HX to the alkene.
42
Describe step two for the addition of HX: make a new bond between a NU and an EL
Reaction of the cation with X- completes the valence shell of carbon
43
What is the rate-determining step for the addition of HX?
add a proton
44
Water, in the presence of an acid-catalyst, converts an alkene to an:
alcohol
45
The mechanism for acid-catalyzed hydration involves:
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
The OH group in acid-catalyzed hydrate becomes bonded to:
the more highly substituted carbon of the alkene
47
Are carbocation rearrangements possible during acid-catalyzed hydration?
yes
48
What is the most common acid used in acid-catalyzed hydration?
H2SO4
49
What are the steps for the mechanism of acid-catalyzed hydration?
1) add a proton
2) make a new bond between a NU and a EL
3) Take a proton away
50
Describe step 1 for the mechanism of acid catalyzed hydration: add a proton
Addition of a proton to the alkene from H3O+ gives a carbocation intermediate
51
Describe step 2 for the mechanism of acid catalyzed hydration: make a new bond between a NU and a EL
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
Describe step 3 for the mechanism of acid catalyzed hydration: take a proton away
Loss of a proton from the relatively acidic oxonium ion to water gives the alcohol and generates a new acid catalyst
53
Cl2 or Br2 is used to convert an alkene to:
a vicinal dihalide
54
The mechanism for the addition of Br2 or Cl2 involves:
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
Can rearrangements occur in the addition of Br2 or Cl2?
no
56
The addition of X2 displays ___ addition
anti
57
The reaction for the addition of X2 is stereospecific because:
Z alkenes give different products than E alkenes
58
Halogenation is generally carried out with ___ reagents or with them being mixed with an ________ such as _____.
pure, inert solvent, CH2Cl2
59
What are the steps for the mechanism of the addition of X2?
1) make a new bond between a NU and an EL
2) make anew bond between a NU and an EL
60
Describe step 1 for the mechanism for the addition of X2:
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
Describe step 2 for the mechanism for the addition of X2:
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
Addition of X2 to cyclohexene and its derivatives gives a _______ product because:
trans diaxial
only axial positions on adjacent atoms of a cyclohexane ring are anti and coplanar
63
Te initial trans diaxial conformation is in equilibrium with the ________ conformation
trans equatorial
64
Cl2 or Br2 in the presence of H2O is used to convert alkenes to
halohydrins
65
Halohydrin
A compound containing a halogen atom and a hydroxyl group on adjacent carbons
66
The mechanism for the addition of HOX involves:
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
Are rearrangements observed in the addition of HOX?
no
68
The addition of HOX displays ___ addition and gives ______________ regioselectivity in that the -OH group becomes bonded to the ____ substituted alkene carbon.
anti, Markovnikov, more highly
69
What are the steps of the mechanism for the addition of HOX?
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
Oxymercuration-reduction is used to convert alkenes to:
alcohols
71
Oxymercuration-reduction displays _________ regioselectivity in that the -OH group bonds to the _____ substituted alkene C.
Markovnikov, more highly
72
Are rearrangements observed in oxymercuration-reduction?
no
73
What is the stereoselectivity for oxymercuration-reduction?
A mixture of syn and anti addition
74
What are the reagents in oxymercuration-reduction?
1. Hg(OAc)2, H2O
2. NaBH4
75
Non-Markovnikov hydration of an alkene can be achieved by:
hydroboration-oxidation
76
BH3 followed by H2O2 converts alkenes into:
alcohols with non-Markovnikov regioselectivity
77
Stereochemistry for hydroboration-oxidation:
syn addition
78
Are rearrangements possible during hydroboration-oxidation?
no
79
Why is hydroboration oxidation syn stereoselective?
The H and B add from the sam face
80
What are the reagents for hydrobroation-oxidation?
1. BH3
2. H2O2, NaOH
81
OsO4 followed by NaHSO3 converts alkenes into:
vicinal diols
82
Oxidation of an alkene to a glycol occurs with ____ stereoselectivity and ______ rearrangement
syn, without
83
What are the reagents involved in oxidation of an alkene to a glycol?
1. OsO4
2. NaHSO3, H2O
84
Ozone followed by dimethylsulfide cleaves the C=C bond of alkenes and gives:
two carbonyl groups in its place
85
In the presence of an appropriate transition metal, H2 reduces alkenes to _______ with __ stereoselectivity and ____ rearrangement
alkanes, syn, without
86
Can trans isomers form during catalytic reduction to form a racemic mixture?
yes
87
What are the reagents for catalytic reduction?
H2/Pt or H2/Pd
88
The reduction of an alkene to an alkane is an _______ process.
exothermic
89
Heats of hydrogenation depend on:
the degree of substitution of the C=C double bond. The greater the substitution, the lower the heat of hydrogenation.
90
The heat of hydrogenation of trans-alkene is ___ than that of a cis-alkene.
lower
91
Optically active product can never be formed by:
the reaction of achiral starting material in an achiral environment
92
Optically active products may be formed by:
achiral starting materials in a chiral environment
93
______ products are optically inactive
achiral
