Woodward-Fieser Rules - Online Tutor, Practice Problems & Exam Prep

The Woodward-Pfizer rules estimate the lambda max of conjugated systems. A conjugated diene has a base value of 217 nm, increasing by 30 nm for each additional double bond. Alkyl oxochromic groups add 5 nm each, while exocyclic double bonds also contribute 5 nm. Homoanular dienes in cis conformation add 39 nm. For example, a system with multiple double bonds and substituents can yield a lambda max of approximately 316 nm, illustrating the cumulative effects of structural features on absorption spectroscopy.

Woodward-Fieser Rules Concept 1

Hey, everyone. In this video, let's take a look at the Woodward-Pfizer rules. Now they themselves represent a set of rules that help to give an estimate for lambda max of conjugated systems. So here we're going to break them down into 4 categories. We have our conjugated diene itself.

We have our alkyl oxochromic group. We have our exocyclic double bond and then our homo angular diene. So coming back up here to our conjugated diene, we're going to say it has a base value of 217 nanometers. And we're going to say each additional double bond, each additional conjugated double bond will increase the lambda max wavelength by 30 nanometers. So if we take a look at this example here, we have our base, we'll say these 2, that's 217, but then we have 2 additional double bonds.

Two additional double bonds, each one increasing my wavelength by 30 nanometers. So together, that's an additional 60 nanometers. So this will come out to be 277 nanometers. Next, we have our alkyl oxochromic groups. Remember, an oxochromic group is just the nonconjugated portion that's connected to our conjugated portion of the molecule.

Not all oxochromic groups are alkyl in nature. For example, OH can be an oxochromic group. But here, we're going to say that the alkyl versions of these have a set value. So here we're going to say that in this case, our oxochromic groups are alkyl carbons attached to a conjugated vanillic carbon. We're going to say each one of these increases my lambda max by 5 nanometers.

So if we take a look here at this, we're going to have our base value of 217 nanometers from our 2 double-bonded double bonds that are conjugated. And they're going to say we have 1, 2, 3, 4 alkyl groups directly connected to that conjugated portion. So there are 4 of them, each one contributing an increase of 5 nanometers. So that's 20 nanometers being added, giving us 237 nanometers at the end. Again, this is just an estimate of what our lambda max would be in terms of this particular molecule.

Now next, we go to the exocyclic double bond. So in an exocyclic double bond, we're going to say a double bond with 1 vanillic carbon as part of the ring, so here, and one vanillic carbon extending outside the ring, here. We're going to say each exocyclic double bond also increases my lambda max by 5 nanometers. So if we took a look here, we'd say we have our base of 217 because it's these 2 double bonds. And then we're going to say we have 2 exochromic groups.

So that's 2 times the plus 5 nanometers that they contribute. And then we have this exocyclic double bond, and that includes another 5 nanometers. Alright. So that's going to give me here 232 nanometers. And then finally here, we take a look at our homoanular diene.

This is just a conjugated diene in the same ring in an s cis conformation. Each one contributes an additional 39 nanometers. So if we take a look here, we're going to say that our homoanular diene is this portion right here. We have a conjugated diene in the same ring and they're both cis. They're pointing in the same direction.

Alright. So here, we're going to say we have our base, which is 217 nanometers. Then we're gonna say next, what do we have? We have an additional double bond here, so that's gonna add an additional 30 nanometers to my wavelength. Then what else are we going to say?

We're going to say here, in addition to that, we have 1, 2, 3, 4, 5, 5 oxochromic groups. So that's 5 times each one contributes an additional 5 nanometers. So that's 25 nanometers. And then we're going to say here that in addition to this, we're going to have our homoanular.

So that's 39 nanometers. And in addition to this, we're going to have here an exocyclic double bond. It's part of a ring, part of it's part of a ring, and it extends outside the ring. That'd be another 5 nanometers. Alright.

So adding all this up together will give us our estimated wavelength, lambda max. So we're going to say 217 plus the 30 gives us 247, plus another 25 here will give us 272, and then another 39 here would give us 311, plus another 5 would give us 316 nanometers roughly. Okay. So that's what we'd say in terms of the lambda max here. This one's a little bit tricky because it's so many moving parts to it.

We have to take into account the fact that we have our base, conjugated diene, the additional double bond, the oxochromic groups, the exocyclic double bond, as well as the homoanular diene. Taking all that into account gives us this rough estimate of the lambda max for this particular compound within the 4th box.

Woodward-Fieser Rules Example 1

Here in this example, it says, using the Woodward Pfizer rules, determine the lambda max for the following compound. Right. So we have a conjugated system. So our base value will be 217 nanometers. And then we can say next is our base of these 2.

And then we have 2 additional conjugated double bonds. So we're going to say the additional ones, that'd be 2 times 30. Each one is 30, so that's equal to 60 nanometers being added. And then we could talk about our oxochromic groups. So there would be 1, 2, 3, 4, 5.

Alright. So 5, and each one contributes 5. So that's an additional 25. When we add that all up, that's going to give us our total, which comes out to λmax=302 nanometers. So this would be our estimated lambda max for the following compound when using the Woodward Pfizer rules.

Which of the following compounds displays the most alkyl (auxochromic) groups? 

• A. A
• B. B
• C. C
• D. D
• E. Both B and C
• F. Both A and D

Which of the following compounds do you expect to show the largest λ_max?

• A.
• B.
• C.
• D.

Rank the following in order of increasing λ_max.

• A. A < C < B < D
• B. C < A < D < B
• C. B < D < A < C
• D. A < C < D < B

Using the Woodward-Fieser Rules, determine the λ_max for the following compound.

• A. 239 nm
• B. 326 nm
• C. 390 nm 
• D. 268 nm 
• E. 235 nm

An image of the visible light spectrum is displayed below. Based on the calculated λ_max for the following compound, which colored region would it reside?

• A. Red
• B. Yellow
• C. Blue
• D. Green
• E. Violet

Here’s what students ask on this topic:

What are the Woodward-Fieser rules in organic chemistry?

The Woodward-Fieser rules are a set of empirical guidelines used to estimate the maximum wavelength (λ_max) of absorption for conjugated systems in UV-Vis spectroscopy. These rules take into account the base value of a conjugated diene, which is 217 nm, and add increments for various structural features. Each additional conjugated double bond adds 30 nm, alkyl oxochromic groups add 5 nm each, exocyclic double bonds add 5 nm each, and homoanular dienes in cis conformation add 39 nm. These cumulative effects help predict the λ_max for complex molecules.

How do you calculate λ_max using the Woodward-Fieser rules?

To calculate λ_max using the Woodward-Fieser rules, start with the base value of 217 nm for a conjugated diene. Add 30 nm for each additional conjugated double bond. Include 5 nm for each alkyl oxochromic group and each exocyclic double bond. If the system contains a homoanular diene in cis conformation, add 39 nm. Sum these values to estimate the λ_max. For example, a molecule with two additional double bonds, three alkyl oxochromic groups, and one exocyclic double bond would have a λ_max of 217 + 60 + 15 + 5 = 297 nm.

What is the base value for a conjugated diene in the Woodward-Fieser rules?

The base value for a conjugated diene in the Woodward-Fieser rules is 217 nm. This value serves as the starting point for calculating the λ_max of a conjugated system. Additional structural features such as extra conjugated double bonds, alkyl oxochromic groups, exocyclic double bonds, and homoanular dienes are added to this base value to estimate the overall λ_max.

How do alkyl oxochromic groups affect the λ_max in the Woodward-Fieser rules?

In the Woodward-Fieser rules, alkyl oxochromic groups increase the λ_max of a conjugated system by 5 nm for each group. An oxochromic group is a non-conjugated substituent attached to the conjugated portion of the molecule. For example, if a molecule has four alkyl oxochromic groups, the λ_max will increase by 4 × 5 = 20 nm. This increment is added to the base value and other contributions to estimate the total λ_max.

What is the significance of exocyclic double bonds in the Woodward-Fieser rules?

Exocyclic double bonds are significant in the Woodward-Fieser rules because they contribute an additional 5 nm to the λ_max of a conjugated system. An exocyclic double bond is a double bond where one of the carbons is part of a ring, and the other extends outside the ring. This structural feature is considered when calculating the overall λ_max of the molecule, adding to the base value and other increments from additional double bonds and substituents.