Ch. 14 – Conjugation, Resonance, Dienes

Question 1

What is conjugation? What does it allow in terms of electrons?

Answer 1

the overlap of p orbitals on 3 or more adjacent carbons

allows electrons to delocalize and makes molecule more stable

Question 2

What are 1,3-dienes?

Answer 2

have two double bonds separated by one single bond (conjugated)

Question 3

What is the hybridization of all carbons in 1,3-dienes?

Answer 3

all carbons are sp2

Question 4

In conjugated systems, how do bond lengths change for double and single bonds?

Answer 4

double bonds = longer than normal
single bonds = shorter than normal
due to resonance

Question 5

Which is better res structure: has more charges or has less charges?

Answer 5

has less charges

Question 6

What makes the better res structure: has atoms without full octets or max number of carbons have full octet?

Answer 6

max number of carbons have full octet

Question 7

What makes a better res structure: negative charge on more EN atoms, negative charge on less EN atoms?

Answer 7

negative charge on more EN atoms

Question 8

What makes a resonance structure a major contributor to the resonance hybrid?

Answer 8

the better (more stable) resonance structure will be major contributor

Question 9

What is the hybridization of a carbon with a lone pair adjacent to a carbon with a double bond?

Answer 9

sp2

Question 10

What are polyenes?

Answer 10

compounds with many pi bonds

Question 11

What is resonance energy?

Answer 11

difference in energy between WHAT due to stabilization of a molecule because of conjugation

Question 12

Which has a more exothermic (more negative) ∆Hº? 1,4-diene or 1,3-diene?

Answer 12

1,4-diene because 1,3-diene is conjugated and is lower energy so it thus releases less energy

Question 13

What type of wavelengths (of max absorbance) do conjugated dienes absorb and does it thus take a lot of energy to excite these?

Answer 13

longer wavelengths

takes less energy to excite

Question 14

What product does electrophilic addition of an isolated diene give (for 1 eq)?

Answer 14

one markovnikov product

Question 15

What happens in hydrobromination of 1,3-dienes?

Answer 15

1,3-diene reacts with HBr to form 1,2 or 1,4 addition product

Question 16

What is most often the kinetic product of hydrobromination of 1,3-dienes?

Answer 16

1,2 product

Question 17

What is most often the thermodynamic product of hydrobromination of 1,3-dienes?

Answer 17

1,4 product

Question 18

What product forms for hydrobromination of 1,3-dienes in cold?

Answer 18

kinetic product

• there is not enough energy to go over higher thermo Ea
• reversibility of KP is slowed so it just stays

Question 19

What product forms for hydrobromination of 1,3-dienes in heat?

Answer 19

thermo product

• there is enough energy to go over high Ea of thermo
• thermo product is not reversible

Question 20

Can the thermo product ever be the kinetic product also?

Answer 20

yes

Question 21

Does thermo or kinetic product have more stability?

Answer 21

thermo

has more stable double bond

Question 22

Why does the kinetic product form? What effect causes this?

Answer 22

the proximity effect

Br just happens to be right by the carbocation

Question 23

What is the Diels-Alder Rxn?

Answer 23

addition reaction between a 1,3-diene and a dienophile alkene to form a new six-mem ring

Question 24

What is the “reactant” for the D-A rxn?

Answer 24

just heat!

Question 25

What type of mechanism is D-A?

Answer 25

concerted rxn of addition

Question 26

Does D-A have intermediates?

Answer 26

no

Question 27

Does the D-A transition state have charges?

Answer 27

no

Question 28

What is the word for the type of reaction for D-A? (It starts with a P)

Answer 28

pericyclic

Question 29

What is the reactive conformer of a 1,3-diene for D-A?

Answer 29

s-cis conformer

Question 30

Will furan undergo D-A?

Answer 30

yes and it does it quickly because it is stuck in the cis conformation

Question 31

What makes a good dienophile for D-A?

Answer 31

electron poor (has electron withdrawing groups)

Question 32

What are two common electron withdrawing groups on dienophiles?

Answer 32

oxygen and nitro (NO2) groups also OMe

Question 33

Why do electron withdrawing groups make dienophiles better?

Answer 33

because they pull electron density from the double bond making ends of double bonds more reactive

Question 34

What is the favored pairing and transition state for DA stereochemistry?

Answer 34

ENDO

Question 35

Will out/in/R(on dieneo) be up/down/ect?

Answer 35

"down and out" out = down (dash) in = up (wedge) R = down (other R's depend if it is cis or trans)

Question 36

When do bridged bicycles form by Diels-Alder?

Answer 36

when the SM is a cyclic diene

Question 37

When will a triangular or square bridge form by Diels-Alder?

Answer 37

``` triangle = 5 mem diene square = 6 mem diene ```