Conjugated unsaturated systems

what is a conjugated unsaturated system?

one that has at least one p orbital adjacent to at least π bond (orbital can be full or empty)
- e.g. allylic cation/anion

allylic system

a group that is attached to a carbon α to a C=C bond is called an allylic group (Br is in allylic position)

SN1 reactions involve what carbons?

tertiary (3°)

why does allyl chloride undergo an SN1 reaction even though its a primary carbon?

due to the stability of the allyl cation by resonance
- shares + charge between more atoms

what is the order of cation stability? (most stable to least stable)

3° > Allyl > 2° > 1° > CH₃

how can the stability of allylic cation / anion be explained using molecular orbital theory? draw and explain

allyl cations have two π electrons in the bonding MO, which is lower energy and is the most stable orbital. the anion has 4 π electrons, 2 in the bonding MO and 2 in the non-bonding MO, which is less stable. The extra stability is due to the bonding electrons in the bonding MO

what are the three types of dienes?

1. *conjugated dienes*: is conjugated because the two C=C double bonds are separated by a single bond
2. *isolated dienes*: is isolated because the two C=C bonds are separated by more than one single bond
3. *Cumulated* dienes: the two C=C double bonds are not separated by single bond

how would the heat of hydrogenation of a conjugated diene compare to that of an alkene or an isolated diene?

alkene and isolated diene would release more heat because they are less stable. the conjugated dienes have extra stability (due to overlap of π bonds)

explain and draw the extra stability of conjugated dienes using molecular orbital theory

since there are 4p orbitals, they combine to form 4 MOs. The lower 2 MOs are bonding MO and top two are antibonding. since there are two π bonds (4 π electrons), they occupy the *bonding* MOs (π₁ and π₂)

what are the conformations of buta-1,3-dienes and which is less stable?

- the *s-trans* conformation (s refers to single bond) where the double bonds are on opposite sides of the single bond
- *s-cis* conformation where the double bonds are on the same side
- *s-cis is less stable* due to steric interference

what is the major product formed in the electrophilic addition of HBr on alkene at -80C?

3-bromobut-1-ene (1,2-addition) 80%
- 20% is 1-bromobut-2-ene (1,4 addition)

what is the major product formed in the electrophilic addition of HBr on alkene at 40C?

4-bromobut-2-ene (1,4 addition) 85%
- 15% 3-bromobut-1-ene (1,2 addition)

what is the more stable product of the electrophilic addition of HBr?

the 1,4 product

what is the more stable carbocation in the electrophilic addition of HBr?

the secondary carbocation

describe the energy diagram for the electrophilic addition of HBr to a an allylic compound

has two peaks with one intermediate (carbocation). depending on which carbocation is formed (primary or secondary), the second peak will be different heights. If the more stable carbocation is formed, the peak is slightly lower. If the less stable carbocation is formed, then the peak is slightly higher. The product from the less stable carbocation will be lower in energy than the other product, and is therefore more stable. The product from the more stable carbocation will be higher in energy, and is therefore more unstable

what is the kinetic vs thermodynamic control in the electrophilic addition of HBr to an allylic compound?

at low temperature, the reaction is kinetic control due to its lower activation energy barrier, it is formed easier and faster to give the 1,2 products at higher temperature, the reaction is thermodynamic control. The driving force for this reaction is due to more stable 1,4 product

what kind of reaction is the Diels-Alder reaction?

1,4 - cycloaddition reactions
- 1,4 addition to a compound with multiple bonds hive 6-membered ring product

describe and draw the mechanism of a Diels-Alder reaction

concerted reaction: happens in one single step. building 2 sigma bonds, breaking 3 pi bonds, and building one pi bond happens simultaneously

Diels-Alder reaction

cycloaddition (electrophilic addition) of a of a dienophile to a diene to form a 6 membered ring. because it involves a system of 4 atoms from the diene and 2 atoms from the alkene, it is called a [4+2] cycloaddition.
- the alkene/alkyne is called a dienophile because it reacts w/ a diene

what are the conditions for a Diels-Alder reaction?

1. diene must be able to form a s-cis conformation (not too bulky, bonds need to be able to rotate)
2. reaction is enhanced if there is a e- present in the leaving group (OR or alkyl group for resonance)
3. the dieneophile MUST have pi electrons and possess an electron-withdrawing group

what are some good electron withdrawing groups?

conjugated carbonyl (ketone), esters, Nitro group, nitrile, conjugates aryl alkene, vinyl ether, halogen

what are three features to recognize Diels-Alder products?

double bond in the ring, six membered ring, electron withdrawing group

is a Diels-Alder reaction stereospecific? why or why not?

yes, it is highly stereospecific. since the reaction is concerted and on the same side, there is not chance for a change in stereochemistry for the dienophile in the reaction. the stereochemistry of BOTH the diene and the dienophile is retained

In a Diels-Alder reaction with two rings reacting, which way will the hydrogens be facing and what is this product called? why does this occur?

the hydrogens will point UP to form the ENDO product rather than the Exo product. This is becuase the secondary overlapping between the C=O p orbitals and the carbon p orbital of the diene which stabilizes the transition state

if the dienophile is non-symmetrical, a pair of what will be formed?

enantiomers. the reaction can take place on both sides of the dienophile

using MO theory, excitation from what oribtals will give light?

from Highest Occupied Molecular Orbital (HOMO) to Lowest Unoccupied Molecular Orbital (LUMO)

with more conjugated double bond, the energy difference between the HOMO and LUMO would get _________ and the wavelength required for excitation would be ________

smaller; longer

what is the general rule regarding light absorption and conjugated systems?

a compound with a longer chain of conjugated double bonds absorbs light at a longer wavelength

how many conjugated double bonds are required to absorb light in the visible region?

at least 6 conjugated double bonds

what is the reason for colour perception?

an object is coloured because absorption of one colour reflects the opposite colour in the colour wheel