Chapter 15: Dienes and Aromaticity

conjugated dienes

cumulenes

non-conjugated dienes

Conjugated dienes have (higher/lower) energies and (higher/lower) heats of formation, thus are (more/less) stable than isolated or cumulated dienes.

lower, lower, more

More nodes → (more/less) destructive interference

more

In a conjugated diene, four adjacent p orbitals overlap to form…

four π molecular orbitals (two bonding and two antibonding)

Resonance structures symbolize electron…

delocalization

Why is electron delocalization stabilizing?

it results in additional bonding via low-energy bonding MOs

Resonance is a ____ effect.

stabilizing

The most stable conformation of dienes is…

s-trans conformation

The second most stable conformation of dienes is…

gauche/skew conformation

The least stable conformation of dienes is…

s-cis conformation, due to van der Waals repulsions

The two π bonds of cumulated dienes are…

mutually perpendicular

Some allenes are chiral even though they lack…

an asymmetric carbon atom

Dienes and ordinary alkenes display similar…

IR and NMR spectra

UV-vis light can probe the extent of conjugation in conjugated systems using…

UV-vis spectroscopy

UV-vis spectrum

graph of radiation absorption by the substance vs the wavelength of radiation

chromophore

the structural feature (atom or group of atoms) of a molecule responsible for its UV-vis absorption

absorbance

measure of the amount of radiant energy absorbed

What is the equation for absorbance?

A = log (I₀÷I)
where I₀ is intensity of radiation entering a sample, I is the intensity of light emerging from the sample

λ_max

wavelength at the maximum of an absorption peak

Beer’s Law

relates the amount of light absorbed by a substance to its concentration
A = Elc
where E is molar extinction coefficient, l is path length, and c is concentration

Larger values of molar extinction coefficients → (greater/lesser) absorbances at a given concentration and path length

greater

Typical alkenes absorb UV at very (large/small) wavelengths.

small (<200 nm)

Conjugated double/triple bonds absorb at wavelengths ____

>200 nm

What is most useful for diagnosis of…

conjugated systems

UV/visible light energy causes an _____ ______ between HOMO and LUMO.

electronic transition

UV (___ - ____ nm) and visible radiations (___ - ____ nm) have enough energy to cause electronic transition from n → π^* and π → π^*.

UV → 190 - 400 nm
vis → 400 - 800 nm

What is the most important part of the UV-vis spectrum?

λ_max

The more consecutive conjugated multiple bonds there are, the (higher/lower) λ_max is.

higher

π-electron promotion occurs from (HOMO/LUMO) to (HOMO/LUMO).

HOMO → LUMO

The HOMO-LUMO gap becomes (bigger/smaller) as the number of conjugated double bonds increases.

smaller

Longer π-system → (higher/lower) λ_max

higher

Each additional conjugated double bond increases λ_max by…

30 - 50 nm

What other factors affect λ_max besides conjugation?

• substituents

• conformation

Dienes locked into ____ conformation have higher λ_max

s-cis

Each additional alkyl group increases λ_max by…

5 nm