Chapter 6 Flashcards
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.
addition reaction, pi, two, two
Reative intermediate
Corresponds to the energy minimum between two transition states
Rate-determining step
The step that corresponds to the highest energy barrier
The most common reactive intermediates are:
carbocations, carbon radicals, and carbanions
Peaks on the reaction coordinate diagrams
transition states
Troughs on reaction coordinate diagrams
intermediates
Calculating the reaction enthalpy can be used to predict the ______ of a reaction, especially when the reaction entropy is relatively ____.
thermodynamics, low
Lewis acid
Has an empty orbital and accepts an electron pair from the lewis base
Lewis base
Has the donor electron pair
Bronsted-Lowry acid-base reactions are often referred to as:
proton transfers
Nucleophile
Electron rich species
Electrophile
Electron deficient species
The region of the molecule that is electron-rich is referred to as:
nucleophilic
The electron-poor region of a molecule is referred to as:
electrophilic
Weak bonds are _____
electrophilic
Weak bonds have low-energy empty antibonding orbitals that can:
accept electron density from a nucleophile
Electrophilic addition reactions occur when:
electrophiles add to a pi bond
Bronsted-Lowry acids can be considered
electrophiles
The rate-determining step in electrophilic addition reactions is:
the making of the first new bond between the alkene pi bond and the electrophile
Carbocations are _____ with bond angles of ____
planar, 120 degrees
Carbocations are stabilized by:
the electron-releasing inductive effect of alkyl groups bonded to the cationic carbon and by hyperconjugation
Hyperconjugation
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
Carbocations can
rearrange
The driving force for carbocation rearrangement is:
conversion to a more stable secondary or tertiary carbocation
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
Regioselective reaction
a reaction in which one direction of bond forming or bond breaking occurs in preference to all other directions
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.
Stereoselective reaction
A reaction in which one stereoisomer is formed in preference to all other that might be formed
Anti addition
Addition of new atoms or groups of atoms from opposite faces of a double bond
In cyclic systems, anti addition is equivalent to:
trans coplanar
Syn addition
The addition of atoms or groups o atoms to the same face of a double bond
The key step in electrophilic addition mechanisms:
Involves the pi electrons of the alkene reacting as a nucleophile with an electrophilic species
HX is used to convert alkenes to ________ in an electrophilic addition reaction
haloalkanes
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
The addition of HX does or does not follow Markovnikov’s rule
does
Are carbocation rearrangements possible in the addition of HX?
yes
What hydrogen halides are often involved in addition of HX reactions?
HCl, HBr, and HI
The addition of HX may be carried out either with ____ reagents or in the presence of ______
pure, polar solvents
HCl reacts _____ compared to HBr and HI
sluggishly
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
Describe step one for the addition of HX: add a proton
Addition begins with the transfer of a froton from HX to the alkene.
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
What is the rate-determining step for the addition of HX?
add a proton
Water, in the presence of an acid-catalyst, converts an alkene to an:
alcohol
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
The OH group in acid-catalyzed hydrate becomes bonded to:
the more highly substituted carbon of the alkene
Are carbocation rearrangements possible during acid-catalyzed hydration?
yes
What is the most common acid used in acid-catalyzed hydration?
H2SO4
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
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
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
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
Cl2 or Br2 is used to convert an alkene to:
a vicinal dihalide
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
Can rearrangements occur in the addition of Br2 or Cl2?
no
The addition of X2 displays ___ addition
anti
The reaction for the addition of X2 is stereospecific because:
Z alkenes give different products than E alkenes
Halogenation is generally carried out with ___ reagents or with them being mixed with an ________ such as _____.
pure, inert solvent, CH2Cl2
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
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
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
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
Te initial trans diaxial conformation is in equilibrium with the ________ conformation
trans equatorial
Cl2 or Br2 in the presence of H2O is used to convert alkenes to
halohydrins
Halohydrin
A compound containing a halogen atom and a hydroxyl group on adjacent carbons
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
Are rearrangements observed in the addition of HOX?
no
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
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
Oxymercuration-reduction is used to convert alkenes to:
alcohols
Oxymercuration-reduction displays _________ regioselectivity in that the -OH group bonds to the _____ substituted alkene C.
Markovnikov, more highly
Are rearrangements observed in oxymercuration-reduction?
no
What is the stereoselectivity for oxymercuration-reduction?
A mixture of syn and anti addition
What are the reagents in oxymercuration-reduction?
- Hg(OAc)2, H2O
- NaBH4
Non-Markovnikov hydration of an alkene can be achieved by:
hydroboration-oxidation
BH3 followed by H2O2 converts alkenes into:
alcohols with non-Markovnikov regioselectivity
Stereochemistry for hydroboration-oxidation:
syn addition
Are rearrangements possible during hydroboration-oxidation?
no
Why is hydroboration oxidation syn stereoselective?
The H and B add from the sam face
What are the reagents for hydrobroation-oxidation?
- BH3
- H2O2, NaOH
OsO4 followed by NaHSO3 converts alkenes into:
vicinal diols
Oxidation of an alkene to a glycol occurs with ____ stereoselectivity and ______ rearrangement
syn, without
What are the reagents involved in oxidation of an alkene to a glycol?
- OsO4
- NaHSO3, H2O
Ozone followed by dimethylsulfide cleaves the C=C bond of alkenes and gives:
two carbonyl groups in its place
In the presence of an appropriate transition metal, H2 reduces alkenes to _______ with __ stereoselectivity and ____ rearrangement
alkanes, syn, without
Can trans isomers form during catalytic reduction to form a racemic mixture?
yes
What are the reagents for catalytic reduction?
H2/Pt or H2/Pd
The reduction of an alkene to an alkane is an _______ process.
exothermic
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.
The heat of hydrogenation of trans-alkene is ___ than that of a cis-alkene.
lower
Optically active product can never be formed by:
the reaction of achiral starting material in an achiral environment
Optically active products may be formed by:
achiral starting materials in a chiral environment
______ products are optically inactive
achiral
