Ch. 14 – Conjugation, Resonance, Dienes Flashcards
What is conjugation? What does it allow in terms of electrons?
the overlap of p orbitals on 3 or more adjacent carbons
allows electrons to delocalize and makes molecule more stable
What are 1,3-dienes?
have two double bonds separated by one single bond (conjugated)
What is the hybridization of all carbons in 1,3-dienes?
all carbons are sp2
In conjugated systems, how do bond lengths change for double and single bonds?
double bonds = longer than normal
single bonds = shorter than normal
due to resonance
Which is better res structure: has more charges or has less charges?
has less charges
What makes the better res structure: has atoms without full octets or max number of carbons have full octet?
max number of carbons have full octet
What makes a better res structure: negative charge on more EN atoms, negative charge on less EN atoms?
negative charge on more EN atoms
What makes a resonance structure a major contributor to the resonance hybrid?
the better (more stable) resonance structure will be major contributor
What is the hybridization of a carbon with a lone pair adjacent to a carbon with a double bond?
sp2
What are polyenes?
compounds with many pi bonds
What is resonance energy?
difference in energy between WHAT due to stabilization of a molecule because of conjugation
Which has a more exothermic (more negative) ∆Hº? 1,4-diene or 1,3-diene?
1,4-diene because 1,3-diene is conjugated and is lower energy so it thus releases less energy
What type of wavelengths (of max absorbance) do conjugated dienes absorb and does it thus take a lot of energy to excite these?
longer wavelengths
takes less energy to excite
What product does electrophilic addition of an isolated diene give (for 1 eq)?
one markovnikov product
What happens in hydrobromination of 1,3-dienes?
1,3-diene reacts with HBr to form 1,2 or 1,4 addition product
What is most often the kinetic product of hydrobromination of 1,3-dienes?
1,2 product
What is most often the thermodynamic product of hydrobromination of 1,3-dienes?
1,4 product
What product forms for hydrobromination of 1,3-dienes in cold?
kinetic product
- there is not enough energy to go over higher thermo Ea
- reversibility of KP is slowed so it just stays
What product forms for hydrobromination of 1,3-dienes in heat?
thermo product
- there is enough energy to go over high Ea of thermo
- thermo product is not reversible
Can the thermo product ever be the kinetic product also?
yes
Does thermo or kinetic product have more stability?
thermo
has more stable double bond
Why does the kinetic product form? What effect causes this?
the proximity effect
Br just happens to be right by the carbocation
What is the Diels-Alder Rxn?
addition reaction between a 1,3-diene and a dienophile alkene to form a new six-mem ring
What is the “reactant” for the D-A rxn?
just heat!
What type of mechanism is D-A?
concerted rxn of addition
Does D-A have intermediates?
no
Does the D-A transition state have charges?
no
What is the word for the type of reaction for D-A? (It starts with a P)
pericyclic
What is the reactive conformer of a 1,3-diene for D-A?
s-cis conformer
Will furan undergo D-A?
yes and it does it quickly because it is stuck in the cis conformation
What makes a good dienophile for D-A?
electron poor (has electron withdrawing groups)
What are two common electron withdrawing groups on dienophiles?
oxygen and nitro (NO2) groups also OMe
Why do electron withdrawing groups make dienophiles better?
because they pull electron density from the double bond making ends of double bonds more reactive
What is the favored pairing and transition state for DA stereochemistry?
ENDO
Will out/in/R(on dieneo) be up/down/ect?
“down and out”
out = down (dash)
in = up (wedge)
R = down (other R’s depend if it is cis or trans)
When do bridged bicycles form by Diels-Alder?
when the SM is a cyclic diene
When will a triangular or square bridge form by Diels-Alder?
triangle = 5 mem diene square = 6 mem diene
