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Chem Oly Pre-Camp > Organic II > Flashcards

Organic II Flashcards

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1
Q

kinetics is about what?

A

reaction rates

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2
Q

define elementary reaction

A

one where bond breaking and making occur during a single collision

eg. proton transfer

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3
Q

requirements of a successful collision

A

collide in the correct orientation

have energy exceeding the activation energy

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4
Q

define transition state

A

the activated complex formed on collision

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

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5
Q

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

A

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

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6
Q

define mechanism

A

the set of steps of the reaction

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7
Q

define ‘rate determining step’

A

the slowest step, with the HIGHEST activation energy

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8
Q

define intermediate

A

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

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9
Q

what is a ‘radical’?

A

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

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10
Q

In radical mechanisms, bond breaking is ______

A

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

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11
Q

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

A

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

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12
Q

radical mechanism bond formation is ____

A

homogenic (-genic as in GENERATING something)

each reactant contributes 1 e-

A* + B* –> A-B

radicals in the reactants

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13
Q

steps in a radical mechanism

A

initiation
propagation
termination

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14
Q

define initiation step in radical mechanism

A

step 1
bond breaking
to FORM radicals in the products

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15
Q

define propagation step in radical mechanism

A

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

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16
Q

define termination step in rad mech

A

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

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17
Q

define polar mechanisms

A

set of steps where all species have paired electrons

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18
Q

bond breaking polar mech

A

heterolytic (-lytic = destruction)

A-B –> A:- + B+

both bonding e- are on one product, A

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19
Q

bond making polar mech

A

heterogenic (-genic = generate)

A:- + B+ –> A-B

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20
Q

polar reactions occur via ….

A

charged intermediates

eg. carbonation

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21
Q

define electrophile

A

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

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22
Q

define nucleophile

A

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

23
Q

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

A

the more positive atom = electron poor

24
Q

which organic group can ONLY act as nucleophiles?

A

alkenes

because of pi e-, high e- density region

25
Q

common nucleophiles

A 
alkenes, alkynes, ethers, alcohols (-O-) 
C=O compounds 
amines and amides 
halide ions 
hydroxide ions
26
Q

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

A

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

27
Q

better nucleophile has…

A

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

28
Q

curly arrows

A

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

29
Q

electrophilic addition mechanism

A

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
30
Q

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

A

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

31
Q

explain the stereospecificity of the addition of bromine to cyclopentene

A

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

32
Q

benzylic carbocations stability relative

A

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

33
Q

haloalkane to alkene

A

possible alkenes = #different groups bonded to the C w/ X

heat with a STRONG BASE dissolved in alcohol (KOH)

the opposite reaction of alkene + ACID HBr –> alkyl bromide,

alkyl bromide + BASE elimination –> alkene

34
Q

Zaitseff’s rule

A

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

35
Q

types of substitution reactions

A

involving free radical intermediates eg. Halogenation, tertiary&raquo_space;> (not in detail)

POLAR MECH: electrophilic or nucleophilic depending on the reagent group in the slow step of the mechanism

36
Q

define nucleophilic substitution

A

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-

37
Q

define electrophilic substitution

A

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

38
Q

what is a good leaving group?

A

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-&raquo_space; F- > CH3COO- > HO-

39
Q

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

A

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

40
Q

define enantiomer

A

stereoisomer with non-superimposable mirror images

C w/ 4 diff substituents (stereogenic, chiral centre)

41
Q

define optically active

A

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

42
Q

chemical differences of enantiomers

A

the rate of reaction with other chiral molecules is different

43
Q

assigning absolute configuration to enantiomers

A

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

44
Q

cycloalkane C-C rotation

A

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

45
Q

cycloalkane enantiomers configurations

A

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

46
Q

define diastereoisomers

A

stereoisomers that are NOT mirror images of each other

47
Q

cycloalkane diastereoisomers

A

have some but not all stereogenic centres with w/ opposite configurations

eg. cis-1,2-dichlorocyclohexane and either enantiomer of trans-1,2-diclorocyclohexane

48
Q

define meso form

A

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

49
Q

alkyne addition reactions

A

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

50
Q

alkyne addition A-B relative orientation for H2 and halogens

A

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

Br2, Cl2, HCl, HBr = trans

51
Q

hydration of alkynes reaction speed and conditions and products

A

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)

52
Q

how to identify an electrophilic carbon

A

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

53
Q

reduction and oxidation for organic

A

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

oxidation = increase in #C-O bonds

Chem Oly Pre-Camp (14 decks)

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