Based on the information presented in the Background section, predict the order

Question

Based on the information presented in the Background section, predict the order in which these chemicals will come through the GC column under optimal conditions for separation. Explain your answer

The compounds we will use in this experiment are methanol, butyl acetate, isopropanol, and 2-butanone

BACKGROUND INFORMATION

Polarity: The way molecules react with other molecules and interact with the world around them is determined by the composition and arrangement of atoms within the molecule. One particular property that has a large influence on molecule-molecule interactions is polarity. Polarity arises out of a difference between the atoms within the molecule based on electronegativity. For example, a molecule such as H2 does not have any polarity and is considered nonpolar because the atoms within the molecule are the same (both are H) and therefore there is no difference. The same would be true for Br2 or I2 or any other diatomic molecule. However, some molecules, such as H2O, are polar, which we will explain in the following paragraphs. Electronegativity is the strength of the attraction of a particular atom to the electrons in a covalent bond. The larger the electronegativity, the stronger the attraction and the more tightly the atom pulls on the electrons in a covalent bond. The most electronegative element on the periodic table is Fluorine with a value of 4.0. As you move across the table towards Fluorine, the electronegativity generally increases. Likewise, electronegativity generally increases as you move up a group (column). The polarity of a bond within a molecule may be determined based on the electronegativity difference between the elements that are bonded together. For example, in the bond between carbon and fluorine shown below, fluorine is the more electronegative atom and therefore the partial negative is on fluorine and the partial positive is on carbon. The delta symbol () indicates a partial charge. Fluorine CHEMISTRY TYPE OF LAB – GAS CHROMATOGRAPHY Creative Commons Attribution 3.0 United States License 3 The polarity of a molecule not only depends on the electronegativity differences (if any) between the atoms in the molecule, but also on the 3-dimensional shape of the molecule. For example, carbon dioxide (shown below on the left) has two polar bonds but the overall molecule is nonpolar because the two bond dipoles cancel each other out. However, the water molecule (shown below on the right), which also has two polar bonds is in fact polar because the bond dipoles do not cancel out due to the tetrahedral electron geometry around the oxygen atom. (Recall that there are two lone pairs of electrons on the oxygen atom). How polar a molecule is will also depend on the strength of the molecular dipole. For example, the net dipole for dichloromethane is smaller than the net dipole in chloroform. Intermolecular forces of attraction: The polarity of a compound influences how strongly the molecules are attracted to each other. Polar compounds are capable of having dipole-dipole interactions which are very strong. Nonpolar compounds can only have London dispersion forces which are very weak. Gas Chromatography: Chromatography separates compounds or particles based on a specific physical property. There are several types of chromatography that are commonly used such as thin layer chromatography, column chromatography, and gas chromatography. The device that is used in this lab activity is called a “gas chromatograph”, or simply a “GC”. In the gas chromatography technique that will be used in this lab, molecules will be separated based on their polarity. In every form of chromatography, there are two phases, a stationary phase and a mobile phase. For the gas chromatography, the stationary phase is a column through which the molecules will pass as they are carried through by the mobile phase which, in this case, is just air. Often, light gases such as helium or hydrogen are used as the mobile phase. The interaction with the column determines how long the molecule will take to get through the column before being detected by the instrument. This causes different molecules to have different retention times (the time it takes for the molecule to get through the column). The stronger the interaction between the molecule and the column, the longer the molecule will stay on the column and therefore come out at a later time. The weaker the interaction, the faster the molecule will come out of the column. Molecules that are similar in polarity to the column will have stronger interactions. In other words, a nonpolar compound will interact more strongly with a nonpolar column and a polar compound will interact more strongly with a polar column. In addition to polarity, the molecule’s volatility (i.e. – boiling point) will impact the retention time. There is a direct relationship between the strength of the intermolecular forces of attraction and the volatility of a substance. The stronger the attraction between the molecules, the less volatile a CHEMISTRY TYPE OF LAB – GAS CHROMATOGRAPHY Creative Commons Attribution 3.0 United States License 4 substance will be. The compound’s molar mass also contributes to the volatility. Molecules that are larger are capable of having stronger intermolecular forces of attraction due to greater magnitude of the temporary dispersion forces and therefore will be less volatile. Substances with lower boiling points (more volatile) will vaporize more quickly than substances with higher boiling points. Therefore, the more volatile molecules might start moving down the column sooner than the less volatile molecules, essentially getting a head-start, depending on what the starting temperature of the column is. For example, if you start the column out at a temperature that is at or above the boiling points of the chemicals, they will all “flash boil” at the same time and start down the column simultaneously. Of course, even if the column never reaches the boiling point of the chemical, it will still evaporate and move down the column (just like water will evaporate at room temperature). It is the combination of volatility and polarity which determines how quickly a molecule will move through the gas chromatography column. The result of all this is that different types of molecules can be separated from each other as they move through the gas chromatography column, if the parameters such as column temperature and gas flow (pressure) are set correctly. In this activity, you will be attempting to completely separate four different chemicals and determine which peaks in the gas chromatogram correspond to which chemicals. As an example, here are the chromatograms produced by three of the four individual chemicals that you will be using in this lab activity, as well as the chromatogram produced by running a mixture of all three chemical compounds through the GC. Chromatogram 1: methanol Chromatogram 2: butyl acetate Chromatogram 3: 2-butanone Chromatogram 4: mixture of all three CHEMISTRY TYPE OF LAB – GAS CHROMATOGRAPHY Creative Commons Attribution 3.0 United States License 5 Note that it is the order in which the compounds come off of the column that we are focusing on in this introductory lab activity. There is a lot of other information that can be gained from detailed analysis of a chromatogram. For example, the area under the peak for each compound indicates the relative amount of that compound in the overall mixture. Also, the shape of the peak can tell us some interesting things about the chemical compound that produced it. However, these more advanced topics will be left for a future lab activity.

Explanation / Answer

according to polarity we can separate that alcohols are more polar than ketones and esters because of presence of OH group.

when we compairng butyl acetate and 2-butanone we can tell that butyl acetate is less polar than 2-butanone.

Hence butyl acetate will come first followed by 2 butanone will come.

though Methanol and isopropanol have same poarity almost methanol have low boiling point than isopropanol.

hence methanol will come as 3rd and isopropanol wil come as 4th

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