Quantcast

Organic chemistry TLC

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

helloboys

Full Member
5+ Year Member
Joined
May 18, 2016
Messages
278
Reaction score
188

Members don't see this ad.
Question: An unknown sample was spotted on a silica gel TLC plate and developed in hexane. The Rf value was calculated to be 0.35. Which of the following statements is true if ethyl acetate was used as the developing solvent instead of hexane?
Answer: The compound would travel further, therefore the Rf value would increase

I don't understand what they mean by "compound". Do they mean the sample would travel further? We don't know if the sample is polar or nonpolar to begin with though. If the sample was nonpolar, wouldn't it travel further if the solvent was hexane because it is nonpolar as well?
 
  • Like
Reactions: 1 user

averageasian

Full Member
5+ Year Member
Joined
Aug 12, 2016
Messages
758
Reaction score
803
So remember that the TLC plate is made of silicon and oxygen (possibility of Hydrogen bonding). when you use a non polar solvent such as hexanes, the compound is more likely to stick to the TLC plate and not move.

Now, when you use a polar solvent such as ethyl acetate, it overpowers the polarity of the TLC plate and is able to carry the compound with the solvent up the plate, resulting in a "spot" higher on your plate and thus a higher Rf value.
 
  • Like
Reactions: 1 users

helloboys

Full Member
5+ Year Member
Joined
May 18, 2016
Messages
278
Reaction score
188
So remember that the TLC plate is made of silicon and oxygen (possibility of Hydrogen bonding). when you use a non polar solvent such as hexanes, the compound is more likely to stick to the TLC plate and not move.

Now, when you use a polar solvent such as ethyl acetate, it overpowers the polarity of the TLC plate and is able to carry the compound with the solvent up the plate, resulting in a "spot" higher on your plate and thus a higher Rf value.

But we are not told if the "compound/sample" is polar or nonpolar. Let's say we had two samples, one polar and the other nonpolar. The solvent is hexane. The nonpolar sample would travel more and have greater Rf value than the polar sample. Now if we have ethyl acetate instead of hexane, the nonpolar sample would travel less and have less of an Rf value.
 
  • Like
Reactions: 1 users

artist2022

DDS 🎓🦷
Administrator
Volunteer Staff
5+ Year Member
Joined
Dec 25, 2016
Messages
3,276
Reaction score
4,494
But we are not told if the "compound/sample" is polar or nonpolar. Let's say we had two samples, one polar and the other nonpolar. The solvent is hexane. The nonpolar sample would travel more and have greater Rf value than the polar sample. Now if we have ethyl acetate instead of hexane, the nonpolar sample would travel less and have less of an Rf value.
I think you're supposed to assume that the same sample is used in both 1) hexane, and 2) ethyl acetate. It didn't travel very far in nonpolar solvent, so in a polar solvent it should/will travel further.
 
  • Like
Reactions: 1 user

averageasian

Full Member
5+ Year Member
Joined
Aug 12, 2016
Messages
758
Reaction score
803
But we are not told if the "compound/sample" is polar or nonpolar. Let's say we had two samples, one polar and the other nonpolar. The solvent is hexane. The nonpolar sample would travel more and have greater Rf value than the polar sample. Now if we have ethyl acetate instead of hexane, the nonpolar sample would travel less and have less of an Rf value.
the compound doesn't matter so much, its the solvent. the solvent determines the Rf value more so than the polarity of the compound itself.
 
  • Like
Reactions: 1 user

DAT Destroyer

Dr. Romano and Nancy
Lifetime Donor
Sponsor
Vendor
15+ Year Member
Joined
Mar 12, 2005
Messages
4,650
Reaction score
3,414
Question: An unknown sample was spotted on a silica gel TLC plate and developed in hexane. The Rf value was calculated to be 0.35. Which of the following statements is true if ethyl acetate was used as the developing solvent instead of hexane?
Answer: The compound would travel further, therefore the Rf value would increase

I don't understand what they mean by "compound". Do they mean the sample would travel further? We don't know if the sample is polar or nonpolar to begin with though. If the sample was nonpolar, wouldn't it travel further if the solvent was hexane because it is nonpolar as well?

Plastic, metal or glass plates are available in general, for TLC. These plates have a coating of silica gel or alumina. This coating is called the “sorbent” and serves as the stationary phase. The sorbent is held tightly to the plate with a binder (plaster, hydrated calcium sulfate). A number of solvents can be used to develop a TLC plate. A mixture of varying ratios of hexanes and ethyl acetate works well in most cases. Other solvents include dichloromethane, ethyl ether, a mixture of methanol with dichloromethane (methanol being 10% or less), etc. The developing solvent used, serves as the mobile phase. In normal phase chromatography (which is more commonly used), the stationary phase is more polar than the mobile phase. There are cases where the polari ties are switched or reversed, which is called reverse phase chromatography, which, for example, is used in separation of DNA or RNA molecules. Organic Chemistry: Techniques and Transformations. Therefore, when developing a plate, the more polar components stick more to the silica gel and the relatively less polar components move further along with the developing solvent. This results in a distribution of spots on the TLC plate, which enables one to determine the order of polarities and hence the identities of the samples spotted. A helpful way of determining the polarities is the use of Rf values. The Rf value is the ratio of the distance moved by the solute to the distance moved by the solvent. The optimal value for Rf is 0.2 - 0.4. The choice of a TLC developing solvent is driven by the goal to achieve optimal R f value, in addition to good separation between spots. In order to help visualize the spots applied to a TLC plate, usually, fluorescent indicators are added to the plate (there are plates without these indicators, but we will use the plates with UV indicators). This indicator absorbs and emits UV light, which appears green. The presence of a UV active compound on the plate prevents the UV light from reaching those parts of the plate. This results in a darker coloration on the plate.


Hope this helps.

Dr. Romano
 

Auntymarkovnikov

Full Member
Joined
Jul 7, 2017
Messages
433
Reaction score
684
if you have a nonpolar solvent like hexane as well as a non polar compound on the tlc plate, youre going to have a small Rf value (like the one given). but if you switch the non polar hexane with a polar ethyl acetate, you're Rf will shoot up since nonpolar compunds elute further than polar compounds when placed in a polar solvent (on a tlc plate of course).
 
  • Like
Reactions: 1 user

Chubz0924

Full Member
2+ Year Member
Joined
Jun 13, 2017
Messages
87
Reaction score
59
For this question, I think it is safe to assume that the compound is polar since it has a low Rf value in the original solvent of hexane (non-polar). When the solvent is non-polar, non-polar molecules tend to travel further and polar molecules. Since the solvent was changed to ethyl acetate (which is polar), it is assumed that the compound will travel the furthest, thus increasing the Rf value.

Besides that, an important thing to remember is that the further it travels, the higher the Rf value! This will eliminate most of the answer choices.
 

Auntymarkovnikov

Full Member
Joined
Jul 7, 2017
Messages
433
Reaction score
684
For this question, I think it is safe to assume that the compound is polar since it has a low Rf value in the original solvent of hexane (non-polar). When the solvent is non-polar, non-polar molecules tend to travel further and polar molecules. Since the solvent was changed to ethyl acetate (which is polar), it is assumed that the compound will travel the furthest, thus increasing the Rf value.

Besides that, an important thing to remember is that the further it travels, the higher the Rf value! This will eliminate most of the answer choices.
non-polar molecules travel farther in polar solvents
 

Chubz0924

Full Member
2+ Year Member
Joined
Jun 13, 2017
Messages
87
Reaction score
59
non-polar molecules travel farther in polar solvents

The samples are spotted onto a silica gel paper=(polar)

A solvent is added in tank (nonpolar)

As solvent moves up via capillary action, it moves up, bringing along with it other non polar compounds, explaining why non-polar compounds move further when in non-polar solvents.

Polar compounds adhere to silica gel paper more, so they do not move upwards as much

High Rf=more nonpolar (in this case) and Low Rf=polar molecules.

Since, in this example, you are switching the solvent and it is polar, the polar substances will travel further because poler molecules tend to stick together, thus raising the Rf value in comparison to being in a non polar substance.



 
Top