Conjugated Pi Systems

Question 1

What is a diene? What are the different types?

Answer 1

a diene is a molecule that has two alkenes

types: cumulated, conjugated (most discussed), isolated

Question 2

What makes conjugated dienes so important?

Answer 2

the electrons found in the double bond travel all the way through the conjugated pi system which means that electron free flow = delocalized

whereas the other types of dienes are not delocalized

Question 3

Can heteroatoms participate in conjugated systems?

Answer 3

yes

Question 4

What can be used to form a diene?

Answer 4

A sterically hindered base can be used to form dienes via elimination reactions

e.g. you can take an allylic halide molecule and treat with a strong bulky base that forms a conjugated diene

Question 5

What does conjugation do to bond length?

Answer 5

single bonds in a conjugated pi system are shorter than single bonds that are not part of a conjugated pi system.

this is because of hybridization. Sp2 hybridized are shorter than sp3 because of more s character. more s character in hybridization means shorter bonds.

Question 6

Which molecule is more stable? (= - - vs = - =)

what amount of kJ/mol of stabilization energy do you get when 2 pi bonds are conjugated?

Answer 6

both end up at the same product which is at the same level of enthalpy, but the starting materials have different starting points so the loss of heat is greater in one than the other

the molecule that gives off the most heat means it is the least stable. why? because the left molecule gives of more heat compared to the right molecule (conjugated diene).

15 kJ/mol

Question 7

What are the two most stable rotational conformations for butadiene?

Answer 7

s-cis and s-trans

s = single

the single bond in a conjugated system can still rotate

Question 8

Which of the two, s-cis and s-trans, has a lower energy and why?

Answer 8

s-trans has a lower energy because of steric hindrance

the transition state from cis to trans has a 15 kJ/mol of energy which is achievable at room temperature

Question 9

What can the molecular orbital theory do as well?

Answer 9

it can also prove why the sp2, conjugated diene has a shorter C-C bond that a sp3 C-C bond

MO shows you that there is partial double bond and partial single bond character in a butadiene, making the bond shorter

increase in energy = increase in the amount of nodes in a MO

atomic orbitals = molecular orbitals

Question 10

In Molecular Orbital Theory, what does reaction occur between?

Answer 10

HOMO and LUMO, the interaction between these two shows how a reaction occurs

HOMO and LUMO are called frontier orbitals

Question 11

What does the Markovnikov rule say?

Answer 11

Hydrogen will go to the carbon that has the most hydrogens already

Question 12

What are the differences of the products that Buta-1,3-diene will get when treated with HCl or HBr or HI?

Answer 12

you will get a 1,2-addition or a 1,4-addition.

number is based off of how far the hydrogen and chlorine/bromine/iodine are.

hydrogens wont be drawn in, that is the trick

Question 13

What are the two steps to from the 3-chlorobut-1-ene and 1-Chlorobut-2-ene?

Answer 13

Buta-1,3-diene is treated with HCl/HBr/HI and the alken takes the proton and adds on to terminal carbon and makes a carbocation that is resonance stabilized (allylic cation)

2: Cl/Br/I attacks the carbocation and forms a product

Question 14

Is there a way to select one product (1,2 vs 1,4) over the other?

Answer 14

Yes, using temperature.

cold temp - favors kinetic product (1,2). formed because it is easier to go over the hill (looking at transition states). intermediate does not have enough energy to go over 1,4 product hill.

hot temp - favors thermodynamic product (1,4). more substituted an alkene is, more stable, less energy. still will go over blue hill, but red hill is favored because it has enough energy to go from blue product back to intermediate and eventually majority of molecules will end up as 1,4 product.

kinetic is irreversible and thermodynamic is reversible

Question 15

What are 3 different types of pericyclic reactions?

Answer 15

Cycloaddition reactions
Electrocyclic reactions
sigmatropic rearrangement

Question 16

What are the 4 characteristic features of a pericyclic reaction?

Answer 16

mechanics is concerted = no intermediates

mechanism involves a ring of electrons

transition state is cyclic

polarity of the solvent generally has no effect on the reaction rate - doesnt matter what solvent you use

Question 17

What is a Diels-Alder reaction and What does it form?

Answer 17

It is a concerted mechanism that has 6 pi electrons that flow in a cyclic fashion. the fact that it has a cyclic transition state makes it a pericyclic reaction

requires two pieces - diene and dienophile
[4+2] 4 on diene 2 on dienophile

forms C-C bonds and can take two separate pieces and form a 6 membered ring

cyclic transition state can be an aromatic transition state. need to know Huckel’s rules to understand

Question 18

What are Huckel’s Rules?

Answer 18

1. Has to be cyclic
2. Has to be planar
3. need to be completely conjugated
4. 4n + 2 when counting how many pi electrons

Question 19

What is the equation for an anti aromatic compound?

Answer 19

4n

Question 20

Are 2+2 cycloadditions allowed?

Answer 20

They are forbidden because they follow all of Huckel’s rules except for the last, in fact it actually follows the anti aromatic equation making it anti aromatic

Question 21

Are 6+2 cycloadditions allowed?

Answer 21

They are forbidden because they follow all of Huckel’s rules except for the last, in fact it actually follows the anti aromatic equation making it anti aromatic

Question 22

What does the conformation of the diene have to be in order for a Diels-Alder reaction to occur?

Answer 22

It must be in the s-cis conformation because this brings the proximity of the carbons close enough to the alkene

trans prevent reaction from occurring because of distance

majority of dienes will be in s-trans (98%) and only 2% will be cis

Question 23

Can all molecules go from s-trans to s-cis to allow a Diels-Alder reaction to occur?

Answer 23

No, some molecules are locked in s-trans, for example, a s-trans diene in a ring is locked due to no free rotation along the single bond, therefore, no DA reaction

sometimes a s-cis is prevented due to sterics from the methly groups on the ends of dienes

Question 24

What can you do to speed up the Diels-Alder reaction?

Answer 24

adding substituent groups to the diene or the dienophile

adding Electron donating groups to the diene (EDG) will make it much faster, e.g. methyls

adding electron withdrawing groups to the dienophile (EWG) will make it much faster, e.g. nitriles

the diene itself is already electron rich, but you can make it even more rich by adding groups with electron density

Question 25

What happens to sp2 carbons on the edges of the diene and dienophile?

Answer 25

they turn into sp3 carbons meaning they have the potential to become tetrahedral stereocenters in the product

Question 26

What happens with stereochemistry in a Diels-Alder reaction?

Answer 26

stereochemistry is always retained

remember the cave example: inside stays on the same side, outside says on the same side

Question 27

What are two types of products we will see with DA reactions?

Answer 27

exo products and endo products

exo: if the EWGs on the dienophile are on the same side on the rings as the inside carbon
endo: if the EWGs on the dienophile are on the opposite side on the rings as the inside carbon

endo is more prominent than exo

Question 28

How do endo and exo products form?

Answer 28

endo product: the EWGs are pointing towards the diene, are underneath the diene

exo product: the EWGs are pointing away from the diene, are above the diene

Question 29

Why are some DA reactions reversible and some irreversible?

Answer 29

Due to the Gibbs Free Energy and Keq numbers

high G and Keq —> irreversible

low G and Keq —> reversible

this is because of the capability of climbing back up the reaction barrier on a graph