A conjugated diene is one in which the double bonds are separated by only one link, causing the 2p orbitals of neighboring p bonds to overlap.

• An unconjugated diene is one with two or more single bonds between the double bonds.

• A cumulated diene has two double bonds that share a sp hybridized carbon.

• Because the 2p orbitals of the p bonds do not overlap in a cumulated diene, they are not conjugated.

Conjugated dienes have two conjugated double bonds that are 14.5–17 kJ (3.5–4.1 kcal)/mol more stable than isomeric nonconjugated dienes, a fact that applies to all conjugated double bonds, not only dienes.

• Delocalization of the four electrons leads in improved conjugated double bond stability.

• Two conjugated double bonds are formed from four p molecular orbitals, according to molecular orbital theory, since the four parallel 2p orbitals overlap in space, including the 2p orbitals on each side of the single bond between the conjugated double bonds.

• The lowest two p molecular orbitals, with zero and one node, respectively, are bonding orbitals with two electrons each.

Each of the lowest two filled p molecular orbitals has a lower energy than isolated p bonds, which accounts for the "additional" stability of conjugated p systems.

• The lowest filled p molecular orbital contains wide lobes that stretch across all four atoms, demonstrating electron density delocalization in conjugated p systems.

• Conjugated dienes undergo 1,2- and 1,4-addition reactions with electrophiles, resulting in combinations of the two types of products.

Temperature affects the ratio of 1,2-addition to 1,4-addition, with 1,2-addition frequently predominating at lower temperatures and 1,4-addition predominating at higher temperatures.

• Because there is normally more positive charge at the 2 position of the allylic cation intermediate, the activation barrier for reaction at this site is lower when added to butadiene at a lower temperature under kinetic (rate) control.

• Because the lower temperature hinders product equilibration, relative product stability is unimportant.

1,4-Addition to butadiene produces the dominating product at higher temperatures under thermodynamic control, since the double bond of the 1,4-addition product is more substituted and hence of lesser quality.

• The ratio of 1,2- to 1,4-addition products is determined by whether the reaction is kinetically or thermodynamically controlled.

• When a conjugated diene reacts with HBr, the initial protonation of one of the double bonds results in a resonancestabilized allylic cation; reaction of bromide with one of the carbons of this intermediate bearing the partial positive charge results in the 1,2-addition product, and reaction at the other results in the 1,4-addition product.

The visible area of the electromagnetic spectrum has wavelengths ranging from 400 nm to 700 nm, whereas the ultraviolet sector has wavelengths ranging from 200 nm to 400 nm.

• Ultraviolet and visible spectral data are displayed as absorbance (A) vs wavelength, where absorbance is determined as the log base 10 of the ratio of (I0/I), where I0 is the intensity of light irradiating a sample at a specific wavelength and I is the light passed through the sample.

• The value (I/Io) 3 100 is referred to as percent transmittance.

The Beer-Lambert Law, A is the connection between absorbance, concentration, and the length of the sample cell (cuvette). 5 «cl where A denotes absorbance and « denotes molar absorptivity

• The molar absorptivity (extinction coefficient) as a function of wavelength is unique to a molecule and is determined by the functional groups included inside the molecule.

• If the molar absorptivity of a particular molecule is known, the BeerLambert rule may be used to compute its concentration in solution.

• The absorbed wavelengths from white light are removed by molecule absorption, and a sample appears to our eyes as a mixture of reflected wavelengths.

Wavelengths that are not absorbed are reflected.

• Using an artist's conventional color wheel, the color of combined reflected wavelengths can be roughly approximated as the complement of the absorbed color.

• Absorption of ultraviolet-visible electromagnetic radiation leads in the promotion of an electron from a lower energy, occupied chemical orbital to a higher energy orbital.

• Bonding electrons are often too low in energy to absorb ultraviolet or visible light.

In the near ultraviolet spectrum, the p S p* transition for unconjugated alkenes is frequently too high in energy (wavelength too short).

• Because conjugation reduces the energy difference between full and empty p orbitals, conjugated p systems feature p S p* transitions that can be detected in the ultraviolet or even visible absorption area.

• The more conjugated p bonds there are, the narrower the p S p* energy gap, and hence the longer the wavelength of absorbed light. Carbonyl groups, like C"C double bonds, can participate in conjugation.

Pericyclic reactions take place in a single step and involve a transition state with a closed loop of orbitals.

Although there are various approaches to understanding these processes, frontier molecular orbital theory is the most frequent and easiest.

In this method, a series of procedures is taken in order to forecast if a response is permitted or prohibited.

• To begin, a response geometry is suggested.

• Second, the responding partners' HOMO and LUMO are recorded.

• Third, the interaction between the partners' HOMO and LUMO is investigated to determine if an even number of phase alterations (usually zero) or an odd number (usually one) exist at the locations of interaction.

In the Diels-Alder reaction, conjugated dienes combine with specific types of molecules that have double or triple bonds to generate two new s bonds and a ring structure, which is an example of a 4 1 2 cycloaddition process.

• A dienophile is a molecule having a double or triple bond that interacts with a diene, and the cyclic result is commonly referred to as the Diels-Alder adduct.

• In the reaction, three p bonds are broken, and two stronger new s bonds, as well as a new p bond, are produced, supplying the driving power.

A Diels-Alder reaction includes the redistribution of six p electrons in a cyclic transition state in a single step with no intermediates.

• The diene and dienophile configurations are conserved.

• The formation of the endo adduct is encouraged.

The Diels-Alder reaction is aided by the presence of electron-withdrawing groups on one reactant (typically the dienophile) and electron-releasing groups on the other (usually the diene).

The diene must be present.