Chapter 15 Flashcards
UV-Vis: determines the amount of _______ in a molecule
conjugation
Heats of hydrogenation are used to compare the relative _______ of C=C
stabilities
- Lower energy = more stable
What are the solvents used in UV-Vis? (2)
1) CH3OH
2) EtOH
When part of a double bond, atoms are _______ hybridized with overlapping __ orbitals
sp2
p
Each p orbital has two lobes with the wave function indicated by _____ and _____ signs
plus
minus
constructive overlap (+ and + or - and -) : _______ MO is formed
pi bonding
destructive overlap (+ and - ) : _______ MO is formed
pi anti-bonding
What does homo stand for?
highest occupied molecular orbital
What does LUMO stand for?
lowest unoccupied molecular orbital
HOMO is usually _____ or _____
bonding
NB
LUMO is usually _____
anti-bonding
Typical excitations occurs from _____ to ______
HOMO to LUMO
Conjugation systems have electronic transitions with _______ energies = wavelengths _______ than 200 nm
lower
larger
UV-Vis spectroscopy can only detect wavelengths higher than _____ nm
200
An increase in the number of pi orbitals leads to a _______ gap between orbitals = _______ energy = _______ wavelength
smaller
lower
longer
What is the formula for Beer’s law?
A = ε c l
- A: absorbance
- ε: molar extinction coef
- c: concentration
- l: path length of cell (1cm)
The values of _____ and _____ are read directly from the spectrum. Other values (_____ and _____) could be calculated
λmax and A
c and εmax
Rules for UV-Vis orbital diagrams:
1) # of ___ bonds = # of bonding π MO’s
2) There are equal and opposite _____ MO’s
3) Any lone pair on a conjugated carbonyl is placed in a NB orbital at E=_____
1) π
2) anti-bonding
3) 0
π —> π* absorptions have _______ ε
higher (5,000 - 30,000)
NB —> π* absorptions have _______ ε
lower (<100)
Interpreting UV-Vis spectra:
addition of conjugated double bond increase λmax by _______ per double bond
30-40 nm
- on top of base molecule
Interpreting UV-Vis spectra: Alkyl groups increase the value of λmax by about _______ per alkyl group
5 nm
- has to be connected to double bond
Absorption of UV-visible energy by a molecule results in ________
electronic transitions
MO theory for conjugated systems:
1) #MOs (lines we draw) = #AOs (# of _______ with p orbitals)
2) Place MOs _______ (never pair them) and _______ around E=O
3) as the # of nodes increases the energy of the MO _______
- # of nodes = _______
4) When drawing nodes, place them as _______ as possible
1) atoms
2) singly; symmetrically
3) increases
- N-1
4) symmetrically
What are the 2 possible planar conformations for butadiene?
1) S-trans (more stable)
2) S-cis
The transition state of the Sn2 mechanism is stabilized through _______ with the p orbitals of the pi bond, which lowers the _______ allowing for the reaction to take place faster
conjugation
Ea
Explain why there is 1,2 and 1,4 addition of HBr to dienes
depending on the temperature, the nucleophile (Br-) will add to the carbon 2 or carbon 4 (resonance allows this to happen)
At what temperatures will the 1,2 addition of HBr to dienes occur?
-78°C, 0°C
At what temperatures will the 1,4 addition of HBr to dienes occur?
Room temp, 40°C
1,2-addition results in the _______ product (@-78°C, 0°C)
kinetic
1,4-addition results in the _______ product (@Room temp, 40°C)
thermodynamic product
The Diels-Alder reaction: important synthetic tool. It involves the [_______] cycloaddition reaction that forms a ____ - member ring with stereochemical control
4+2
6
What is the name of the two parts that are combined in a Diels-Alder reaction?
1) diene
2) dienophile
Reactivity of a Diels-Alder reaction: to enhance the flow of electrons, _______ are often used on the dienophile, and _______ are used on the diene
EWG (dienophile)
EDG (diene)
the mechanism of the Diels-Alder reaction is a ________ cyclic movement of 6 electrons
concerted
(Diels-Alder RXN)
To get proper orbital overlap, the reactants add _______, resulting in the _______ stereochemistry for the products as was found in the starting material
syn
same
a 2,3 substituted diene spends more time in s-cis than s-trans = leads to _____ faster reaction than unsubstituted diene
10x
(Diels-Alder RXN)
The _______: when substituents are on the dienophile, two possible products can be theoretically formed, proving that the starting dene was already a ring. However, only the _______ product is formed
Endo Rule
endo
(Diels-Alder RXN)
Unsymmetrical reagents: when both diene and dienophile are asymmetric, the _____ or _____ products are the major product
ortho
para
- never make meta product!!!
(Diels-Alder RXN for diene)
2 Rin = _____
2 Rout = _____
1 Rin 1 Rout =_____
cis
cis
trans
The Diels-Alder reaction is an example of a _______ reaction: involves the _______ forming and breaking of bonds within a closed ring of interacting orbitals
pericyclic
concerted
Pericyclic reactions require _______ or _______ conditions
thermal (heat)
photochemical (hv)
the two different sets of conditions give different pericyclic products. Woodward and Hoffmann predicted the reaction products using their theory of _______
conservation of orbital symmetry
cycloaddition reactions: these reactions can be viewed as electrons flowing form the _______ of one molecule to the _______ of the other reactant
HOMO
LUMO
ALWAYS use 1 homo and 1 lumo if the ends of the pi system meeting have the _______ mathematical orbital signs, the reactions is _______ using these conditions
same
allowed
usually you will use the HOMO of the _______ and the LUMO of the _______
nucleophile
electrohpile
In photochemical reactions, the lifetime of excited state molecules is short, resulting in the chance of 2 excited state molecules reacting to be small. therefor only use _______ and _______ where only one is excited
1 HOMO
1 LUMO
- this will allow constructive overlap to occur
What is a cumulated diene?
C=C=C
