Organic II

Question 1

kinetics is about what?

Answer 1

reaction rates

Question 2

define elementary reaction

Answer 2

one where bond breaking and making occur during a single collision

eg. proton transfer

Question 3

requirements of a successful collision

Answer 3

collide in the correct orientation

have energy exceeding the activation energy

Question 4

define transition state

Answer 4

the activated complex formed on collision

very short life time, unstable, maximum energy, cannot be isolated

Question 5

what does the graph of a reaction with several steps look like?

Answer 5

energy vs reaction pathway

like hills, the dips are the places where intermediates are formed
the peaks are the transition states

sum of steps = overall reaction

Question 6

define mechanism

Answer 6

the set of steps of the reaction

Question 7

define ‘rate determining step’

Answer 7

the slowest step, with the HIGHEST activation energy

Question 8

define intermediate

Answer 8

a reactive species produced in one step and consumed in a later step.
reactant –> intermediate
intermediate –> product

may be an unstable molecule, may be stable.
represented by the dip in the graph

Question 9

what is a ‘radical’?

Answer 9

uncharged species having an unpaired electron

NOTE that radicals have NO charge!!
eg *CH2CH3
C has 3 bonding e- and one unpaired, total 4 e-, same as non bonded C hence no charge

Question 10

In radical mechanisms, bond breaking is ______

Answer 10

homolytic (-lysis as in DESTRUCTION)

one atom at each of the products has one e- of the bond cleaved

A-B –> A* + B*
or Cl-C –> 2Cl with 8 e-

form radicals in the products

Question 11

what type of arrow is used to show e- movement?

Answer 11

a fish hook arrow - single pronged
from a pi bond or nonbonding pair to an atom gives a sigma bond
from a sigma bond results in cleavage

tail more +ve, head more -ve

Question 12

radical mechanism bond formation is ____

Answer 12

homogenic (-genic as in GENERATING something)

each reactant contributes 1 e-

A* + B* –> A-B

radicals in the reactants

Question 13

steps in a radical mechanism

Answer 13

initiation
propagation
termination

Question 14

define initiation step in radical mechanism

Answer 14

step 1
bond breaking
to FORM radicals in the products

Question 15

define propagation step in radical mechanism

Answer 15

step 2 (and 3)
radicals in both the reactants and the products
these steps add to give the overall equation

Question 16

define termination step in rad mech

Answer 16

the final step
two radicals react to give a molecule
radical reactants only

Question 17

define polar mechanisms

Answer 17

set of steps where all species have paired electrons

Question 18

bond breaking polar mech

Answer 18

heterolytic (-lytic = destruction)

A-B –> A:- + B+

both bonding e- are on one product, A

Question 19

bond making polar mech

Answer 19

heterogenic (-genic = generate)

A:- + B+ –> A-B

Question 20

polar reactions occur via ….

Answer 20

charged intermediates

eg. carbonation

Question 21

define electrophile

Answer 21

electron lover
accepts both bonding e- from reaction partner (nucleophile)

metal cation / H+
the more electropositive atom in the reagent

also known as a Lewis Acid

Question 22

define nucleophile

Answer 22

nucleon lover
the e- rich site (pi bonds, non-bonding pairs)
donates both bonding e-

may have a negative charge or no charge

Also a Lewis Base

Question 23

in a polar bond, which atom is the electron rich/poor site?

Answer 23

the more positive atom = electron poor

Question 24

which organic group can ONLY act as nucleophiles?

Answer 24

alkenes

because of pi e-, high e- density region

Question 25

common nucleophiles

Answer 25

``` alkenes, alkynes, ethers, alcohols (-O-) C=O compounds amines and amides halide ions hydroxide ions ```

Question 26

why is C=O more electrophilic than C-O?

Answer 26

because pi e- can be delocalised into O, giving O -ve and C +ve, so C is more electron deficient

Question 27

better nucleophile has...

Answer 27

the non-bonding pair more available for reaction with the electrophile. hence NH3 > H2O or HF, as N is less electronegative, so non-bonding e- more available

Question 28

curly arrows

Answer 28

e- movement the charge on an atom that has the same number of bonds in reactants and products (one arrow head, one tail) does not change

Question 29

electrophilic addition mechanism

Answer 29

the process for adding a diatomic molecule across the C=C 1. bond forms between C and the more electropositive atom of the reagent (electrophile) breaking bond in C=C forming C-C results in 2 CHARGED INTERMEDIATES (including a carbocation, the electrophile) 2. bond forms between two intermediates to form overall product

Question 30

for an asymmetrical alkene, what does the major product depend on?

Answer 30

the relative energies of the two possible carbocation intermediates. species are of LOWER energy (more stable) if the charge is DELOCALISED over more atoms MAJOR product arises from LOWER energy intermediate (on graph, the lower dip) greater delocalisation: tertiary>secondary>primary

Question 31

explain the stereospecificity of the addition of bromine to cyclopentene

Answer 31

the intermediate has a bridged bromium ion (+) blocks the approach of Br- from one side one of these bonds is broken and another formed to the Br- ion to get a TRANS product

Question 32

benzylic carbocations stability relative

Answer 32

major product on addition arises from Secondary carbocation, as charge can be delocalised on the ring by resonance Br attached to the benzylic C benzylic carbocations are MORE stable than tertiary carbocations with alkyl substituents

Question 33

haloalkane to alkene

Answer 33

heat with a STRONG BASE dissolved in alcohol (KOH) the opposite reaction of alkene + ACID HBr --> alkyl bromide, alkyl bromide + BASE elimination --> alkene #possible alkenes = #different groups bonded to the C w/ X

Question 34

Zaitseff's rule

Answer 34

generally alkenes w/ fewer H (more substituted) on the double bond are more stable, hence faster rate of formation, produced in greater amounts.

Question 35

types of substitution reactions

Answer 35

involving free radical intermediates eg. Halogenation, tertiary >>> (not in detail) POLAR MECH: electrophilic or nucleophilic depending on the reagent group in the slow step of the mechanism

Question 36

define nucleophilic substitution

Answer 36

substitution at a saturated C by a NUCLEOPHILE if the C of the nucleophile bears a good leaving group eg. RX + Y:- --> RY + X:- Y:- is the nucleophile - an atom w/ nonbonding e-

Question 37

define electrophilic substitution

Answer 37

in the slow step of the mechanism, a small electrophile reacts w/ the nucleophile eg. reaction of benzene with reagents (HNO3, RCl) results in the substitution of a ring H (substituted NO3 or R group, and H2O or HCl) catalyst required due to stability of delocalised pi system of aromatic hydrocarbons

Question 38

what is a good leaving group?

Answer 38

in elimination and nucleophilic sub ``` good leaving groups give rise to ANIONS capable of supporting charge - atom bearing charge is large (eg I-) - charge delocalised/resonance stabilised weaker base (have stronger conj acids) thus most elimination occurs in base ``` stable anions or small molecules I- > Br- > Cl- >> F- > CH3COO- > HO-

Question 39

relative extents when elimination / substitution compete for reacts with alkyl halides

Answer 39

generally at elevated temp: - primary: sub>elim - tertiary: elim>sub - secondary: mixture of sub/elim products eg. tertiary at high temp in KOH elim --> alkene vs primary + KOH warm --> alcohol sub

Question 40

define enantiomer

Answer 40

stereoisomer with non-superimposable mirror images | C w/ 4 diff substituents (stereogenic, chiral centre)

Question 41

define optically active

Answer 41

enantiomers differ in one physical property - direction in which a solution of the enantiomer rotates in PLANE-POLARISED LIGHT rotates in equal and opposite directions

Question 42

chemical differences of enantiomers

Answer 42

the rate of reaction with other chiral molecules is different

Question 43

assigning absolute configuration to enantiomers

Answer 43

assign priorities to substituents for the lowest priority group (usually H) facing "away" (dashed bond) highest to lowest for the remaining three groups in anticlockwise direction = S clockwise direction = R

Question 44

cycloalkane C-C rotation

Answer 44

restricted rotation due to the ring, do not interconvert at room temp

Question 45

cycloalkane enantiomers configurations

Answer 45

the have OPPOSITE configuration at ALL stereogenic centres R, R becomes S, S R, S becomes S, R

Question 46

define diastereoisomers

Answer 46

stereoisomers that are NOT mirror images of each other

Question 47

cycloalkane diastereoisomers

Answer 47

have some but not all stereogenic centres with w/ opposite configurations eg. cis-1,2-dichlorocyclohexane and either enantiomer of trans-1,2-diclorocyclohexane

Question 48

define meso form

Answer 48

compounds with TWO stereogenic centres that are NOT optically active due to symmetry planes

Question 49

alkyne addition reactions

Answer 49

in sufficient reagent A-B, a second molar equivalent can be added to give the saturated compound. 1st mol A-B, RC(A)=(B)CR this can be isolated 2nd mol A-B, RC(A2)-(B2)CR Markovnikoff's applies for asymmetric reagents to a terminal carbon

Question 50

alkyne addition A-B relative orientation for H2 and halogens

Answer 50

H2 w/ Pt/Pd, or poisoned catalyst = cis H2 w/ Li/NH3 (liquid metal ammonia reduction) = trans Br2, Cl2, HCl, HBr = trans

Question 51

hydration of alkynes reaction speed and conditions and products

Answer 51

slower than hydration of alkenes requires catalyst and dil. H2SO4 produces an enol (triple --> double, new bond with OH) rearranges by TAUTOMERISM to give a ketone (=O on the C with the -OH in the enol, by H moving and another bond going to O)

Question 52

how to identify an electrophilic carbon

Answer 52

C is electrophilic if it is at the POSITIVE end of a Polar bond

Question 53

reduction and oxidation for organic

Answer 53

reduction = increase in #C-H bonds, decrease in #C-O bonds oxidation = increase in #C-O bonds