Ch 6- Reactions of Alkenes

Question 1

addition reaction

Answer 1

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

Question 2

What happens in an addition reaction?

Answer 2

the pi bond is broken, and in its place, sigma bonds form to 2 new atoms or groups of atoms

Question 3

reactive intermediate

Answer 3

a high-energy species, formed between 2 successive reaction steps, that lies in an energy minimum between the 2 transition states

Question 4

What are three reactive intermediates common in organic chemistry transformations?

Answer 4

carbocation, radical, carboanion

Question 5

rate-determining step

Answer 5

the step in a multistep reaction sequence that crosses the highest energy barrier

Question 6

On a reaction coordinate diagram, peaks represent ___.

Answer 6

transition states

Question 7

On a reaction coordinate diagram, troughs represent ___.

Answer 7

reaction intermediates

Question 8

Reaction intermediates have lifetimes ___ than the time it takes for a bond to vibrate.

Answer 8

longer

Question 9

Transition states have lifetimes that are ___.

Answer 9

on the order of the lifetime of a bond vibration; for practical purposes, they can be considered as having no measurable lifetime

Question 10

Equilibrium in a chemical reaction usually favors the side with ___ chemical bonds

Answer 10

stronger

Question 11

bond dissociation enthalpy

Answer 11

the amount of energy required to break a bond into two radicals in the gas phase at 25°C

Question 12

How do you calculate ΔH°?

Answer 12

Subtract the bond dissociation enthalpies of the products from the bond dissociation enthalpies of the reactants

Question 13

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°.

Answer 13

negative

Question 14

electrophilic addition

Answer 14

a common type of reaction with alkenes in which an electrophilic species adds to a pi bond

Question 15

What are three characteristic features that can make a molecule or ion electrophilic?

Answer 15

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

Question 16

regioselective reaction

Answer 16

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

Question 17

Markovnikov’s rule

Answer 17

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

Question 18

carbocation

Answer 18

a species in which a carbon atom has only six electrons in its valence shell and bears a positive charge

Question 19

What are the bond angles on a carbocation?

Answer 19

120°

Question 20

What orbitals does a carbocation use?

Answer 20

sp²

Question 21

What is the vacant orbital on a carbocation and where does it lie?

Answer 21

2p; lies perpendicular to the sigma bond framework

Question 22

Carbocations are highly reactive ___ because ___.

Answer 22

electrophiles; of their positive charge and the lack of an octet at carbon, and they commonly have lifetimes of microseconds or less

Question 23

List in order of increasing stability the four types of alkyl carbocations.

Answer 23

methyl cation, ethyl cation, isopropyl cation, and tert-butyl cation

Question 24

A system bearing a charge is more stable if the charge is ___.

Answer 24

delocalized

Question 25

Explain why a carbocation is stabilized as more alkyl groups are added to it.

Answer 25

Alkyl groups bonded to a positively charged carbon release electron density toward that carbon and thereby help delocalize the positive charge on the cation.

Question 26

inductive effect

Answer 26

the polarization of the electron density of a covalent bond resulting from the electronegativity of a nearby atom

Question 27

The larger the volume over which the positive charge on a carbocation is delocalized, the greater the ___.

Answer 27

stability of the cation

Question 28

hyperconjugation

Answer 28

interaction of electrons in a sigma-bonding orbital with the vacant 2p orbital of an adjacent positively charged carbon

Question 29

In a 2-step electrophilic addition, what is the slow, rate-determining step?

Answer 29

addition of a proton from HX to the carbon-carbon double bond

Question 30

hydration

Answer 30

the addition of water

Question 31

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

Answer 31

greater number of hydrogen; the fewer hydrogens; Markovnikov

Question 32

oxonium ion

Answer 32

an ion in which oxygen bears a positive charge

Question 33

rearrangement

Answer 33

a change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material

Question 34

1,2 shift

Answer 34

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

Question 35

What is the driving force for a 1,2 shift?

Answer 35

the less stable 2° carbocation is converted to a more stable 3° carbocation

Question 36

The addition of bromine or chlorine to a cycloalkene gives a ___ formed as a ___.

Answer 36

trans-dihalocycloalkane; racemic mixture

Question 37

stereoselective reaction

Answer 37

a reaction in which one stereoisomer is formed in preference to all others may be enantioselective or diastereoselective

Question 38

antistereoselectivity

Answer 38

the addition of atoms or groups of atoms to opposite faces of a carbon-carbon double bond

Question 39

Addition of chlorine or bromine to cyclohexane and its derivatives gives a ___ because ___.

Answer 39

transdiaxial product; only axial positions on adjacent atoms of a cyclohexane ring are anti and coplanar (more stable)

Question 40

halohydrin

Answer 40

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

Question 41

T/F: Addition of HOCl and HOBr is not regioselective.

Answer 41

FAAAALSEEEEEEE

Question 42

T/F: Addition of HOCl and HOBr is antistereoselective

Answer 42

TWUUUUU

Question 43

oxymercuration-reduction

Answer 43

a method for converting an alkene to an alcohol; the alkene is then treated with mercury (II) acetate followed by reduction with sodium borohydride

Question 44

hydroboration

Answer 44

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

Question 45

Addition of borane to alkenes ___(is/isn't) regioselective.

Answer 45

is! the boron becomes bonded predominantly to the less-substituted carbon of the double bond (non-Markovnikov)

Question 46

Addition of borane to alkenes ___(is/isn't) stereoselective.

Answer 46

is! hydrogen and boron add from the same face of the double bond; the rxn is syn stereoselective

Question 47

syn stereoselective

Answer 47

the addition of atoms or groups of atoms to the same face of a carbon-carbon double bond

Question 48

oxidation

Answer 48

loss of electrons/loss of hydrogens/gain of oxygens

Question 49

reduction

Answer 49

gain of electrons/gain of hydrogens/loss of oxygens

Question 50

What happens when you treat an alkene with ozone?

Answer 50

The carbon-carbon double bond is cleaved and 2 carbonyl (C=O) groups are formed in its place

Question 51

Is ozone nucleophilic or electrophilic?

Answer 51

electrophilic

Question 52

Most alkenes are reduced quantitatively by ___ in the presence of ___ to give an ___.

Answer 52

H_2; transition metal catalyst; alkane

Question 53

The addition of hydrogen to an alkene is ___(endothermic/exothermic).

Answer 53

exothermic

Question 54

Catalytic reduction is also called ___.

Answer 54

catalytic hydrogenation