Ch 6- Reactions of Alkenes Flashcards
addition reaction
a reaction in which 2 atoms or ions react with a double bond, forming a compound with the 2 new groups bonded to the carbon of the original double bond
What happens in an addition reaction?
the pi bond is broken, and in its place, sigma bonds form to 2 new atoms or groups of atoms
reactive intermediate
a high-energy species, formed between 2 successive reaction steps, that lies in an energy minimum between the 2 transition states
What are three reactive intermediates common in organic chemistry transformations?
carbocation, radical, carboanion
rate-determining step
the step in a multistep reaction sequence that crosses the highest energy barrier
On a reaction coordinate diagram, peaks represent ___.
transition states
On a reaction coordinate diagram, troughs represent ___.
reaction intermediates
Reaction intermediates have lifetimes ___ than the time it takes for a bond to vibrate.
longer
Transition states have lifetimes that are ___.
on the order of the lifetime of a bond vibration; for practical purposes, they can be considered as having no measurable lifetime
Equilibrium in a chemical reaction usually favors the side with ___ chemical bonds
stronger
bond dissociation enthalpy
the amount of energy required to break a bond into two radicals in the gas phase at 25°C
How do you calculate ΔH°?
Subtract the bond dissociation enthalpies of the products from the bond dissociation enthalpies of the reactants
Reactions in which the new bonds made in the product(s) are stronger than the bonds broken in the reactants will have a ___ (positive/negative) value of ΔH°.
negative
electrophilic addition
a common type of reaction with alkenes in which an electrophilic species adds to a pi bond
What are three characteristic features that can make a molecule or ion electrophilic?
1) a region of low electron density, reflected by a partial or full positive charge; 2) lack of an octet of an atom; 3) a relatively weak bond to an atom that can depart as a stable ion or molecule
regioselective reaction
an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed
Markovnikov’s rule
in the addition of HX, H2O, or ROH to an alkene, hydrogen adds to the carbon of the double bond having the greater number of hydrogens
carbocation
a species in which a carbon atom has only six electrons in its valence shell and bears a positive charge
What are the bond angles on a carbocation?
120°
What orbitals does a carbocation use?
sp²
What is the vacant orbital on a carbocation and where does it lie?
2p; lies perpendicular to the sigma bond framework
Carbocations are highly reactive ___ because ___.
electrophiles; of their positive charge and the lack of an octet at carbon, and they commonly have lifetimes of microseconds or less
List in order of increasing stability the four types of alkyl carbocations.
methyl cation, ethyl cation, isopropyl cation, and tert-butyl cation
A system bearing a charge is more stable if the charge is ___.
delocalized
Explain why a carbocation is stabilized as more alkyl groups are added to it.
Alkyl groups bonded to a positively charged carbon release electron density toward that carbon and thereby help delocalize the positive charge on the cation.
inductive effect
the polarization of the electron density of a covalent bond resulting from the electronegativity of a nearby atom
The larger the volume over which the positive charge on a carbocation is delocalized, the greater the ___.
stability of the cation
hyperconjugation
interaction of electrons in a sigma-bonding orbital with the vacant 2p orbital of an adjacent positively charged carbon
In a 2-step electrophilic addition, what is the slow, rate-determining step?
addition of a proton from HX to the carbon-carbon double bond
hydration
the addition of water
In a hydration, H adds to the carbon of the double bond with ___ and OH adds to the carbon with ___, thus being in accordance with ___’s rule
greater number of hydrogen; the fewer hydrogens; Markovnikov
oxonium ion
an ion in which oxygen bears a positive charge
rearrangement
a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material
1,2 shift
a type of rearrangement in which an atom or a group of atoms moves with its bonding electrons from one atom to an adjacent electron-deficient atom
What is the driving force for a 1,2 shift?
the less stable 2° carbocation is converted to a more stable 3° carbocation
The addition of bromine or chlorine to a cycloalkene gives a ___ formed as a ___.
trans-dihalocycloalkane; racemic mixture
stereoselective reaction
a reaction in which one stereoisomer is formed in preference to all others may be enantioselective or diastereoselective
antistereoselectivity
the addition of atoms or groups of atoms to opposite faces of a carbon-carbon double bond
Addition of chlorine or bromine to cyclohexane and its derivatives gives a ___ because ___.
transdiaxial product; only axial positions on adjacent atoms of a cyclohexane ring are anti and coplanar (more stable)
halohydrin
a compound containing a halogen atom and a hydroxyl group on adjacent carbons
T/F: Addition of HOCl and HOBr is not regioselective.
FAAAALSEEEEEEE
T/F: Addition of HOCl and HOBr is antistereoselective
TWUUUUU
oxymercuration-reduction
a method for converting an alkene to an alcohol; the alkene is then treated with mercury (II) acetate followed by reduction with sodium borohydride
hydroboration
a method for converting an alkene to an alcohol; the alkene is treated with borane to give a trialkylborane, which is then oxidized with alkaline hydrogen peroxide to give the alcohol
Addition of borane to alkenes ___(is/isn’t) regioselective.
is! the boron becomes bonded predominantly to the less-substituted carbon of the double bond (non-Markovnikov)
Addition of borane to alkenes ___(is/isn’t) stereoselective.
is! hydrogen and boron add from the same face of the double bond; the rxn is syn stereoselective
syn stereoselective
the addition of atoms or groups of atoms to the same face of a carbon-carbon double bond
oxidation
loss of electrons/loss of hydrogens/gain of oxygens
reduction
gain of electrons/gain of hydrogens/loss of oxygens
What happens when you treat an alkene with ozone?
The carbon-carbon double bond is cleaved and 2 carbonyl (C=O) groups are formed in its place
Is ozone nucleophilic or electrophilic?
electrophilic
Most alkenes are reduced quantitatively by ___ in the presence of ___ to give an ___.
H_2; transition metal catalyst; alkane
The addition of hydrogen to an alkene is ___(endothermic/exothermic).
exothermic
Catalytic reduction is also called ___.
catalytic hydrogenation
